Chemistry · Alcohols, Phenols And Ethers · NEET
Tertiary alcohols dehydrate the fastest. The correct order of ease of dehydration is 3 degree > 2 degree > 1 degree. NCERT says tertiary alcohols dehydrate under the mildest conditions, while ethanol (a primary alcohol) needs concentrated H2SO4 at 443 K. So the more substituted the carbon holding the OH group, the easier the dehydration.
Acid dehydration goes by an E1 path. First the OH is protonated, then water leaves to make a carbocation. This carbocation step is the slow, rate-deciding step. A tertiary carbocation has three alkyl groups pushing electrons toward the positive carbon, so it is the most stable and forms most easily. Easier carbocation means faster dehydration. That is why 3 degree beats 2 degree beats 1 degree.
If the first carbocation formed is not the most stable one it could be, it can rearrange to a more stable one. A hydrogen with its electron pair can shift from a neighbouring carbon (a 1,2-hydride shift), or an alkyl group can shift. For example, a secondary carbocation can turn into a more stable tertiary carbocation by a hydride shift from the next carbon. NCERT gives exactly this hint. After rearranging, the alkene forms from the more stable carbocation, so you may get a different alkene than expected.
A tertiary alcohol forms the most stable carbocation because the positive carbon is surrounded by three electron-donating alkyl groups (hyperconjugation and inductive effect stabilise the charge). Order of carbocation stability: 3 degree > 2 degree > 1 degree > methyl. This is the exact same order as ease of dehydration, because dehydration speed depends on how easily the carbocation forms.
Two quick checks. First, see which alcohol gives the most stable carbocation after losing water (tertiary is best, and allylic or benzylic positions add extra stability by resonance). Second, make sure there is at least one beta-hydrogen on a neighbouring carbon so an alkene can actually form. Electron-withdrawing groups like NO2 near the carbocation slow dehydration; the farther that group is, the less it hurts. Pick the option with the most stable carbocation and enough beta-hydrogens.
Which amongst the following would be most readily dehydrated under acidic conditions? (Four substituted alcohols are shown; the correct one forms the most stabilised carbocation and has enough alpha/beta hydrogens for elimination.)
When the vapours of a secondary alcohol are passed over heated copper at 573 K, the product formed is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, the order is the same: 3 degree > 2 degree > 1 degree, and both are decided by carbocation stability. Tertiary alcohols make the most stable carbocation, so they react fastest in both dehydration and the Lucas test. Only the reagents differ.
Not always. If the first carbocation can rearrange (by a hydride or alkyl shift) to a more stable one, the alkene forms from the rearranged carbocation. When more than one alkene is possible, the more substituted (Saytzeff) alkene is the major product.
NO2 is strongly electron-withdrawing (-I effect). It pulls electron density away from the positive carbon, making the carbocation less stable and harder to form. The closer NO2 sits to the carbocation, the more it slows dehydration; the farther away, the less it matters.
Ethanol needs concentrated H2SO4 at about 443 K to give ethene. It is a primary alcohol, so it forms only a poor primary carbocation, which needs strong acid and high heat. Secondary and tertiary alcohols dehydrate under much milder conditions.
