To help students develop a better understanding of the acidity concept, we present a detailed, quantitative analysis of the factors that govern both the absolute and relative acidities of methanol and water in aqueous solution and in dipolar aprotic DMSO. In particular, the preferential solvation of hydroxide over methoxide in aqueous solution, which leads to the nearly identical acidity of water and methanol in water, is examined. View Author Information. Cite this: J. Article Views Altmetric -.
Citations Abstract The relative acidities of water and methanol have been a nagging issue. Cited By. This article is cited by 12 publications. Elvin V. Payal Singh 4 years, 9 months ago.
The methyl group slightly takes away electron density from the oxygen atom, unlike the hydrogen in water, which actually contributes more electron distribution. When considering the acidity of an compound, consider these four elements: 1. Attached electronegative atom 2. Resonance structures 3. Size of molecule 4. Induction proximity to other electron-withdrawing groups 4th is the reason why methanol is more acidic than water, but as you probably know, it doesn't contribute much.
Shruti Dadhwal 4 years, 10 months ago. Hence H2O is more basic than methanol. Methanol is acidic than water because its conjugate base methoxide is weaker than waters conjugate base hydroxide. Methanol is more acidic than water because conjugate base of methanol methanoxide ion is weaker base than hydroxide ion ,methyl slightely withdraw electron density from oxygen atom unlike hydrogen which donates electron to oxygen atom. Mohammed Kasib Siddiqui Points. Other Related Questions on Organic Chemistry.
View all Questions ». You will get reply from our expert in sometime. We will notify you when Our expert answers your question. To View your Question Click Here. View courses by askIITians. These lessen the amount of inductive withdrawal that the alpha carbon can do.
The problem with only considering the acidities of these molecules in aqueous solution is that it assumes only one explanation for acidity. The inversion of the order of acidity in DMSO indicates that there are multiple factors at play.
The larger size of the alkyl substituents in the conjugate bases allows for increased distribution of negative charge over a larger volume. This reduces the charge density, and thereby the Coulombic repulsion between the conjugate base and the solvent. In DMSO, water is much less acidic than even methanol, which is also consistent with the relative polarizability of a methyl group and a hydrogen atom, but fails to explain the relative acidity of t-butanol and methanol.
The trend in alcohol acidity is explained by anionic hyperconjugation, which is weakened in larger alkyl substituents. Although DMSO does solvate the solute to an extent, because is an aprotic solvent, it is incapable of hydrogen bonding with the solute.
This allows DMSO acidities better reflect the intrinsic acidities of the molecules. In aqueous solution, however, the conjugate bases are stabilized by hydrogen bonding. Because smaller ions are better solvated by water, water's acidity is increased more than methanol's, but because methanol is intrinsically more acidic due to its polarizability, it is still slightly more acidic than water, even in aqueous solution.
As the size of the alkyl substituent increases, the alcohol becomes weaker due to the decreasing ability of water to solvate the conjugate base. This has a greater affect on the alcohol's acidity than the dispersion of the negative charge, and overrides the the intrinsic acidity, inverting the order of acidity.
Since oxygen is more electronegative than both carbon and hydrogen So a methyl group cannot fulfill the electron needs of oxygen which is why oxygen takes electrons from Hydrogen and a hydrogen ion is released easily. If a more electron donating group was present it could have fulfilled the electron needs of oxygen and hydrogen ion wouldn't have been released easily thus making the compound less acidic.
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