For which of the following reactions would you expect elimination to be more favored than substitution?
(d)

Verified step by step guidance

1. Step 1: Analyze the structure of the alkyl halide (benzyl bromide) in both reactions. Benzyl bromide contains a benzylic carbon, which is adjacent to the benzene ring. This benzylic position is highly reactive due to resonance stabilization of the carbocation formed during substitution or elimination.
2. Step 2: Examine the base used in each reaction. In the first reaction, LiNH₂ is a strong, small base. Small bases typically favor elimination (E2 mechanism) because they can easily abstract a proton from the β-carbon. In the second reaction, the base is a bulky amine (lithium diisopropylamide, LDA). Bulky bases strongly favor elimination over substitution because steric hindrance prevents them from effectively attacking the electrophilic carbon in substitution reactions.
3. Step 3: Consider the solvent (THF) used in both reactions. THF is an aprotic solvent, which generally supports elimination reactions, especially when strong bases are involved. Aprotic solvents do not stabilize carbocations as well as protic solvents, making substitution less favorable.
4. Step 4: Compare the steric effects of the bases. The bulky base (LDA) in the second reaction will strongly favor elimination because it is sterically hindered and cannot easily approach the electrophilic carbon for substitution. The smaller base (LiNH₂) in the first reaction may allow for some substitution, but elimination is still likely to be favored due to the strength of the base.
5. Step 5: Conclude that elimination is more favored in the reaction with the bulky base (LDA) compared to the reaction with the smaller base (LiNH₂). The steric hindrance of LDA and the use of an aprotic solvent make elimination the dominant pathway in the second reaction.

Key Concepts

Elimination vs. Substitution Reactions

In organic chemistry, elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of a double bond or a ring structure. In contrast, substitution reactions involve replacing one atom or group with another. The preference for elimination over substitution often depends on factors such as the structure of the substrate, the nature of the leaving group, and the reaction conditions.

Regioselectivity in Reactions

Regioselectivity refers to the preference of a chemical reaction to occur at one location over another in a molecule. In the context of elimination and substitution reactions, the regioselectivity can influence whether a reaction will favor the formation of a double bond or the substitution of a functional group. Understanding the regioselectivity helps predict the major products formed in a reaction.

Role of Solvents in Organic Reactions

The choice of solvent can significantly affect the outcome of organic reactions. Polar protic solvents tend to stabilize ions and can favor substitution reactions, while polar aprotic solvents can promote elimination reactions by stabilizing the transition state. In the given question, the use of THF (tetrahydrofuran) as a solvent may influence whether elimination or substitution is favored in the reactions depicted.