कार्बनिक रसायन

Analysis of acidic strength:

• Aliphatic alcohols (Propan-1-ol, A) are much weaker acids than phenols due to the absence of resonance stabilization of the alkoxide ion $\rightarrow$ lowest acidity.
• In 4-methylphenol (E), the electron-donating $+I$ and hyperconjugative methyl group decreases the acidity of phenol $\rightarrow$ less acidic than phenol.
• Phenol (D) is more acidic than 4-methylphenol.
• In 3-nitrophenol (B), the electron-withdrawing nitro group ($-I$ effect at meta position) stabilizes the phenoxide ion $\rightarrow$ more acidic than phenol.
• In 3,5-dinitrophenol (C), two electron-withdrawing nitro groups exert strong cumulative $-I$ inductive stabilization on the phenoxide ion $\rightarrow$ highest acidity.

In alkyl aryl ethers such as ethoxybenzene ($\text{C}_6\text{H}_5-\text{O}-\text{C}_2\text{H}_5$):

• The oxygen atom is first protonated by $\text{HBr}$ to form an oxonium ion: $\text{C}_6\text{H}_5-\text{O}^+(\text{H})-\text{C}_2\text{H}_5$.
• The bond between oxygen and the phenyl ring ($sp^2$ hybridized carbon) has partial double bond character due to resonance and is significantly stronger than the alkyl $\text{C-O}$ bond ($sp^3$ hybridized carbon).
• Nucleophilic attack by $\text{Br}^-$ occurs exclusively via an $\text{S}_\text{N}2$ mechanism at the smaller ethyl carbon, cleaving the alkyl-oxygen bond.
• The products formed are phenol ($\text{C}_6\text{H}_5\text{OH}$) and ethyl bromide ($\text{C}_2\text{H}_5\text{Br}$).

Reactivity towards nucleophilic addition is governed by steric hindrance and $+I$ electron donation:

• Ketones are less reactive than aldehydes because two alkyl groups hinder attack and reduce partial positive charge on carbonyl carbon.
• Between the two ketones, butanone ($\text{CH}_3\text{COCH}_2\text{CH}_3$) has greater steric hindrance than propanone ($\text{CH}_3\text{COCH}_3$). Thus: Butanone (D) < Propanone (B).
• Between the two aldehydes, propanal ($\text{CH}_3\text{CH}_2\text{CHO}$) has larger steric hindrance than ethanal ($\text{CH}_3\text{CHO}$). Thus: Propanal (C) < Ethanal (A).
  Increasing order: Butanone (D) < Propanone (B) < Propanal (C) < Ethanal (A).