[kelaklub]

The answer to the following question is stated as choice B. I am having trouble understanding why molecule 2 is more acidic than molecule 3. Please shed some light.

[Will]

If I had to order them in increasing acidity then I would say its:
4, 1, 2, 3.
I think there may be a typo in the 'answers'/question.
As far as I'm aware formic acid is stronger than acetic acid, as acetic acid has electron density pushed to the OH group from the methyl group (PIE), making the O-H bond stronger than in formic acid, so acetic acid dissociates less (as I'm sure you know, thats why you got confused with the question as none of the answers seem right!).

[kelaklub]

Yeah I was thinking along those lines. I guess I confused myself more trying to convince myself why there wrong answer was right. Doesn't surprise me, because these sample tests already had a bunch of typos.

[barcrphd]

will17
just a small doubt. is it correct to say, the inductive effect of methyl group in acetic acid strengthens the OH bond? infact it destabilizes the carbanion, isnt it? i mean, i can understand what you mean, but the word "strengthens" may mislead beginers.

[Winga]

After searching the pKa values of formic acid and acetic acid, fomic acid is more acidic!
formic acid: pKa = 3.75
acetic acid: pKa = 4.75

So, there should be something wrong with your answer.

In addition, for acetic acid, beside the positive inductive effect destabilizes the conjugate base, the base can be stabilized by hyperconjugation from methyl group. (but +ve inductive effect > hyperconjugation)

[swati]

According to me the correct order of acidity should be  4 < 1 < 2 < 3
My answer is supported by pKa values
Ethanol       :   15.9
Phenol        :    9.95
CH₃COOH   :    4.75
HCOOH       :    3.73

REASONING
Acidity of carboxyilic acid  >  phenol  >  aliphatic alcohols

In Alcohols
R-OH   +   H₂O    ------>   R-O^-    +  H₃O⁺

Due to inductive effect electron density will shift towards O making the release of H⁺ difficult. Electron donating groups destabilize alkoxide ion ( conjugate base ) . So it is a weak acid .

In Phenol
It's conjugate base i.e. phenoxide ion is stabilized by resonance , so it is a better acid than alcohol

In carboxylic acids
 C=O is directly attached to OH , so the release of H⁺ is much more easier in this case .Also it's conjugate base i.e. RCOO^- is stabilized by resonance , so it is a better acid than phenol.
Also Phenoxide, the conjugate base of phenol, is less stable than acetate because acetate has two
electronegative O atoms upon which to delocalize the negative charge, whereas phenoxide has only one.

   Out of CH₃COOH  &  HCOOH  ,  CH₃COOH is weaker acid . Due to +I effect of CH₃ it will increase the electron density at O thereby decreasing the stability of conjugate base

[Yggdrasil]

will17
just a small doubt. is it correct to say, the inductive effect of methyl group in acetic acid strengthens the OH bond? infact it destabilizes the carbanion, isnt it? i mean, i can understand what you mean, but the word "strengthens" may mislead beginers.

I think it is still correct to say that the inductive effect strengthens the O-H bond.  The inductive effect from the methyl group will disfavor an accumulation of electron density on the oxygen, so it promotes more sharing of electrons between the O and H.  This increases electron density between the two nuclei and strenghtens the bond.

[movies]

will17
just a small doubt. is it correct to say, the inductive effect of methyl group in acetic acid strengthens the OH bond? infact it destabilizes the carbanion, isnt it? i mean, i can understand what you mean, but the word "strengthens" may mislead beginers.

I think it is still correct to say that the inductive effect strengthens the O-H bond.  The inductive effect from the methyl group will disfavor an accumulation of electron density on the oxygen, so it promotes more sharing of electrons between the O and H.  This increases electron density between the two nuclei and strenghtens the bond.

I like to think of this effect as the donation of electron density from the Me into the carbonyl C=O bond make the carbonyl less electron withdrawing.  Therefore, there is less driving force to pull electrons from the OH oxygen and ultimately less driving force to break the O-H bond.  So maybe the conclusion is (as I just realized, but everyone seems to be ahead of me on this) that the O-H bond doesn't really get "stronger" but it's just less likely to break because the conjugate base gets higher in energy when there are donating groups attached.