Lesson 12: Reaction Kinetics, Transition States, and Carbocation Stability

Vocabulary flashcards covering reaction kinetics, transition states, reaction coordinate diagrams, carbocation stability, hyperconjugation, and electrophilic addition mechanisms.

Transition State

A non-isolable, high-energy state occurring at the peak energy point of a reaction step, characterized by partially formed and partially broken bonds.

Intermediate

A chemical species formed during a multi-step reaction that is the product of one step and the reactant for the subsequent step, possessing fully formed bonds.

Rate-Determining Step (RDS)

The slowest step in a reaction mechanism, which corresponds to the step with the highest activation energy barrier ($\Delta G^{\circ\ddagger}$) and dictates the overall reaction rate.

Regioselectivity

The preference of a chemical reaction to form one constitutional isomer over another, typically driven by the faster formation of the more stable intermediate.

Hyperconjugation

The stabilization of a carbocation caused by the overlap and donation of electron density from adjacent $\sigma$ bond orbitals into the vacant $p$ orbital.

Carbocation Stability Trend

The relative stability ranking of alkyl carbocations, ordered as tertiary ($3^{\circ}$) > secondary ($2^{\circ}$) > primary ($1^{\circ}$) > methyl cation, where greater alkyl substitution provides increased charge stabilization.

Hammond Postulate

The principle stating that the structure of a transition state most closely resembles the structure of the species (reactant, intermediate, or product) to which it is closest in free energy.

Vinyl Group and Allyl Group

Commonly named structural arrangements where a vinyl group is attached directly to a double-bonded carbon, whereas an allyl group contains a methylene spacer ($-CH_2-$) adjacent to the double bond.

Vinyl Group

An alkenyl substituent or structural arrangement consisting of a $-CH=CH_2$ group attached directly to the main molecular framework.

Allyl Group

A substituent or functional arrangement consisting of a $-CH_2-CH=CH_2$ group attached to a carbon atom adjacent to a double bond.

Phenyl Group

An aromatic substituent formed by removing a single hydrogen atom from a benzene ring, represented structurally as $-C_6H_5$.

Benzyl Group

A substituent consisting of a phenyl ring attached to a methylene group, represented structurally as $-CH_2-C_6H_5$.

Kinetic Product

The product of a reaction that forms the fastest because it proceeds through a transition state with a lower activation energy barrier ($\Delta G^{\circ\ddagger}$).

Thermodynamic Product

The product of a reaction that is overall the most stable because it has the lowest free energy ($\Delta G^{\circ}$).

Electrophilic Addition Rate-Determining Step

Step 1 of electrophilic addition to an alkene, where the nucleophilic alkene $\pi$ bond attacks an electrophile to generate a carbocation intermediate.

Resonance Stabilization of Carbocations

The dispersal and delocalization of a carbocation's positive charge across multiple atoms due to overlap between an empty $p$ orbital and adjacent $\pi$ bonds or lone pairs.