Question

Write the expected substitution product(s) for each reaction and predict the mechanism by which each product is formed. (a) CC(C)(C)C1CCC(Br)CC1 (b) CCC(C)Cl ( \(R)\)-2-Chlorobutane

Short answer

Question: Briefly describe the expected substitution products and their mechanisms for the following reactions: (a) A compound with a bromine atom attached to a cyclohexane ring with a tert-butyl group. (b) (\(R\))-2-Chlorobutane. Answer: (a) The expected substitution product would be a molecule with the same skeleton as the reactant, but with the nucleophile replacing the bromine atom through an S_N1 mechanism involving a carbocation intermediate. (b) The expected substitution product would be a molecule with the same skeleton as the reactant, but with the nucleophile replacing the chlorine atom and inverted stereochemistry at the reactive center, following an S_N2 mechanism.

Step-by-step solution

Identify the reactive site and possible nucleophiles

The reactive site in this molecule is the carbon bonded to the bromine atom. Since bromine is a good leaving group, a nucleophile (usually a negatively charged ion or a molecule with a lone pair of electrons) can attack the carbon and displace bromine.

Predict the mechanism and substitution product

Since the carbon attached to the bromine atom is tertiary (3°), the most likely mechanism for the substitution reaction is the S_N1 mechanism. In the S_N1 mechanism, the carbon-bromine bond breaks first, forming a carbocation intermediate. The nucleophile then attacks the carbocation to form the substitution product. The expected substitution product is generally a molecule with the same skeleton, but with the nucleophile replacing the bromine atom. (b)

Identify the reactive site and possible nucleophiles

The reactive site in (\(R\))-2-Chlorobutane is the carbon bonded to the chlorine atom. Since chlorine is also a good leaving group, a nucleophile can attack the carbon and displace the chlorine atom.

Predict the mechanism and substitution product

In this case, the carbon attached to the chlorine atom is secondary (2°). Both S_N1 and S_N2 mechanisms are possible for secondary substrates, but for 2-chlorobutane, the S_N2 mechanism is more likely due to a less congested transition state. In S_N2 reactions, the nucleophile attacks the carbon from the opposite side of the leaving group, leading to stereochemical inversion of the product. The expected substitution product is a molecule with the same skeleton as the reactant, but with the nucleophile replacing the chlorine atom and inverted stereochemistry at the reactive center.