9.5
Preparation of Alkynes: Dehydrohalogenation
Introduction
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liqu…
In addition to the alkylation pathway, alkynes can also be prepared by dehydrohalogenation of vicinal or geminal dihalides.
Vicinal dihalides are compounds with halogens on adjacent carbons, whereas compounds with halogens on the same carbon are called geminal dihalides.
For example, a vicinal dichloride can be synthesized from an alkene by the addition of chlorine in the presence of an inert solvent like dichloromethane. While a geminal dichloride can be prepared by treating a ketone with phosphorous pentachloride.
In the presence of a strong base like sodium amide in liquid ammonia, the dihalides lose two equivalents of hydrogen halide through two successive E2 elimination reactions. Hence the name double dehydrohalogenation.
The first elimination reaction proceeds with the abstraction of a proton by sodium amide and simultaneous departure of the halide leaving group to form a haloalkene.
In the second elimination reaction, another equivalent of the base reacts with the haloalkene to yield the desired alkyne. Thus, at least two equivalents of sodium amide are required for the reaction to go to completion.
Similarly, treatment of geminal dihalides with sodium amide gives alkynes through two consecutive E2 elimination reactions.
However, if the product is a terminal alkyne, the acidic hydrogen is deprotonated by the strong base to form an acetylide ion. Thus, a third equivalent of the base is required to complete the dehydrohalogenation of the remaining haloalkene. Protonation of the acetylide ion with water or a weak acid drives the reaction to completion.
If the first elimination step gives a haloalkene with hydrogen on adjacent carbons, subsequent elimination can yield an allene as a side product in addition to the alkyne. However, the presence of adjacent double bonds in an allene makes them more unstable, thereby favoring the formation of alkynes.
Lastly, the reaction can be terminated at the first elimination step using weaker bases like sodium hydroxide to give an alkene as the final product.
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Q1: What is the difference between vicinal and geminal dihalides?
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.
Q2: How does dehydrohalogenation convert dihalides to alkynes?
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.
Q3: Why do terminal alkynes require a third equivalent of base?
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.
Q4: What is the role of the E2 mechanism in alkyne formation?
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.
Q5: How can allenes form as side products during dehydrohalogenation?
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.
Q6: Can dehydrohalogenation be stopped at the alkene stage?
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.
Q7: What is a practical example of converting an alkene to an alkyne?
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.