AS-Level Organic Reaction Mechanisms

15 question-and-answer flashcards summarizing key concepts, examples, and rules for AS-level organic reaction mechanisms.

What is a reaction mechanism in organic chemistry?

A detailed, step-by-step description of how a chemical reaction occurs at the molecular level, showing bond breaking/forming and electron movement.

In mechanisms, what do curly arrows represent?

The movement of electron pairs; the arrow starts at the electron source (lone pair or bond) and points to the destination.

Name the three main types of mechanisms covered in AS-level organic chemistry.

Free radical substitution, electrophilic addition, and nucleophilic substitution.

Define free radical substitution.

A reaction where a halogen replaces a hydrogen in an alkane via a radical chain mechanism (initiation, propagation, termination).

What happens during the initiation stage of free radical substitution?

UV light homolytically splits a Cl₂ or Br₂ molecule into two halogen radicals.

Describe the propagation stage of free radical substitution.

A halogen radical abstracts a hydrogen from the alkane forming HX and an alkyl radical, which then reacts with another halogen molecule to regenerate the halogen radical.

What occurs in the termination stage of free radical substitution?

Two radicals combine to form a stable molecule, removing radicals from the reaction.

What is electrophilic addition?

A reaction where an electrophile adds across an alkene’s double bond, breaking the π-bond and forming a saturated product.

Outline the electrophilic addition of ethene with HBr.

The C=C bond attacks H of HBr forming a carbocation; Br⁻ then bonds to the carbocation, yielding bromoethane.

State Markovnikov’s rule.

When a hydrogen halide adds to an asymmetric alkene, hydrogen attaches to the carbon already bearing more hydrogens, predicting the major product.

Define nucleophilic substitution.

A reaction where a nucleophile (electron-pair donor) replaces a leaving group (often a halide) in a molecule.

Give an example equation of nucleophilic substitution with bromoethane.

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻; hydroxide ion substitutes for bromide.

List three common nucleophiles used in substitution reactions.

OH⁻ (hydroxide), CN⁻ (cyanide), and NH₃ (ammonia).

How can reaction conditions influence whether substitution or elimination occurs?

Solvent, temperature, and substrate structure can steer the mechanism toward substitution or elimination and determine whether it proceeds via SN1 or SN2 pathways.

Why is understanding reaction mechanisms important in organic chemistry?

It allows chemists to explain reaction pathways, predict products, and design efficient synthetic routes.