REACTION MECHANISM AND KINETICS

What is a Reaction Mechanism?
A reaction mechanism is a detailed, step-by-step description of how a chemical reaction proceeds from reactants to products. It shows:

Which bonds break and which form.

The order of steps.

The formation of intermediates (temporary, unstable compounds).

Why do Medical and Health Students Need to Know This?

To understand drug synthesis — how active pharmaceutical ingredients are made.

To predict drug metabolism in the body.

To grasp how enzymes control biochemical reactions critical to health.

 

Common Examples of Mechanisms:

 Nucleophilic Substitution (SN1 and SN2)

SN2: Single step, nucleophile directly displaces leaving group; seen in lab synthesis of alcohols from alkyl halides.

SN1: Two-step; leaving group leaves first → carbocation intermediate → nucleophile attacks carbocation; important in understanding drug stability, since carbocations can rearrange and lead to side products.

 Example in chemistry:
Making an alcohol from an alkyl halide using a hydroxide ion.

1. SN2 Mechanism (for primary alkyl halides)

One-step, concerted reaction.

Steps:

Nucleophilic attack:
The hydroxide ion (–OH), a strong nucleophile, attacks the electrophilic carbon bonded to the halogen from the opposite side of the leaving group.

Simultaneous bond changes:

The C–X bond breaks, releasing the halide ion (X⁻).

The C–O bond forms at the same time.

Inversion of configuration:
If the carbon is chiral, the stereochemistry inverts (like an umbrella flipping in the wind).

Reaction:

R–X + ^-OH→R–OH + X^-

2. SN1 Mechanism (for tertiary alkyl halides)

Two-step reaction.

Steps:

Carbocation formation (rate-determining step):
The halide leaves, forming a carbocation (R⁺).

Nucleophilic attack:
The hydroxide ion attacks the carbocation, forming the alcohol.

Possible racemization:
Since the carbocation is planar, attack can happen from both sides, leading to a mix of stereoisomers.

Reaction:

R–X → R⁺ + X⁻ R⁺ + ⁻OH → R–OH