Why is an alkenyl hydrogen more acidic than the alkyl analog?

Asked Jan 20 '17 at 16:11

Active Jan 20 '17 at 16:27

Viewed 137 times

For example, in ethene the sigma bonds are stronger overall than in ethane.

Ethene (pKa = 44)
C=C 108 kcal mol-1 (+18)
C-H 111 kcal mol-1 (+10)

Ethane (pKa = 50)
C-C 90 kcal mol-1
C-H 101 kcal mol-1

This is because the degree of s character is higher in the sp2 hybridized bonds, giving more compact and stronger bonds and thus reducing acidity.

This greater s character also makes the carbon more electron withdrawing, polarizing the bond and making the alkenyl hydrogen more acidic.

It seems counter intuitive that the stronger bonded ethene H is more acidic than the weaker bonded ethane H. I thought the increased bond strength would outweigh the increased bond polarity. It is like saying: "this bond is stronger, but it is also easier to break."

edited Jan 20 '17 at 16:27

asked Jan 20 '17 at 16:11

ttdijkstra

1

Duplicate of Relative acidities of alkanes, alkenes, and alkynes? – orthocresol Jan 20 '17 at 16:30
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You need to be careful here. "Stronger bond" here means higher homolytic dissociation energy, which is the energy needed to produce two radicals. Acidity is based on heterolytic dissociation. – DSVA Jan 20 '17 at 16:35

Why is an alkenyl hydrogen more acidic than the alkyl analog?

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