Organic Chemistry

Analysis of statements on physical properties of alcohols:

• As the number of carbon atoms increases, the molecular surface area increases, strengthening van der Waals dispersion forces and increasing the boiling point $\rightarrow$ Statement A is correct.
• With branching, the molecule becomes more spherical, surface area decreases, van der Waals forces weaken, and boiling point decreases (not increases) $\rightarrow$ Statement B is false.
• Due to strong intermolecular hydrogen bonding, boiling points of alcohols are substantially higher than those of haloalkanes and hydrocarbons of comparable molecular mass $\rightarrow$ Statement C is false; Statement D is correct.
• The exceptionally high boiling points of alcohols are primarily due to the extensive intermolecular hydrogen bonding between hydroxyl groups $\rightarrow$ Statement E is correct.

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 ($\text{C}_2\text{H}_5\text{Br}$).

The nitro group ($-\text{NO}_2$) is strongly electron-withdrawing ($-I$ and $-M$ effects), stabilizing the phenoxide anion and greatly increasing acidity:

• Phenol (D) has no electron-withdrawing groups ($\text{p}K_a \approx 10.0$, least acidic).
• nitrophenol (A) has one meta-nitro group operating via $-I$ effect ($\text{p}K_a \approx 8.3$).
• 3,5-dinitrophenol (B) has two meta-nitro groups operating via additive $-I$ effects ($\text{p}K_a \approx 6.7$).
• 2,4,6-trinitrophenol (C, picric acid) has three nitro groups operating at ortho and para positions with powerful $-M$ resonance stabilization ($\text{p}K_a \approx 0.38$, most acidic).
  Increasing order: Phenol (D) < 3-nitrophenol (A) < 3,5-Dinitrophenol (B) < 2,4,6-Trinitrophenol (C).