iknow d basic app. but im not able to understand jee 2009 paper1 qus in which acidic order of benzene with diff.group attached...give exact exp. please....thanks

Dear student,

please go thru this:

The strengths of weak acids are measured on the pK_a scale. The smaller the number on this scale, the stronger the acid is.

Three of the compounds we shall be looking at, together with their pK_a values are:

carboxylic acids are highly acidic that alcohols ...actually alchols are basic in nature but

phenol is acidic only due to resonace stablised phenoxide ion....

+I effect of CH₃ is very weak , so its presence  would not decrease  acitidty of benzoic acid more  ...

so it will be somewhat lesser acidic than benzoic acid but will be more acidic than remaining two...

now  , structure 2 is more acidic than 1 because Cl has -I effect which makes p chloro phenol

more acidic than phenol ...

so acidic character will be 3>4>2>1

approve my ans if u like & also give ur responce to previously asked questions...

generally the acidic nature of organic compounds is as carboxylic acids> phenols> alcohols

now in given options carboxylic acids are more acidic and phelols are less acidic in nature.

between Carboxylate ion and phenoxide ion , the first one is more stable due to more resonance stabilisation.

phenoxide ion also exbits resonance structure but there is charge seperation occures so it is less stable than the one which does not posses charge seperation like carboxylate ion. so highest acidic nature is for benzoic acid or p-methyl benzoic acid.

if any +I efeective group attaches to the benzene ring it decreases the acidic nature of the compound so now most acidic nature compound is benzoic acid in given options.

halogen group is -I effective group which increases the acidic nature of compound so least acidic among the following is phenol

therefore the order of acidic nature is  benzoic acid > p-methyl benzoic acid>p-chlorophenol>phenol