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Chapter 8: Problem 4

Explain why alkynes are less reactive than alkenes towards electrophilic addition reactions?

Short Answer

Expert verified
Alkynes are less reactive than alkenes due to their stronger and more stable triple bonds and less favorable intermediate formation during electrophilic additions.

Step by step solution

01

Understand the Structural Differences

Alkynes have a triple bond between carbon atoms while alkenes have a double bond. The triple bond consists of one sigma (蟽) bond and two pi (蟺) bonds, whereas a double bond consists of one 蟽 bond and one 蟺 bond.
02

Analyze Bond Strength

The 蟺 bonds in alkynes are more tightly held due to the linear structure of the molecules and reduced electron repulsion, making them less available for reaction compared to the 蟺 bond in alkenes.
03

Electrophilic Attack on Pi Bond

Electrophiles are attracted to areas of high electron density. In alkenes, the 蟺 bond provides a region of high electron density that is easily accessible to electrophiles. In alkynes, the additional 蟺 bond and the tightly held electrons reduce the electron density available for electrophiles.
04

Consideration of Intermediate Stability

In electrophilic addition reactions, the formation of carbocation intermediates is crucial. Alkenes can stabilize these intermediates better than alkynes, making the reaction more favorable with alkenes.
05

Reactivity Summary

Due to the stable nature of alkynes' triple bond and its linear geometry, along with the less favorable formation of carbocation intermediates, alkynes are less reactive towards electrophilic addition reactions than alkenes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alkynes
Alkynes are fascinating hydrocarbons characterized by the presence of at least one triple bond between carbon atoms. This triple bond is composed of one sigma (蟽) bond and two pi (蟺) bonds. The unique structure of alkynes gives them distinct properties. Because the triple bond is linear, electrons in 蟺 bonds are held more closely to the axis of the bond compared to other hydrocarbon bonds. This tight hold affects the reactivity of alkynes in certain chemical reactions, particularly electrophilic addition reactions.
Alkenes
Alkenes, like alkynes, are a type of unsaturated hydrocarbon, but they feature a double bond instead of a triple bond. This double bond is made up of one sigma (蟽) bond and one pi (蟺) bond. The pi bond in alkenes is highly reactive because it represents a region of high electron density. Electrophiles, which are electron-loving species, are drawn to these dense areas to initiate reactions. Alkenes' ability to readily participate in such reactions makes them more reactive than alkynes overall.
Triple Bond Structure
The triple bond in alkynes contributes to its unique chemical behavior. The linear arrangement in a triple bond involves one sigma (蟽) bond, which provides strength, and two pi (蟺) bonds, which are responsible for electronic interactions. These pi bonds are more tightly held and closer to the carbon nuclei compared to the pi bond in alkenes. This configuration results in a reduced electron density around the triple bond, making it less reactive toward electrophilic addition because electrophiles find it harder to access these electrons.
Carbocation Intermediate Stability
During electrophilic addition reactions, one significant factor is the stability of the carbocation intermediate that forms. A carbocation is a positively charged ion with a carbon atom bearing a vacant p orbital. In alkenes, the pi bond is able to favorably delocalize charge to stabilize this intermediate. In contrast, alkynes do not stabilize carbocations as efficiently due to their linear triple bond and closer-held electrons. This lack of stabilization makes electrophilic additions to alkynes less favorable, contributing to their decreased reactivity.

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Most popular questions from this chapter

. Explain with suitable examples, the electrophilic and nucleophilic reactions of acetylene.

How will you prepare alkynes by (a) Dehydrohalogenation of 1,2 -dihalides (vic-dihalide). (b) Dehalogenation of tetrahalides. (c) Reaction of acetylides with haloalkanes.

What happens when (a) But-2-yne is treated with hydrogen in presence of Lindlar's catalyst. (b) But- 2 -yne is treated with sodium in liquid ammonia. (c) But-1-yne is treated with (a) ammonical \(\mathrm{AgNO}_{3}\) and (b) ammonical \(\mathrm{Cu}_{2} \mathrm{Cl}_{2}\) solution

Write a short note on hydroboration-oxidation reactions of alkynes.

Draw the structures of all isomers of alkynes having molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8}\) and give their IUPAC names.
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