achiral the opposite of chiral; also called nonchiral. An achiral molecule can be superimposed on its mirror image.
acid see Arrhenius theory, Brønsted-Lowry theory of acids and bases, and Lewis theory of acids and bases.
acid-base reaction a neutralization reaction in which the products are a salt and water.
activated complex molecules at an unstable intermediate stage in a reaction.
activation energy the energy that must be supplied to chemicals to initiate a reaction; the difference in potential energy between the ground state and the transition state of molecules. Molecules of reactants must have this amount of energy to proceed to the product state.
addition a reaction that produces a new compound by combining all of the elements of the original reactants.
adduct the product of an addition reaction.
alcohol an organic chemical that contains an — OH group.
aldehyde an organic chemical that contains a — CHO group. Water addition to terminal alkynes forms aldehydes.
alicyclic compound an aliphatic cyclic hydrocarbon, which means that an alicyclic compound contains a ring but not an aromatic benzene ring (a six-carbon ring with three double bonds).
aliphatic compound a straight- or branched-chain hydrocarbon; an alkane, alkene, or alkyne.
alkane a hydrocarbon that contains only single covalent bonds. The alkane general formula is CnH2n+2.
alkene a hydrocarbon that contains a carbon-carbon double bond. The alkene general formula is CnH2n.
alkoxide ion an anion formed by removing a proton from an alcohol; the RO-ion.
alkoxy free radical a free radical formed by the homolytic cleavage of an alcohol — OH bond; the RO· free radical.
alkyl group an alkane molecule from which a hydrogen atom has been removed. Alkyl groups are abbreviated as "R" in structural formulas.
alkyl halide a hydrocarbon that contains a halogen substituent, such as fluorine, chlorine, bromine, or iodine.
alkyl-substituted cycloalkane a cyclic hydrocarbon to which one or more alkyl groups are bonded. (Compare with "cycloalkyl alkane.")
alkyne a hydrocarbon that contains a triple bond. The alkyne general formula is CnH2n-2.
allyl group the H2C = CHCH2 — group.
allylic carbocation the H2C = CHCH2+ ion.
analogue in organic chemistry, chemicals that are similar to each other, but not identical. For example, the hydrocarbons are all similar to each other, but an alkane is different from the alkenes and alkynes because of the types of bonds they contain. Therefore, an alkane and an alkene are analogues.
angle of rotation (α) in a polarimeter, the angle right or left in which plane-polarized light is turned after passing through an optically active compound in solution.
angle strain the strain created by the deformation of bond angles from their normal values.
angular momentum quantum number (l) the second number in Schrödinger's electron wave equation, which tells the shape of the orbital.
anion a negatively charged ion.
anti addition the addition of atoms to opposite sides of a molecule. (Compare with "syn addition.")
antibonding molecular orbital a molecular orbital that contains more energy than the atomic orbitals from which it was formed; in other words, an electron is less stable in an antibonding orbital than it is in its original atomic orbital.
anti-Markovnikov addition a reaction in which the hydrogen atom of a hydrogen bromide bonds to the carbon of a double bond that is bonded to fewer hydrogen atoms. The addition takes place via a free-radical intermediate rather than a carbocation. (Compare with "Markovnikov rule.")
Arrhenius theory a theory (limited to aqueous systems) that defines an acid as a compound that liberates hydrogen ions and a base as a compound that liberates hydroxide ions. A neutralization is the reaction of a hydrogen ion with a hydroxide ion to form water.
asymmetric center see stereogenic center.
atom the smallest amount of an element; a nucleus surrounded by electrons.
atomic mass (A) the sum of the weights of the protons and neutrons in an atom. (A proton and neutron each have a mass of 1 atomic mass unit.)
atomic number (Z) the number of protons or electrons in an atom.
atomic 1s orbital the spherical orbital nearest the nucleus of an atom.
atomic orbital a region in space around the nucleus of an atom where the probability of finding an electron is high.
atomic p orbital an hourglass-shaped orbital, oriented on x, y, and z axes in three-dimensional space.
atomic s orbital a spherical orbital.
aufbau buildup the order in which electrons fill atomic orbitals according to energy factors.
axial bond a bond positioned perpendicularly to the general plane of a cyclohexane ring.
axial position the position a group occupies in an axial bond. (See axial bond.)
Baeyer reagent cold, dilute potassium permanganate, which is used to oxidize alkenes and alkynes.
base see Arrhenius theory, Brønsted-Lowry theory of acids and bases, and Lewis theory of acids and bases.
β elimination a group of reactions that form double or triple bonds through the loss of atoms or groups from adjacent carbon atoms; included are dehydrations, dehydrogenations, dehydrohalogenations, and double dehydrohalogenations.
bicyclic ring system a molecule made up of two carbon rings that share common sides.
boat conformation one of the conformations of the cyclohexanes, which resembles a boat. The boat conformation has high energy because of interactions between eclipsed hydrogen atoms or groups.
bond angle the angle formed between two adjacent bonds on the same atom.
bond-dissociation energy the amount of energy needed to homolytically fracture a bond.
bonding electron see valence electron.
bonding molecular orbital the orbital formed by the overlap of adjacent atomic orbitals.
bond length the equilibrium distance between the nuclei of two atoms or groups that are bonded to each other.
bond strength the amount of energy needed to homolytically fracture a bond (also called bond-dissociation energy).
branched-chain alkane an alkane with alkyl groups bonded to the central carbon chain.
Brønsted-Lowry theory of acids and bases a Brønsted-Lowry acid is a compound capable of donating a proton (a hydrogen ion), and a Brønsted-Lowry base is capable of accepting a hydrogen ion. In a neutralization, an acid donates a proton to a base, creating a conjugate acid and a conjugate base.
Cahn-Ingold-Prelog notational system a system that labels the three-dimensional arrangement of atoms around a stereogenic center of a molecule by ranking their atomic weights. The letter R indicates a clockwise decline in rank, and S indicates a counterclockwise decline in rank.
carbanion a carbon atom bearing a negative charge; a carbon anion.
carbene an electrically uncharged molecule that contains a carbon atom with only two single bonds and just six electrons in its valence shell.
carbenoid a chemical that resembles a carbene in its chemical reactions.
carbocation a carbon cation; a carbon atom bearing a positive charge (sometimes referred to as a "carbonium ion").
carbonyl group the group.
carboxylic acid the group.
catalyst a substance that affects the rate of a reaction in which it participates; however, it is not altered or used up in the process. Platinum metal is a catalyst in alkyne hydrogenation, for example.
cation a positively charged ion.
cationic polymerization see polymerization; cationic polymerization occurs via a cation intermediate and is less efficient than free-radical polymerization.
chain reaction a reaction that, once started, produces sufficient energy to keep the reaction running. These reactions proceed by a series of steps, which produce intermediates, energy, and products.
chair conformation a conformation of cyclohexane that resembles a chair and has less energy than a boat conformation.
chiral describes a molecule that is not superimposable on its mirror image; like the relationship of a left hand to a right hand.
chiral center see stereogenic center.
chiral molecule a molecule that has a chiral center and rotates plane-polarized light.
cis isomer a stereoisomer in which substituents are located on the same side of a double bond. (Compare with "trans isomer.")
competing reactions two reactions that start with the same reactants but form different products.
concerted taking place at the same time without the formation of an intermediate.
condensation reaction a reaction in which two molecules join to form a product and release a small molecule such as water.
condensed formula a formula in which the single bonds between the atoms are not shown with lines.
configuration the specific arrangement of atoms and groups in three-dimensional space. A configuration is characteristic of a specific stereoisomer.
conformation a specific three-dimensional shape of a molecule at any given time. Conformation can change by rotation around a single bond.
conjugate acid the acid that results when a Brønsted-Lowry base accepts a hydrogen ion.
conjugate base the base that results when a Brønsted-Lowry acid loses a hydrogen ion.
conjugated double bonds carbon-carbon double bonds that are separated from one another by just one single bond; for example,
conjugation the overlapping in all directions of a series of p orbitals. This process usually occurs in a molecule with alternating double and single bonds.
conjugation energy see resonance energy.
constitutional isomer see structural isomer.
covalent bond a bond formed by the sharing of electrons between atoms.
cyano group the group.
cyclization the formation of ring structures.
cycloaddition a reaction that forms a ring.
cycloalkane a ring hydrocarbon made up of carbon and hydrogen atoms joined by single bonds.
cycloalkyl alkane an alkane to which a cyclical structure is bonded.
cyclohydrocarbon an alkane, alkene, or alkyne formed in a ring structure rather than a straight or branched chain.
debye unit (D) the unit of measure for a dipole moment. One debye equals 1.0 × 10-18 esu · cm. (See dipole moment.)
dehalogenation the elimination reaction in which two halogen atoms are removed from adjacent carbon atoms to form a double bond.
dehydration the elimination reaction in which a molecule of water is removed from a molecule.
dehydrohalogenation the elimination reaction in which a hydrogen atom and a halogen atom are removed from a molecule to form a double bond.
delocalization the spreading of electron density or electrostatic charge across a molecule.
delocalization energy see resonance energy.
deprotonation the loss of a proton (hydrogen ion) from a molecule.
dextrorotatory describes the clockwise rotation of plane-polarized light (from Latin dextro, "to the right"). A lowercase "d" or a "+" is the notation used before an isomer's name to indicate that it is dextrorotatory; for example, d-2-butanol. (Compare with "levorotatory.")
diastereomer a stereoisomer of an optical isomer that has more than one stereogenic center and is not a mirror image of one of the other enantiomers of the molecule. Diastereomers have the same configuration at one or more stereogenic centers but opposite configurations at others.
Dieckmann condensation a condensation is a reaction in which two molecules join to form a new product, eliminating water or some other small molecule in the process. A Dieckmann condensation takes place within one molecule, resulting in a new cyclical molecule and the elimination of a small molecule.
Diels-Alder reaction a cycloaddition reaction between a conjugated diene and an alkene that produces a 1,4-addition product.
diene an organic compound that contains two double bonds.
dienophile the alkene that adds to the diene in a Diels-Alder reaction.
dihalide a compound that contains two halogen atoms; also called a "dihaloalkane."
diol a compound that contains two hydroxyl (— OH) groups; also called a "dihydroxy alkane."
dipole moment a measure of the polarity of a molecule; it is the mathematical product of the charge in electrostatic units (esu) and the distance that separates the two charges in centimeters (cm). For example, substituted alkynes have dipole moments caused by differences in electronegativity between the triple-bonded and single-bonded carbon atoms.
distillation the separation of components of a liquid mixture based on differences in boiling points.
double bond a multiple bond composed of one σ bond and one π bond. Rotation is not possible around a double bond. Hydrocarbons containing one double bond are called alkenes, and hydrocarbons with two double bonds are called dienes.
duet two electrons. Helium, the simplest noble gas, has a duet of electrons. The gaining of an electron by a hydrogen atom adds stability because it achieves the helium duet. (Compare with "octet.")
E (entgegen) the notation for the stereochemical arrangement in which the higher-ranked substituent groups are on opposite sides of the double bond. (See E-Z notation.)
eclipsed conformation one of the possible orientations of atoms or groups around two carbon atoms joined by a single bond. Atoms and groups bonded to an eclipsed conformer are positioned in line with one another, creating repulsive forces that give the molecule a high energy state. (Compare with "staggered conformation.")
electromagnetic wave the type of wave found in visible, infrared, and ultraviolet light as well as radio signals and X-rays.
electron negatively charged particles of little weight that exist in quantized probability areas around the atomic nucleus.
electron affinity the amount of energy liberated when an electron is added to an atom in the gaseous state.
electron dot structure a system in which the entire structure of the atom, except its valence electrons, is represented by the symbol for the element. The valence electrons are represented by dots (also known as a Lewis structure).
electronegativity the measure of an atom's ability to attract electrons toward itself in a covalent bond. The halogen fluorine is the most electronegative element.
electronegativity scale an arbitrary scale by which the electronegativity of individual atoms can be compared.
electrophile an "electron seeker;" an atom that seeks an electron to stabilize itself.
electrophilic addition a reaction in which the addition of an electrophile to an unsaturated molecule results in the formation of a saturated molecule.
electrostatic attraction the attraction of a positive ion for a negative ion.
element of unsaturation a π bond; a multiple bond or ring in a molecule.
enantiomer a stereoisomer that cannot be superimposed on its mirror image.
enantiomorphic pair in optically active molecules with more than one stereogenic center, the two structures that are mirror images of each other are enantiomorphic pairs.
energy of reaction the difference between the total energy content of the reactants and the total energy content of the products. The greater the energy of reaction, the more stable the products.
enol an unstable compound (for example, vinyl alcohol) in which a hydroxide group is attached to a carbon bearing a carbon-carbon double bond. These compounds tautomerize to form ketones, which are more stable.
enthalpy of activation see activation energy.
enzyme a stereospecific chiral protein that acts as a biological catalyst.
epoxide a three-membered ring that contains oxygen.
equatorial bond a bond attached to a ring structure that roughly parallels the equator of the ring.
equatorial position the position a group occupies in an equatorial bond.
equilibrium constant a measure of the degree of completion of an equilibrium reaction.
equivalent orbitals orbitals of the same principal level and type, such as the three p orbitals.
ester the functional group.
ether an organic compound in which an oxygen atom is bonded to carbon atoms. The general formula is R — O — R′. Epoxyethane, an epoxide, is a cyclic ether.
excited state a higher energy state than the ground state, achieved by adding energy to an atom or molecule in its ground state.
exothermic describes the giving off of energy as heat.
E-Z notation a notation, somewhat like the cis and trans system, that is used for alkenes with more than two substituents. The atoms or groups on either side of the double bond are ranked by atomic weight. If the heavier atoms are on the same side of the molecule, it is labeled Z, and if the heavier atoms are on opposite sides of the molecule, it is labeled E.
Fischer projection a projection that uses perpendicular lines to depict the absolute configuration of chiral molecules on a planar surface.
formal charge a charge on an atom created by the loss or gain of electrons.
four-center interaction a reaction in which bonds are simultaneously formed and broken between four atoms. For example, the reaction A — A + B — B could form two A — B molecules by simultaneously forming the new bonds while breaking the old bonds.
free radical an atom or group that has a single unshared electron.
free-radical chain reaction a reaction that proceeds by a free-radical intermediate in a chain mechanism, which is a series of self-propagating, interconnected steps. (Compare with "free-radical reaction.")
free-radical polymerization a polymerization initiated by a free radical.
free-radical reaction a reaction in which a covalent bond is formed by the union of two radicals. (Compare with "free-radical chain reaction.")
functional group a set of bonded atoms that displays a specific molecular structure and chemical reactivity when bonded to a carbon atom in the place of a hydrogen atom.
geminal a term that describes the location of two identical atoms or groups as being bonded to the same carbon atom; a geminal dihalide, for example. (Compare with "vicinal.")
glycol a class of alcohols that contain two — OH groups; CnH2n(OH)2.
Grignard reagent an organometallic reagent in which magnesium metal inserts between an alkyl group and a halogen; for example, CH3MgBr.
ground state the most stable electron configuration for an atom; this configuration has the least energy associated with it.
haloalkane an alkane that contains one or more halogen atoms; also called an alkyl halide.
halogen an electronegative, nonmetallic element in Group VII of the periodic table, including fluorine, chlorine, bromine, and iodine. Halogens are often represented in structural formulas with an "X."
halogenation a reaction in which halogen atoms are bonded to an alkene at the double bond.
halonium ion a halogen atom that bears a positive charge. This ion is highly unstable.
heat of combustion the energy released when an alkane is completely oxidized.
heat of hydrogenation the energy released when two hydrogen atoms bond to the carbon atoms of a former double bond.
hetero atom in organic chemistry, an atom other than carbon.
heterocyclic compound a class of cyclic compounds in which one of the ring atoms is not carbon; epoxyethane, for example.
heterogenic bond formation a type of bond formed by the overlap of orbitals on adjacent atoms. One orbital of the pair donates both of the electrons to the bond.
heterolytic cleavage the fracture of a bond in such a manner that one of the atoms receives both electrons. In reactions, this asymmetrical bond rupture generates carbocation and carbanion mechanisms.
homogenic bond formation a type of bond formation in which each atom donates one electron to the bond.
homologous series a set of compounds with common compositions; for example, the alkanes.
homologue one of a series of compounds in which each member differs from the next by a constant unit.
homolytic cleavage the fracture of a bond in such a manner that both of the atoms receive one of the bond's electrons. This symmetrical bond rupture forms free radicals; in reactions, it generates free-radical mechanisms.
hybrid atomic orbital a probability area created by a linear combination of atomic orbitals.
hybridization the changing, or mixing, of orbitals to form new atomic or molecular orbitals that are lower in energy.
hybrid orbital an orbital formed by the linear combination of atomic orbitals in the ground state.
hybrid orbital number rule the hybrid orbital number is equal to the sum of a molecule's σ bonds plus the number of unshared electron pairs. A hybrid orbital number of 2 indicates sp hybridization; 3 indicates sp2 hybridization; 4 indicates sp3 hybridization.
hydration the addition of the elements of water to a molecule.
hydride shift the movement of a hydride ion, a hydrogen atom with a negative charge, to form a more inductively stabilized carbocation.
hydroboration-oxidation the addition of borane (BH3) or an alkyl borane to an alkene and its subsequent oxidation to produce the anti-Markovnikov indirect addition of water.
hydrocarbon a molecule that contains exclusively carbon and hydrogen atoms. The central bond may be a single, double, or triple covalent bond, and it forms the backbone of the molecule.
hydrogenation the addition of hydrogen to an unsaturated compound.
hydrohalogenation a reaction in which a hydrogen atom and a halogen atom are added to a double bond to form a saturated compound.
hydrolyze to cleave a bond via the elements of water.
inductive effect the electron-donating or electron-withdrawing effect that is transmitted through σ bonds. It can also be defined as the ability of an alkyl group to "push" electrons away from itself. The inductive effect gives stability to carbocations and makes tertiary carbocations the most stable.
initiation step the first step in the mechanism of a reaction.
initiator a material capable of being easily fragmented into free radicals, which in turn initiate a free-radical reaction.
insertion placing between two atoms.
in situ from Latin, meaning "in place."
intermediate a species that forms in one step of a multistep mechanism; intermediates are unstable and cannot be isolated.
ion a charged atom; an atom that has either lost or gained electrons.
ionic bond a bond formed by the transfer of electrons between atoms, resulting in the formation of ions of opposite charge. The electrostatic attraction between these ions is the ionic bond.
ionization energy the energy needed to remove an electron from an atom.
isoelectronic having the same number of electrons. For example, a sodium atom that is lacking one electron is isoelectronic with neon, an inert gas.
isolated double bond a double bond that is more than one single bond away from another double bond in a diene.
isomer compounds that have the same molecular formula but different structural formulas.
IUPAC nomenclature a systematic method for naming molecules based on a series of rules developed by the International Union of Pure and Applied Chemistry. IUPAC nomenclature is not the only system in use, but it is the most common.
keto-enol tautomerization the process by which an enol equilibrates with its corresponding aldehyde or ketone.
ketone a compound in which an oxygen atom is bonded to a carbon atom, which is itself bonded to two more carbon atoms. Water addition to nonterminal alkynes forms ketones.
kinetically controlled reaction a reaction in which the rate of formation of the competing products accounts for the major product.
kinetic control reactions that have a major product that forms the fastest are under kinetic control. These reactions follow the lowest activation-energy pathway.
kinetics the study of reaction rates.
levorotatory describes the counterclockwise rotation of plane-polarized light (from Latin, levo, "to the left"). A lowercase "l" or "-" is the notation used before an isomer's name to indicate it is levorotatory; for example, l-2-butanol. (Compare with "dextrorotatory.")
Lewis structure see electron dot structure.
Lewis theory of acids and bases a Lewis acid is a compound capable of accepting an electron pair, and a Lewis base is capable of donating an electron pair.
Lindlar catalyst a particular poisoned catalyst used in alkyne reactions; it is finely divided palladium coated with quinoline and absorbed on barium sulfate.
linear the shape of a molecule with sp hybrid orbitals; an alkyne.
linear combination of atomic orbitals the process of combining atomic orbitals to form new orbitals. Linear combination can occur between orbitals in a single atom, creating hybrid atomic orbitals, or between the orbitals of two atoms, creating molecular orbitals. In either case, the number of orbitals always remains constant.
line-bond structure a representation of a molecule that shows covalent bonds as lines between atoms.
lone-pair electrons a nonbonding pair of electrons, which occupy the valence orbitals.
magnetic quantum number (m) the third number in Schrödinger's electron wave equation, which tells the orientation of the orbital in space.
major product the product that forms in the greatest amount in a reaction.
Markovnikov rule states that the positive part of a reagent (a hydrogen atom, for example) adds to the carbon of the double bond that already has more hydrogen atoms attached to it. The negative part adds to the other carbon of the double bond. Such an arrangement leads to the formation of the more stable carbocation over other less-stable intermediates.
mass number the total number of protons and neutrons in an atom.
mechanism the series of steps that reactants go through during their conversion into products.
meso compound a compound that has a stereogenic center but is optically inactive because it also has a plane of symmetry.
methylene group a — CH2 group.
minor product the product that forms in the least amount in a reaction.
molecular formula a chemical formula that shows the number and kinds of atoms in a molecule but not their arrangement; for example, C2H6.
molecular orbital an orbital formed by the linear combination of two atomic orbitals.
molecule a covalently bonded collection of atoms that has no electrostatic charge.
monomer the smallest molecule that reacts with itself to form a polymer.
multiple bond a double or triple bond; multiple bonds involve the atomic p orbitals in side-to-side overlap, preventing rotation.
neutralization the reaction of an acid and a base. The products are a salt and water.
neutron an uncharged particle in the atomic nucleus that has the same weight as a proton. Additional neutrons do not change an element but convert it to one of its isotopic forms.
Newman projection a drawing of a molecule that shows a head-on view of a carbon-carbon bond. The front carbon is represented by a dot, and the rear carbon is represented by a circle. Substituents are shown as spokes radiating from the dot or circle. This projection is used to show the possible interactions of substituents bonded to adjacent carbon atoms.
node a region of zero electron density in an orbital; a point of zero amplitude in a wave.
nonbonding electrons valence electrons that are not used for covalent bond formation.
nonchiral see achiral.
nonterminal alkyne an alkyne in which the triple bond is located somewhere other than the 1 position.
nucleophile a species that is capable of donating a pair of electrons to a nucleus.
nucleus the central core of an atom; the location of the protons and neutrons.
octet eight electrons. Carbon, oxygen, and the halogens either share, lose, or gain electrons to have eight electrons in their valence shells. (Compare with "duet.")
optical activity the ability of some chemicals to rotate plane-polarized light.
optical isomer another name for an enantiomer.
optical purity a number equal to the angle of rotation of a solution divided by the rotation of the pure enantiomer (x 100).
orbit an area around an atomic nucleus where there is a high probability of finding an electron; also called a shell. An orbit is divided into orbitals, or subshells.
orbital an area in an orbit where there is a high probability of finding an electron; a subshell. All of the orbitals in an orbit have the same principal and angular quantum numbers.
organometallic reaction a reaction in which a metallic element adds between a carbon atom and an electronegative atom in an organic molecule.
outer-shell electron see valence electron.
overlap region the region in space where atomic or molecular orbitals overlap, creating an area of high electron density.
oxidation the loss of electrons by an atom in a covalent bond. In organic reactions, this occurs when a compound accepts additional oxygen atoms.
oxirane a three-membered ring that contains oxygen; also called an epoxide ring.
oxonium ion a positively charged oxygen atom.
ozonide a compound formed by the addition of ozone to a double bond.
ozonolysis the cleavage of double and triple bonds by ozone, O3.
paired spin the spinning in opposite directions of the two electrons in a bonding orbital.
parent name the root name of a molecule according to the IUPAC nomenclature rules; for example, hexane is the parent name in trans-1,2-dibromocyclohexane.
Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers.
pericyclic process a step in a reaction in which the bonding electrons are redistributed through a cyclic structure.
peroxide a compound that contains an oxygen-oxygen single covalent bond.
phase sign the positive and negative symbols assigned to the upward and downward displacement, respectively, of the standing wave that describes the orbitals about an atom's nucleus. Each upward and downward displacement is called a phase.
π (pi) bond a bond formed by the side-to-side overlap of atomic p orbitals. A π bond is weaker than a σ bond because of poor orbital overlap caused by nuclear repulsion. Unsaturated molecules are created by π bonds.
π complex an intermediate formed when a cation is attracted to the high electron density of a π bond.
π molecular orbital a molecular orbital created by the side-to-side overlap of atomic p orbitals.
plane of symmetry an imaginary plane that bisects a molecule, producing two halves that are mirror images of each other.
plane-polarized light ordinary light that has had all oscillations of the electromagnetic field filtered out but one. The remaining oscillation exists in only one plane.
poisoned catalyst a deactivated catalyst; one that is less effective in reactions than the nonpoisoned material.
polar covalent bond a bond in which the shared electrons are not equally available in the overlap region, leading to the formation of partially positive and partially negative ends on the molecule.
polarimeter a device that first polarizes light and then passes the polarized light through a chemical solution. An analyzer shows the degree and direction of rotation of the plane-polarized light if the chemical is optically active.
polarity the asymmetrical distribution of electrons in a molecule, leading to positive and negative ends on the molecule.
polarizer a filter that blocks light waves in all planes except one; a polarizer creates plane-polarized light.
polymer a very large molecule composed of repeating smaller units.
polymerization the process by which an organic compound reacts with itself to form a high-molecular-weight compound from repeating units of the original compound. Polymerization occurs by either cationic or free-radical mechanisms.
potential energy the energy a substance has due to its position or composition.
precursor the substance from which another compound is formed.
preparation a reaction in which a desired chemical is produced; for example, the dehydration of an alcohol is a preparation for an alkene.
primary carbocation a carbocation to which one alkyl group is bonded.
primary (1°) carbon a carbon atom that is attached to one other carbon atom.
principal quantum number (n) the first number in Schrödinger's electron wave equation, which tells the location of an orbital relative to the atom's nucleus.
priority rules the rules in the Cahn-Ingold-Prelog notational system that allow atoms or groups around a chiral carbon atom to be ranked by atomic weight.
product the substance that forms when reactants combine in a reaction.
projection a drawing of a molecule.
propagation step the step in a free-radical reaction in which both a product and energy are produced. The energy keeps the reaction going.
proton a positively charged particle in the nucleus of an atom.
protonation the addition of a proton (a hydrogen ion) to a molecule.
pure covalent bond a bond in which the shared electrons are equally available to both bonded atoms.
pyrolysis the application of high temperatures to a compound.
quantum mechanics the study of the mathematical formulas that describe the electronic structure of atoms.
quaternary carbon a carbon atom that is directly attached to four other carbon atoms.
racemate another name for "racemic mixture"; a 1:1 mixture of enantiomers.
racemic mixture a 1:1 mixture of enantiomers.
rate constant the proportionality constant of a reaction that reflects the concentration of reactants.
rate-determining step the step in a reaction's mechanism that requires the highest activation energy and is therefore the slowest.
rate equation the mathematical formula that relates the rate of a reaction to the concentration of the reactants.
rate of reaction the speed with which a reaction proceeds.
reactant a starting material.
reaction the process of converting reactants into products.
reaction energy the difference between the energy of the reactants and that of the products.
reagent the chemicals that ordinarily produce reaction products.
rearrangement reaction a reaction that causes the skeletal structure of the reactant to undergo change in converting to the product.
recrystallization a process based on solubility in which a substance is dissolved in a minimum amount of hot solvent, which is then cooled to allow new, purer crystals to form.
reduction the gaining of electrons by an atom or molecule. In organic compounds, a reduction is an increase in the number of hydrogen atoms in a molecule.
resolution to resolve; the process of separating enantiomers from a racemic mixture.
resonance the process by which a substituent either removes electrons from or gives electrons to a π bond in a molecule; a delocalization of electrical charge in a molecule.
resonance energy the difference in energy between the calculated energy content of a resonance structure and the actual energy content of the hybrid structure.
resonance hybrid the actual structure of a molecule that shows resonance. A resonance hybrid possesses the characteristics of all possible drawn structures (and consequently cannot be drawn). It is lower in energy than any structure that can be drawn for the molecule and thus more stable than any of them.
resonance structures various intermediate structures of one molecule that differ only in the location of the electrons.
R group see alkyl group.
ring-opening reaction a reaction that causes a cyclic structure to form a straight chain.
ring structure a molecule in which the end atoms have bonded, forming a ring rather than a straight chain.
rotation the ability of carbon atoms attached by single bonds to freely turn, which gives the molecule an infinite number of conformations.
R,S notational system see Cahn-Ingold-Prelog notational system.
saturated compound a compound containing all single bonds.
saturation the condition of a molecule containing the most atoms possible; a molecule made up of single bonds.
sawhorse projection a line drawing that is centered on two of the carbon atoms in a molecule and that shows, through perspective, the three-dimensional structure about them. Carbon atoms are represented by the intersection of bond lines. The arrangement resembles a carpenter's sawhorse.
secondary carbocation a carbocation to which two alkyl groups are bonded.
secondary (2°) carbon a carbon atom that is directly attached to two other carbon atoms.
separation technique a process by which products are isolated from each other and from impurities.
sequence rules the rules for establishing the order of atoms or groups in the Cahn-Ingold-Prelog notational system.
σ (sigma) antibonding molecular orbital a σ molecular orbital in which one or more of the electrons are less stable than when localized in the isolated atomic orbitals from which the molecular orbital was formed.
σ bond a bond formed by the linear combination of orbitals in such a way that the maximum electron density is along a line joining the two nuclei of the atoms.
σ (sigma) bonding molecular orbital a σ molecular orbital in which the electrons are more stable than when they are localized in the isolated atomic orbitals from which the molecular orbital was formed.
Simmons-Smith reaction the formation of a cyclopropane molecule via reaction of an alkene with iodomethylzinc iodide (ICH2ZnI), the Simmons-Smith reagent.
skeletal structure the carbon backbone of a molecule.
sp hybrid orbital a molecular orbital created by the combination of wave functions of an s and a p orbital.
sp2 hybrid orbital a molecular orbital created by the combination of wave functions of an s and two p orbitals.
sp3 hybrid orbital a molecular orbital created by the combination of wave functions of an s and three p orbitals.
spin quantum number (ms) the fourth number in Schrödinger's electron wave equation, which tells the direction of spin on an electron.
staggered conformation one of the possible orientations of atoms or groups around two carbon atoms joined by a single bond. Atoms and groups bonded to the two carbons of a staggered conformer are positioned so there is maximum separation between them and therefore minimum interaction. (Compare with "eclipsed conformation.")
stereochemistry the study of the three-dimensional structure of molecules and how it affects their interactions.
stereogenic center a central atom that has four different atoms or groups bonded to it; also called chiral center or asymmetric center.
stereoisomers compounds with the same molecular formula but different arrangement of their atoms in three-dimensional space.
stereospecific the requirement of a specific stereochemical shape for a reaction to occur.
steric hindrance the blocking of one side of a molecule by a substituent so that any further bonding must occur on the opposite side. Steric hindrance causes the formation of trans stereoisomers.
straight-chain alkane a saturated hydrocarbon that has no carbon-containing side chains.
structural formula a chemical formula that shows not only the number and kind of atoms in a molecule but also their arrangement.
structural isomer also known as a constitutional isomer, structural isomers have the same molecular formula but different bonding arrangements among their atoms. For example, C4H10 can be butane or 2-methylpropane, and C4H8 can be 1-butene or 2-butene.
subatomic particles a component of an atom; either a proton, neutron, or electron.
substituent group any atom or group that replaces a hydrogen atom on a hydrocarbon.
substitution the replacement of an atom or group bonded to a carbon atom with a second atom or group.
symmetry plane see plane of symmetry.
syn addition the addition of atoms to the same side of a molecule. (Compare with "anti addition.")
tautomers structural isomers that easily interconvert.
terminal alkyne an alkyne whose triple bond is located between the first and second carbon atoms of the chain.
terminal carbon the carbon atom on the end of a carbon chain.
termination step the step in a reaction mechanism that ends the reaction, often a reaction between two free radicals.
tertiary carbocation a carbocation to which three alkyl groups are bonded.
tertiary (3°) carbon a carbon atom that is directly attached to three other carbon atoms.
tetrahaloalkane an alkane that contains four halogen atoms on the carbon chain. The halogen atoms can be located on vicinal or nonvicinal carbon atoms.
tetrahedral angle an angle of 109°28″, or approximately 110°. All of the bond angles in methane, CH4, are tetrahedral angles.
thermodynamically controlled reaction a reaction in which conditions permit two or more products to form. The products are in an equilibrium condition, allowing the more stable product to predominate.
torsional strain strain caused by repulsion between groups in an eclipsed conformation.
trans isomer a stereoisomer in which substituents are located on opposite sides of a double bond. (Compare with "cis isomer.")
transition state the point in a reaction at which the system has the most energy.
trigonal planar the shape of a molecule with an sp2 hybrid orbital. In this arrangement, the σ bonds are located in a single plane separated by 60° angles.
triple bond a multiple bond composed of one σ bond and two π bonds. Rotation is not possible around a triple bond. Hydrocarbons that contain triple bonds are called alkynes.
unsaturated compound a compound that contains one or more multiple bonds; for example, alkenes and alkynes.
unsaturation refers to a molecule containing less than the maximum number of single bonds possible because of the presence of multiple bonds.
valence electrons the outermost electrons of an atom. The valence electrons of the carbon atom occupy the 2s, 2px, and 2py orbitals, for example.
valence shell the outermost electron orbit.
van der Waals forces the forces of attraction or repulsion between two unbonded groups or atoms due to unbalanced electrical forces.
van't Hoff rule predicts the maximum number of enantiomers that an optically active molecule can have; 2 raised to the nth power, where n equals the number of stereogenic centers.
vicinal a term that describes the location of two identical atoms or groups as being bonded to adjacent carbon atoms; a vicinal dihalide, for example. (Compare with "geminal.")
vinyl group the
wedge-and-dash projection a drawing of a molecule in which three types of lines are used to represent the three-dimensional structure of a molecule: 1) solid lines are bonds in the plane of the paper, 2) dashed lines are bonds extending away from the viewer, and 3) wedge-shaped lines are bonds oriented toward the viewer.
Wurtz reaction the coupling of two alkyl halide molecules to form an alkane.
X group "X" is often used as the abbreviation for a halogen substituent in the structural formula of an organic molecule.
Z (zusammen) the notation for the stereochemical arrangement in which the higher-ranked substituent groups are on the same side of the double bond. (See E-Z notation.)
Zaitsev rule states that the major product in the formation of alkenes by elimination reactions will be the more highly substituted alkene, or the alkene with more substituents on the carbon atoms of the double bond.