In: Chemistry
1) Why do we use H3PO4 instead of H2SO4 as a catalyst for the synthesis of cyclohexene?
2) Why do we use H3PO4 instead of HCl as a catalyst for the synthesis of cyclohexene?
3) What alkene(s) would be produced on dehydration of each of the following alcohols? If more than one product is possible, use Zaitsev’s rule to predict which product would be formed in greater amounts.
a) 2-methylcyclohexanol
b) 2,2-dimethylcyclohexanol
c) 1,2-cyclohexanediol
1. H2SO4 is strong oxidising agent it can oxidise the cyclohexanol to carbondioxide. Phosphoric acid has no such effect. That is why phosphoric acid is preffered over sulfuric acid in dehydration reaction.
2. Concentrated HCl is 37 % and concentrated phosphoric acid is 85% . Hence concentrated HCl contains large concentration of water, presence of water disfavors the elimination reaction as one of the product in dehydration reaction is water. Again concentrated HCl can reacts with alcohol to fom alkyl halide.
Hence phosphoric acid is preffered over hydrochloric acid in dehydration reaction.
3.
Zaitsev's rule: In elimination reaction alkene fromed as major product is the one that corresponds to removal of Beta hydrogen having less number of hydrogen ( from more substituted carbon).
a) Dehydration of 2-methylcyclohexanol.
B. 2,2-dimethylcyclohexanol.
C. 1,2-cyclohexanediol.