Physical properties and acidity

Boiling points. Alcohols and phenols boil much higher than hydrocarbons, ethers and haloalkanes of similar size. The reason is hydrogen bonding: the H of one –OH is attracted to the O of another, and extra energy is needed to pull the molecules apart. Ethers have no O–H, so they cannot hydrogen-bond to each other.

Two more rules: the boiling point rises as the carbon chain gets longer, and falls with branching. A straight chain has more surface contact with its neighbours than a compact, branched one.

Solubility. Small alcohols dissolve in water because their –OH forms hydrogen bonds with water. As the carbon chain grows, the oily, non-polar part takes over, and solubility falls.

Acidity. Alcohols and phenols both have an O–H that can lose H⁺. Alcohols are very weakly acidic: they react with sodium metal, giving hydrogen gas, but not with NaOH. Phenol is much more acidic (pKa 10, against about 16 for ethanol) and does react with NaOH.

Why? When phenol loses H⁺, the negative charge on the phenoxide ion can spread into the benzene ring by resonance. A spread-out charge is a stable charge, and the more stable the ion left behind, the stronger the acid. In an alkoxide ion such as ethoxide, the charge stays stuck on the oxygen.

Groups on the ring change phenol's acidity:

• Electron-withdrawing groups such as –NO₂ pull charge away, stabilise the phenoxide, and increase acidity. They work best at the ortho and para positions. Of the two, p-nitrophenol is a little more acidic, because in o-nitrophenol the OH is held by an intramolecular hydrogen bond to the nearby NO₂.
• Electron-releasing groups such as –CH₃ and –OCH₃ push charge in and decrease acidity.