CBSE Class 12 Chemistry Alcohols Phenols And Ethers Board Exam Notes

3. Oxidation of Alcohols: (1°), (2°) & (3°) Using p otassium d i chromate & sulphuric Acid as
catalysts In the oxidation [O] of a primary alcohol( 1°) , aldehyde is produced
Primary alcohol → aldehyde →carboxylic acid
On oxidation of 20 alcohols a ketone is formed. Secondary alcohol → ketone
Tertiary alcohols do not oxidize.
Tertiary alcohol → No reaction

CONCEPTUAL QUESTIONS
Q1) Preparation of ethers by acid dehydration of secondary or 30 alcohols is not a suitable method?
Ans. The formation of ethers by dehydration of alcohol is a bimolecular reaction (SN2) group is hindered. As a result elimination dominates substitution as 30 carbocation is more stable. Hence in place of ethers, alkenes are formed.

Q2) Phenols do not give protonation reactions readily. Why?
Ans. The lone pair on oxygen of O‐H in phenol is being shared with benzene ring through resonance.
Thus, lone pair is not fully present on oxygen and hence phenols do not undergo protonation reactions.

REASONING QUESTIONS
Q1. Explain why propanol has higher boiling point than that of the hydrocarbon, butane?
Ans. The molecules of butane are held together by weak van der Waal's forces of attraction while those of propanol are held together by stronger intermolecular hydrogen bonding.

Q2. Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.
Ans. Alcohols can form hydrogen bonds with water and break the hydrogen bonds already existing between water molecules Therefore they are soluble in water, whereas hydrocarbons cannot form hydrogen bonds with water and hence are insoluble in water.

Q3 While separating a mixture of ortho and para nitro phenols by steam distillation, name the isomer which will be steam volatile. Give reason.
Ans. O‐nitrophenol is steam volatile due to intramolecular hydrogen bonding and hence can be separated by steam distillation from p‐nitrophenol which is not steam volatile because of inter‐molecular hydrogen bonding.

Q4. Explain why is ortho- nitro phenol more acidic than ortho -- methoxyphenol?
Ans. The nitro‐group is an electron‐withdrawing group. Therefore decreases the electron density of the ring as well as oxygen.. As a result, it is easier to lose a proton. Also, the o‐nitrophenoxide ion formed after the loss of proton is stabilized by resonance. Hence, o‐ nitro phenol is stronger acid. On the other hand, methoxy group is an electron‐releasing group. Thus, it increases the electron density of the oxygen atom and hence, the proton cannot be given out easily. Therefore, o‐nitrophenol is more acidic than o-methoxyphenol.

Q5. Write chemical reaction for the preparation of phenol from chlorobenzene.
Ans. Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced.