There are four compounds with the
formulac4h9br. list them in order
ofdecreasing activity in an sn2 reaction.

The four compounds with formula C4H9Br are

1. 1- Bromobutane

2. 2-Bromobutane

3. 1-bromo-2-methyl propane

4. 2-Bromo-2-methyl propane

Reactivity towards SN2 in decreasing order is :

1- Bromobutane >  1-bromo-2-methyl propane > 2-Bromobutane >

2-Bromo-2-methyl propane

Explanation:

SN2 reaction : This is also known as second order nucleophilic Substitution Reaction because :

Primary > Secondary > tertiary

Among the isomers of C4H9Br

(Br is attached to primary carbon)

but 1- Bromobutane is less hindered than 1-bromo-2-methyl propane

So,1- Bromobutane >  1-bromo-2-methyl propane

Hence order is :

1- Bromobutane >  1-bromo-2-methyl propane > 2-Bromobutane >

2-Bromo-2-methyl propane

a electronegativity decreases beacuse the size of nucleus is bigger.