7.2 Properties and Reactions of Alcohols and Phenols

Alcohols react with active metals such as sodium, potassium and aluminium, giving alkoxides and hydrogen:

• Alcohols are weaker acids than water: an alkoxide ion is a stronger base than hydroxide and takes a proton from water, $\mathrm{RO^{-}+H_{2}O\rightarrow ROH+OH^{-}}$. So sodium ethoxide cannot exist in water, and alcohols do not react with aqueous sodium hydroxide.
• Acid strength: 1° $\gt$ 2° $\gt$ 3°. Alkyl groups release electrons (+I effect), which increases the electron density on oxygen and makes the alkoxide ion less stable. More alkyl groups on the carbinol carbon, weaker acid.
• Alcohols are also weak bases: a lone pair on oxygen accepts a proton from a strong acid, $\mathrm{ROH+H^{+}\rightleftharpoons R\overset{+}{O}H_{2}}$. This protonation starts most C–O-cleavage reactions below.

Phenols react with sodium and also with aqueous sodium hydroxide, which alcohols cannot do:

• Why phenol is more acidic than an alcohol. (i) The $\mathrm{OH}$ is on an $sp^{2}$ carbon, which is more electronegative than an $sp^{3}$ carbon and pulls electrons from oxygen, so the $\mathrm{O{-}H}$ bond is more polar. (ii) More importantly, the negative charge of the phenoxide ion is delocalised by resonance over the oxygen and the ortho and para carbons of the ring (figure below). In phenol itself the resonance structures need a separation of charge and contribute less, so ionisation is favoured. An alkoxide ion has no resonance: its charge sits on one oxygen.
• Electron-withdrawing groups such as $\mathrm{{-}NO_{2}}$ increase the acidity, most strongly at the ortho and para positions, where the nitro group can take part in the delocalisation (–R effect as well as –I). At the meta position only the weaker –I effect operates. Two or three nitro groups have a large cumulative effect: 2,4,6-trinitrophenol (picric acid) is about as strong as a mineral acid.
• Electron-releasing groups such as $\mathrm{{-}CH_{3}}$ (+I, hyperconjugation) decrease the acidity slightly: the cresols are weaker acids than phenol.

Acid strength is measured by $K_{a}$ for $\mathrm{ArOH\rightleftharpoons ArO^{-}+H^{+}}$, or by $\mathrm{p}K_{a}=-\log K_{a}$. The smaller the $\mathrm{p}K_{a}$, the stronger the acid; one unit of $\mathrm{p}K_{a}$ is a factor of $10$ in $K_{a}$.

Alcohols and phenols react with carboxylic acids, acid anhydrides and acid chlorides to form esters; the $\mathrm{O{-}H}$ bond of the alcohol breaks:

With a carboxylic acid the reaction is reversible; concentrated $\mathrm{H_{2}SO_{4}}$ catalyses it and removing the water drives it forward. With an acid chloride, pyridine neutralises the $\mathrm{HCl}$ formed and shifts the equilibrium to the right. Introducing an acetyl group, $\mathrm{CH_{3}CO{-}}$, this way is called acetylation; acetylating salicylic acid (2-hydroxybenzoic acid) gives aspirin (acetylsalicylic acid):

• Reactivity of the acid: $\mathrm{HI\gt HBr\gt HCl}$. Reactivity of the alcohol: 3° $\gt$ 2° $\gt$ 1°, the order of stability of the carbocations formed after protonation and loss of water. A tertiary alcohol reacts with concentrated $\mathrm{HCl}$ simply on shaking at room temperature; primary and secondary alcohols need anhydrous $\mathrm{ZnCl_{2}}$ as a catalyst, and primary alcohols also need heating.
• Phosphorus and sulphur halides give alkyl halides from all classes of alcohol: $\mathrm{3ROH+PBr_{3}\rightarrow3RBr+H_{3}PO_{3}}$, $\ \mathrm{ROH+PCl_{5}\rightarrow RCl+POCl_{3}+HCl}$, $\ \mathrm{ROH+SOCl_{2}\rightarrow RCl+SO_{2}+HCl}$ (Chapter 6).
• Phenols do not react with $\mathrm{HX}$ in this way: their C–O bond has partial double-bond character and the ring carbon will not form a carbocation.

Heated with a protic acid (concentrated $\mathrm{H_{2}SO_{4}}$ or $\mathrm{H_{3}PO_{4}}$), or passed as vapour over heated alumina, an alcohol loses water and gives an alkene. The ease of dehydration is 3° $\gt$ 2° $\gt$ 1°, shown by the much milder conditions needed:

Mechanism (ethanol). Step 1 (fast): the alcohol is protonated, $\mathrm{CH_{3}CH_{2}OH+H^{+}\rightleftharpoons CH_{3}CH_{2}\overset{+}{O}H_{2}}$. Step 2 (slow, rate-determining): the protonated alcohol loses water, $\mathrm{CH_{3}CH_{2}\overset{+}{O}H_{2}\rightleftharpoons CH_{3}\overset{+}{C}H_{2}+H_{2}O}$. Step 3 (fast): a proton is lost from the carbon next to the positive one, $\mathrm{CH_{3}\overset{+}{C}H_{2}\rightleftharpoons CH_{2}{=}CH_{2}+H^{+}}$, regenerating the acid.

Because step 2 makes a carbocation, the ease of dehydration follows carbocation stability. When the proton can be lost from more than one carbon, the more substituted alkene is the main product (Saytzeff's rule, Chapter 6). At a lower temperature ($413\ \mathrm{K}$) with excess ethanol, a second alcohol molecule attacks the protonated alcohol instead and the product is an ether (Section 7.3).

Oxidation of an alcohol removes the hydrogen of $\mathrm{O{-}H}$ and a hydrogen from the carbinol carbon, forming $\mathrm{C{=}O}$. So the result depends on how many hydrogens the carbinol carbon carries.

• Primary alcohols give aldehydes, which are easily oxidised further to carboxylic acids: $\mathrm{RCH_{2}OH\xrightarrow{[O]}RCHO\xrightarrow{[O]}RCOOH}$. Strong oxidising agents (acidified $\mathrm{KMnO_{4}}$ or $\mathrm{K_{2}Cr_{2}O_{7}}$) go all the way to the acid. To stop at the aldehyde, use $\mathrm{CrO_{3}}$ in an anhydrous medium or pyridinium chlorochromate (PCC, $\mathrm{C_{5}H_{5}NH^{+}CrO_{3}Cl^{-}}$) in dichloromethane.
• Secondary alcohols give ketones: $\mathrm{R_{2}CHOH\xrightarrow{CrO_{3}}R_{2}C{=}O}$.
• Tertiary alcohols have no H on the carbinol carbon and resist oxidation; under drastic conditions (hot acidified $\mathrm{KMnO_{4}}$) C–C bonds break and a mixture of acids with fewer carbon atoms forms.

Dehydrogenation. Alcohol vapour passed over heated copper at $573\ \mathrm{K}$ loses $\mathrm{H_{2}}$: primary alcohols give aldehydes, secondary give ketones, and tertiary alcohols, which cannot lose $\mathrm{H_{2}}$ this way, are dehydrated to alkenes:

In the body, enzymes oxidise ethanol to ethanal and then ethanoic acid. Methanol is oxidised in the same way to methanal and then methanoic acid, which damage the optic nerve and can kill; a treatment is a slow infusion of ethanol, which competes for the same enzyme so that the methanol is excreted unchanged.

The $\mathrm{{-}OH}$ group is activating and ortho/para directing.

(a) Nitration. With dilute nitric acid at $298\ \mathrm{K}$ phenol gives a mixture of 2-nitrophenol and 4-nitrophenol:

The isomers are separated by steam distillation: 2-nitrophenol is steam-volatile because its $\mathrm{OH}$ forms a hydrogen bond with the neighbouring nitro group inside the molecule (intramolecular), whereas 4-nitrophenol forms hydrogen bonds between molecules (intermolecular), which makes it much less volatile. With concentrated nitric acid phenol gives 2,4,6-trinitrophenol (picric acid), in poor yield because much of the phenol is oxidised; it is better made by first sulphonating phenol with concentrated $\mathrm{H_{2}SO_{4}}$ (to phenol-2,4-disulphonic acid) and then treating with concentrated $\mathrm{HNO_{3}}$.

(b) Halogenation. No Lewis-acid catalyst is needed. In a solvent of low polarity ($\mathrm{CS_{2}}$ or $\mathrm{CHCl_{3}}$) at low temperature, phenol gives monobromophenols, mainly 4-bromophenol with some 2-bromophenol. With bromine water, where phenol is partly ionised to the even more reactive phenoxide and $\mathrm{Br_{2}}$ is polarised by water, all three o/p positions are substituted and a white precipitate of 2,4,6-tribromophenol forms at once, a test for phenol:

• Zinc dust: heating phenol with zinc dust removes the oxygen and gives benzene, $\mathrm{C_{6}H_{5}OH+Zn\rightarrow C_{6}H_{6}+ZnO}$.
• Oxidation: with chromic acid ($\mathrm{Na_{2}Cr_{2}O_{7}}$ and $\mathrm{H_{2}SO_{4}}$) phenol is oxidised to benzo-1,4-quinone ($p$-benzoquinone), $\mathrm{C_{6}H_{4}O_{2}}$, a yellow conjugated diketone. Slow oxidation in air gives dark-coloured mixtures containing quinones, which is why old samples of phenol turn pink or brown.