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

(c) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) hydrolyzes fastest due to the electron-withdrawing oxygen atom increasing its reactivity.

Step-by-step solution

Identify the Reaction Type

The problem involves identifying which compound hydrolyzes fastest. Hydrolysis typically involves a reaction with water, often facilitated by a nucleophile with the substrate replacing a leaving group. Here, each of the given compounds is an alkyl chloride, where the chlorine atom can be the leaving group.

Consider Steric Effects

Consider the steric hindrance around the chlorine atom. Less steric hindrance generally results in faster hydrolysis because the nucleophile has easier access to the reactive center. Among the options, \(\mathrm{CH}_{3} \mathrm{Cl}\) has the least steric hindrance, followed by \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\), \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\), and finally \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) which has the most hindrance.

Consider Electronic Effects and Inductive Effects

Check the electronic environment and inductive effects that may affect hydrolysis. In \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\), the presence of the electronegative oxygen atom will increase the positive character on the carbon attached to the chlorine, making it more susceptible to nucleophilic attack. Thus, \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) is likely to hydrolyze faster than other compounds.

Combine Effects to Determine Fastest Hydrolyzing Compound

Combining the steric and electronic considerations, even though \(\mathrm{CH}_{3} \mathrm{Cl}\) has the least steric hindrance, the inductive effect of \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) due to the oxygen atom outweighs it, leading \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) to hydrolyze the fastest.

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental chemical reaction where a nucleophile, a chemical species with a free pair of electrons, seeks out a positively charged center to form a new chemical bond. In the context of hydrolysis, which in this exercise involves the interaction with water, the nucleophile (often the hydroxide ion from water) substitutes another atom or group in the molecule, which is called a leaving group.

The compound being considered in this exercise is an alkyl chloride, which means the leaving group is the chlorine atom.

• The nucleophile initiates the reaction by attacking the carbon atom to which chlorine is attached.
• This carbon atom is deficient in electrons, making it susceptible to nucleophilic attack.
• Once the nucleophile binds to this carbon, the chlorine atom, the leaving group, departs.

The efficiency of this process is influenced by various factors like sterics and electronics, which will further be discussed.

Steric Hindrance

Steric hindrance is the physical obstruction that prevents a nucleophile from efficiently attacking the reactive carbon atom in the substitution process. It is mainly caused by the bulkiness of groups attached to this reactive carbon.In our compounds:

• e.g., **Methane Group (\(\mathrm{CH}_{3} \mathrm{Cl}\)):** The simplest structure here provides the least steric hindrance, allowing the nucleophile almost unhindered access.
• e.g., **Ethane Group (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\)):** Slightly bulkier than methane but still provides relatively easy nucleophile access.
• e.g., **Isopropyl Group (\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\)):** More significant crowding around the reactive center makes nucleophilic attack more challenging.

Greater steric hindrance decreases reaction speed because it physically blocks incoming nucleophiles from reaching the reaction site, thereby slowing hydrolysis.

Inductive Effect

The inductive effect refers to the electron-withdrawing or electron-donating nature of atoms in a molecule, affecting how the nucleophile attacks the substrate. Atoms like oxygen, due to their electronegativity, can attract electrons through the sigma bonds, leading to a positive charge buildup on the carbon atom attached to the chlorine.In \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\), the nearby oxygen atom causes an electron-withdrawing inductive effect:

• This increases the positive character of the adjacent carbon atom, making it a more attractive target for nucleophiles.
• The nucleophile is more effectively drawn towards this carbon, enhancing the speed of hydrolysis, surpassing the reaction rate in compounds without such electron-withdrawing effects.
• This inductive effect outweighs steric considerations when comparing \(\mathrm{CH}_{3} \mathrm{Cl}\) and \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) \mdash;the latter hydrolyzes faster due to the electron-withdrawing oxygen completing the reaction more effectively.

The interplay of these factors guides the prediction of reaction speed in nucleophilic substitution reactions, with the inductive effect significantly impacting the likelihood and speed of these reactions.