Why Heating Ethanol With Acid Can Give Either Ethene or Ether

Ethanol in hand sanitiser, glycerol in soaps and the menthol that cools mint all carry an -OH group on a saturated carbon. How many carbons surround that carbon decides whether an alcohol is primary, secondary or tertiary — and that one detail changes how it is made, how it reacts and which tests it passes.

This lesson covers naming and preparing alcohols, their dehydration and esterification reactions, and chemical tests that tell the three classes apart.

Alcohols are named with the suffix -ol and classified by how many carbons are bonded to the carbon carrying the -OH group, and they are prepared by hydrating alkenes, reducing carbonyl compounds, or adding Grignard reagents to aldehydes and ketones.

Classes:

- Primary — the -OH carbon is attached to one carbon, as in propan-1-ol, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$
- Secondary — attached to two carbons, as in propan-2-ol, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_3$
- Tertiary — attached to three carbons, as in 2-methylpropan-2-ol, $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{COH}$

Methods of preparation:

- Acid-catalysed hydration of alkenes, following Markovnikov's rule: $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}^{+}}{\to }\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_3$
- Hydroboration-oxidation gives the anti-Markovnikov alcohol, so propene gives propan-1-ol
- Reduction with $\mathrm{NaB}{\mathrm{H}}_4$, $\mathrm{LiAl}{\mathrm{H}}_4$ or catalytic hydrogen turns aldehydes into primary alcohols and ketones into secondary alcohols
- Grignard reagents add to carbonyl compounds and hydrolysis gives the alcohol: methanal gives a primary alcohol, other aldehydes give secondary alcohols, and ketones give tertiary alcohols

Worked example. To make 2-methylpropan-2-ol, add methylmagnesium bromide to propanone and hydrolyse:

$$(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CO}+\mathrm{C}{\mathrm{H}}_3\mathrm{MgBr}\to (\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{COMgBr}\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{COH}$$

An everyday example. Ethanol for blending into petrol in India is made largely by fermenting sugarcane molasses, as yeast enzymes turn sugars into ethanol and carbon dioxide.

The substance. Reduction can never make a tertiary alcohol — a ketone gives only a secondary alcohol, so tertiary alcohols need a Grignard route.

Alcohols lose water when heated with concentrated sulphuric acid, giving alkenes at higher temperature or ethers at lower temperature, and they react with carboxylic acids in the presence of an acid catalyst to form esters.

Dehydration to alkenes:

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\stackrel{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4,443\mathrm{K}}{\to }\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}$$

- Mechanism: the -OH is protonated, water leaves to form a carbocation, and loss of ${\mathrm{H}}^{+}$ forms the double bond
- Ease of dehydration: tertiary > secondary > primary, following carbocation stability; tertiary alcohols need only dilute acid at about 358 K
- Saytzeff's rule applies, so butan-2-ol gives mainly but-2-ene

Dehydration to ethers:

$$2C{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\stackrel{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4,413\mathrm{K}}{\to }\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}$$

- At the lower temperature, a second alcohol molecule attacks the protonated alcohol
- This works for primary alcohols; secondary and tertiary alcohols give alkenes instead

Esterification:

$$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\stackrel{{\mathrm{H}}^{+}}{\rightleftharpoons }\mathrm{C}{\mathrm{H}}_3\mathrm{COO}{\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{H}}_2\mathrm{O}$$

- The reaction is reversible; removing water or using excess alcohol raises the yield
- The -OR part of the ester comes from the alcohol, while the -OH lost as water comes from the acid
- Acid chlorides and anhydrides react faster and irreversibly

Worked example. 6.0 g of ethanoic acid (0.10 mol) is heated with an equal number of moles of ethanol, and two-thirds of the acid is converted at equilibrium. The mass of ethyl ethanoate ($M=88$ g mol${}^{-1}$) formed is

$$0.10\times \frac{2}{3}\times 88=5.9\text{ g}$$

An everyday example. The fruity smell of pineapple- and banana-flavoured sweets often comes from synthetic esters such as ethyl butanoate and isopentyl acetate.

The substance. Temperature decides the dehydration product — the same ethanol and sulphuric acid give diethyl ether at 413 K but ethene at 443 K.

The Lucas test separates the three classes by how quickly they turn cloudy with zinc chloride in concentrated hydrochloric acid, and oxidation shows that primary alcohols give aldehydes and acids, secondary alcohols give ketones, and tertiary alcohols resist oxidation.

Lucas test — anhydrous $\mathrm{ZnC}{\mathrm{l}}_2$ in concentrated HCl:

- Tertiary — cloudiness appears at once, as an insoluble alkyl chloride forms through a stable carbocation
- Secondary — cloudiness appears within about five minutes
- Primary — no cloudiness at room temperature

Oxidation:

- Primary — acidified ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ turns from orange to green, giving an aldehyde and then a carboxylic acid; pyridinium chlorochromate stops at the aldehyde
- Secondary — oxidised to a ketone, which resists further oxidation
- Tertiary — not oxidised, so the dichromate stays orange

Heated copper at 573 K. Primary alcohols give aldehydes, secondary alcohols give ketones, and tertiary alcohols are dehydrated to alkenes.

**Worked example — three isomers of ${\mathrm{C}}_4{\mathrm{H}}_{10}\mathrm{O}$:

- Butan-1-ol: no Lucas cloudiness, and oxidation gives butanal then butanoic acid — primary
- Butan-2-ol: cloudiness after a few minutes, and oxidation gives butanone — secondary
- 2-Methylpropan-2-ol: instant cloudiness and no oxidation — tertiary

An everyday example. Toddy and other fermented drinks turn sour when left open, because microbes oxidise their ethanol, a primary alcohol, into ethanoic acid.

The substance. Oxidation needs a hydrogen atom on the carbon carrying the -OH group** — tertiary alcohols have none, which is why they resist oxidation.