Question 13.18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.
Class XI Hydrocarbons Page 397
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we know, acidity is directly proportional to S - character , means higher the S - character, higher is the acidity . so, first of all we have to find out hybridisation of all the given compounds and then find then compare the acidity by comparison of S - character .
increasing order of S - character :
sp³ - hybridized ( 25% , s - character ) < sp² - hybridized ( 33.33%, S - character ) < sp - hybridized ( 50% , S - character )
benzene : all C - atoms are sp² hybridized so, S - character is only 33.33 % .
n - hexane : all C - atoms are sp³ hybridized so, S - character is only 25% .
Ethyne : {H-CΞC - H } , hence triple bond between C- atoms . so, 50 % , S - character in Ethyne .
hence, it is clear that increasing order of acidity : Ethyne < benzene < n-Hexane
increasing order of S - character :
sp³ - hybridized ( 25% , s - character ) < sp² - hybridized ( 33.33%, S - character ) < sp - hybridized ( 50% , S - character )
benzene : all C - atoms are sp² hybridized so, S - character is only 33.33 % .
n - hexane : all C - atoms are sp³ hybridized so, S - character is only 25% .
Ethyne : {H-CΞC - H } , hence triple bond between C- atoms . so, 50 % , S - character in Ethyne .
hence, it is clear that increasing order of acidity : Ethyne < benzene < n-Hexane
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