Chromic acid oxidation of an alcohol (Section 11-2A) occurs in two steps: formation of the chromate ester, followed by an elimination of H⁺ and chromium. Which step do you expect to be rate-limiting? Careful kinetic studies have shown that Compound A undergoes chromic acid oxidation over 10 times as fast as Compound B. Explain this large difference in rates.

Verified step by step guidance

1. Step 1: Understand the chromic acid oxidation mechanism. Chromic acid oxidation of alcohols involves two key steps: (1) formation of a chromate ester intermediate, and (2) elimination of H+ and chromium to form the oxidized product. The rate-limiting step is typically the formation of the chromate ester, as it involves bond formation and rearrangement.
2. Step 2: Analyze the structures of Compound A and Compound B. Compound A has a secondary alcohol group, while Compound B has a primary alcohol group. The steric and electronic environment around the alcohol group influences the rate of chromate ester formation.
3. Step 3: Consider steric hindrance. Compound A has a methyl group adjacent to the alcohol, which can stabilize the transition state during chromate ester formation. In contrast, Compound B lacks this stabilizing group, making the transition state less favorable and slowing down the reaction.
4. Step 4: Evaluate electronic effects. The methyl group in Compound A can donate electron density through hyperconjugation, stabilizing the intermediate formed during chromate ester formation. Compound B does not have this stabilizing effect, contributing to its slower reaction rate.
5. Step 5: Conclude the explanation. The large difference in rates between Compound A and Compound B is due to the steric and electronic stabilization provided by the methyl group in Compound A, which facilitates faster formation of the chromate ester intermediate, the rate-limiting step in the reaction.

Key Concepts

Chromic Acid Oxidation Mechanism

Chromic acid oxidation of alcohols involves a two-step mechanism. The first step is the formation of a chromate ester, where the alcohol reacts with chromic acid to form a transient intermediate. The second step involves the elimination of H+ and chromium species, leading to the formation of the carbonyl compound. Understanding this mechanism is crucial for identifying which step may be rate-limiting.

Rate-Limiting Step

The rate-limiting step in a reaction mechanism is the slowest step that determines the overall reaction rate. In the context of chromic acid oxidation, identifying whether the formation of the chromate ester or the elimination step is rate-limiting is essential for predicting reaction kinetics. This concept is fundamental in understanding how different factors can influence the speed of chemical reactions.

Kinetic Studies and Reactivity

Kinetic studies involve measuring the rates of reactions to understand how different compounds react under similar conditions. The observation that Compound A undergoes chromic acid oxidation over 10 times faster than Compound B suggests significant differences in their structures or steric effects, which can influence the stability of intermediates and transition states. This concept helps explain variations in reactivity among different alcohols.