Chapter 11 Alcohols Phenols and Ethers Solutions
Question - 21 : - Give the equations of reactions for the preparation ofphenol from cumene.
Answer - 21 : -
To prepare phenol, cumene is first oxidized in thepresence of air of cumene hydro-peroxide.
Then, cumene hydroxide is treated with dilute acid toprepare phenol and acetone as by-products.
Question - 22 : - Write chemical reaction for the preparation of phenol fromchlorobenzene.
Answer - 22 : -
Chlorobenzene is fused with NaOH (at 623 K and 320 atmpressure) to produce sodium phenoxide, which gives phenol on acidification.
Question - 23 : - Write the mechanism of hydration of ethene to yieldethanol.
Answer - 23 : -
The mechanism of hydration of ethene to form ethanolinvolves three steps.
Step 1:
Protonation of ethene to form carbocation byelectrophilic attack of H3O+:
Step 2:
Nucleophilic attack of water on carbocation:
Step 3:
Deprotonation to form ethanol:
Question - 24 : - You are given benzene, conc. H2SO4 andNaOH. Write the equations for the preparation of phenol using these reagents.
Answer - 24 : - 
Question - 25 : - Show how will you synthesize:
Answer - 25 : -
(i) 1-phenylethanol from a suitable alkene.
(ii) cyclohexylmethanol using an alkyl halide by an S_N2 reaction.
(iii) pentan-1-ol using a suitable alkyl halide?
Answer
(i) Byacid-catalyzed hydration of ethylbenzene (styrene), 1-phenylethanol can besynthesized.
(ii) Whenchloromethylcyclohexane is treated with sodium hydroxide, cyclohexylmethanol isobtained.
(iii) When1-chloropentane is treated with NaOH, pentan-1-ol is produced.
Question - 26 : - Give two reactions that show the acidic nature of phenol.Compare acidity of phenol with that of ethanol.
Answer - 26 : -
The acidic nature of phenol can be represented by thefollowing two reactions:
(i) Phenolreacts with sodium to give sodium phenoxide, liberating H2.
(ii) Phenolreacts with sodium hydroxide to give sodium phenoxide and water as by-products.
The acidity of phenol is more than that of ethanol. Thisis because after losing a proton, the phenoxide ion undergoes resonance andgets stabilized whereas ethoxide ion does not.
Question - 27 : - Explain why is ortho nitrophenolmore acidic than ortho methoxyphenol?
Answer - 27 : -
The nitro-group is an electron-withdrawinggroup. The presence of this group in the ortho position decreases the electrondensity 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 isstabilized by resonance. Hence, ortho nitrophenol is astronger acid.
On the other hand, methoxy group is an electron-releasinggroup. Thus, it increases the electron density in the O−H bond and hence, theproton cannot be given out easily.
Question - 28 : - Explain how does the −OH group attached to a carbon ofbenzene ring activate it towards electrophilic substitution?
Answer - 28 : -
The −OH group is an electron-donating group. Thus, itincreases the electron density in the benzene ring as shown in the givenresonance structure of phenol.
As a result, the benzene ring is activated towardselectrophilic substitution.
Question - 29 : - Give equations of the following reactions:
Answer - 29 : -
(i) Oxidation of propan-1-ol with alkaline KMnO_4 solution.
(ii) Bromine in CS_2 with phenol.
(iii) Dilute HNO_3 with phenol.
(iv) Treating phenol with chloroform in presence of aqueous NaOH.
Answer
(iv)
Question - 30 : - Explain the following with an example.
Answer - 30 : -
(i) Kolbe’s reaction.
(ii) Reimer-Tiemann reaction.
(iii) Williamson ether synthesis.
(iv) Unsymmetrical ether.
Answer
(i) Kolbe’sreaction:
When phenol is treated with sodium hydroxide, sodiumphenoxide is produced. This sodium phenoxide when treated with carbon dioxide,followed by acidification, undergoes electrophilic substitution to giveortho-hydroxybenzoic acid as the main product. This reaction is known asKolbe’s reaction.
(ii) Reimer-Tiemannreaction:
When phenol is treated with chloroform (CHCl3) inthe presence of sodium hydroxide, a −CHO group is introduced at the orthoposition of the benzene ring.
This reaction is known as the Reimer-Tiemann reaction.
The intermediate is hydrolyzed in the presence of alkalisto produce salicyclaldehyde.
(iii) Williamsonether synthesis:
Williamson ether synthesis is a laboratory method toprepare symmetrical and unsymmetrical ethers by allowing alkyl halides to reactwith sodium alkoxides.
This reaction involves SN2attack of the alkoxide ion on the alkyl halide. Better results are obtained incase of primary alkyl halides.
If the alkyl halide is secondary or tertiary, thenelimination competes over substitution.
(iv) Unsymmetricalether:
An unsymmetrical ether is an ether where twogroups on the two sides of an oxygen atom differ (i.e., have an unequal numberof carbon atoms). For example: ethyl methyl ether (CH3−O−CH2CH3).