Alcohols, Phenols and Ethers

Question 1: What are alcohols? How are they classified?
Answer.

Alcohols: The organic compounds containing at least one hydroxyl group attached to saturated carbon atom are called alcohols. Alcohols are generally classified on the basis of hydroxyl groups.

Monohydric alcohol: An alcohol that has only hydroxyl group are known as monohydric alcohol. For example, methanol, ethanol, propanol, the etc.

Polyhydric alcohols: An alcohol that has two or more than two hydroxyl groups is Polyhydric alcohol. For example, ethylene glycol (two hydroxyl groups) , glycerol (three hydroxyl groups) etc.

Question 2: How are monohydric alcohols classified?

Answer. Monohydric alcohols are classified into primary, secondary (sec-, s-) and tertiary (ter-, t-) alcohols depending upon number of carbons attached to a carbon that is bonded to hydroxyl group.

i. Primary alcohols:

Primary alcohols are those monohydric alcohols where one or no other carbon is attached to a carbon that is bonded with hydroxyl group. e. g.

ii. Secondary alcohols: Secondary alcohols are those monohydric alcohols where two other carbons are attached to a carbon that is bonded with hydroxyl group. e. g.

iii. Tertiary alcohols: Tertiary alcohols are those monohydric alcohols where three other carbons are attached to a carbon that is bonded with hydroxyl group. e. g.

Question 3: Compare the acidity of primary, secondary and tertiary alcohols.
Answer.

Primary alcohols are more acidic than secondary alcohols which are more acidic than tertiary alcohols.
This is due to fact that acidic strength of an alcohol as an acid is dependent on stability of its conjugate base (alkoxide ion). Greater the stability of a conjugate base, stronger is the acid and vice versa.
There are two factors which usually affect stability of conjugate base.
Inductive effect: Due to positive inductive effect of more alkyl groups attached to hydroxyl carbon, electron density is increased on oxygen of alkoxide and hence alkoxide will become less stable.
Stearic effect: Due to stearic effect of more alkyl groups attached to the hydroxy carbon, solvation of oxygen of alkoxide is decreased, and hence alkoxide become less stable.
Therefore, due to these both factors, tertiary alcohol produce least stable alkoxide while primary alcohol produce most stable alkoxide as conjugate base. So, tertiary alcohols are least acidic and primary alcohols are most acidic.

Question 4: Alcohols and phenols have OH group but phenols are more acidic than alcohols, why?
Answer:
In fact, acidic strength of an acid depends on stability of its conjugate base. Greater the stability of the conjugate base of the acid, stronger is the acid and vice versa.
Phenols are more acidic as their conjugate bases (phenolate ions) are stablized due to delocalized negative charge on entire aromatic ring.
On the other hand, alcohols are less acidic as their conjugate bases (alkoxide ions) are not stablized due to localised negative charge on oxygen of alkoxide ions.

Question 5: How will you differentiate between an alcohol and phenol?
Answer
Alcohols and phenol can be differentiated by following method
Litmus test: Phenol is acidic in nature and it turns blue Litmus paper red while alcohol has no effect.
Bromine water test: Phenol gives deep blue, blue green or voilet colour with bromine water while alcohol does not show any reaction with bromine water.
Reaction with NaOH: Phenol reacts with NaOH while alcohol does not react.

Question 6. Why is phenol more soluble in water than toluene?
Answer. Although both phenol and toluene have same larger phenyl rings in their molecular structures, yet phenol is more soluble in water than toluene. This is due to fact that phenol has a polar OH substituent, which makes it soluble in water due to hydrogen bonding while toluene has non polar methyl substituent which makes it water repellent and less soluble in water.

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