Chapter 4 Glossary

Front 1

activation energy (Ea)

Back 1

The energy difference between the reactants and the transition state; the minimum energy the reactants must have for the reaction to occur. (p. 139)

Front 2

bond-dissociation enthalpy (BDE)

Back 2

The amount of enthalpy required to break a particular bond homolytically, to give radicals. (p. 134)

Front 3

carbanion

Back 3

A strongly nucleophilic species with a negatively charged carbon atom having only three bonds. The carbon atom has a nonbonding pair of electrons. (p. 158)

Front 4

carbene

Back 4

A highly reactive species with only two bonds to an uncharged carbon atom and with a nonbonding pair of electrons. The simplest carbene is methylene, :CH₂. (p. 159)

Front 5

carbocation (carbonium ion, carbenium ion)

Back 5

A strongly electrophilic species with a positively charged carbon atom having only three bonds. (p. 155)

Front 6

catalyst

Back 6

A substance that increases the rate of a reaction (by lowering Ea) without being consumed in the reaction. (p. 141)

Front 7

chain reaction

Back 7

A multistep reaction where a reactive intermediate formed in one step brings about a second step that generates the intermediate needed for the following step. (p.126)

Front 8

endothermic

Back 8

Consuming heat (having a positive ∆H⁰).

Front 9

exothermic

Back 9

Giving off heat (having a negative ∆H⁰).

Front 10

enthalpy (heat content; H)

Back 10

A measure of the heat energy in a system. In a reaction, the heat absorbed or evolved is called the heat of reaction, ∆H⁰. A decrease in enthalpy (negative ∆H⁰) is favorable for a reaction. (p. 133)

Front 11

entropy (S)

Back 11

A measure of disorder or freedom of motion. An increase in entropy (positive ∆S⁰) is favorable for a reaction. (p. 133)

Front 12

equilibrium

Back 12

A state of a system such that no more change is taking place; the rate of the forward reaction equals the rate of the reverse reaction. (p. 131)

Front 13

equilibrium constant

Back 13

A quantity calculated from the relative amounts of the products and reactants present at equilibrium. (p. 131)

Front 14

free energy (Gibbs free energy; G)

Back 14

A measure of a reaction’s tendency to go in the direction written. A decrease in free energy (negative ∆G) is favorable for a reaction. (p. 131)

Front 15

Hammond postulate

Back 15

Related species (on a reaction-energy diagram) that are closer in energy are also closer in structure. In an exothermic reaction, the transition state is closer to the reactants in energy and in structure. In an endothermic reaction, the transition state is closer to the products in energy and in structure. (p. 149)

Front 16

heterolytic cleavage (ionic cleavage)

Back 16

The breaking of a bond in such a way that one of the atoms retains both of the bond’s electrons. A heterolytic cleavage forms two ions. (p. 134)

Front 17

homolytic cleavage (radical cleavage)

Back 17

The breaking of a bond in such a way that each atom retains one of the bond’s two electrons. A homolytic cleavage produces two radicals. (p. 134)

Front 18

inductive effect

Back 18

A donation (or withdrawal) of electron density through sigma bonds. (p. 156)

Front 19

initiation step

Back 19

The preliminary step in a chain reaction, where the reactive intermediate is first formed.

Front 20

intermediate

Back 20

A molecule or a fragment of a molecule that is formed in a reaction and exists for a finite length of time before it reacts in the next step. An intermediate corresponds to a relative minimum (a low point) in the reaction-energy diagram. (p. 140)

Front 21

kinetic order

Back 21

The power of a concentration term in the rate equation.

Front 22

kinetics

Back 22

The study of reaction rates. (p. 137)

Front 23

mechanism

Back 23

The step-by-step pathway from reactants to products showing which bonds break and which bonds form in what order. The mechanism should include the structures of all intermediates and arrows to show the movement of electrons. (p. 126)

Front 24

potential-energy diagram

Back 24

See reaction-energy diagram (p. 141)

Front 25

propagation steps

Back 25

The steps in a chain reaction that are repeated over and over to form the product. The sum of the propagation steps should give the net reaction.

Front 26

radical (free radical)

Back 26

A highly reactive species in which one of the atoms has an odd number of electrons. Most commonly, a radical contains a carbon atom with three bonds and an "odd" (unpaired) electron. (p. 157)

Front 27

radical inhibitor

Back 27

A compound added to prevent the propagation of free-radical chain reactions. In most cases, the inhibitor reacts to form radical that is too stable to propagate the chain. (p. 153)

Front 28

rate constant

Back 28

The proportionality constant, k, in the rate equation.

Front 29

rate equation (rate law)

Back 29

The relationship between the concentrations of the reagents and the observed reaction rate (p. 137)

Front 30

rate of a reaction

Back 30

The amount of product formed or reactant consumed per unit of time. (p. 137)

Front 31

rate-limiting step (rate-determining step)

Back 31

The slowest step in a multistep sequence of reactions. In general, the rate-limiting step is the step with the highest-energy transition state. (p. 142)

Front 32

reaction-energy diagram (potential-energy diagram)

Back 32

A plot of potential-energy changes as the reactants are converted to products. The vertical axis is potential energy (usually free energy, but occasionally enthalpy). The horizontal axis is the reaction coordinate, a measure of the progress of the reaction. (p. 141)

Front 33

reactive intermediate

Back 33

A short-lived species that is never present in high concentration because it reacts as quickly as it is formed. (p. 155)

Front 34

resonance stabilization

Back 34

Stabilization that takes place by delocalization of electrons in a pi bonded system. Cations, radicals, and anions are often stabilized by resonance delocalization. (p. 156)

Front 35

standard Gibbs free energy change (∆G⁰)

Back 35

The free-energy change corresponding to reactants and products in their standard states (pure substances in their most stable states) at 25oC and 1 atm pressure. (p. 131)

Front 36

substitution

Back 36

A reaction in which one atom replaces another, usually as a substitution on a carbon atom. (p.127)

Front 37

termination steps

Back 37

Any steps where a reactive intermediate is consumed without another one being generated.

Front 38

thermodynamics

Back 38

The study of the energy changes accompanying chemical transformations. Thermodynamics is generally concerned with systems at equilibrium. (p. 130)

Front 39

transition state (activated complex)

Back 39

The state of highest energy between reactants and products. A relative maximum (high point) on the reaction-energy diagram. (p. 140)