Question

explain why dehydration of alcohol to form alkenes is always carried out with concentrated H2SO4 and not with concentrated HCl..​

Answer 1

Dehydration of alcohols to/from alkenes is always carried out with conc. H2SO4 and not with conc. HCL or HNO3, why?

Mostly for dehydration we use concentrated acids which remove water from given substances and they become less concentrated.

Firstly we need to take such a acid it can not react with the substance.

Here if we take HCl it will dehydrated the alcohol but it react with alcohol to form alkyl chloride.

HCl + ROH ——— RCl + H2O.

Similarly if we take HNO3 it will dehydrated but it will form nitro ester with alcohol.

HNO3 + ROH ——— RNO2 + H2O.

But if we use H2SO4 it will dehydrated and it will form some by products depending on temperatures.

H2SO4 + ROH ———- ROSO3 + H2O. At temperature below 413 k.

If we increase the temperature ROSO3 react with acid to form ester

H2SO4 + ROH ——— ROR + H2O at temperature 413 k.

But if increase further it will form alkene

H2SO4 + ROH ——— RR+ H2O. At temperature 443 k.

But the products formed with HCl and HNO3 can not be reduces further with increasing temperatures .

So for dehydration of alcohol we use only H2SO4 .