Under second-order conditions (strong base/nucleophile), SN2 and E2 reactions may occur simultaneously and compete with each other. Show what products might be expected from the reaction of 2-bromo-3-methylbutane (a moderately hindered 2° alkyl halide) with sodium ethoxide. HINT: Zaitsev's rule usually applies in E2 reactions unless the base and/or the leaving group are unusually bulky.

Verified step by step guidance

1. Identify the structure of 2-bromo-3-methylbutane and recognize it as a secondary alkyl halide, which is susceptible to both SN2 and E2 reactions under strong base/nucleophile conditions.
2. Consider the nature of the base/nucleophile, sodium ethoxide (NaOEt). It is a strong base and a good nucleophile, which can promote both SN2 (nucleophilic substitution) and E2 (elimination) reactions.
3. For the SN2 reaction, predict the substitution product by replacing the bromine atom in 2-bromo-3-methylbutane with the ethoxide group (OEt) from sodium ethoxide. The reaction involves a backside attack mechanism, leading to inversion of configuration at the carbon attached to the leaving group (bromine).
4. For the E2 reaction, consider Zaitsev's rule, which states that the more substituted alkene (having more alkyl groups attached to the double-bonded carbons) will be the major product. Identify the β-hydrogens relative to the leaving group (bromine) and predict the formation of the most substituted alkene by elimination of H and Br.
5. Evaluate the steric hindrance and the strength of the base to determine the likelihood of each reaction pathway (SN2 vs E2) and the major products expected from each. Given that sodium ethoxide is a strong, small base and not excessively bulky, both SN2 and E2 reactions are feasible, with E2 possibly being favored due to the formation of a more stable, substituted alkene.

Key Concepts

SN2 Mechanism

The SN2 mechanism is a type of nucleophilic substitution reaction where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction occurs in a single concerted step, leading to an inversion of configuration at the carbon center. It is favored by primary and some secondary alkyl halides, especially in polar aprotic solvents, and is characterized by its dependence on steric accessibility.

E2 Mechanism

The E2 mechanism is a bimolecular elimination reaction where a strong base abstracts a proton from a β-carbon, leading to the formation of a double bond while simultaneously expelling a leaving group. This reaction occurs in a single concerted step and is favored by strong bases and more hindered substrates. The regioselectivity of E2 reactions often follows Zaitsev's rule, which states that the more substituted alkene is typically the major product unless steric factors dictate otherwise.

Zaitsev's Rule

Zaitsev's rule is a principle in elimination reactions that predicts the formation of the more stable, more substituted alkene as the major product. This rule applies primarily to E2 reactions, where the stability of the alkene is influenced by hyperconjugation and the degree of substitution. However, if the base or leaving group is bulky, the less substituted alkene may be favored, highlighting the importance of sterics in determining the outcome of elimination reactions.