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NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Page No. 195 - Intext Questions

Q7.1: Classify the following as primary, secondary and tertiary alcohols:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans: Primary alcohol → (i), (ii), (iii)

Secondary alcohol → (iv), (v)

Tertiary alcohol → (vi)


Q7.2: Identify allylic alcohols in the above examples.

Ans: The alcohols given in (ii) and (vi) are allylic alcohols.

Page No. 198 - Intext Questions

Q7.3: Name the following compounds according to IUPAC system.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans: (i) 3-Chloromethyl-2-isopropylpentan-1-ol

(ii) 2, 5-Dimethylhexane-1, 3-diol

(iii) 3-Bromocyclohexanol

(iv) Hex-1-en-3-ol

(v) 2-Bromo-3-methylbut-2-en-1-ol

Page No. 203 - Intext Questions

Q7.4: Show how are the following alcohols prepared by the reaction of a suitable Grignard's reagent on methanal?

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers                           

Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.5: Write structures of the products of the following reactions:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans: (i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

(iii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Page No. 213 - Intext Questions

Q7.6: Give structures of the products you would expect when each of the following alcohol reacts with (a) HCl-ZnCl2 (b) HBr and (c) SOCl2.

(i) Butan-1-ol (ii) 2-Methylbutan-2-ol

Ans:

(a)           

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Primary alcohols do not react appreciably with Lucas’ reagent (HCl-ZnCl2) at room temperature.

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Tertiary alcohols react immediately with Lucas’ reagent.

(b)       

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(c)      

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.7: Predict the major product of acid catalysed dehydration of

(i) 1-methyl cyclohexanol and                              

(ii) butan-1-ol

Ans:

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

  NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.8: Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of the corresponding phenoxide ions.

Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Resonance structure of the phenoxide ion

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Resonance structures of p-nitrophenoxide ion

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Resonance structures of o-nitrophenoxide ion

It can be observed that the presence of nitro groups increases the stability of phenoxide ion.


Q7.9: Write the equations involved in the following reactions:
(i) Reimer-Tiemann reaction
(ii) Kolbe’s reaction

Ans: Reimer-Tiemann reaction

   NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Kolbe’s reaction

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Page No. 220 - Intext Questions

Q7.10: Write the reactions of Williamson synthesis of 2-ethoxy-3-methyl pentane starting from ethanol and 3-methylpentan-2-ol.

Ans: In Williamson synthesis, an alkyl halide reacts with an alkoxide ion. Also, it is an SN2 reaction. In the reaction, alkyl halides should be primary having the least steric hindrance. Hence, an alkyl halide is obtained from ethanol and alkoxide ion from 3-methylpentan-2-ol.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.11: Which of the following is an appropriate set of reactants for the preparation of 1-methoxy-4-nitrobenzene and why?

(i)       

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers                                                  

AnsSet (ii) is an appropriate set of reactants for the preparation of 1-methoxy-4-nitrobenzene.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

In set (i), sodium methoxide (CH3ONa) is a strong nucleophile as well as a strong base. Hence, an elimination reaction predominates over a substitution reaction.

Page No. 221 - Intext Questions

Q7.12: Predict the products of the following reactions:

(i) 

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)       

 

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii)    

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers                               

(iv)

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans:

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iv)

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Exercises

Q7.1: Write IUPAC names of the following compounds:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans: (i) 2, 2, 4-Trimethylpentan-3-ol
(ii) 5-Ethylheptane-2, 4-diol
(iii) Butane-2, 3-diol
(iv) Propane-1, 2, 3-triol
(v) 2-Methylphenol  
(vi) 4-Methylphenol
(vii) 2, 5-Dimethylphenol  
(viii) 2, 6-Dimethylphenol
(ix) 1-Methoxy-2-methylpropane    
(x) Ethoxybenzene
(xi) 1-Phenoxyheptane      
(xii) 2-Ethoxybutane


Q7.2: Write structures of the compounds whose IUPAC names are as follows:

(i) 2-Methylbutan-2-ol  
 (ii) 1-Phenylpropan-2-ol
 (iii) 3,5-Dimethylhexane −1, 3, 5-triol
 (iv) 2,3 − Diethylphenol
 (v) 1 – Ethoxypropane    
 (vi) 2-Ethoxy-3-methylpentane
 (vii) Cyclohexylmethanol
 (viii) 3-Cyclohexylpentan-3-ol
 (ix) Cyclopent-3-en-1-ol  
 (x) 3-Chloromethylpentan-1-ol.

Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.3: (i) Draw the structures of all isomeric alcohols of molecular formula C5H12O and give their IUPAC names.
(ii) Classify the isomers of alcohols in question 11.3 (i) as primary, secondary and tertiary alcohols.
Ans:
(i) The structures of all isomeric alcohols of molecular formula, C5H12O are shown below:

(a)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Pentan-1-ol (1°)

(b)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

2-Methylbutan-1-ol (1°)

(c)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

3-Methylbutan-1-ol (1°)

(d)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

2, 2-Dimethylpropan-1-ol (1°)

(e)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Pentan-2-ol (2°)

(f)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

3-Methylbutan-2-ol (2°)

(g)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Pentan-3-ol (2°)

(h)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

2-Methylbutan-2-ol (3°)

(ii) Primary alcohol: Pentan-1-ol; 2-Methylbutan-1-ol; 3-Methylbutan-1-ol; 

2, 2−Dimethylpropan-1-ol

Secondary alcohol: Pentan-2-ol; 3-Methylbutan-2-ol; Pentan-3-ol

Tertiary alcohol: 2-methylbutan-2-ol


Q7.4: Explain why propanol has higher boiling point than that of hydrocarbon, butane?
Ans: Propanol undergoes intermolecular H-bonding because of the presence of the −OH group. On the other hand, butane does not

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Therefore, extra energy is required to break hydrogen bonds. For this reason, propanol has a higher boiling point than hydrocarbon butane.


Q7.5: Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.
Ans: Alcohols form H-bonds with water due to the presence of −OH group. However, hydrocarbons cannot form H-bonds with water.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

As a result, alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses.


Q7.6: What is meant by the hydroboration-oxidation reaction? Illustrate it with an example.
Ans: The addition of borane followed by oxidation is known as the hydroboration-oxidation reaction. For example, propan-1-ol is produced by the hydroboration-oxidation reaction of propene. In this reaction, propene reacts with diborane (BH3)2 to form trialkyl borane as an additional product. This additional product is oxidized to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.7: Give the structures and IUPAC names of monohydric phenols of molecular formula, C7H8O.
Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.8: While separating a mixture of ortho and para nitrophenols by steam distillation, name the isomer which will be steam volatile. Give reason.
Ans: Intramolecular H-bonding is present in o-nitrophenol. In p-nitrophenol, the molecules are strongly associated due to the presence of intermolecular bonding. Hence, o-nitrophenol is steam volatile.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.9: Give the equations of reactions for the preparation of phenol from cumene.
Ans: To prepare phenol, cumene is first oxidized in the presence of air of cumene hydro-peroxide.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Then, cumene hydroxide is treated with dilute acid to prepare phenol and acetone as by-products.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.10: Write a chemical reaction for the preparation of phenol from chlorobenzene.
Ans: Chlorobenzene is fused with NaOH (at 623 K and 320 atm pressure) to produce sodium phenoxide, which gives phenol on acidification. NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.11: Write the mechanism of hydration of ethene to yield ethanol.
Ans: The mechanism of hydration of ethene to form ethanol involves three steps.

Step 1:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Protonation of ethene to form carbocation by the electrophilic attack of H3O :

Step 2:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Nucleophilic attack of water on carbocation:

Step 3:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Deprotonation to form ethanol:


Q7.12: You are given benzene, conc. H2SO4 and NaOH. Write the equations for the preparation of phenol using these reagents.
Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.13: Show how will you synthesize:

(i) 1-phenylethanol from a suitable alkene.

(ii) cyclohexylmethanol using an alkyl halide by an SN2 reaction.

(iii) pentan-1-ol using a suitable alkyl halide?

Ans: (i) By acid-catalyzed hydration of ethylbenzene (styrene), 1-phenylethanol can be synthesized.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 (ii) When chloromethylcyclohexane is treated with sodium hydroxide, cyclohexyl methanol is obtained.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 (iii) When 1-chloropentane is treated with NaOH, pentan-1-ol is produced.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.14: Give two reactions that show the acidic nature of phenol. Compare the acidity of phenol with that of ethanol.
Ans: The acidic nature of phenol can be represented by the following two reactions:

(i) Phenol reacts with sodium to give sodium phenoxide, liberating H2.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii) Phenol reacts with sodium hydroxide to give sodium phenoxide and water as by-products.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

The acidity of phenol is more than that of ethanol. This is because after losing a proton, the phenoxide ion undergoes resonance and gets stabilized whereas the ethoxide ion does not.
NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.15: Explain why is ortho nitrophenol more acidic than ortho methoxy phenol?
Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

The nitro-group is an electron-withdrawing group. The presence of this group in the ortho position decreases the electron density in the O−H bond. As a result, it is easier to lose a proton. Also, the o-nitrophenoxide ion formed after the loss of protons is stabilized by resonance. Hence, ortho nitrophenol is a stronger acid.

On the other hand, the methoxy group is an electron-releasing group. Thus, it increases the electron density in the O−H bond and hence, the proton cannot be given out easily.

For this reason, ortho-nitrophenol is more acidic than ortho-methoxy phenol.

 

Q7.16: Explain how does the −OH group attached to a carbon of benzene ring activate it towards electrophilic substitution?
Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

The −OH group is an electron-donating group. Thus, it increases the electron density in the benzene ring as shown in the given resonance structure of phenol.

As a result, the benzene ring is activated towards electrophilic substitution.


Q7.17: Give equations of the following reactions:

(i) Oxidation of propan-1-ol with alkaline KMnO4 solution.

(ii) Bromine in CS2 with phenol.

(iii) Dilute HNO3 with phenol.

(iv) Treating phenol with chloroform in presence of aqueous NaOH.

Ans:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers  

 

Q7.18: Explain the following with an example.

(i) Kolbe’s reaction.
 (ii) Reimer-Tiemann reaction.
 (iii) Williamson ether synthesis.  
 (iv) Unsymmetrical ether.

Ans: (i) Kolbe’s reaction:

When phenol is treated with sodium hydroxide, sodium phenoxide is produced. This sodium phenoxide when treated with carbon dioxide, followed by acidification, undergoes electrophilic substitution to give ortho-hydroxybenzoic acid as the main product. This reaction is known as Kolbe’s reaction.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii) Reimer-Tiemann reaction:

When phenol is treated with chloroform (CHCl3) in the presence of sodium hydroxide, a −CHO group is introduced at the ortho position of the benzene ring.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

This reaction is known as the Reimer-Tiemann reaction.

The intermediate is hydrolyzed in the presence of alkalis to produce salicylaldehyde.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii) Williamson ether synthesis:

Williamson ether synthesis is a laboratory method to prepare symmetrical and unsymmetrical ethers by allowing alkyl halides to react with sodium alkoxides.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

This reaction involves an SN2 attack of the alkoxide ion on the alkyl halide. Better results are obtained in the case of primary alkyl halides.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

If the alkyl halide is secondary or tertiary, then elimination competes over substitution.

(iv) Unsymmetrical ether:

An unsymmetrical ether is an ether where two groups on the two sides of an oxygen atom differ (i.e., have an unequal number of carbon atoms). For example ethyl methyl ether (CH3−O−CH2CH3).


Q 7.19: Write the mechanism of acid-catalysed dehydration of ethanol to yield ethene.

Ans: The mechanism of acid dehydration of ethanol to yield ethene involves the following three steps:

Step 1:

Protonation of ethanol to form ethyl oxonium ion:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 2:

Formation of carbocation (rate-determining step):

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 3:

Elimination of a proton to form ethene:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

The acid consumed in step 1 is released in Step 3. After the formation of ethene, it is removed to shift the equilibrium in a forward direction.


Q7.20: How are the following conversions carried out?

(i) Propene → Propan-2-ol

(ii) Benzyl chloride → Benzyl alcohol

(iii) Ethyl magnesium chloride → Propan-1-ol.

(iv) Methyl magnesium bromide → 2-Methylpropan-2-ol.

Ans: (i) If propene is allowed to react with water in the presence of an acid as a catalyst, then propan-2-ol is obtained.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii) If benzyl chloride is treated with NaOH (followed by acidification) then benzyl alcohol is produced.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii) When ethyl magnesium chloride is treated with methanal, an adduct is produced which gives propan-1-ol on hydrolysis.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iv) When methyl magnesium bromide is treated with propane, an adduct is a product that gives 2-methylpropane-2-ol on hydrolysis.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.21: Name the reagents used in the following reactions:

(i) Oxidation of primary alcohol to carboxylic acid.

(ii) Oxidation of primary alcohol to aldehyde.

(iii) Bromination of phenol to 2,4,6-tribromophenol.

(iv) Benzyl alcohol to benzoic acid.

(v) Dehydration of propan-2-ol to propene.

(vi) Butan-2-one to butan-2-ol.

Ans: (i) Acidified potassium permanganate

(ii) Pyridinium chlorochromate (PCC)

(iii) Bromine water

(iv) Acidified potassium permanganate

(v) 85% phosphoric acid

(vi) NaBH4 or LiAlH4


Q7.22: Give a reason for the higher boiling point of ethanol in comparison to methoxymethane.
Ans: Ethanol undergoes intermolecular H-bonding due to the presence of the −OH group, resulting in the association of molecules. Extra energy is required to break these hydrogen bonds. On the other hand, methoxymethane does not undergo H-bonding. Hence, the boiling point of ethanol is higher than that of methoxymethane.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.23: Give IUPAC names of the following ethers:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans: (i) 1-Ethoxy-2-methylpropane

(ii) 2-Chloro-1-methoxyethane

(iii) 4-Nitroanisole

(iv) 1-Methoxypropane

(v) 1-Ethoxy-4, 4-dimethylcyclohexane

(vi) Ethoxybenzene

Q7.24: Write the names of reagents and equations for the preparation of the following ethers by Williamson’s synthesis:

(i) 1-Propoxypropane                                
 (ii) Ethoxybenzene
 (iii) 2-Methoxy-2-methylpropane            
 (iv) 1-Methoxyethane

Ans:

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iv)

  NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.25: Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.
Ans: The reaction of Williamson synthesis involves the SN2 attack of an alkoxide ion on a primary alkyl halide. NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

But if secondary or tertiary alkyl halides are taken in place of primary alkyl halides, then elimination would compete over substitution. As a result, alkenes would be produced. This is because alkoxides are nucleophiles as well as strong bases. Hence, they react with alkyl halides, which results in an elimination reaction.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.26: How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.
Ans: 1-propoxypropane can be synthesized from propan-1-ol by dehydration.

Propan-1-ol undergoes dehydration in the presence of protic acids (such as H2SO4, H3PO4) to give 1-propoxypropane.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

The mechanism of this reaction involves the following three steps:

Step 1: Protonation

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 2: Nucleophilic attack

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 3: Deprotonation

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.27: Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.
Ans: The formation of ethers by dehydration of alcohol is a bimolecular reaction (SN2) involving the attack of an alcohol molecule on a protonated alcohol molecule. In the method, the alkyl group should be unhindered. In the case of secondary or tertiary alcohols, the alkyl group is hindered. As a result, elimination dominates substitution. Hence, in place of ethers, alkenes are formed.


Q7.28: Write the equation of the reaction of hydrogen iodide with:
(i) 1-propoxypropane                                
(ii) Methoxybenzene and
(iii) Benzyl ethyl ether

Ans: (i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Q7.29: Explain the fact that in aryl alkyl ethers

(i) The alkoxy group activates the benzene ring towards electrophilic substitution and

(ii) It directs the incoming substituents to ortho and para positions in the benzene ring.

Ans:

(i)

  NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

In aryl alkyl ethers, due to the R effect of the alkoxy group, the electron density in the benzene ring increases as shown in the following resonance structure.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

 

Thus, benzene is activated towards electrophilic substitution by the alkoxy group.

(ii) It can also be observed from the resonance structures that the electron density increases more at the ortho and para positions than at the meta position. As a result, the incoming substituents are directed to the ortho and para positions in the benzene ring.


Q7.30: Write the mechanism of the reaction of HI with methoxymethane.

Ans: The mechanism of the reaction of HI with methoxymethane involves the following steps:

Step1: Protonation of methoxymethane:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step2: Nucleophilic attack of I−:

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step3:

When HI is in excess and the reaction is carried out at a high temperature, the methanol formed in the second step reacts with another HI molecule and gets converted to methyl iodide

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.31: Write equations of the following reactions:

(i) Friedel-Crafts reaction−alkylation of anisole.

(ii) Nitration of anisole.

(iii) Bromination of anisole in ethanoic acid medium.

(iv) Friedel-Craft’s acetylation of anisole.

Ans: (i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iv)

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.32: Show how would you synthesise the following alcohols from appropriate alkenes?

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Ans: The given alcohols can be synthesized by applying Markovnikov’s rule of acid-catalyzed hydration of appropriate alkenes.

(i)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(ii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

(iii)

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Acid-catalyzed hydration of pent-2-ene also produces pentan-2-ol but along with pentan-3-ol.

NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Thus, the first reaction is preferred over the second one to get pentan-2-ol.

(iv)

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


Q7.33: When 3-methylbutan-2-ol is treated with HBr, the following reaction takes place:

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Give a mechanism for this reaction.

(Hint: The secondary carbocation formed in step II rearranges to a more stable tertiary carbocation by a hydride ion shift from the 3rd carbon atom.

Ans: The mechanism of the given reaction involves the following steps:

Step 1: Protonation

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 2: Formation of 2° carbocation by the elimination of a water molecule

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 3: Re-arrangement by the hydride-ion shift

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

Step 4: Nucleophilic attack

 NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers


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FAQs on NCERT Solutions Class 12 Chemistry Chapter 7 - Alcohols, Phenols & Ethers

1. What are the physical properties of alcohols, phenols, and ethers?
Ans. Alcohols, phenols, and ethers exhibit properties such as solubility in water, boiling points, and odors, which are determined by their chemical structures.
2. How are alcohols, phenols, and ethers classified based on their structure?
Ans. Alcohols are classified as primary, secondary, or tertiary based on the number of carbon atoms bonded to the carbon atom bearing the hydroxyl group. Phenols are classified based on the position of the hydroxyl group in the benzene ring. Ethers are classified based on the alkyl groups bonded to the oxygen atom.
3. What are some common uses of alcohols, phenols, and ethers in daily life?
Ans. Alcohols are commonly used as antiseptics, disinfectants, and solvents. Phenols are used in the production of plastics, disinfectants, and pharmaceuticals. Ethers are used as solvents and anesthetics.
4. How do alcohols, phenols, and ethers react with different reagents?
Ans. Alcohols can undergo oxidation, dehydration, and substitution reactions. Phenols react with metals, acids, and alkalis to form salts. Ethers are relatively inert but can undergo cleavage reactions under specific conditions.
5. What are some important industrial applications of alcohols, phenols, and ethers?
Ans. Alcohols are used in the production of paints, varnishes, and fuels. Phenols are used in the production of disinfectants, resins, and pharmaceuticals. Ethers are used as solvents in various chemical processes.
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