The following compounds are given to you :

2-Bromopentane, 2-Bromo-2-methylbutane, 1-Bromopentane

(a) Write the compound which is most reactive towards reaction.

(b) Write the compound which is optically active.

(c) Write the compound which is most reactive towards β-elimination reaction.

Structure of 1-Bromopentane:

Structure of 2-Bromo-2-methylbutane:

Structure of 2-bromopentane:

(a) The compound which is most reactive towards reaction is 1-Bromopentane ()

Explanation: In S_n2 reaction, the attack of the nucleophile occurs from the back side on the α-carbon(carbon carrying the halogen). Therefore, presence of bulky substituents on or the near the α-carbon atom tends to hinder (or block) the approach of the nucleophile to the α-carbon due to the steric hindrance and thus makes the reaction difficult to occur. Hence the overall order of reactivity in S_N2 reactions is :

Primary halides > Secondary halides > Tertiary halides

∴ 1-bromopentane is the most reactive towards S_n2 reaction

Because it is a primary halide (halogen attached on the first carbon)

(b) The compound which is optically active is 2-Bromopentane.

Explanation: 2-bromopentane is an optically active compound because it contains an asymmetric carbon (chiral carbon). An asymmetric carbon is carbon atom that is attached to four different types of atoms or groups of atoms. In 2-Bromopentance, the central carbon atom is attached to four different types of atoms (CH₂, CH₃, Br, H), hence it is an optically active compound.

On the other hand, 1-Bromopentane and 2-Bromo-2-methylbutane do not contain asymmetric carbon, hence they are not optically active.

(c) The compound which is most reactive towards β-elimination reaction is: 2-Bromo-2-methylbutane

Explanation: β-elimination reaction is a reaction in which two groups (usually H and X-halogen) are eliminated from the adjacent carbon atom, hydrogen atom is lost from β-carbon(carbon next to that which carries the halogen) and halogen is lost from the α-carbon(carbon carrying the halogen).

This reaction is also called dehydrohalogenation (removal of hydrogen and halogen) reaction.

The ease of β-elimination of alkyl halides increases as the number of β-Hydrogen increases. Thus, the reactivity order is:

Tertiary (has three β-Hydrogen) > Secondary (has two β-hydrogen) > Primary (has one β-hydrogen)

Hence 2-Bromo-2-methylbutane is most reactive towards β-elimination reaction.