on dehydration gives

Concept:

Dehydration reaction of alcohols:

• Alcohols when heated at high temperatures in presence of conc acids, undergo dehydration.
• They lose a molecule of water and form alkenes.
• The reaction proceeds via a carbocationic intermediate.
• The ease of dehydration follows the order tertiary>secondary>primary.
• There is a formation of unexpected products because the rearrangement of carbocations takes place in the reaction.
• Carbocationic rearrangement takes place whenever there is a possibility of a more stable intermediate to be formed by rearrangement.
• More stable carbonation gives a more saturated alkene as the product.

Explanation:

• In the first step, the nucleophilic oxygen of the OH group of alcohol takes up a proton from the acid.
• In the next step elimination of water molecule takes place and gives the carbocationic intermediate. 
• In the third step Rearrangement of this intermediate takes place to give a more stable carbocation.
• Finally, elimination of proton occurs giving more substituted alkenes.
• The mechanism is:

Hence,  on dehydration is .