Alcohols Phenols and Ethers Class 12 Chemistry Exam Questions

Please refer to the below Alcohols, Phenols and Ethers important questions for Class 12 Chemistry. These questions and answers have been prepared as per the latest NCERT Book for Class 12 Chemistry. Students should go through chapter wise Class 12 Chemistry Important Questions designed as per the latest examination pattern issued by CBSE.

Very Short Answer Questions :

Question. Write the main product(s) in the following reaction :

Answer : 

Question. Explain the mechanism of the following reactions :
(i) Addition of Grignard’s reagent to the carbonyl group of a compound forming an adduct followed by hydrolysis.
(ii) Acid catalysed dehydration of an alcohol forming an alkene.
(iii) Acid catalysed hydration of an alkene forming an alcohol.  
Answer : 

(ii) Acid catalysed dehydration of alcohols at high temperature takes place with formation of an alkene.

(iii) 

Question. Explain the following giving one example :
Friedel Craft’s acetylation of anisole. 
Answer : Reagents : Acetyl chloride and Lewis acid catalyst.

Question. Name a substance that can be used as an antiseptic as well as a disinfectant. 
Answer : Phenol : 0.2% solution of phenol is an antiseptic while 2% solution is used as disinfectant.

Question. Why is the preparation of ether by acid dehydration of secondary alcohol not a suitable method?  
Answer : Acid dehydration of 2° and 3° alcohols give alkenes rather than ethers.
Due to steric hindrance the nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not occur.
The protonated 2° and 3° alcohols lose water molecules to form stable 2° and 3° carbocations.

Question. How would you convert the following :
(i) Phenol to benzoquinone
(ii) Propanone to 2-methylpropan-2-ol
(iii) Propene to propan-2-o1.  
Answer : (i)

(ii) 

(iii)

Question. How is the following conversion carried out?
Anisole to p-bromoanisole  
Answer : 

Question. Name the reagents which are used in the following conversions :
(i) A primary alcohol to an aldehyde
(ii) Butan-2-one to butan-2-ol
(iii) Phenol to 2, 4, 6-tribromophenol 
Answer : (i) Copper at 573 K
(ii) Sodium borohydride (NaBH₄)
(iii) Bromine water (Br₂(aq))

Question. Account for the following :
Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method.  
Answer : Acid dehydration of 2° and 3° alcohols give alkenes rather than ethers.
Due to steric hindrance the nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not occur.
The protonated 2° and 3° alcohols lose water molecules to form stable 2° and 3° carbocations.

Question. Write the equations involved in the following reaction :
Williamson synthesis 
Answer : Williamson ether synthesis : Alkyl halide when treated with sodium alkoxide gives dialkyl ether.
C₂H₅ONa + C₂H₅Cl → C₂H₅—O—C₂H₅ + NaCl

Question. Draw the structure and name of the product formed if the following alcohols are oxidized. Assume that an excess of oxidising agent is used.
(i) CH₃CH₂CH₂CH₂OH
(ii) 2-butenol
(iii) 2-methyl-1-propanol 
Answer :

Question. Illustrate the following name reaction :
Williamson Synthesis 
Answer : Williamson ether synthesis : Alkyl halide when treated with sodium alkoxide gives dialkyl ether.
C₂H₅ONa + C₂H₅Cl → C₂H₅—O—C₂H₅ + NaCl

Question. (i) Describe the mechanism of acid dehydration of ethanol to yield ethene.
(ii) Describe a chemical test to distinguish between ethanol and phenol. 
Answer : (i) Ethanol undergoes dehydration by heating it with concentrated H₂SO₄ at 453 K.

(ii) Distinction between ethanol and phenol.
FeCl₃ test : Phenol gives a violet colouration with FeCl₃ solution while ethanol does not.
      3C₆H₅OH + FeCl₃ → (C₆H₅O)3Fe + 3HCl
                 Phenol             Violet colouration
      C2H₅OH + FeCl₃ → No violet colouration.

Question. Give reasons for the following :
(i) Boiling point of ethanol is higher in comparison to methoxymethane.
(ii) (CH₃)₃C—O—CH₃ on reaction with HI gives CH₃OH and (CH₃)₃C—I as the main products and not (CH₃)₃C—OH and CH₃I. 
Answer : (i) Ethanol has higher boiling point because of strong intermolecular hydrogen bonding whereas in methoxymethane, molecules are held by dipoledipole interaction.
(ii) When one alkyl group is a tertiary group the halide formed is tertiary halide.
In step II the departure of leaving group (CH₃—OH) creates a more stable carbocation (3°) and the reaction follows S_N1 mechanism.

Short Answer Questions :

Question. Write IUPAC name of the following :

Answer : 1-Ethoxy-2-nitrocyclohexane.

Question. Give mechanism of preparation of ethoxy ethane from ethanol. 
Answer : Mechanism : The formation of ether is nucleophilic bimolecular reaction.

Question. Describe the following :
Unsymmetrical ether 
Answer : Unsymmetrical ether : Ethers in which two alkyl groups are different are known as unsymmetrical ether. e.g., CH₃OCH₂CH₃ Methoxyethane.

Question. (a) Describe the mechanism of hydration of ethene to yield ethanol.
(b) Write Kolbe’s reaction with an example. 
Answer : (a)

(b) Kolbe’s reaction : When sodium phenoxide is heated with carbon dioxide under pressure, it gives salicylic acid.

Question. Name the reagents and write the chemical equations for the preparation of the following compounds by Williamson’s synthesis :
(i) Ethoxybenzene
(ii) 2-Methyl-2-methoxypropane 
Answer : 

Question. Account for the following :
The boiling points of ethers are lower than isomeric alcohols. 
Answer : The boiling points of ethers are much lower than, those of alcohol of comparable molar masses because like alcohols they cannot form intermolecular hydrogen bonds.

Question. Acid catalysed dehydration of t-butanol is faster than that of n-butanol. Explain.  
Answer : Acid catalysed dehydration of alcohols follows carbocation mechanism.
Hence, dehydration of t-butanol which form 3° carbocation is faster than n-butanol which form primary carbocation.

Question. Explain the following with an example :
Williamson ether synthesis. 
Answer : Williamson ether synthesis : Alkyl halide when treated with sodium alkoxide gives dialkyl ether.
C₂H₅ONa + C₂H₅Cl → C₂H₅—O—C₂H₅ + NaCl

Question. (a) Write the IUPAC name of the following :

(b) Give reasons for the following :
(i) Phenol is a stronger acid than alcohol.
(ii) Alcohols are comparatively more soluble in water than the corresponding hydrocarbons. 
Answer : (a) 2,5-Dimethylphenol
(b) (i) Phenols are more acidic than alcohols. It can be explained on the basis that alcohol on losing H+ ions form alkoxide ion and phenol forms phenoxide ion. 
The greater acidity of phenol is due to the stability of the phenoxide ion which is resonance stabilized as shown below.

On the other hand, alkoxide ion shows no such resonance stabilisation and is unstable.
(ii) The solubility of alcohols in water is due to their ability to form hydrogen bonds with water molecules. Hydrocarbons cannot form such hydrogen bonds, hence they are insoluble in water.

Question. The boiling points of ethers are much lower than those of the alcohols of comparable molar masses.  
Answer : The boiling points of ethers are much lower than, those of alcohol of comparable molar masses because like alcohols they cannot form intermolecular hydrogen bonds.

Question. How is 1-propoxypropane synthesised from propan-1-ol?  
Answer : Propan-1-ol on treatment with conc. H₂SO₄ at 413 K would yield 1-propoxypropane. In this method, the alcohol is continuously added to keep its concentration in excess.

Question. Phenylmethyl ether reacts with HI to give phenol and methyl iodide and not iodobenzene and methyl alcohol. Why? 
Answer : Protonation of anisole (Phenyl methyl ether) gives methyl phenyl oxonium ion.

In this ion, the stronger bond is O—C₆H₅. Therefore, attack by I– ion exclusively breaks the weaker O—CH₃ bond forming methyl iodide and phenol. The phenol formed does not react further to give aryl halides.

QuestionWrite the mechanism of the following reaction :
2CH3CH2OH →Conc. H2SO4413 K CH3CH2—O—CH2—CH3 
Answer : Mechanism : The formation of ether is nucleophilic bimolecular reaction.