What is the possible arrangement of the isomers of molecular formula C4H9Cl in order of decreasing rate of reaction with sodium iodide in acetone?

1 Answer
Feb 7, 2018

The order is butyl > isobutyl > sec-butyl > tert-butyl.

Explanation:

The reaction is an #"S"_text(N)2# displacement reaction:

#"I"^"-" + "R-Cl" → "I-R" + "Cl"^"-"#

Thus, a major factor affecting relative reaction rates is steric hindrance to attack by the nucleophile #"I"^"-"#.

Let's examine ball-and-stick and space-filling models of the isomeric chlorides.

In the space-filling models, I have marked the backside of the carbon bearing the leaving group with a yellow dot.

Butyl chloride

Bu1
Bu2

This is a primary alkyl halide.

It has the least steric hindrance to approach by the nucleophile, so it should have the fastest rate.

Isobutyl chloride

iBu1
iBu2

This is also a primary alkyl halide.

However, the branched isopropyl group provides more steric hindrance than the linear propyl group in butyl chloride.

This is the next fastest reacting chloride.

sec-Butyl chloride

sBu1
sBu2

The methyl and ethyl groups severely hinder approach by the nucleophile. This is the third fastest reacting chloride.

tert-Butyl chloride

tBu1
tBu2

The carbon bearing the #"Cl"# atom is almost completely shielded by the three methyl groups. This is the slowest reacting of the four chlorides.