Alcohols, Phenols and Ethers

Alcohols contain the hydroxyl ($-\text{OH}$) group attached to an $sp^3$ carbon, with the general formula $\text{R–OH}$. They are classified by the carbon bearing the $-\text{OH}$ as primary ($1^\circ$), secondary ($2^\circ$) or tertiary ($3^\circ$) — a distinction that controls their oxidation and the Lucas test — and by the number of $-\text{OH}$ groups as mono-, di- (e.g. ethylene glycol) or trihydric (e.g. glycerol). The C–O and O–H bonds are both polar, which is the source of all their characteristic chemistry.

Several preparations are essential. From alkenes, acid-catalysed hydration adds water across the double bond following Markovnikov's rule (the $-\text{OH}$ goes to the more substituted carbon), while hydroboration–oxidation (with $\text{B}_2\text{H}_6$ then $\text{H}_2\text{O}_2$/$\text{OH}^-$) gives the anti-Markovnikov alcohol — a frequently tested contrast. From carbonyl compounds, reduction gives alcohols: aldehydes give primary alcohols and ketones give secondary alcohols, using $\text{NaBH}_4$, $\text{LiAlH}_4$ or catalytic $\text{H}_2$.

Two more routes complete the picture. Grignard reagents add to carbonyl compounds and then to water to give alcohols — methanal gives a $1^\circ$ alcohol, other aldehydes give $2^\circ$, and ketones give $3^\circ$ alcohols, a neat way to control the class of product. Carboxylic acids and esters can also be reduced (with $\text{LiAlH}_4$) to primary alcohols, and alkyl halides give alcohols on hydrolysis with aqueous alkali.

The physical properties of alcohols are dominated by hydrogen bonding through the O–H group. This makes their boiling points much higher than those of comparable hydrocarbons or ethers, and the lower alcohols (methanol, ethanol, propanol) are completely miscible with water because they hydrogen-bond to it; solubility falls as the carbon chain lengthens and the hydrocarbon part dominates. Boiling point rises with chain length and falls with branching — the same trends seen for alkanes, and a reliable NEET comparison.

Alcohols react in two broad ways — at the O–H bond and at the C–O bond. Because the O–H bond is polar, alcohols are weakly acidic: they react with reactive metals such as sodium to liberate hydrogen and form alkoxides ($\text{R–ONa}$). The acidity order of alcohols is $1^\circ \gt 2^\circ \gt 3^\circ$ (electron-donating alkyl groups destabilise the alkoxide), and alcohols are weaker acids than water — a contrast NEET likes to set against phenol in the next module.

Acting as nucleophiles or substrates, alcohols undergo esterification with carboxylic acids (warm, with concentrated $\text{H}_2\text{SO}_4$, giving a fragrant ester and water) and react with $\text{HX}$, $\text{PCl}_3$, $\text{PCl}_5$ or $\text{SOCl}_2$ to form haloalkanes (the link back to the previous chapter). The reaction with $\text{HCl}$/$\text{ZnCl}_2$ is the basis of the Lucas test, the classic way to tell the three classes apart: a tertiary alcohol gives an immediate cloudy turbidity, a secondary alcohol turns turbid in about five minutes, and a primary alcohol shows no change at room temperature.

Dehydration removes water to give an alkene. Heating an alcohol with concentrated $\text{H}_2\text{SO}_4$ (around 443 K) eliminates water; when more than one alkene is possible the major product follows Saytzeff's rule (more substituted alkene), and the ease of dehydration is $3^\circ \gt 2^\circ \gt 1^\circ$. This is intramolecular elimination, distinct from the intermolecular dehydration (at lower temperature) that forms ethers.

Oxidation is the most diagnostic reaction. A primary alcohol is oxidised first to an aldehyde and then to a carboxylic acid (strong oxidants such as acidified $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$ go all the way; the milder reagent PCC stops at the aldehyde). A secondary alcohol is oxidised to a ketone. A tertiary alcohol has no H on the C–OH carbon, so it resists oxidation under normal conditions (it only breaks down under very harsh acidic oxidation). Knowing the oxidation product therefore identifies the class — a guaranteed NEET question.

Phenols have the $-\text{OH}$ group attached directly to a benzene ring; the simplest is phenol itself, $\text{C}_6\text{H}_5\text{OH}$. Although it shares the $-\text{OH}$ group with alcohols, its proximity to the aromatic ring changes its behaviour dramatically — most importantly, it is distinctly acidic. Phenol is prepared industrially by the cumene process (oxidation of isopropylbenzene, which also yields acetone), and in the lab from chlorobenzene (the Dow process), from benzenesulphonic acid (fusion with NaOH), and from benzenediazonium salts (warming with water).

The acidity of phenol is the central NEET idea. When phenol loses its proton it forms the phenoxide ion, in which the negative charge is delocalised over the ortho and para positions of the ring by resonance. This stabilises the ion, so phenol gives up its proton far more readily than an alcohol does. Phenol is therefore more acidic than water and alcohols, but less acidic than carboxylic acids (whose carboxylate ion is even more stabilised). Electron-withdrawing groups like $-\text{NO}_2$ increase acidity by further stabilising the phenoxide — 2,4,6-trinitrophenol (picric acid) is strongly acidic — while electron-donating groups decrease it.

A direct consequence is that phenol reacts with sodium hydroxide to give sodium phenoxide and water, whereas ordinary alcohols do not react with NaOH — a simple chemical test that separates the two. Phenol also gives a characteristic violet colour with neutral $\text{FeCl}_3$, a standard identification test.

Because $-\text{OH}$ is a strongly activating, ortho/para-directing group, the ring of phenol is very reactive towards electrophilic substitution. Phenol reacts with bromine water even without a catalyst to give a white precipitate of 2,4,6-tribromophenol, and is readily nitrated and sulphonated. Two named reactions are NEET essentials: Kolbe's reaction (sodium phenoxide + $\text{CO}_2$ under pressure $\rightarrow$ salicylic acid, the basis of aspirin) and the Reimer–Tiemann reaction (phenol + $\text{CHCl}_3$ + $\text{NaOH}$ $\rightarrow$ salicylaldehyde, introducing a $-\text{CHO}$ group at the ortho position).

Ethers have two alkyl or aryl groups joined to an oxygen, $\text{R–O–R'}$; they are symmetrical when the two groups are identical (e.g. diethyl ether) and unsymmetrical/mixed when they differ (e.g. ethyl methyl ether). The C–O–C linkage is bent and the oxygen carries two lone pairs, but there is no O–H, which shapes both their physical properties and their relative inertness.

The most important preparation is the Williamson ether synthesis: a sodium alkoxide reacts with an alkyl halide ($\text{R–ONa} + \text{R'–X} \rightarrow \text{R–O–R'}$). Because the step is SN2, the alkyl halide should be primary — a secondary or tertiary halide would instead undergo elimination to an alkene. This method is versatile: it can make both symmetrical and mixed ethers, and to make a mixed ether you choose the combination that puts the bulkier group on the alkoxide and the simpler primary group on the halide. Symmetrical ethers can also be made by acid dehydration of a primary alcohol at a moderate temperature (about 413 K), but this works only for primary alcohols.

In their physical properties, ethers cannot hydrogen-bond to one another (no O–H), so their boiling points are much lower than those of isomeric alcohols and similar to comparable alkanes. However, the oxygen lone pairs can accept hydrogen bonds from water, so ethers are slightly soluble in water — about as soluble as an alcohol of the same size. Diethyl ether's volatility and low reactivity made it a famous early anaesthetic and a common laboratory solvent.

Ethers are fairly unreactive (they resist bases, oxidising agents and reducing agents), but they are cleaved by hot concentrated halogen acids, especially HI and HBr. One C–O bond breaks to give an alkyl halide and an alcohol; with excess acid the alcohol is converted to a second alkyl halide. With aromatic ethers such as anisole ($\text{C}_6\text{H}_5\text{–O–CH}_3$), the aryl–oxygen bond is not broken (a phenyl cation would be too unstable), so cleavage with HI gives phenol and methyl iodide, not iodobenzene — a favourite NEET point. Aromatic ethers also undergo electrophilic substitution, with the $-\text{OR}$ group acting as an activating, ortho/para director.