Alcohols, Phenols and Ethers

Written and reviewed by Akanksha Tiwari, Physics & Chemistry faculty at UniDrill (M.Sc. Physics, Banaras Hindu University · 8 years teaching Class XI–XII science). Based on NCERT Chemistry Class XII, Chapter 7.

Two compounds, one formula, a hundred degrees apart

Ethanol (CH₃CH₂OH) and methoxymethane (CH₃OCH₃) both have the formula C₂H₆O and the same mass. Yet ethanol is a liquid that boils at 351 K, while methoxymethane is a gas that boils at 249 K. The only difference is where the hydrogen sits. In ethanol it is bonded to oxygen, so molecules can grip each other by hydrogen bonds. In the ether it is bonded only to carbon, so they cannot. That one fact, the –OH group makes hydrogen bonds, explains the boiling points, the solubility and much of the reactivity of this chapter. The second big idea is that the oxygen is bonded to a different kind of carbon in each family, which decides how acidic and how reactive the compound is.

The three families and their structures

Alcohols by the number of –OH groups: monohydric (one), dihydric (two, such as ethane-1,2-diol), trihydric (three, such as propane-1,2,3-triol or glycerol) and polyhydric. Monohydric alcohols by the carbon: primary (1°), secondary (2°) and tertiary (3°) alcohols have the –OH on a carbon with one, two or three other carbons attached. An allylic alcohol has –OH on an sp³ carbon next to C=C, a benzylic alcohol has it next to a benzene ring, and a vinylic alcohol (CH₂=CH–OH) has it on an sp² carbon of a double bond. Ethers are simple (symmetrical) if both groups are the same (CH₃CH₂OCH₂CH₃) and mixed (unsymmetrical) if not (C₂H₅OC₆H₅).

Naming them

In IUPAC names an alcohol takes the ending -ol, replacing the final "e" of the alkane. The chain is numbered to give –OH the lowest locant. In polyhydric alcohols the "e" is kept (ethane-1,2-diol). Phenols are named as substituted phenols. Ethers are named as alkoxy alkanes: the smaller group becomes the alkoxy prefix on the larger parent chain.

Worked example: naming. Name CH₃CH(CH₃)CH₂CH₂OH and (CH₃)₂CHOCH₂CH₃.

Making alcohols

Acid catalysed hydration happens in three steps. The key is the middle one, which is why the rule is Markovnikov's.

Examples from Grignard reagents: CH₃MgBr with HCHO gives ethanol, with CH₃CHO gives propan-2-ol, and with CH₃COCH₃ gives 2-methylpropan-2-ol (a tertiary alcohol) after hydrolysis.

Making phenols

Beyond the textbook. For the exam: the cumene process gives phenol and acetone together. In reality: this is the main industrial route to both, and its economics depend on acetone, which is a by-product that is always made in a fixed amount alongside the phenol: C₆H₅CH(CH₃)₂ + O₂ → C₆H₅C(CH₃)₂OOH, followed by C₆H₅C(CH₃)₂OOH → C₆H₅OH + CH₃COCH₃. Why it matters: it is a good example of a cheap starting material, benzene and propene, converted into two valuable products without waste.

Physical properties: it all comes back to hydrogen bonding

For the PYQ ordering: butane < ethoxyethane < pentanal < pentan-1-ol. Pentanal is polar (dipole–dipole forces) but cannot hydrogen bond to itself, so it sits between the ether and the alcohol.

Boiling points rise with chain length and fall with branching: butan-1-ol (391 K) > butan-2-ol (373 K) > 2-methylpropan-2-ol (356 K).

Solubility in water. The lower alcohols mix with water in all proportions because they hydrogen bond to it. As the non-polar alkyl part grows, solubility falls. Ethers are slightly soluble, since the oxygen can accept H-bonds from water but has none to give. Phenol is moderately soluble because the large non-polar ring offsets the –OH.

Acidity: why phenol is acidic and alcohols barely are

Alcohols and phenols are both weak Brønsted acids. They react with active metals to give hydrogen:

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂ and 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂

Only phenol also reacts with aqueous sodium hydroxide: C₆H₅OH + NaOH → C₆H₅ONa + H₂O. Alcohols do not.

Why phenol is more acidic than ethanol. After losing H⁺, an alkoxide (RO⁻) keeps its negative charge on oxygen alone. A phenoxide ion can spread the charge over the oxygen and the ring carbons through five resonance structures, so it is much more stable. A more stable conjugate base means a stronger acid. The difference is large: phenol has pKa about 10 and ethanol 15.9, so phenol is about a million times more acidic.

Why 1° alcohols are more acidic than 3°. Alkyl groups push electrons towards oxygen. In a tertiary alkoxide there are more such groups, so the negative charge is less stable and the alcohol is less willing to lose H⁺. The order is phenol > water > 1° > 2° > 3° alcohols. Alcohols are weaker acids than water, so alkoxides are stronger bases than hydroxide.

Substituents on phenol. An electron withdrawing group helps spread the negative charge on the phenoxide and makes the phenol more acidic. An electron releasing group does the opposite.

Worked example: ranking acidity. Arrange phenol, 3-nitrophenol, 3,5-dinitrophenol and 2,4,6-trinitrophenol in increasing acid strength.

The common wrong step is to think that a meta nitro group has no effect. It still withdraws electrons through the sigma framework, only less than at ortho or para.

Pause and predict: will ethanol dissolve in aqueous NaOH to form sodium ethoxide?

Check your answer: no. Ethanol is a weaker acid than water, so hydroxide cannot deprotonate it appreciably. Sodium ethoxide must be made with metallic sodium.

Reactions of alcohols that break the O–H bond

Esterification. An alcohol (or a phenol) reacts with a carboxylic acid, an acid anhydride or an acid chloride to give an ester.

Acetylation of salicylic acid with acetic anhydride gives aspirin: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH.

Reactions that break the C–O bond

With hydrogen halides. R–OH + HX → R–X + H₂O. Reactivity is 3° > 2° > 1°, and HI > HBr > HCl. The Lucas test uses this to tell the classes of alcohol apart. Lucas reagent is concentrated HCl with ZnCl₂. The alkyl chloride formed is insoluble, so the clear solution turns cloudy.

Other reagents: PBr₃ gives R–Br, and SOCl₂ gives R–Cl cleanly.

Dehydration. The same alcohol gives different products depending on temperature and which reagent is in excess.

Ease of dehydration to alkenes is 3° > 2° > 1°. The temperature needed falls from 443 K for primary alcohols to 440 K for secondary and 358 K for tertiary alcohols. The mechanism is:

Oxidation. The product depends on the class of alcohol and the reagent.

Passing alcohol vapour over copper at 573 K removes hydrogen instead: CH₃CH₂OH → CH₃CHO + H₂ (1°), (CH₃)₂CHOH → (CH₃)₂CO + H₂ (2°). A tertiary alcohol dehydrates to an alkene instead, (CH₃)₃COH → (CH₃)₂C=CH₂ + H₂O.

Beyond the textbook. For the exam: methanol is toxic and causes blindness, and ethanol is made by fermentation. In reality: methanol is poisonous because the body's liver enzyme converts it into formaldehyde and then formic acid. Ethanol is used as an antidote, because the enzyme prefers ethanol, which slows the conversion of methanol and lets it be excreted. Why it matters: it explains why denatured alcohol is made undrinkable, with CuSO₄ (colour) and pyridine (foul smell), rather than harmless.

Phenols: reactions of the ring

The –OH group is strongly activating and ortho and para directing, because its lone pair feeds electron density into the ring. So phenol reacts under milder conditions than benzene.

The two nitrophenols can be separated by steam distillation. o-Nitrophenol has an intramolecular hydrogen bond between the –OH and the neighbouring –NO₂, so it is a single, less polar molecule that evaporates easily and is steam volatile. p-Nitrophenol can only hydrogen bond to other molecules, so it forms associated molecules, is less volatile and stays behind.

Pause and predict: phenol is treated with bromine water and a white precipitate forms. What is it, and why is a Lewis acid not needed?

Check your answer: 2,4,6-tribromophenol. The –OH group activates the ring so strongly that it brominates all three positions with no catalyst, in a polar solvent that also helps ionise Br₂.

Worked example: a reaction chain. Phenol is treated in turn with (1) Zn dust, (2) CH₃Cl and anhydrous AlCl₃, (3) K₂Cr₂O₇ with H₂SO₄, and (4) HNO₃ with H₂SO₄. Find the final product.

The common wrong step is to give an ortho or para product at the end. A –COOH group is meta directing.

Commercial alcohols

Methanol (wood spirit) is now made from synthesis gas: CO + 2H₂ → CH₃OH over a ZnO–Cr₂O₃ catalyst at high temperature and pressure. It boils at 337 K and is highly toxic.

Ethanol is made by fermentation of sugars. Invertase turns sucrose into glucose and fructose, and zymase turns these into ethanol and carbon dioxide, without air:

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆, then C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Zymase stops working when the ethanol content reaches about 14 percent. Ethanol is also made industrially by hydrating ethene.

Ethers: making them

Williamson with a tertiary halide fails. An alkoxide is also a strong base, and with a bulky halide it takes a proton and makes an alkene instead.

Worked example: choosing the Williamson route. Prepare (a) tert-butyl methyl ether and (b) anisole.

Ethers: reactions

Ethers are quite unreactive, which is why they are good solvents. The C–O bond is cleaved only by strong acids.

Cleavage by HX. The order of reactivity is HI > HBr > HCl. Excess HI converts a dialkyl ether to two alkyl iodides: C₂H₅OC₂H₅ + 2HI → 2C₂H₅I + H₂O. The mechanism is: the ether oxygen is protonated, and then iodide attacks one carbon.

Electrophilic substitution of anisole. The –OR group is activating and ortho, para directing, like –OH, so anisole is more reactive than benzene.

Beyond the textbook. For the exam: ethers are inert and are good solvents. In reality: diethyl ether slowly reacts with oxygen in air on standing and forms explosive peroxides, so laboratory bottles are stored in the dark, with stabilisers, and old bottles must not be distilled to dryness. Why it matters: inertness to acids and bases does not mean inertness to oxygen.

Exam traps

Reaction sheet

60-second revision

Where this chapter connects

• Before this: Haloalkanes and Haloarenes supplies SN1, SN2, the Williamson mechanism and the idea of a good leaving group, which are used again for the Lucas test and ether cleavage.
• Next: Aldehydes, Ketones and Carboxylic Acids is where alcohols oxidise to and where Grignard reagents add, so the products here become the starting materials there.
• Class XI link: Hydrocarbons gives the Markovnikov and anti Markovnikov addition to alkenes that start the preparation of alcohols.