Question

The decreasing order of rate of esterification of the following acids with MeOH is (I) \(\mathrm{Me}-\mathrm{CH}_{2} \mathrm{COOH}\) (II) Me \(_{2} \mathrm{CHCOOH}\) (III) \(\mathrm{Me}_{3} \mathrm{C}-\mathrm{COOH}\) (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (B) III \(>\mathrm{II}>\mathrm{I}\) (C) II \(>\mathrm{III}>\mathrm{I}\) (D) II > I > III

Short answer

The decreasing order of rate of esterification of the given carboxylic acids with methanol is (A) I > II > III, based on the number of methyl groups attached to the alpha-carbon and their electron-donating properties.

Step-by-step solution

Identify the functional groups in each carboxylic acid

The given carboxylic acids have different numbers of methyl groups attached to the alpha-carbon (the carbon adjacent to the carbonyl group). These groups play a role in the acidity, electrophilicity, and esterification rates of each carboxylic acid. 1. \(\mathrm{Me}-\mathrm{CH}_{2} \mathrm{COOH}\) : 1 methyl group 2. \(\mathrm{Me}_{2} \mathrm{CHCOOH}\) : 2 methyl groups 3. \(\mathrm{Me}_{3} \mathrm{C}-\mathrm{COOH}\) : 3 methyl groups

Analyze the effect of the attachments on esterification rates

Methyl groups are electron-donating groups, which increase electron density around the carbonyl group, making it less electrophilic (i.e., less reactive). As the number of methyl groups increases, the carbonyl group becomes less electrophilic, and the rate of esterification decreases. Therefore, the order of esterification rates can be determined based on the number of methyl groups attached to the alpha-carbon.

Determine the order of esterification rates for the given carboxylic acids

According to our analysis, carboxylic acids with fewer methyl groups will have faster esterification rates. Thus, the order of esterification rates is: 1. \(\mathrm{Me}-\mathrm{CH}_{2} \mathrm{COOH} > \mathrm{Me}_{2} \mathrm{CHCOOH} > \mathrm{Me}_{3} \mathrm{C}-\mathrm{COOH}\) 2. This corresponds to the option (A) I > II > III

Key concepts

Carboxylic Acids

Carboxylic acids are organic compounds that contain a carboxyl group (\(-\text{COOH}\)). They are widely found in nature and play a crucial role in various chemical reactions, including esterification.
One key feature of carboxylic acids is their acidity, which is due to the presence of the hydrogen atom in the carboxyl group. This hydrogen can be released as a proton (\(\text{H}^+\)), leading to the formation of a carboxylate ion, which is stabilized by resonance.
The structure of carboxylic acids makes them reactive, especially in proton transfer reactions. They can form esters when reacting with alcohols, particularly under acidic conditions, which is known as esterification. Carboxylic acids have varying strengths of acidity, which can be influenced by the rest of the molecular structure present.
The acidity affects their reactivity and the rate of such reactions significantly.

Methyl Groups

Methyl groups (\(\text{CH}_3\)) are a type of alkyl group consisting of one carbon atom bonded to three hydrogen atoms. They are one of the simplest substituents and often play an important role in altering the properties of organic molecules, such as carboxylic acids.
Methyl groups are electron-donating groups, which means they can increase the electron density within a molecule. In the case of carboxylic acids, the presence of methyl groups on the alpha-carbon (the carbon adjacent to the carboxyl group) affects the molecule's reactivity.

• Increase in methyl groups typically decreases electrophilicity, as they push electrons towards the carbonyl carbon.
• This electron donation can hinder reactions that rely on the electrophilicity of the carbonyl group, such as esterification.

Thus, acids with more methyl groups tend to show slower esterification rates compared to those with fewer methyl groups due to reduced electrophilicity.

Electrophilicity

Electrophilicity refers to the tendency of a chemical species to attract electrons, making it an electron-deficient area that is highly reactive towards nucleophiles.
In organic chemistry, the carbon atom in the carbonyl group of carboxylic acids is a common electrophilic site. Its ability to react with nucleophiles, such as alcohols in esterification reactions, depends on the surrounding molecular environment.
Factors such as the electron-withdrawing or electron-donating nature of adjacent groups can significantly influence electrophilicity.

• Electron-donating groups like methyl groups, decrease electrophilicity by increasing the electron density around the carbonyl carbon.
• This decrease makes the carbonyl group less susceptible to nucleophilic attacks, leading to slower chemical reactions.

Therefore, in the context of carboxylic acid esterification, a decrease in electrophilicity due to an increase in methyl groups can slow down the reaction rate.