Question 1

What is the difference between vicinal and geminal dihalides?

Accepted Answer

Vicinal dihalides have halogens on adjacent carbons, while geminal dihalides have both halogens on the same carbon. Vicinal dichlorides are synthesized from alkenes by chlorine addition in an inert solvent like dichloromethane. Geminal dichlorides are prepared by treating ketones with phosphorous pentachloride.

Question 2

How does dehydrohalogenation convert dihalides to alkynes?

Accepted Answer

Dehydrohalogenation uses a strong base like sodium amide in liquid ammonia to remove two equivalents of hydrogen halide through two successive E2 elimination reactions. The first elimination forms a haloalkene, and the second elimination yields the desired alkyne. This double elimination process requires at least two equivalents of base.

Question 3

Why do terminal alkynes require a third equivalent of base?

Accepted Answer

Terminal alkynes have an acidic hydrogen that is deprotonated by the strong base to form an acetylide ion. The third equivalent of sodium amide is needed to complete this deprotonation. Protonation of the acetylide ion with water or a weak acid then drives the reaction to completion.

Question 4

What is the role of the E2 mechanism in alkyne formation?

Accepted Answer

E2 elimination follows a concerted pathway where proton abstraction and halide departure occur simultaneously. The base abstracts a proton oriented anti to the leaving group, forming a new double bond. Two successive E2 reactions convert dihalides to alkynes through this mechanism.

Question 5

How can allenes form as side products during dehydrohalogenation?

Accepted Answer

If the first elimination step produces a haloalkene with hydrogen on adjacent carbons, the second elimination can yield an allene as a side product. However, allenes are less stable due to their adjacent double bonds, so alkynes are thermodynamically favored and form as the major product.

Question 6

Can dehydrohalogenation be stopped at the alkene stage?

Accepted Answer

Yes, using weaker bases like sodium hydroxide instead of sodium amide allows the reaction to terminate after the first elimination step, yielding an alkene as the final product. This provides an alternative synthetic route when alkenes are the desired target rather than alkynes.

Question 7

What is a practical example of converting an alkene to an alkyne?

Accepted Answer

Chlorination of 1-propene produces 1,2-dichloropropane, a vicinal dihalide. Double dehydrohalogenation of this compound with sodium amide yields 1-propyne. This demonstrates how alkenes can be converted to alkynes through dihalide intermediates.

Préparation des alcynes : déshydrohalogénation

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