Question

Given below are two statements: 
Statement (I): In the case of formaldehyde, \( K \) is about 2280, due to small substituents, hydration is faster. 
 
Statement (II): In the case of trichloroacetaldehyde, \( K \) is about 2000 due to the -I effect of Cl. 
 
In the light of the above statements, choose the correct answer from the options given below:

• A. Statement I true but Statement II is false
• B. Both Statement I and Statement II are true
• C. Statement I is false but Statement II is true
• D. Both Statement I and Statement II are false

Hint:

The rate of hydration and the equilibrium constant depend on the electron-withdrawing or electron-donating effects of substituents on the carbonyl group. Small substituents favor faster hydration.

Answer 1

The Correct Option is B

To determine the correct answer, we need to analyze both statements regarding their veracity based on chemical principles:

1. Statement I: "In the case of formaldehyde, \( K \) is about 2280, due to small substituents, hydration is faster."
   • Formaldehyde (\( \text{HCHO} \)) is a simple aldehyde where the carbonyl group (\( \text{C=O} \)) is bound to one hydrogen atom and one aldehyde hydrogen. Because of its small size, there is less steric hindrance, which increases its reactivity towards nucleophilic attack by water, leading to faster hydration. 
   • The hydration of aldehydes involves the addition of water across the carbonyl bond to form a geminal diol. This reaction is influenced by both steric factors and electronic factors.
   • The given hydration constant \( K \) is a reflection of this high reactivity. Therefore, Statement I is true.
2. Statement II: "In the case of trichloroacetaldehyde, \( K \) is about 2000 due to the -I effect of Cl."
   • Trichloroacetaldehyde, also known as chloral (\( \text{CCl}_3\text{CHO} \)), has three chlorine atoms attached to the carbon adjacent to the carbonyl group.
   • The chlorine atoms exert a strong electron-withdrawing inductive effect (denoted as the -I effect), which polarizes the carbonyl group, making the carbon more electrophilic.
   • This increased electrophilicity enhances the reaction with nucleophiles such as water, thus favoring the formation of the geminal diol.
   • The hydration constant \( K \) being around 2000 supports this assertion. Therefore, Statement II is true.

Based on the analysis above, both Statement I and Statement II are true about the hydration constants and the effects impacting the reactivity of formaldehyde and trichloroacetaldehyde.

 

Answer 2

Statement (I): Formaldehyde (\( CH_2O \)) has small substituents, and the small size of the substituents leads to faster hydration due to less steric hindrance around the carbonyl group. The equilibrium constant \( K \) for the hydration of formaldehyde is about 2280, indicating that hydration is relatively fast.
 - Statement (II): Trichloroacetaldehyde (\( CCl_3CHO \)) has three chlorine atoms attached to the alpha-carbon. The chlorine atoms withdraw electron density from the carbonyl carbon through the -I effect, making the carbonyl carbon less electrophilic and slowing down hydration. The equilibrium constant \( K \) for trichloroacetaldehyde is around 2000, indicating slower hydration. Both statements are true.