Question

The following statements are true experimental observations. Explain the reason behind each observation. (a) The reaction of acetic acid with ammonia in water does not give any amide products. (b) The reaction of acetyl chloride with water causes the \(\mathrm{pH}\) to decrease. (c) The hydrolysis of an amide at neutral pH takes seven years at room temperature, while the hydrolysis of an acid chloride takes a few minutes.

Short answer

#tag_title# Summary #tag_content# In summary, the observed reactions can be explained by understanding the nature and reactivity of the involved functional groups and the reaction mechanisms: 1. The reaction of acetic acid with ammonia does not produce amides because an ammonium salt is formed instead, due to the acid-base reaction favored in the presence of water. 2. The reaction of acetyl chloride with water results in a decrease in pH due to the formation of acetic acid and hydrochloric acid, which release H+ ions, increasing the acidity of the solution. 3. The difference in hydrolysis rates between amides and acid chlorides is attributed to the stability of amides due to resonance and the high reactivity of acid chlorides due to their electron-withdrawing chloride group. Therefore, the hydrolysis of amides takes significantly longer compared to the hydrolysis of acid chlorides under neutral pH and room temperature conditions.

Step-by-step solution

Observation (a) - Reaction of Acetic Acid with Ammonia

The observation states that the reaction of acetic acid with ammonia in water does not give any amide products. To explain this, let's first write down the reaction between acetic acid (CH3COOH) and ammonia (NH3): CH3COOH + NH3 → CH3CONH2 + H2O However, this reaction does not form any amides. Instead, it forms an ammonium salt—CH3COO- + NH4+. Since the acid-base reaction is favored in the presence of water due to its high polarity, this reaction proceeds via proton transfer from the acidic proton of acetic acid to the lone pair of electrons on nitrogen in ammonia, forming the corresponding ammonium salt: CH3COOH + NH3 → CH3COO- + NH4+ Therefore, no amides are formed in this process.

Observation (b) - Reaction of Acetyl Chloride with Water

The observation states that the reaction of acetyl chloride (CH3COCl) with water causes the pH to decrease, meaning the solution becomes more acidic. Acetyl chloride reacts with water in hydrolysis forming acetic acid and hydrochloric acid. Therefore, the presence of these acids in the solution will cause the pH to decrease. Reaction can be represented as: CH3COCl + H2O → CH3COOH + HCl As HCl is a strong acid, it ionizes completely, releasing H+ ions which lowers the pH: HCl → H+ + Cl-

Observation (c) - Hydrolysis of Amide vs Acid Chloride at Neutral pH

The observation states that the hydrolysis of an amide at neutral pH takes seven years at room temperature, while the hydrolysis of an acid chloride takes a few minutes. This difference in reaction time can be attributed to the stability and reactivity of the functional groups involved. Amide (RCONH2) is considerably stable due to resonance stabilization, causing the reaction with water to be slow and take a considerably longer time to occur under neutral pH and room temperature. The reaction is as follows: RCONH2 + H2O → RCOOH + NH3 On the other hand, acid chlorides (RCOCl) are highly reactive and easily hydrolyzed by water due to the strong electron-withdrawing effect of the chloride group, which increases the electrophilicity of the carbonyl carbon. The reaction occurs as follows: RCOCl + H2O → RCOOH + HCl Since acid chlorides are more reactive in comparison to amides, they also hydrolyze at a faster rate. This explains the difference in the reaction times of hydrolysis of amides and acid chlorides at neutral pH and room temperature.