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

(N/A) The acidic character of a species is defined by the ease with which it can lose its $H^+$ ions.
The hybridization states of carbon in the given compounds are:
$1$. Ethyne $(HC \equiv CH)$: $sp$ hybridized carbon ($50\% \ s-$character).
$2$. Benzene $(C_6H_6)$: $sp^2$ hybridized carbon ($33.3\% \ s-$character).
$3$. $n-$Hexane $(CH_3(CH_2)_4CH_3)$: $sp^3$ hybridized carbon ($25\% \ s-$character).
As the $s-$character increases,the electronegativity of the carbon atom increases,causing the electrons of the $C-H$ bond to be held more strongly by the carbon atom. This increases the polarity of the $C-H$ bond,making the $H$ atom more acidic (easier to release as $H^+$).
The $s-$character increases in the order: $sp^3 < sp^2 < sp$.
Therefore,the decreasing order of acidic behaviour is: $\text{Ethyne} > \text{Benzene} > n-\text{Hexane}$.