The ion is called the phenoxide ion and it gets stability by resonance which is not found in aliphatic alcohol. Further, phenol is more acidic than water too, because water has more polar OH-group in H-OH than in phenol, because, the alkyl group releases electrons and minimizes polarity of -OH group so, the water can have more stable hydroxide ion.
Phenol is more reactive and acidic than alcohol aliphatic as the phenoxide ion formed in the product is more stable due to resonance than alkoxide ion formed in the product side and hence the reaction shifts in forward direction in phenol and is slightly in backward direction in alcohols.
Therefore, phenol is more acidic than water. Phenols are stronger acids than alcohols bcoz of resonance a partial -ve charge weakens the OH bond and makes it easier to release a proton. 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.
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:. Phenol is a very weak acid and the position of equilibrium lies well to the left. Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent.
The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring. This overlap leads to a delocalization which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localized on the oxygen, but is spread out around the whole ion. Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen.
However, oxygen is the most electronegative element in the ion and the delocalized electrons will be drawn towards it. 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:. Phenol is a very weak acid and the position of equilibrium lies well to the left.
Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring. This overlap leads to a delocalization which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localized on the oxygen, but is spread out around the whole ion.
Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen. However, oxygen is the most electronegative element in the ion and the delocalized electrons will be drawn towards it.
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