Synthesis of Alkynes

Summary

This video explains two distinct methods for synthesizing alkynes: elimination of HBr using a strong base, and nucleophilic substitution of terminal alkynes with alkyl halides. The first method involves creating a triple bond through the removal of two moles of HBr, while the second method forms an acetylide anion by removing a proton from a terminal alkyne, which then reacts with alkyl halides. Both methods are explored with practical examples, showing how to build carbon-carbon bonds to form larger molecules from simpler ones, with a focus on retrosynthetic analysis for planning organic syntheses.

Takeaways

• 😀 Elimination of HBr can be used to form alkynes, similar to the process for making alkenes, but requires the elimination of two moles of HBr.
• 😀 Strong bases like sodium amide are used to facilitate the formation of alkynes from vinyl bromides.
• 😀 In the first step, a vinyl bromide is formed, which then undergoes dehydrohalogenation to create a triple bond.
• 😀 A good way to make dibromides, necessary for this reaction, is through the electrophilic addition of bromine to an alkene.
• 😀 An alternative method for making alkynes is using a strong base to remove a proton from a terminal alkyne, forming a nucleophilic acetylide ion.
• 😀 The acetylide ion is highly nucleophilic and can react in SN2 reactions with alkyl halides to form carbon-carbon bonds.
• 😀 A reaction between an acetylide ion and an alkyl halide (e.g., ethyl iodide) adds an alkyl group to the alkyne, producing a larger molecule.
• 😀 Two methods for making a specific alkynes are: converting alkenes to alkynes via dibromides or using a terminal acetylene and adding alkyl groups via SN2 reactions.
• 😀 In retrosynthetic analysis, a target molecule can often be synthesized by transforming alkenes or alkynes through a series of steps involving bromine and strong bases.
• 😀 The order of adding alkyl groups to an acetylene can be flexible, as both methods lead to the same final product, showcasing versatility in organic synthesis.

Q & A

What are the two main methods discussed for making alkynes?

The two main methods for making alkynes discussed are: 1) The elimination of HBr from a dibromoalkene using a strong base, which forms a triple bond; and 2) Deprotonating a terminal alkyne with a strong base to form an acetylide ion, which can then react with alkyl halides in an SN2 reaction to form a new alkyne.

How does the elimination of HBr lead to the formation of an alkyne?

The elimination of HBr from a dibromoalkene involves using a strong base, such as sodium amide (NaNH2), to remove two moles of HBr, resulting in the formation of a triple bond. This process requires excess base to ensure both moles of HBr are eliminated.

Why is sodium amide often used in these reactions?

Sodium amide (NaNH2) is a strong base that is capable of deprotonating terminal alkynes and promoting eliminations to form alkynes. It is also effective in creating acetylide ions that can participate in nucleophilic substitution reactions (SN2).

What is an acetylide ion and how is it formed?

An acetylide ion is a negatively charged ion formed when a terminal alkyne is treated with a strong base like sodium amide. The base removes a proton (H+) from the terminal carbon, creating the acetylide ion, which is a strong nucleophile.

How does the acetylide ion participate in SN2 reactions?

The acetylide ion, being a good nucleophile, can react in SN2 reactions with alkyl halides. The acetylide displaces the halide ion from the alkyl halide, forming a new carbon-carbon bond and resulting in an alkyne with an alkyl group attached.

What is the significance of making carbon-carbon bonds in organic synthesis?

Making carbon-carbon bonds is crucial in organic synthesis as it allows chemists to build complex molecules from simpler ones. These bonds are foundational to the construction of a wide range of organic compounds used in pharmaceuticals, materials, and other applications.

What is the retrosynthetic approach to planning the synthesis of a disubstituted alkyne?

The retrosynthetic approach involves analyzing the target molecule and identifying possible precursor molecules that can be transformed into the desired product. In this case, the retrosynthesis involves converting an alkene into a di-bromide, which can then be converted into the alkyne using sodium amide.

What are the two methods for synthesizing a disubstituted alkyne and when would you choose one over the other?

The two methods for synthesizing a disubstituted alkyne are: 1) Converting an alkene into a dibromoalkene and then using sodium amide to form the alkyne; and 2) Starting with a terminal acetylene and adding alkyl groups via SN2 reactions. The choice of method depends on the availability of starting materials.

Can the order in which alkyl halides are added to a terminal acetylene affect the outcome of the reaction?

No, the order in which alkyl halides are added to a terminal acetylene does not affect the outcome. Both primary alkyl halides can be added in any sequence, and the same product will be formed in both cases.

What is the role of bromine in the formation of dibromides during the synthesis of alkynes?

Bromine is used to add to the double bond of an alkene in an electrophilic addition reaction, forming a di-bromide. The di-bromide can then undergo elimination with sodium amide to form a triple bond and create the alkyne.