Q.Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

Concept: Acidity trend determined by stability of conjugate base (anion stability after losing H⁺).

Acidity depends on how well the conjugate base can stabilise the negative charge left behind when a proton is removed.

Ethyne ($\ce{HC≡CH}$): The acidic hydrogen is attached to an $sp$-hybridised carbon (50% s-character). Electrons in orbitals with higher s-character are held closer to the nucleus, so the conjugate base $\ce{HC≡C^-}$ is relatively stable. Ethyne is weakly acidic ($\mathrm{p}K_\mathrm{a} \approx 25$).

Benzene ($\ce{C6H6}$): Hydrogen is on an $sp^2$ carbon (33% s-character). Removing H⁺ gives a phenyl anion, which is much less stable than the acetylide ion. Benzene is extremely weakly acidic ($\mathrm{p}K_\mathrm{a} \approx 43$). …

The acidity of a hydrocarbon is determined by how readily it donates a proton ($\ce{H+}$), which in turn depends on how stable the resulting conjugate base (the carbanion) is. The more stable the anion, the more acidic the parent compound.

The key insight here is that s-character of the carbon orbital holding the lone pair in the conjugate base dictates stability. Electrons in orbitals with higher s-character are held closer to the nucleus and are more stable. This is because s-orbitals are spherical and centreed on the nucleus, while p-orbitals have lobes extending away from it.

Let's examine each compound:

1. Ethyne ($\ce{HC≡CH}$): The terminal carbon is $sp$-hybridised (50% s-character). When ethyne loses a proton, the conjugate base is $\ce{HC≡C^-}$, where the negative charge resides in an $sp$ orbital. This high s-character means the electrons are held tightly, making the anion very stable. Ethyne is therefore the most acidic of the three, with $\text{p}K_a \approx 25$.

2. Benzene ($\ce{C_6H_6}$): The carbon atoms are $sp^2$-hybridised (33% s-character). Removing a proton from benzene gives the phenyl anion $\ce{C_6H_5^-}$, where the negative charge is in an $sp^2$ orbital. This is less stable than the ethynide ion but more stable than an $sp^3$ carbanion. Benzene has $\text{p}K_a \approx 43$.

3. n-Hexane ($\ce{CH_3(CH_2)_4CH_3}$): All carbons are $sp^3$-hybridised (25% s-character). The conjugate base would be an alkyl anion with the negative charge in an $sp^3$ orbital. This is the least stable arrangement because the electrons are held furthest from the nucleus. n-Hexane is extremely weakly acidic, with $\text{p}K_a \approx 50$.