Question

A researcher attempted this reaction with H2SO4 instead of H3PO4 and noticed a 3rd product. How did this product form from just switching the acid catalyst?

the E1 reaction for the lab:

2-methylcyclohexanol with H3PO4 to make 2-methycyclohexane product. The H3PO4 solution will instead be H2SO4.

Answer 1

Actually this E₁ reaction follows the following steps with 2-methyl hexanol -

1) The hydroxyl group is protonated by H₂SO₄ or even with H₃ PO₄

2) Water leaves giving a carbo cation,

3) A proton is lost from adjacent carbon atom giving the alkene product when H₃ PO₄ is used. In this case 1-methyl cyclohexene is the major product ( according to Zaitsev's rule ) as it has most substituted double bond. However, 3-methyl cyclohexene is also obtained as a minor poduct ,along with limitted / very little amount of methylene cyclohexane.

But when sulphuric acid is used instead of phosphoric acid , a number of dehydration products are reported to be formed . These products depend upon the isomeric forms of 2-methyl-cyclohexenol (ie. cis- & trans- isomers ) The products include alkenes : 1-, 3-,&4-methyl cyclohexenes, and ethylcyclopentene as well as the alcohol (1-, 3-, 4-methylcyclohexanol and the residual starting material,This has been experimentally verified by performing independent reactions under identical conditions with pure cis- and trans-methyl cyclo hexanol. the results reveal that cis- isomer reacts predominantly to form 1-methyl cyclohexene where as trans isomer react to give a complicated mixture of 1-, 3-, 4--methyl cyclohexanol, 2-, 3-,4-methyl cyclohexene and a ring contraction product,1-ethyl cyclopentene.