CBSE Class 12 Chemistry Alcohols Phenols Ethers Questions Answers

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ALCOHOLS, PHENOLS AND ETHERS.

Q.1 Give the IUPAC name of the following.

CH3-C (CH3) =CH-CH2OH

Ans:-3-Methyl But-2-en 1-ol

Q.2 Phenols are much more acidic than alcohols. Why

Ans:-Phenoxide ion is more resonance stabilised than alkoxide ion.

Q.3 Give the IUPAC name of the following compound:

Solution :

2 − Bromo-3-methyl-but-2-enol.

Q.4 What happen when phenol is treated with excess of bromine(aq).

Ans:- It gives 2,4,6-tribromo phenol.

Q.5 Write chemical equation Williamson synthesis.

Ans:- R-X+R- O- Na --------- R-O-R + NaCl

Q.6 Mention one uses of methanol.

Ans:– (i) As a denaturant for ethanol

Q.7 The boiling point of ethanol is higher than that methoxy methane.

Ans:-Ethanol has inter molecular hydrogen bonding, methoxymehane does not have H-bonding.

Q.8 Name a substance that can be used as an antiseptic as well as a disinfectant.

Ans:-Phenol can be used as an antiseptic as well as a disinfectant. 0.1% Soln of phenol is used as an antisepticn & 1% Soln of phenol is used as a disinfectant.

Q.9 Write the IUPAC name.

(CH3)3 C-OH

Ans:- 2-methylpropan-2-ol

Q.10 What is Nucleophiles.

Ans:- The species which has high electrons density.

Q.11 Which catalyst are used in Friedel craft reaction?

Ans:- Anh.AlCl3.

Q.12 Write a test to distinguish between primary, secondary and alcohols?

Ans:- Lucas test.

Q.13 Write the IUPAC name of CH₃OH?

Ans:- Methanol.

Q.14 Write the IUPAC name of CH₃-O-CH₃?

Ans:- Methoxy methane.

Q.15.Write the IUPAC name of CH₃ CH₂-O-CH₃?

Ans:- Methoxy ethane. 

Short answers questions (2marks)

Q.1 Phenols are as a smaller dipole moment than methanol.

Ans:- Due to electron with drawning nature of ph-group-O bond is less polar.in case of methanol methyl group is electron releasing group So C-O bond is more polar.

Q.2 Explain why Phenol do not undergo substitution of OH group like alcohol.

Ans:-C-O bond in phenol has some double bond characters due to resonance an hence cannot be easily replaced by Nu. In contrast the C-O bond in alcohol is pure single bond an hence can be easily released by Nu.

Q.3 Give a test to distinguish between phenol and Benzyl alcohol.

Ans:- Phenols give violet colour with ferric chloride while benzyl alcohol does not give this coloure.

Q.4 Give a test to distinguish ethanol and phenol.

Ans:- phenol turns blue litmus red,but Ethanol donot have effect on litmus paper.

Q.5Write theWilliamson synthesis reaction

Ans:- R-X +R-ONa----------R-O-R +NaX

3marks questions

Q.1 Write the reaction of phenol with Zn.

Ans:- ph-OH +Zn-----------------------C₆H₆ +ZnO

Q.2 (i) Explain the mechanism of Addition of Grignard’s reagent to the carbonyl group of a compound forming an adduct followed by hydrolysis.

(ii) Explain the mechanism of Acid catalysed dehydration of an alcohol forming an alkene.

(iii) Explain the mechanism of Acid catalysed hydration of an alkene forming an alcohol.

Ans:- (i) Grignard’s reagent is an alkyl magnesium halide. The alkyl group has a partial negative charge, whereas the magnesium group has a partial positive charge. The alkyl group attacks the carbon of the carbonyl group to form an addition compound.

Grignard’s reagent acts as a nucleophilic agent & attacks electrophilic carbon atoms to yield a carbon − carbon bond. The addition to the nucleophile is an irreversible process due to the high pka value of the alkyl group.

(ii) When heated with concentrated sulphuric acid, phosphoric acid or boric acid, alcohols undergo dehydration to form alkenes. The mechanism of this reaction involves the protonation of alcohol, followed by loss of a water molecule & a proton.

During the dehydration of alcohol, the intermediate carbocation may undergo re-arrangement, resulting in the formation of a stable carbocation.

(iii) Some reactive alkenes like 2 − methyl propene undergo direct hydration in the presence of mineral acids which act as catalysts. The addition of water to the double bond takes place in accordance with Markonikoff’s rule. 

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