[rajajidwivedi]

as we know that alcohols are weaker acid than water but why methanol is slightly stronger than water. why is it so?

[enahs]

In a pure form, H₂O   H₃O⁺ + OH^-, where as CH₃OH   CH₃O⁺H₂ + CH₃O^-. I would guess it has to do with the donating ability of the small alkyl group to better stabalize the positive charge on the oxygen in CH₃O⁺H₂.

Acidity is a wonderful tool and probably the most important fundamental concept in organic that is under-taught, as it can help understanding and predicting in SO many ways.

But do NOT get to focused on values and slight differences. Just a change of a few degrees in temperature can completely reverse what is more acidic then the other in solution, etc.

[nox]

^See the problem is that if what you said was true, then we'd expect t-butanol to be substantially more acidic than H2O, but in fact the opposite is true.

But maybe it involves some fine balance between alkyls stabilizing the cation, versus it destabilizing the anion.

[movies]

There is also an important effect from the ability to solvate a charged intermediate.  Molecules ionize easily in water because there are many small dipolar molecules around to negate the negative Coulombic effect of separating charges.  The effect is ultimately just smearing out the charges over a network of different molecules.  As this relates to alcohols, of course you would expect methanol to be the most like water and then decreasing solvation ability as the alkyl portion of the alcohol gets larger.  I suspect you are using the pK_a values in water (pK_a is strongly dependent on solvent).  If you instead consider acidity in the gas phase, then you observe the order of decreasing acidity as follows: neopentyl alcohol, t-BuOH, i-PrOH, EtOH, MeOH, H₂O (see JACS 1970, 92, 5986).

[enahs]

^See the problem is that if what you said was true, then we'd expect t-butanol to be substantially more acidic than H2O, but in fact the opposite is true.

But maybe it involves some fine balance between alkyls stabilizing the cation, versus it destabilizing the anion.

No, we would not expect t-Butanol to be more acidic. When we add more alkyl groups we also are adding more to the alkoxide ion, especially when the R's are bulky there is even less stabilization of the alkoxide ion, thus shifting equilibrium to the alcohol and not the acid.

Just my opinion...

[rajajidwivedi]

<a href="http://" target="_blank" rel="noopener noreferrer" class="bbc_link bbc_flash_disabled new_win">http://</a> same problem i think thank as we know that alkyl is a +I group so it should destablize the alkoxide ion and same happens in the case of other alcohols but not in the case of MeOH so it is the problem.

[orgopete]

This is an excellent question, however I think water is the outlier. The alcohols seem to me to follow a predictable pattern. Water (or hydrogen) seems to not follow that pattern.

If you compare the acidity of protonated dimethyl ether (-3.8 )  with protonated THF (-2.1), this seems consistent with methanol (15.5), ethanol (16), isopropanol (16.5), and t-butanol (17). If methyl groups are replaced with hydrogen, the protonated methanol (-2.2) and hydronium (-1.7) seem to have a trend counter to the electron donation of substituted carbons. In virtually all other circumstances, carbon is a better electron donor than hydrogen.

I agree with the suggestion of movies to consider the gas phase acidities in which the acidities do follow the predicted trend. I am wary of gas phase acidities (probably because I do not understand them). However, I do think that we can learn from the gas phase acidities as to how they might differ from solution pKa values in that solvent effects are removed. If I accept this consequence to the measurement of pKa values, then I would contemplate whether hydrogen bonding might reverse the electron withdrawing properties of protons. If pKa measurements are solvent dominated, that is only a small amount of acids are present, then we could consider the bulk hydrogen bonding present could stretch the O-H bonds and make the non-bonded electrons of oxygen more available. This would be an effect opposite of a proton being an electron withdrawing group, but consistent with hydrogen bonding.

[rajajidwivedi]

but why same concepts is not used in case of other alcohols.