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

The hybridization state of carbon in these three compounds is 

Since, s-electrons are closer to the nucleus, therefore, as the s-character of the orbital making the C—H bond increases, the electrons of C—H bond lie closer and closer to the carbon atom. In other words, the partial + ve charge on the H-atom increases and hence the acidic character increases as the s-character of the orbital increases. Thus, the acidic character decreases in the order : Ethyne > Benzene > Hexane