Question

Arrange the following compounds in increasing order of their property as indicated: (i) Acetaldehyde. Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone (reactivity towards HCN) (ii) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Br}) \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}_{2} \mathrm{COOH} .\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}\). \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (acid strength) (iii) Benzoic acid. 4-Nitrobenzoic acid, 3.4-Dinitrobenzoic acid. 4-Methoxybenzoic acid (acid strength)

Short answer

(i) Di-tert-butyl ketone < Methyl tert-butyl ketone < Acetone < Acetaldehyde (ii) (CH_3)_2 CHCOOH < CH_3 CH_2 CH_2 COOH < CH_3 CH_2 CH(Br)COOH < CH_3 CH(Br)CH_2 COOH (iii) 4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid

Step-by-step solution

Understanding Reactivity towards HCN

Reactivity towards HCN depends on the electrophilicity of the carbonyl carbon. Steric hindrance and electronic effects play a crucial role here. Smaller or less hindered ketones and aldehydes generally exhibit higher reactivity. In this list, acetaldehyde is the most reactive due to its simple structure and lack of steric hindrance. Di-tert-butyl ketone would be the least reactive due to the bulky groups that create steric hindrance. Thus, arrange: Di-tert-butyl ketone < Methyl tert-butyl ketone < Acetone < Acetaldehyde.

Evaluating Acid Strength for Carboxylic Acids

Acid strength in carboxylic acids is influenced by electron-withdrawing groups and the stability of the conjugate base. In number (ii), the presence of bromine, an electron-withdrawing halogen, enhances acid strength. The molecule with bromine closest to the carboxyl group will be the strongest acid: 1. H_3 CH(Br)CH_2 COOH 2. CH_3 CH_2 CH(Br)COOH 3. CH_3 CH_2 CH_2 COOH 4. (CH_3)_2 CHCOOH Thus, strongest to weakest acid strength: H_3 CH(Br)CH_2 COOH > CH_3 CH_2 CH(Br)COOH > CH_3 CH_2 CH_2 COOH > (CH_3)_2 CHCOOH.

Comparing Acid Strength in Benzoic Acid Derivatives

In benzoic acid derivatives, electron-withdrawing groups increase acidity, while electron-donating groups decrease it. For number (iii), the nitro group is a strong electron-withdrawing group. 3,4-Dinitrobenzoic acid has two nitro groups and is the strongest acid. 4-Nitrobenzoic acid follows due to one nitro group. Benzoic acid without substituents is next. 4-Methoxybenzoic acid, containing an electron-donating methoxy group, is the weakest acid. Order: 4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid.

Key concepts

Reactivity Towards HCN

When considering the reactivity of compounds, particularly aldehydes and ketones, towards hydrogen cyanide (HCN), several factors come into play. The primary determinant is the electrophilicity of the carbonyl carbon, which is a carbon double-bonded to an oxygen. Electrophilic carbons are more likely to react with nucleophiles like HCN.
\[\text{Electrophilicity} \propto \text{Reactivity} \text{ Towards HCN}\]
Smaller aldehydes and ketones with minimal steric hindrance are more reactive. This is because larger groups around the carbonyl carbon impede the approach of nucleophiles. In our given list, acetaldehyde, with its simple structure, stands out as the most reactive due to having no bulky groups around its carbonyl carbon. Acetone, slightly larger, follows.

• Acetaldehyde: Most reactive
• Acetone: Moderately reactive
• Methyl tert-butyl ketone: Less reactive
• Di-tert-butyl ketone: Least reactive, due to bulky tert-butyl groups causing steric hindrance

This order highlights the influence of steric and electronic effects on reactivity with HCN.

Acid Strength in Carboxylic Acids

Acid strength in carboxylic acids is a fascinating aspect of chemistry that depends on several factors, the most influential being the presence of electron-withdrawing groups. These groups stabilize the conjugate base by drawing electron density away, increasing acidity.
In our exercise, bromine (Br) is an electron-withdrawing halogen. Its position relative to the carboxyl group has a substantial effect on acid strength.

• Bromine nearest to carboxyl: \( \mathrm{H}_3 \mathrm{CH(Br)CH}_2 \mathrm{COOH} \) is the strongest acid since electron-withdrawing effects are maximized.
• Bromine one carbon away: \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH(Br)COOH} \) is less pronounced in effect.
• \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{COOH} \) lacks electron-withdrawing substituents entirely, further reducing acid strength.
• \((\mathrm{CH}_3)_2 \mathrm{CHCOOH}\)

Therefore, the order from strongest to weakest acid is:
\( \mathrm{H}_3 \mathrm{CH(Br)CH}_2 \mathrm{COOH} > \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH(Br)COOH} > \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{COOH} > (\mathrm{CH}_3)_2 \mathrm{CHCOOH}\).

Role of Electron-Withdrawing Groups in Acid Strength

In benzoic acid derivatives, the acidity is notably influenced by electron-withdrawing groups. These groups, like nitro (NO\(_2\)), can delocalize negative charge over the aromatic ring when the acid donates a proton, enhancing the stability of the conjugate base.
In our case:

• 3,4-Dinitrobenzoic acid: Possesses two nitro groups, making it the strongest acid due to compounded electron-withdrawing effects.
• 4-Nitrobenzoic acid: With one nitro group, it is a strong acid but less so compared to the dinitro counterpart.
• Benzoic acid: Lacks electron-withdrawing substituents and is thus weaker in acidity.
• 4-Methoxybenzoic acid: Features a methoxy group, which donates electrons and reduces acidity further.

The presence and number of electron-withdrawing groups fundamentally dictate the strength of these acids. The hierarchy becomes:
\( \mathrm{4-Methoxybenzoic \ acid} < \mathrm{Benzoic \ acid} < \mathrm{4-Nitrobenzoic \ acid} < \mathrm{3,4-Dinitrobenzoic \ acid} \). This order demonstrates how electron-withdrawing and donating properties of substituents affect acidic behavior.