Wade:OrganicChemistryCh4
Terms
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- energy difference between the reactants and the transition state; the minimum energy the reactants must have for the reaction to occur: Ea
- activation energy
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the amount of energy required to break a particularbond homolytically, to give radicals: BDE
A:B --> A. + B.
delta H = BDE - bond-dissociation energy
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a strongly nucleophilic species (reactive intermediate)with a negatively charged carbon atom having only three bonds; the carbon atom has a nonbonding pair of electrons
-:CH3 nucleophilic strong bases - carbanion
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a highly reactive species (reactive intermediate) with only two bonds to an uncharged carbon atom and with a nonbinding pair of electrons
:CH2 (methylene) both nucleophilic and electrophilic - carbene
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a strongly electrophilic species (reactive intermediate) with a positively charged carbon atom having only three bonds
+CH3 electrophilic strong acids -
carbocation or
carbonium ion or
carbenium ion - substance that increases the rate of a reaction (by lowering the Ea) without being consumed in the reaction
- catalyst
- 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
- chain reaction
- the preliminary step in a chain reaction, where the reactive intermediate is first formed
- initiation step
- the steps in a chain reaction that are repeated over and over to form the product; the sum of these steps should give the net reaction
- propagation steps
- any steps where a reactive intermediate is consumed without another one being generated
- termination steps
- a measure of the heat energy in a system. In a reaction, the heat absorbed or evolved is called the heat of reaction (delta H). A negative delta H is favorable for reaction to occur
- enthalpy
- reaction consuming heat (increase in enthalpy); having a positive delta H; does not favor reaction
- endothermic
- reaction giving off heat (a decrease in enthalpy); a negative delta H; favorable for reaction
- exothermic
- measure of diorder or freedom of motion; a positive value, or increase in the delta S is favorable for the reaction to occur.
- entropy
- 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
- equilibrium
- a quantity calculated from the relative concentrations of the products over the concentration of the reactants at equilibrium
- equilibrium constant (Keq)
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a measure of a reactions tendency to go in the direction written; a decrease or negative delta G is favorable for a reaction to occur
delta G = delta H - T delta S -
free energy or
Gibbs free energy - related species on a reaction-energy diagram that are closer in energy are also closer in structure; exothermic reaction, the transition state is closer to the reactants; endothermic reaction, the transition state is closer to the products in energy and
- Hammond postulate
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breaking of a bond in such a way that one of the atoms retains both of the bonds electrons; forms two ions
A:B --> A:- + B+ -
heterolytic cleavage or
ionic cleavage -
breaking of a bond in such a way that each atom retains one of the bonds two electrons; produce two radicals
A:B --> A. + B. - homolytic cleavage
- a donation (or withdrawel) of electron density through sigma bonds
- inductive effect
- molecule or 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; it corresponds to relative minimum (a low point) in the reaction-energy diagram
- intermediate
- a short lived species that is never present in high concentration because it reacts as quickly as it is formed
- reactive intermediate
- the study of reaction rates
- kinetics
- the step-by-step pathway from reactants to productsshowing which bonds break and which bonds form in what order; includes the structures of all intermediates and arrows to show the movement of electrons
- mechanism
- a plot of potential energy changes as the reactants are converted to products; vertical axis is all potential energy and the horizontal is reaction progress
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reaction-energy diagram or
potential-energy diagram - highly reactive speciesin which one of the atoms has an odd number of electrons; most commonly a carbon atom with three bonds and an unpaired electron
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radical or
free radical - compound added to prevent the propagation of free radical chain reactions; reacts to form a radical that is too stable to propagate the chain
- radical inhibitor
- the amount of product formed or reactant consumed per unit of time
- reaction rate
- relationship between the concentrations of the reagents and the observed reaction rate
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rate equation or
rate law - the power of a concentration term in the rate equation; overall is the sum of the individual concentration orders
- kinetic order
- the constant in the rate equation (kr)
- rate constant
- the slowest step in a multi step sequence of reactions; step with the highest-energy transition state
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rate-limiting step or
rate-determining step - stabilization that takes place by delocalizationof electrons in a pi-bonded system; stabilizes cations, radicals, and anions
- resonance stabilization
- a reaction in which one atom replaces another, usually as a substituent on a carbon atom
- substitution
- study of the energy changes that accompany a chemical transformations; generally concerned with systems at equilibrium
- thermodynamics
- position of the activated complex; the state of highest energy between reactants and products; a relative maximum (high point) on the reaction-energy diagram
- transition state