Chapter 17: Q 1P (page 803)

Explain why the pK_aof a hydrogen bonded to the sp³carbon of propene is greater (pK_a= 42) than that of any of the carbon acids listed in Table 17.1, but is less than the pK_aof an alkane (pK_a> 60).

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Hydrogen bonded to propene

Hydrogen and carbon have similar electronegativities, which means that the two atoms share the electrons that bond them together almost equally. Consequently, a hydrogen-bonded to carbon is usually not acidic. This is particularly true for hydrogens bonded to sp³ carbons because these carbons are the most similar to hydrogen in electronegativity.

Acidic nature of carbon

The compound has acidic hydrogen bonded to an sp³carbon is called a carbon acid. Lesser is the pK_avalue more is the acidic nature.. The pK_a of allylic hydrogen of propene is 42. It has more pK_a than all carbon acids as shown which is attributed to the less stabilized resonance hybrid than the other carbon acids. The resonance stabilization of conjugate base from allyl derivative is shown as:

Resonance hybrid between carbons

Other carbon acids have lower pK_aand are due to the attachment of aldehyde or ketone groups, ester groups, nitro groups, and more carbonyls. More electron-withdrawing groups increase the strength of resonance. More is the resonance more is the stability of the conjugate base. Hence, the carbon acids which belong to carbonyl derivative and nitro groups, etc, are more acidic and contain lesser pK_a.

Moreover, the resonance hybrids which are with carbon and more electronegative atoms are more stabilized than the resonance hybrids between the same electronegative elements like carbon.

Resonance hybrid between carbon and oxygen

If we compare alkenes with alkanes, the alkene has lesser pK_athan alkanes because when the proton is removed from an alkane, the formed negative charge is not capable of involving in any resonance like a conjugate base of alkene derivative. Hence, the conjugate base of an alkane is not stable. Therefore, it has a very high pK_a value and is less acidic.