If that doesn't help, please let us know. Unable to load video. Please check your Internet connection and reload this page. If the problem continues, please let us know and we'll try to help. An unexpected error occurred. Previous Video The oxygen atom of an alcohol polarizes both its adjacent bonds. While the polarization of the O—H bond makes alcohol a weak acid, the lone pairs on oxygen make it both basic and nucleophilic.
The conjugate base of an aliphatic alcohol is an alkoxide ion, and that of a phenol is a phenoxide or phenolate ion. A negative charge is more stable on an oxygen atom than on nitrogen or carbon but most stable on a halogen atom. Recall that conjugate base stability is a measure of acidity. Therefore, alcohols are more acidic than terminal alkynes, amines, and alkanes but less acidic than hydrogen halides.
Typical alcohols like methanol or ethanol are almost as acidic and basic as water. However, phenols are more acidic than alcohols but less acidic than carboxylic acids.
Unlike alkoxide ions, the negative charge on the oxygen atom of phenoxide is stabilized by resonance and polarization via its aromatic ring. Therefore, phenols can be deprotonated by a relatively weak base like hydroxide.
However, alcohols can only be deprotonated by stronger bases like sodium metal or potassium metal or sodium hydride with hydrogen evolution. The acidity of an alcohol is further influenced by the inductive effect and the positions of its substituents, as seen here with fluoride.
Trifluoro- and trichloroethanol are stronger acids than ethanol, since their conjugate bases are stabilized by the electron-withdrawing effect of the adjacent halogen atoms. In phenols, while electron-withdrawing nitro and chloro substituents enhance the acidity, methoxy and alkyl substituents only change it marginally. Naphthols are slightly more acidic than unsubstituted phenol. This trend is reversed in the solution phase due to the solvation effect, which makes a branched tert -butanol molecule less acidic than unhindered ethanol.
Like water, alcohols are weak acids and bases. This is attributed to the polarization of the O—H bond making the hydrogen partially positive. How does carbon dioxide change the acid-base condition of a solution? Why do strong acids have a low pH? Is neutralization a double replacement reaction? Why is NaH a strong base? Is HBr a strong acid? Is HF a strong acid? Why does neutralization occur? Do strong acids completely dissociate in water?
Aqueous solutions of sodium hydroxide can NOT deprotonate alcohols to a high enough concentration to be synthetically useful.
In solutions of organic solvents, more extreme reaction conditions can be created. Sodium metal can be added to an alcohol in an organic solvent system to fully deprotonate the alcohol to form alkoxide ions.
Several important chemical reactions of alcohols involving the O-H bond or oxygen-hydrogen bond only and leave the carbon-oxygen bond intact. An important example is salt formation with acids and bases. Alcohols, like water, are both weak bases and weak acids. It is convenient to employ sodium metal or sodium hydride, which react vigorously but controllably with alcohols:. By this we mean that the equilibrium position for the proton-transfer reaction lies more on the side of ROH as R is changed from primary to secondary to tertiary; therefore, tert-butyl alcohol is considered less acidic than ethanol:.
However, in the gas phase the order of acidity is reversed, and the equilibrium position for lies increasingly on the side of the alkoxide as R is changed from primary to secondary to tertiary, tert-butyl alcohol is therefore more acidic than ethanol in the gas phase.
This seeming contradiction appears more reasonable when one considers what effect solvation or the lack of it has on equilibria. In solution, the larger alkoxide ions, probably are less well solvated than the smaller ions, because fewer solvent molecules can be accommodated around the negatively charged oxygen in the larger ions:. Acidity of alcohols therefore decreases as the size of the conjugate base increases.
They do this by polarization of their bonding electrons, and the bigger the group, the more polarizable it is. Alcohols are bases similar in strength to water and accept protons from strong acids. An example is the reaction of methanol with hydrogen bromide to give methyloxonium bromide, which is analogous to the formation of hydroxonium bromide with hydrogen bromide and water:.
Compounds like alcohols and phenol which contain an -OH group attached to a hydrocarbon are very weak acids. Alcohols are so weakly acidic that, for normal lab purposes, their acidity can be virtually ignored. However, phenol is sufficiently acidic for it to have recognizably acidic properties - even if it is still a very weak acid.
A hydrogen ion can break away from the -OH group and transfer to a base. For example, in solution in water:. There are many sites on the internet with explanations of the relative ordering of alcohol acidities in aqueous solution. The general explanation is that the larger substituents are better electron donors, which destabilize the resulting alkoxide anions. Because hydrogen is least donating of the substituents, water is the strongest acid.
Unfortunately, although this belief persists, it is incomplete because it does not account for the gas-phase results. The problem with the electron donation explanation is that it suggests that the order of acidity is due solely to the intrinsic electronic effects of the substituents.
However, if that were the case, the electron donating effect should also be evident in the gas-phase data. However, the relative acidities in the gas phase are opposite to those in aqueous solution. Consequently, any interpretation of the acidities of alcohols must take the gas phase data into account. The inversion of the acidities of alcohols between the gas phase and aqueous solution was pointed out by Brauman and Blair in Polarizibility almost completely accounts for the trend in gas-phase acidities.
As the size of the substituent increases, the acid becomes stronger due to the ability for the charge to be distributed over a larger volume, thereby reducing the charge density and, consequently, the Coulombic repulsion.
Therefore, in the gas-phase, t-butanol is the most acidic alcohol, more acidic than isopropanol, followed by ethanol and methanol.
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