Organic Chemistry Relative Rate(s)

Question

Predict the relative rates of reaction for 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane with sodium iodide.

1: Fastest

2:

3: Lowest

The possible answers are 1-bromobutane, 2-bromobutane, and 2-bromo-2methylpropane.

Answer 1

NaI contains a potential nucleotide, iodide (I^–). The alkyl halide given each have a potential electrophilic carbon that is bonded to a halogen. This polar bond provides a partial (+) charge on carbon that draws the (–) charged nucleophile.

The less branching at the alpha carbon (i.e. the electrophilic carbon bonded to the halogen), the faster the nucleophile will be able to attack.

Thus, the order of rates with NaI:

(1) 1-bromobutane (Sn2, fast)

(2) 2-bromobutane (Sn2, slow)

(3) 2-bromo-2-methylpropane (Sn1)