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Organic Chemistry
Found in: Page 378

Short Answer

Question: Although alcohol V gives a single alkene W when treated with POCl3 and pyridine, three isomeric alkenes (X–Z) are formed on dehydration with H2SO4. Draw a stepwise mechanism for each reaction and explain why the difference occurs.


In the treatment of alcohol with POCl3 , only one product is formed as only one carbon has a β hydrogen. The product formed on the dehydration with H2SO4 contains several carbons with β hydrogen leading to the formation of multiple products.

See the step by step solution

Step by Step Solution

Step-by-Step SolutionStep 1: Reaction of alcohol with  

Tertiary alcohols can undergo the elimination reaction in the presence of reagents, such as POCl3 in pyridine. Other hindered alcohols, such as the secondary alcohol, can also proceed by the elimination reaction.

Step 2: Reaction of alcohol with  H2SO4

The combination of alcohol with H2SO4 is a dehydration reaction that results in the creation of an alkene. A specific quantity of temperature is necessary for this reaction to take place.

Step 3: Reaction mechanism for the formation of isomeric alkenes

When alcohol is treated with POCl3 (pyridine), elimination takes place by the E2 mechanism. Only one product is formed in this particular reaction as only one carbon has a β hydrogen. The reaction mechanism can be given as follows:

Treatment of alcohol with POCl3

The dehydration of alcohol with H2SO4 proceeds via the E1 elimination. The secondary carbocation formed in the reaction undergoes a 1,2-methyl shift, leading to the formation of a tertiary carbocation.

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