Question

Which of the following gives correct rank order from fastest to slowest of the relative rates in SN2 reaction ofmethyl bromide, tert-butyl bromide, isopropyl bromide and ethyl bromide?

• A. Methyl bromide > Ethyl Bromide > Isopropyl bromide> tert-Butyl bromide
• B. tert-Butyl bromide > Isopropyl bromide > EthylBromide > Methyl bromide
• C. Ethyl Bromide > Methyl bromide > Isopropyl bromide> tert-Butyl bromide >
• D. Methyl bromide > Ethyl Bromide > tert-Butyl bromide> Isopropyl bromide

Answer 1

The Correct Option is A

To understand the rank order for the SN2 reaction rates of the compounds listed, we need to consider the factors that affect the SN2 reaction mechanism. SN2 reactions (substitution nucleophilic bimolecular) are characterized by a backside attack, which results in the inversion of stereochemistry at the carbon atom.

A key factor influencing SN2 reactions is steric hindrance. As steric hindrance increases, the reaction rate decreases because the approach of the nucleophile to the carbon center becomes more difficult. Let's analyze the steric hindrance for each compound:

• Methyl Bromide (CH₃Br): No steric hindrance from additional carbon groups, allowing for quick nucleophile attack.
• Ethyl Bromide (C₂H₅Br): Slightly more steric hindrance than methyl bromide due to the presence of one extra carbon.
• Isopropyl Bromide ((CH₃)₂CHBr): Increased steric hindrance as two additional methyl groups (CH₃) are present.
• tert-Butyl Bromide ((CH₃)₃CBr): Most steric hindrance due to three additional methyl groups, severely hindering nucleophilic attack.

Considering the steric hindrance, the order of the SN2 reaction rate from fastest to slowest is:

Methyl Bromide > Ethyl Bromide > Isopropyl Bromide > tert-Butyl Bromide

This explains why the correct answer is: Methyl bromide > Ethyl Bromide > Isopropyl bromide > tert-Butyl bromide.

Other options do not match the logic based on steric hindrance and the nature of SN2 reactions:

• tert-Butyl bromide > Isopropyl bromide > Ethyl Bromide > Methyl bromide: This order would imply an increase in reaction rate with increased hindrance, which contradicts SN2 reaction principles.
• Ethyl Bromide > Methyl bromide > Isopropyl bromide > tert-Butyl bromide: This incorrectly places ethyl before methyl bromide, whereas methyl bromide should be faster due to less hindrance.
• Methyl bromide > Ethyl Bromide > tert-Butyl bromide > Isopropyl bromide: This mixes the order for tert-Butyl and isopropyl bromide, which is not accurate based on steric considerations.