Question

Which of the following hydrolyzes fastest? (a) \(\mathrm{CH}_{3} \mathrm{Cl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) (c) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\)

Short answer

(d) \((\mathrm{CH}_{3})_{2} \mathrm{CHCl}\) hydrolyzes fastest due to forming a more stable secondary carbocation.

Step-by-step solution

Understand the Question

We need to determine which of the given compounds undergoes hydrolysis the fastest. Hydrolysis is a chemical reaction where a molecule reacts with water or hydroxide ions to break down into smaller components. For these compounds, the ease of hydrolysis depends largely on the stability of the formed carbocation intermediate.

Identify Carbocation Formation

Consider how each compound forms a carbocation. Primary carbocations are less stable than secondary carbocations, and secondary carbocations are less stable than tertiary carbocations. Thus, the more substituted the carbocation, the faster the hydrolysis.

Analyze Options

(a) \(\mathrm{CH}_{3} \mathrm{Cl}\) forms a primary carbocation (least stable). (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) also forms a primary carbocation.(c) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) forms a primary carbocation but the presence of electronegative oxygen could destabilize it further.(d) \((\mathrm{CH}_{3})_{2} \mathrm{CHCl}\) forms a secondary carbocation on hydrolysis, which is more stable compared to a primary carbocation.

Conclusion on Stability and Rate

The compound \( (\mathrm{CH}_{3})_{2} \mathrm{CHCl} \) forms the most stable intermediate (a secondary carbocation) upon hydrolysis, which corresponds to a faster hydrolysis rate compared to the primary carbocations formed by the other compounds.

Key concepts

Carbocation Stability

Carbocation stability plays a crucial role in determining the speed of hydrolysis reactions. A carbocation is an ion with a positively charged carbon atom. The stability of this intermediate directly impacts how quickly a reaction can proceed in organic chemistry. Generally, the order of carbocation stability is as follows: tertiary (3°) > secondary (2°) > primary (1°).

The rationale behind this is that more substituted carbocations, like tertiary carbocations, are stabilized by neighboring alkyl groups through hyperconjugation and the inductive effect. In hyperconjugation, adjacent C-H bonds donate electron density to the positively charged carbon, dispersing the charge and making the carbocation more stable. Similarly, the inductive effect involves electron donation through sigma bonds from surrounding carbon atoms.

In summary, a secondary or tertiary carbocation is more stable than a primary one, leading to faster hydrolysis reactions due to a lower energy barrier for the transition state.

Substitution Effect on Hydrolysis

The substitution pattern of a compound greatly affects its hydrolysis rate. In organic chemistry, hydrolysis can proceed via a mechanism that involves carbocation formation, such as the S_N1 reaction mechanism. The number and type of substituents around the reactive site directly influence the hydrolysis.

Substituents can stabilize carbocations through:

• Electronic effects, such as hyperconjugation and the inductive effect, where electron-rich groups can donate electron density, enhancing stability.
• Steric effects, where bulkier groups can prevent certain reaction pathways but can also stabilize through spatial arrangements.

To illustrate, a compound like (0CH_3)0_2CHCl0, which forms a secondary carbocation upon hydrolysis, hydrolyzes faster compared to primary carbocations. This is because the secondary carbocation is more stable, favoring rapid transition to products.

Organic Reaction Mechanisms

Understanding organic reaction mechanisms is essential for predicting reaction outcomes, such as hydrolysis speed. A reaction mechanism explains the step-by-step sequence of elementary reactions that lead from reactants to products.

In the context of hydrolysis, particularly for alkyl halides, several mechanisms can play a role, such as:

• S_N1 Mechanism: Involves a two-step process where a carbocation intermediate is formed. It is favorable for compounds that can form stable carbocations.
• S_N2 Mechanism: A single-step process in which the nucleophile attacks the substrate, requiring a primary substrate due to less steric hindrance.

For a compound to undergo hydrolysis fastest, such as ((CH_3)0_2CHCl), an S_N1 mechanism might occur because the formation of a stable secondary carbocation facilitates a rapid reaction. Whether a reaction follows an S_N1 or S_N2 mechanism depends on various factors, including carbocation stability and solvent effects, influencing the speed of hydrolysis.