IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

Nithy

which is more acidic?
m-nitro benzoic acid and p-fluoro benzoic acid.

destinationIIT09

p-fluoro benzoic acid

dream

p-fluoro benzoic acid

see. in case of m- nitro benzoic acid, both -COOH and -NO2 groups are e repelling and so they will create high e density at carbon atom equidistant from them.........................ie .................the ortho position

on contrary , p-acid will not create high e density at any particular atom and so removing of e shud be easy in here then at m- acid

rtiit

u r rite rohit.........

tarinbansal

I think the answer is m-nitrobenzoic acid.

First of all,
@Dream
ur explanation is wrong. NO2 is not electron repelling, it is electron withdrawing group by both -I, -R.

Now to decide acidic character I am using the stability of conjugate base. Means in the case in which the -ve charge on O wud be more delocalised, it wud be more acidic.

Now, COOH is at meta position of NO2, means its -R effect wont work here and only -I wud work. And in case of F, COOH is at para position of F. So we can consider its very feeble +R effect and -I effect of F.
Now in case of inductive effect, we see the distance. More the distance, less the inductive effect.
Now, NO2 is nearer to COOH than F. It means its -I > -I of F. Means it wud help decreasing electron density from O in the Conjugate base more than F.
Secondly if we consider a feeble +R effect of F, it wud increase the electron density on O rather than decreasing it.

SO in either way, whether u consider F's +R effect or not, NO2 is acting as a better electron pulling group than F and hence m-nitrobenzoic acid's conjugate will be more stable as compared to the other one.

So, M-nitro benzoic acid is more acidic.

Hope U get it and plz tell me if I hav made some mistake.

harsha_27

No.Tarin is right.Metanitrobenzoic acid is stronger.

The explanation given by Tarin is absolutely right.In addition to that there is one more thing.Here p-flourobenzoicacid will have strong intermolecular hydrogen bonding which decreases the acidity.

dream

@ tarin

thanx for showing my mistake

ya

m-nitro benzoic acid is stronger acid

as NO2 attracts the electron towards itself, it creates attracts the e from C to which it is attached..................................which indeed attracts the e from other carbon atom and in this way, -COOH becomes a better H repelling group

the same thing happens with fluoro group also

but now another criterion comes into picture

that is the distance

NO2 is more closer to COOH then F

so its -I effect is more

also NO2 is better e attracting group as compared to F

so considering both the facts, m-nitro benzoic acid is a stronger acid

so here the two cases important in deciding the acid strngth are

distance from COOH

relative -I effect of NO2 and F

coolriya

m-nitro benzoic acid is stronger acid I suppose !

saurabh_reincarnated

oye i know NO2 is electron withdrawing...

and yes i saw conjugate base structures and resonance of it.......

dekha jaaye to aftr donating H+ .... - charges appear at P and O positn and not meta......

so any electron withdrawing grp prsent thre ud help this procedure and remove H+ from benzoic acid......

and F is also an electron withdrawing grp .... so it can help in gving up acidic H

inductive effect shud be considered last...... if anythng is not getting into

secondly R+ effect ...... yes it supports NO2......... par dekha jaaye to F is very small and is very reluctant to donate electron pair....

but overall if seeing we shud alws give more priority to R+ effect thn any other....... watever may b the grp present ..... F or Cl

so by close contest yes.... meta nitro benzoic acid wins.......

but the situation is close and ......... i dun thnk iit will ask such close cases

mukool.sitara

m-nitro benzoic aicd is more acidic than p-fluoro benzoic acid

explaination given by TARIN is absolutely correct

Nithy

thanks everyone....

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