19.5 Alcohols, Phenols & Ethers: Preparation and Reactions

Alcohols, Phenols & Ethers: Preparation and Reactions

Alcohols ($\text{R}-\text{OH}$), phenols ($\text{Ar}-\text{OH}$) and ethers ($\text{R}-\text{O}-\text{R}'$) all contain oxygen bonded by single bonds, but they behave very differently. Section 19.4 already covered the Lucas test, the iodoform test, Grignard synthesis of alcohols and the acidity of phenol. This section adds the preparation methods and the reactions you need to predict products.

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1. Classification and Physical Properties of Alcohols

Alcohols with two OH groups are diols (ethane-1,2-diol, used as antifreeze) and with three are triols (glycerol, propane-1,2,3-triol).

• Boiling points are much higher than those of alkanes or ethers of similar molar mass because alcohol molecules are hydrogen bonded. Ethanol boils at $78^\circ\text{C}$, while dimethyl ether (an isomer) boils at $-24^\circ\text{C}$.
• Solubility: lower alcohols mix completely with water (they hydrogen bond with it); solubility falls as the hydrocarbon chain grows.

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2. Preparation of Alcohols

Ethanol by Fermentation

Molasses, the syrup left after sugar crystallization (an important by-product of Pakistan's sugar industry), contains sucrose. Yeast supplies two enzymes:

$\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{invertase}} \text{C}_6\text{H}_{12}\text{O}_6\ (\text{glucose}) + \text{C}_6\text{H}_{12}\text{O}_6\ (\text{fructose})$

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{zymase}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

Fermentation works best at about $25$–$35^\circ\text{C}$ and stops when the alcohol concentration reaches roughly 12–15%, because the yeast is killed. Fractional distillation gives rectified spirit (about 95% ethanol, a constant-boiling azeotrope with water). Absolute alcohol (about 100%) is made by removing the remaining water, for example with quicklime. Methylated spirit is ethanol made unfit to drink by adding methanol and other substances.

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3. Reactions of Alcohols

(a) Reactions Breaking the O–H Bond

• With sodium: $2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2$. The reaction is much gentler than that of sodium with water, showing that alcohols are weaker acids than water.
• Esterification: an alcohol and a carboxylic acid form an ester and water in a reversible reaction catalysed by concentrated $\text{H}_2\text{SO}_4$: $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \underset{\text{conc. H}_2\text{SO}_4}{\rightleftharpoons} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$ Ethyl acetate has a pleasant fruity smell.

(b) Reactions Breaking the C–O Bond (Replacing OH by Halogen)

• $\text{R}-\text{OH} + \text{HX} \rightarrow \text{R}-\text{X} + \text{H}_2\text{O}$ (reactivity $\text{HI} > \text{HBr} > \text{HCl}$; HCl needs $\text{ZnCl}_2$, as in the Lucas test)
• $\text{R}-\text{OH} + \text{PCl}_5 \rightarrow \text{R}-\text{Cl} + \text{POCl}_3 + \text{HCl}$ (white fumes of HCl: a test for the OH group)
• $3\text{R}-\text{OH} + \text{PCl}_3 \rightarrow 3\text{R}-\text{Cl} + \text{H}_3\text{PO}_3$
• $\text{R}-\text{OH} + \text{SOCl}_2 \xrightarrow{\text{pyridine}} \text{R}-\text{Cl} + \text{SO}_2 + \text{HCl}$ (thionyl chloride is preferred because both by-products are gases, leaving a pure product)

(c) Dehydration

$\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4,\ 170^\circ\text{C}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \quad (\text{excess acid})$

$2\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4,\ 140^\circ\text{C}} \text{C}_2\text{H}_5-\text{O}-\text{C}_2\text{H}_5 + \text{H}_2\text{O} \quad (\text{excess alcohol})$

The ease of dehydration is $3^\circ > 2^\circ > 1^\circ$, because the reaction proceeds through a carbocation. Unsymmetrical alcohols follow Saytzeff's rule.

(d) Oxidation

With acidified potassium dichromate (orange $\text{Cr}_2\text{O}_7^{2-}$ turns green $\text{Cr}^{3+}$):

To stop at the aldehyde, distil it off as it forms, because it boils at a lower temperature than the alcohol. This colour change is the principle of the breath-alcohol test.

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4. Phenol

Preparation

• From sodium benzenesulphonate: fusing with sodium hydroxide at about $300^\circ\text{C}$ gives sodium phenoxide, which is acidified to phenol.
• From chlorobenzene (Dow process): heating with aqueous NaOH at about $350^\circ\text{C}$ under high pressure, then acidifying.
• From cumene: air oxidation of isopropylbenzene (cumene) and acid treatment gives phenol and acetone; this is the main modern industrial route.

Reactions

The OH group is an ortho/para director because its lone pair is donated into the ring by resonance (Section 19.3). Phenol is used to make Bakelite (with formaldehyde), antiseptics, dyes, aspirin and nylon intermediates.

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5. Ethers

Ethers have the general formula $\text{R}-\text{O}-\text{R}'$ and are functional-group isomers of alcohols (for example, ethanol and dimethyl ether are both $\text{C}_2\text{H}_6\text{O}$).

Williamson synthesis: an alkoxide reacts with a primary alkyl halide by $\text{S}_\text{N}2$:

$\text{C}_2\text{H}_5\text{ONa} + \text{CH}_3\text{I} \rightarrow \text{C}_2\text{H}_5-\text{O}-\text{CH}_3 + \text{NaI}$

Ethers can also be made by dehydrating excess alcohol at $140^\circ\text{C}$, as shown above.

• Ethers have no O–H bond, so their molecules cannot hydrogen bond with each other; their boiling points are close to those of alkanes of similar mass.
• They are fairly unreactive towards bases, sodium and oxidizing agents, which makes diethyl ether a good solvent (it is used for Grignard reagents and for solvent extraction).
• Concentrated HI cleaves them: $\text{R}-\text{O}-\text{R} + 2\text{HI} \rightarrow 2\text{R}-\text{I} + \text{H}_2\text{O}$ (on heating).
• Diethyl ether is very volatile and highly flammable and was once used as an anaesthetic.