Alcohols Phenols And Ethers – JEE Main ChemistryPractice Questions & PYQs

Protonation of the hydroxyl group by $H^+$ ions dissociated from HCl transforms the poor $-OH$ group into a highly reactive $-OH_2^+$ species. Since substitution pathways are inherently favored over elimination processes in this reaction, the elimination route leads only to a minor product. The major product 'A' is therefore represented by the substitution structure shown in option D.

Propyne reacts with $\mathrm{NaNH}_2$ to form an acetylide ion, which acts as a nucleophile to generate the intermediate hept-5-yn-2-ol upon reacting with a carbon chain. When this molecule is treated with $\mathrm{H}_2/\mathrm{Pd-C}$, the triple bond undergoes complete hydrogenation to yield the saturated secondary alcohol, heptan-2-ol. Finally, oxidation of this secondary alcohol with $\mathrm{CrO}_3$ selectively transforms the hydroxyl group into a ketone, resulting in heptan-2-one.

Electrophilic aromatic substitution governs the reactivity of phenol, as the electron-donating hydroxyl group directs incoming substituents to specific positions on the benzene ring. When phenol reacts with $\mathrm{Br}_2$ in the non-polar solvent $\mathrm{CS}_2$ at $273\,\mathrm{K}$, the $\mathrm{Br}^+$ electrophile selectively substitutes the para-hydrogen to produce 4-bromophenol as the major product. Conversely, the Reimer-Tiemann reaction utilizes $\mathrm{CHCl}_3$ and $\mathrm{NaOH}$ to generate the electrophilic dichlorocarbene intermediate, $\mathrm{CCl}_2$, which attacks the ring to ultimately yield salicylaldehyde upon subsequent hydrolysis.

The synthesis of phthalein dyes is based on an electrophilic aromatic substitution reaction between phthalic anhydride and a phenolic compound. In the presence of concentrated $H_2SO_4$, the phthalic anhydride molecule undergoes protonation, which generates a reactive carbocation intermediate that acts as a potent electrophile. This intermediate attacks the aromatic ring of 2-propylphenol to create a substituted phthalein structure. When the reaction mixture is subsequently treated with $NaOH$, the hydroxyl groups on the resulting molecule are deprotonated to form a highly conjugated dianion, which absorbs visible light and gives rise to the characteristic pink color.

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The $S_N2$ mechanism operates through a concerted single-step pathway where the nucleophile approaches the electrophilic center from the side opposite to the leaving group, entirely bypassing the formation of any carbocation intermediate. In this specific reaction, the phenoxide nucleophile executes a backside attack on the carbon atom linked to the bromide ion, resulting in the structure shown in option D. This direct displacement of the bromide ion in a single transition state determines the final stereochemical configuration of the product.

Heating ethylene glycol with oxalic acid initiates an esterification process that yields an unstable cyclic diester intermediate known as ethylene oxalate. When this intermediate is subjected to a temperature of 210 °C, it undergoes rapid decarboxylation, losing carbon dioxide to form the alkene $\text{CH}_2=\text{CH}_2$. This substance represents the final product X in the reaction.

The hydrolysis of a geminal dihalide to form a carbonyl compound provides the key to identifying the reactants in this transformation. Since the product B possesses the molecular formula $\mathrm{C_4H_8O}$ and reacts with $\mathrm{NH_2OH}$, it is established as a carbonyl compound; furthermore, the fact that it does not react with the Tollen's reagent confirms it is a ketone and not an aldehyde. Given this formula, the only possible ketone is 2-butanone. Therefore, the starting material A must be the corresponding gem-dichloride, 2,2-dichlorobutane, which undergoes hydrolysis at 373 K to yield this specific ketone.

The formation of colored phthalein dyes during the condensation of a phenol with phthalic anhydride relies on an electrophilic substitution reaction occurring specifically at the para position of the phenol ring. For this reaction to produce a visible color, the para position must be unoccupied to allow the phthalic anhydride molecule to bond effectively. In the case of para-cresol, a substituent group already occupies this critical site, creating an obstruction that prevents the necessary electrophilic attack from taking place in the presence of $\mathrm{H}_2\mathrm{SO}_4$. Because the reaction cannot proceed at this blocked position, the molecule fails to generate the colored condensation product observed with other phenols.

The regioselectivity of this reaction is determined by the relative stability of the intermediate species formed during the rate-determining step, where the pathway leading to the more stable intermediate-such as a more substituted or resonance-stabilized carbocation-possesses a lower activation energy. Because the transition state for the formation of Compound A is lower in energy compared to that of Compound B, it is produced more rapidly, making it the kinetically favored major product.

When propene reacts with bromine water, a cyclic bromonium ion forms across the double bond, which is subsequently attacked by water acting as a nucleophile. Because the more substituted carbon atom can better stabilize the partial positive charge in the transition state, the water molecule adds to this position in accordance with Markownikoff's rule, resulting in the formation of 1-bromopropan-2-ol. The overall transformation is represented by the equation $CH_3CH=CH_2 + Br_2 + H_2O \rightarrow CH_3CH(OH)CH_2Br$. This mechanism confirms that both the assertion concerning the product and the reason regarding the regioselectivity of the reaction are correct.

The reaction proceeds via the formation of the most stable carbocation intermediate, such as $R_3C^+$, in accordance with Markovnikov's rule, which dictates that the nucleophile adds to the more substituted carbon atom. This regioselective pathway minimizes the transition state energy, ultimately yielding the thermodynamically favored product configuration represented in D.

The acid-catalyzed dehydration of 3,3-dimethylbutan-2-ol occurs when the hydroxyl group is protonated by concentrated $\mathrm{H}_{2}\mathrm{SO}_{4}$ and subsequently lost as water, leaving behind a secondary carbocation. This intermediate quickly undergoes a rearrangement via a methyl shift to achieve greater stability before losing a proton to form the resulting alkene. Consequently, this pathway drives the reaction to favor the formation of but-2,3-diene as the major product.

Grignard reagents react with esters through a two-step nucleophilic addition-elimination mechanism to produce tertiary alcohols, requiring a stoichiometric ratio of two equivalents of the reagent per mole of ester. The first equivalent of $CH_3MgBr$ attacks the carbonyl carbon of ethyl ethanoate to displace the ethoxide leaving group, generating the ketone intermediate acetone.

$CH_3COOCH_2CH_3 + CH_3MgBr \rightarrow CH_3COCH_3 + CH_3CH_2OMgBr$

The second equivalent of $CH_3MgBr$ immediately reacts with the acetone intermediate via nucleophilic addition to form the tertiary magnesium alkoxide species, which yields 2-methylpropan-2-ol upon aqueous acidic workup.

$CH_3COCH_3 + CH_3MgBr \rightarrow (CH_3)_3COMgBr$

Because one equivalent is consumed in the formation of the ketone and a second is consumed in the formation of the alcohol, the synthesis of 1.0 mole of 2-methylpropan-2-ol requires exactly 2 equivalents of $CH_3MgBr$.

Ceric ammonium nitrate serves as a specific diagnostic reagent used to confirm the presence of an alcoholic $-\mathrm{OH}$ group, which is visually indicated by the formation of a red-colored complex. In contrast, the mixture of $\mathrm{CHCl}_{3}$ and alcoholic $\mathrm{KOH}$ is employed in the carbylamine test, a process that uniquely identifies primary amines by converting them into isocyanides, which are easily detected by their distinctively foul odor.

The Reimer-Tiemann reaction of phenol ($C_6H_6O$) with chloroform ($CHCl_3$) and potassium hydroxide ($KOH$) yields $2$-hydroxybenzaldehyde, which confirms A as phenol due to its characteristic color reaction with ferric chloride. Pyridinium chlorochromate (PCC) selectively oxidizes primary alcohols to aldehydes, identifying C as $2$-hydroxybenzyl alcohol, which produces B upon oxidation. Thus, the sequence identifies A as phenol, B as $2$-hydroxybenzaldehyde, and C as $2$-hydroxybenzyl alcohol.

The reaction proceeds via an $S_N1$ mechanism in a polar protic solvent, involving the formation of a secondary carbocation followed by a 1,2-methyl shift to generate a more stable tertiary carbocation. The initial secondary carbocation formed at the C3 position rearranges to a tertiary carbocation at the C2 position of the 2,3-dimethylbutane skeleton through the migration of a methyl group. Ethanol subsequently acts as a nucleophile, attacking this more stable tertiary carbocation to yield 2-ethoxy-2,3-dimethylbutane as the major product.

The formation of a tertiary alcohol from excess ${CH}_{3} {MgBr}$ involves two successive nucleophilic additions to a carbonyl carbon, a process characteristic of esters rather than aldehydes or ketones. The Grignard reagent first attacks the carbonyl group of the ester to displace the alkoxy group, forming a ketone intermediate that is highly reactive and susceptible to a second nucleophilic attack by another equivalent of the reagent. Following the final acidic hydrolysis step, this double addition yields a tertiary alcohol with two methyl groups originating from the Grignard reagent, which correctly identifies the ester compound provided in option A.

In ortho-nitrophenol, the hydroxyl ($-OH$) and nitro ($-NO_2$) groups occupy adjacent positions on the benzene ring, facilitating the formation of a stable five-membered chelate ring through intramolecular hydrogen bonding. This internal association prevents the molecules from linking with one another, allowing the compound to exist in a monomeric state that leads to steam volatility. Because these hydrogen bonds are confined within individual molecules rather than building an extended network, the intermolecular cohesive forces are weaker, which results in a lower melting point rather than a high one. This behavior stands in stark contrast to the isomer para-nitrophenol, which engages in intermolecular hydrogen bonding to create stronger molecular aggregates, thereby exhibiting significantly higher boiling and melting points.

Electrophilic aromatic substitution on a monosubstituted benzene ring is primarily governed by the electronic and steric nature of the existing substituent, which directs incoming electrophiles $E^+$ preferentially to the para position. This regioselectivity arises because the para orientation significantly minimizes steric repulsion between the existing substituent and the incoming group, unlike the more crowded ortho position. Consequently, the transition state leading to the para-substituted product is of lower energy, making it the major product through the formation of a more stabilized $\sigma$-complex intermediate.