Q.Which one of the following compounds is more reactive towards $S_N2$ reaction and why ?
$CH_3CH(Cl)CH_2CH_3$ or $CH_3CH_2CH_2Cl$

The Core Idea: Two Ways to Swap a Group

Imagine you have a molecule with a leaving group (like a halogen) attached to a carbon. You want to replace that leaving group with a nucleophile (something that loves positive charge). There are two fundamentally different ways this can happen — like two different ways to replace a lightbulb.

SN2 is like unscrewing the old bulb and screwing in the new one in one smooth motion. SN1 is like first pulling the old bulb out completely, leaving an empty socket, and then putting the new bulb in.

That empty socket — the carbocation — is the key difference.

SN2: One Step, Backside Attack

The name says it all: Substitution, Nucleophilic, Bimolecular. "Bimolecular" means two molecules (the nucleophile and the substrate) are involved in the rate-determining step.

The Mechanism

The nucleophile attacks the carbon from the backside — directly opposite the leaving group. As the nucleophile approaches, the leaving group starts to leave. At the transition state, the nucleophile is partially bonded and the leaving group is partially detached. Then the leaving group departs completely, and the nucleophile is fully bonded.

All of this happens in one step — no intermediate.

The Stereochemistry: Inversion

Because the nucleophile attacks from the back, the configuration at the carbon inverts — like an umbrella turning inside out in a strong wind. If you start with an R configuration, you get S (and vice versa). This is called Walden inversion.

What Favours SN2?

• Primary carbon (least steric hindrance — the backside is wide open)
• Strong nucleophile (needs to push its way in)
• Good leaving group (but not too good — it needs to wait for the nucleophile)
• Polar aprotic solvent (doesn't solvate the nucleophile too tightly)

SN1: Two Steps, Carbocation Intermediate

Substitution, Nucleophilic, Unimolecular. "Unimolecular" means only one molecule (the substrate) is involved in the rate-determining step.

The Mechanism

Step 1 (slow, rate-determining): The leaving group leaves on its own, forming a carbocation (a carbon with only six electrons — positively charged and very unstable).

Step 2 (fast): The nucleophile attacks the carbocation. Since the carbocation is flat (trigonal planar), the nucleophile can attack from either side with equal probability.

The Stereochemistry: Racemisation

Because the nucleophile can attack from either face of the flat carbocation, you get a racemic mixture — equal amounts of R and S. If the starting material is optically pure, the product will be optically inactive.

What Favours SN1?

• Tertiary carbon (the carbocation is stabilised by three alkyl groups — hyperconjugation and inductive effect)
• Weak nucleophile (doesn't need to force its way in — the carbocation is desperate for electrons)
• Excellent leaving group (must be able to leave on its own)
• Polar protic solvent (stabilises the carbocation and the leaving group)

The Big Comparison Table …