CBSE Class 12 Chemistry Chapter 11: Alcohols, Phenols and Ethers NCERT Solutions

Solution: The reaction begins with the monochlorination of toluene in the presence of sunlight. This reaction is a free radical substitution that preferentially occurs at the benzylic position, forming benzyl chloride ($C\_6H\_5CH\_2Cl$). Subsequently, benzyl chloride undergoes hydrolysis with aqueous sodium hydroxide ($NaOH$). This is a nucleophilic substitution reaction where the chlorine atom is replaced by a hydroxyl ($-OH$) group. The product formed is benzyl alcohol ($C\_6H\_5CH\_2OH$).

$$C\_6H\_5CH\_3\; \backslash xrightarrow\{Cl\_2,\; \backslash text\{sunlight\}\}\; C\_6H\_5CH\_2Cl\; \backslash xrightarrow\{aq.\; NaOH\}\; C\_6H\_5CH\_2OH$$

Therefore, the correct option is (d) benzyl alcohol.

Solution: To determine the number of chiral alcohols, we first need to identify the structural isomers of alcohols with the molecular formula $C\_4H\_\{10\}O$. A chiral molecule is one that is non-superimposable on its mirror image, which typically occurs when a carbon atom is bonded to four different groups (a chiral center).

The possible structural isomers of butanol ($C\_4H\_\{10\}O$) are:

1. Butan-1-ol: $CH\_3CH\_2CH\_2CH\_2OH$. None of the carbon atoms are bonded to four different groups.
2. Butan-2-ol: $CH\_3CH\_2CH(OH)CH\_3$. The second carbon atom is bonded to a hydrogen atom, a hydroxyl group, a methyl group ($-CH\_3$), and an ethyl group ($-CH\_2CH\_3$). Since these four groups are different, this carbon is a chiral center, making butan-2-ol chiral.
3. 2-Methylpropan-1-ol: $(CH\_3)\_2CHCH\_2OH$. None of the carbon atoms are bonded to four different groups.
4. 2-Methylpropan-2-ol: $(CH\_3)\_3COH$. The carbon atom bonded to the hydroxyl group is bonded to three methyl groups and no hydrogen, so it cannot be chiral.

Out of these isomers, only butan-2-ol possesses a chiral center.

Therefore, there is only one chiral alcohol with the molecular formula $C\_4H\_\{10\}O$. The correct option is (a).

Solution: The given reaction is a nucleophilic substitution where the hydroxyl ($-OH$) group of the alcohol is replaced by a chlorine ($-Cl$) atom. This reaction is typically carried out using concentrated hydrochloric acid ($HCl$) in the presence of anhydrous zinc chloride ($ZnCl\_2$) as a Lewis acid catalyst (Lucas reagent for distinguishing alcohols). The mechanism involves the formation of a carbocation intermediate.

The stability of carbocations follows the order: 3^{\circ} > 2^{\circ} > 1^{\circ}. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects from the alkyl groups. Secondary carbocations are less stable, and primary carbocations are the least stable.

Since the reaction proceeds through carbocation intermediates, the reactivity of alcohols will be directly proportional to the stability of the carbocation they form. Therefore, tertiary alcohols react fastest, followed by secondary alcohols, and then primary alcohols.

The order of reactivity is: 3^{\circ} \text{ alcohol} > 2^{\circ} \text{ alcohol} > 1^{\circ} \text{ alcohol}.

Thus, the correct option is (c).

Solution: The conversion of ethanol ($CH\_3CH\_2OH$), a primary alcohol, into ethanal ($CH\_3CHO$), an aldehyde, requires an oxidation reaction. We need to select an oxidizing agent that can stop the oxidation at the aldehyde stage.

(a) Catalytic hydrogenation is a reduction process, not oxidation.

(b) Lithium aluminum hydride (LiAlH_4) is a strong reducing agent and would not convert an alcohol to an aldehyde.

(c) Pyridinium chlorochromate (PCC) is a mild oxidizing agent specifically used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. It is dissolved in a suitable solvent like dichloromethane (CH_2Cl_2).

The reaction is: CH3CH2OH⟶PCCCH3CHO\mathrm{CH_3CH_2OH} \xrightarrow{\mathrm{PCC}} \mathrm{CH_3CHO}

(d) Potassium permanganate ($KMnO\_4$) is a strong oxidizing agent. In neutral, acidic, or basic conditions, it would oxidize primary alcohols all the way to carboxylic acids ($CH\_3COOH$).

Therefore, the correct reagent to convert ethanol to ethanal is pyridinium chlorochromate (PCC).