Question

Arrange the compounds of each set in order of reactivity towards \(\mathrm{S}_{N} 2\) displacement: (i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane (ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane

Short answer

(i) 1-Bromopentane > 2-Bromopentane > 2-Bromo-2-methylbutane; (ii) 1-Bromo-3-methylbutane > 2-Bromo-3-methylbutane > 2-Bromo-2-methylbutane.

Step-by-step solution

Understand the basics of SN2 reactions

SN2 reactions involve a nucleophile attacking a carbon atom bonded to a leaving group. The reaction is second-order, meaning it depends on both the concentration of the substrate and the nucleophile. For SN2 reactions, less steric hindrance (less bulky groups around the reaction center) leads to higher reactivity.

Analyze compounds from set (i)

For this set, consider the degree of substitution at the reaction centers: - **2-Bromo-2-methylbutane** is a tertiary halide, having three bulky groups attached to the carbon bearing the bromine. This makes it highly hindered, thus least reactive. - **1-Bromopentane** is a primary halide, where the bromine is attached to a carbon with only one alkyl group. It has low steric hindrance, making it highly reactive. - **2-Bromopentane** is a secondary halide with moderate steric hindrance. Therefore, the order of reactivity will be 1-Bromopentane > 2-Bromopentane > 2-Bromo-2-methylbutane.

Analyze compounds from set (ii)

For this set: - **1-Bromo-3-methylbutane** is a primary halide like 1-bromopentane and is highly reactive due to low steric hindrance. - **2-Bromo-2-methylbutane** is a tertiary halide, similar to the previous set, and is the least reactive due to high steric hindrance. - **2-Bromo-3-methylbutane** is a secondary halide, having some steric hindrance but less than the tertiary halide. Therefore, the reactivity order is 1-Bromo-3-methylbutane > 2-Bromo-3-methylbutane > 2-Bromo-2-methylbutane.

Key concepts

Steric Hindrance in SN2 Reactions

Steric hindrance is a fundamental concept when considering the reactivity of SN2 reactions. When a nucleophile approaches a molecule to displace a leaving group in an SN2 reaction, it prefers less crowded environments. Steric hindrance refers to the resistance that bulky groups present against a nucleophile's approach. This obstruction slows down or even prevents the reaction.

In simpler terms, imagine trying to fit a key into a lock that has extra big pieces around it—a little tricky, right? That's what bulky groups do to nucleophiles. The less additional bulkiness around the reaction center, the easier it is for the nucleophile to move in confidently and effectively.

Therefore, compounds with less steric hindrance (fewer large groups attached to the reacting carbon) exhibit higher reactivity in SN2 reactions than those with more bulkiness. To predict which compounds will react faster, always look for that less crowded pathway for the nucleophile.

Primary, Secondary, and Tertiary Halides

The classification of halides into primary, secondary, and tertiary is crucial in understanding SN2 reactivity. It determines the degree of steric hindrance the nucleophile will face.

• **Primary halides**: These have a carbon attached to only one other carbon, meaning minimal steric hindrance. They are usually the most reactive in SN2 reactions because the nucleophile can quickly access the attacking carbon.


• **Secondary halides**: These have a carbon connected to two other carbons. This results in moderate steric hindrance, offering a medium level of reactivity toward SN2 reactions.


• **Tertiary halides**: With a carbon connected to three other carbon atoms, tertiary halides present the most steric hindrance. This makes them the least reactive in SN2 reactions due to the significant obstruction faced by nucleophiles.

Understanding this classification helps predict reactivity. For instance, primary halides like 1-bromopentane are much more reactive than tertiary counterparts like 2-bromo-2-methylbutane in SN2 reactions.

Nucleophilic Substitution

Nucleophilic substitution is a reaction where a nucleophile replaces a leaving group in a molecule. In the context of SN2 reactions, it is a concerted and bimolecular process. This means the nucleophile attacks the substrate at the same time that the leaving group departs.

The magic of SN2 reactions lies in their simplicity and efficiency. Because of their bimolecular nature, they depend equally on the presence and concentration of the nucleophile and the substrate. Thus, they don't proceed in steps but rather occur in a single transitional phase.

Factors such as the strength of the nucleophile and the nature of the leaving group also influence these reactions. Strong nucleophiles and good leaving groups, which can easily depart the molecule, enhance the speed and completion of SN2 reactions. Therefore, understanding both the molecular framework and environment can significantly guide predictions about rates and success in chemical syntheses and reactions.