Chemistry · Alcohols, Phenols And Ethers · NEET
The alcohol loses one molecule of water (H2O) and forms a C=C double bond, giving an alkene. You heat it with concentrated H2SO4 (or H3PO4). Example: ethanol heated with conc. H2SO4 at about 443 K gives ethene. So dehydration = remove water = make an alkene. It is the exact reverse of the acid hydration that makes alcohols from alkenes.
Conc. H2SO4 is a strong acid that first protonates the -OH group. This turns the poor leaving group -OH into a good leaving group -OH2+ (water). Then water leaves easily. Dilute acid has too much water around, which pushes the reaction backward (toward hydration, making the alcohol again). So NEET answer: concentrated acid + heat drives water OUT.
For most alcohols (especially 2° and 3°) it is E1. Three steps: (1) the -OH is protonated to -OH2+; (2) water leaves, forming a carbocation; (3) a beta-hydrogen (H on the carbon next to the positive carbon) is removed and the C=C double bond forms. The slow, rate-deciding step is losing water to make the carbocation. Because a carbocation forms, the more stable the carbocation, the easier the dehydration.
Tertiary (3°) > secondary (2°) > primary (1°). A 3° alcohol gives a stable 3° carbocation, so it dehydrates most easily and at the lowest temperature. A 1° alcohol gives an unstable 1° carbocation, so it needs the harshest conditions (hot conc. H2SO4). Remember: more branching on the carbon holding -OH = easier dehydration.
The more substituted alkene (the one with more carbon groups on the double bond carbons) is the major product. This is Saytzeff's (Zaitsev's) rule, because the more substituted alkene is more stable. Example: butan-2-ol gives mainly but-2-ene (more substituted) and only a little but-1-ene.
They give different products. Dehydration (conc. H2SO4, heat) removes H2O and makes an ALKENE. Dehydrogenation (copper catalyst at 573 K) removes H2 (two hydrogen atoms) and makes a carbonyl: 1° alcohol gives an aldehyde, 2° alcohol gives a ketone. Watch the word: dehydratION = lose water; dehydrogenatION = lose hydrogen. NEET mixes these two on purpose.
When the vapours of a secondary alcohol are passed over heated copper at 573 K, the product formed is:
Which amongst the following would be most readily dehydrated under acidic conditions?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ethanol with excess concentrated H2SO4 at about 443 K (170 C) gives ethene. Lower temperatures with acid instead form diethyl ether.
Yes. Hot concentrated phosphoric acid (H3PO4) also dehydrates alcohols and is sometimes preferred because it does not oxidise or char the alcohol the way H2SO4 can.
A primary alcohol would form an unstable primary carbocation in the E1 step. Because that carbocation is high in energy, the reaction is slow and needs the hottest, most concentrated acid conditions.
A beta-hydrogen is a hydrogen on the carbon next to the carbon carrying the positive charge (or the -OH). This beta-H is removed to form the C=C double bond. No beta-H means no alkene can form.
Yes. Acid hydration adds water across an alkene to make an alcohol; acid dehydration removes water to give back the alkene. They are opposite reactions, so conditions (dilute vs concentrated acid, temperature) decide the direction.
