Alcohols Phenols And Ethers – JEE Main ChemistryPractice Questions & PYQs

The mass of a substance relates to the quantity in moles through the formula $w = n \times M$, where $w$ is the mass in grams and $M$ is the molar mass. Given that the molar mass of compound S is 130 g/mol and the sample contains 0.1 moles, we calculate the total weight by multiplying these values. This operation is defined by the equation:

$w = 0.1 \times 130 = 13$

Consequently, the sample weighs 13 g.

The hydroxyl group attached to the phenol ring provides strong activation, significantly increasing the electron density of the aromatic system and allowing bromination to occur effectively without a Lewis acid catalyst in solvents such as $\mathrm{CHCl}_3$ or $\mathrm{CS}_2$. Because the reaction proceeds readily due to this inherent activation, the assertion that a catalyst is required for this specific process is inaccurate. Conversely, the second statement correctly characterizes the general mechanism of electrophilic halogenation, where a Lewis acid facilitates the generation of the active electrophilic species $\mathrm{Br}^{+}$ by polarizing the bromine molecule.

Successfully pairing items in this exercise depends on verifying the distinct properties that unify each element in the first list with its specific definition in the second. Through this process, one discovers that (A) corresponds to (II) and (B) correlates with (IV), while (C) relates directly to (I) and (D) functions as the definition for (III). These specific links are essential to uphold the conceptual framework and satisfy the requirements of each category pairing.

The fundamental principle governing this transformation is the oxidation of a primary alcohol to a carboxylic acid, followed by the classic soda lime decarboxylation reaction. Treating ethanol with strong oxidizing agents, specifically Jones reagent and subsequently $\mathrm{KMnO}_4$, converts the hydroxyl group into a carboxyl group to yield ethanoic acid, $\mathrm{CH}_3\mathrm{COOH}$. Upon heating the resulting sodium salt of this acid with soda lime, which consists of $\mathrm{NaOH}$ and $\mathrm{CaO}$, the molecule loses $\mathrm{CO}_2$ to yield the final alkane product, methane, $\mathrm{CH}_4$.

The synthesis of picric acid serves as a clear illustration of electrophilic aromatic substitution, starting with the reaction of phenol and concentrated sulfuric acid to generate phenol-2,4-disulfonic acid ($C_6H_3(OH)(SO_3H)_2$) as Product A. This intermediate contains seven oxygen atoms in total, consisting of one from the hydroxyl group and six from the two sulfonic acid groups. Further treatment with concentrated nitric acid replaces these substituents to form 2,4,6-trinitrophenol ($C_6H_2(OH)(NO_2)_3$) as Product B. Since this final product also features seven oxygen atoms, derived from one in the hydroxyl group and two in each of the three nitro groups, the combined sum of oxygen atoms across both Product A and Product B is 14.

The outcome of this reaction is primarily determined by the stability of intermediates formed in the rate-determining step, as processes producing lower-energy carbocations or radicals are kinetically prioritized. Consistent with Markovnikov's rule, the electrophile attaches to the less substituted carbon, which places the positive charge on a more substituted, stable carbon to generate an intermediate of the form $R-C^+H-CH_3$. This pathway passes through the most stable transition state, leading directly to B as the major product.

When two hydroxyl groups are attached to adjacent carbon atoms on a benzene ring, the structure forms an ortho-substituted isomer represented by the chemical formula $C_6H_4(OH)_2$. In organic chemistry nomenclature, this specific configuration is universally recognized as catechol. This structural arrangement is distinct from the other dihydroxybenzene isomers, specifically resorcinol, which occupies the meta-position, and quinol, which is found in the para-position.

The reaction described is the Reimer-Tiemann reaction, a classic chemical process used to install a formyl group directly onto the phenol ring. Interaction between chloroform ($CHCl_3$) and sodium hydroxide ($NaOH$) produces a reactive dichlorocarbene ($:CCl_2$) electrophile, which selectively attacks the ortho position of the phenol molecule. Following this step, treatment with an acid ensures the hydrolysis of the intermediate to yield an aldehyde, culminating in the production of 2-hydroxybenzaldehyde.

p-Nitrophenol exhibits the highest acidity among these isomers because the nitro group at the para position exerts a strong electron-withdrawing effect, which provides maximum stabilization to the phenoxide conjugate base through resonance. Conversely, ethanol is a primary alcohol, and such compounds react very slowly with the mixture of $HCl$ and $ZnCl_2$, meaning they do not produce immediate turbidity.

The chemical behavior of phenol is defined by its ability to undergo specific reduction and electrophilic substitution reactions based on the reagents employed. Treating phenol with $\mathrm{Zn}/\Delta$ causes reduction, effectively stripping the hydroxyl group to generate benzene, whereas the reaction with $\mathrm{CHCl}_3$ and aqueous $\mathrm{NaOH}$, known as the Reimer-Tiemann reaction, introduces a formyl group to create salicylaldehyde. Introducing $\mathrm{CO}_2$ under basic conditions through the Kolbe-Schmidt reaction yields salicylic acid, while intense nitration using concentrated $\mathrm{HNO}_3$ converts the ring into 2,4,6-trinitrophenol, which is widely recognized as picric acid.

Chemical identification of functional groups relies on the use of specific reagents that produce distinct visual changes characteristic of the group present. The phthalein dye test serves as the diagnostic method for phenols, as it involves a condensation reaction with phthalic anhydride that yields a colored dye. In contrast, the remaining options serve different analytical purposes: the Lucas test differentiates between primary, secondary, and tertiary alcohols, Tollen's test is used to detect aldehydes, and the carbylamine test detects primary amines.

The Reimer-Tiemann reaction centers on the formation of the highly electrophilic dichlorocarbene species, $:CCl_2$, which is generated from chloroform in the presence of a strong base. When the phenoxide ion is attacked by this carbene at the ortho position, the conjugation of the aromatic ring is momentarily interrupted to accommodate the new substituent. This leads to the formation of an anionic dichloromethyl-substituted cyclohexadienone intermediate, where the $-CHCl_2$ group and the oxygen anion reside on a non-aromatic, six-membered ring, which aligns with the structure provided in option D. These transient states are crucial precursors that undergo subsequent hydrolysis and elimination to eventually yield the final salicylaldehyde product.

The Kolbe-Schmidt reaction involves the carboxylation of phenol to generate salicylic acid, whereas the Reimer-Tiemann process introduces a formyl group to produce salicylaldehyde. Treating phenol with strong oxidizing agents results in the formation of p-benzoquinone, while the neutralization of phenol with sodium hydroxide is a straightforward acid-base reaction yielding water and the phenoxide ion, $\mathrm{PhO}^{-}$. These distinct reaction pathways illustrate the characteristic chemical behavior of phenol, systematically matching each specific transformation to its corresponding product.

The process begins with a Friedel-Crafts alkylation where benzene reacts with methyl chloride in the presence of anhydrous $AlCl_3$ to undergo electrophilic aromatic substitution, resulting in toluene as intermediate $A$. Treating this alkylated product with alkaline $KMnO_4$ triggers a robust side-chain oxidation that converts the methyl group into a carboxyl group, ultimately yielding benzoic acid as the final product $B$.

Acidity is fundamentally governed by an inverse relationship with $pK_a$ values, meaning that a lower $pK_a$ value corresponds to a stronger acid. The assertion correctly identifies the experimental $pK_a$ of phenol as 10.0 and ethanol as 15.9, accurately reflecting the data for these compounds. However, because phenol possesses the lower value, it is chemically the stronger acid of the two, which directly contradicts the claim stated in the reason regarding ethanol.

To react with a weak base like dilute $\mathrm{NaOH}$, a molecule must be sufficiently acidic to readily donate its proton. Phenol ($\mathrm{C_6H_5OH}$) is uniquely capable of this because the phenoxide ion ($\mathrm{C_6H_5O^-}$) formed after dissociation gains significant stability through resonance with the aromatic benzene ring. Since the alkoxide ions derived from aliphatic alcohols like ethanol, benzyl alcohol, and tert-butyl alcohol lack this essential resonance stabilization, they are too weakly acidic to react with dilute $\mathrm{NaOH}$.

The reaction proceeds via electrophilic addition, where a proton initially binds to the double bond to generate a secondary carbocation. This intermediate rapidly rearranges through a 1,2-hydride shift to yield a more stable tertiary carbocation, which adopts an $sp^2$ hybridized planar geometry. Since this carbocation is planar, the nucleophilic bromide ion is able to attack from either face with equal probability, resulting in the formation of a racemic mixture due to the generation of a chiral center.

Alcohols demonstrate a dual chemical nature because they function as nucleophiles using the oxygen atom's lone pairs and as electrophiles when the $C-O$ bond undergoes cleavage, which makes the assertion true. Simultaneously, the reaction with active metals such as sodium or potassium illustrates the acidity of the $O-H$ bond, where alkoxides are formed and $H_2$ gas is evolved, confirming the reason is also valid. Since the acidic behavior described in the reason is a separate property and does not define the mechanism behind the nucleophilic and electrophilic characteristics, the reason is not the correct explanation for the assertion.

Nuclear interactions are governed by fundamental principles, including the requirement that conservation laws for energy, momentum, charge, and specific quantum numbers like the baryon number must be upheld. A reaction is only physically allowed if the total baryon number on the left side of the equation equals the total on the right. In the reaction presented in option B, evaluating these values confirms that $B_{reactants} \neq B_{products}$, indicating a failure to preserve this quantity. This violation makes the process impossible according to the established laws of nature.

The acidity of an -OH group is determined by the stability of its conjugate base, where electron-withdrawing groups increase acidity by stabilizing the negative charge on the oxygen atom through resonance ($-R$) and inductive ($-I$) effects, while electron-donating groups destabilize it via inductive ($+I$) and hyperconjugative effects. Ethanol (A) is the least acidic due to the absence of resonance stabilization, followed by $p$-cresol (C) which is less acidic than phenol (D) due to the electron-donating methyl group. Phenol (D) is less acidic than $p$-chlorophenol (B) and $p$-nitrophenol (E), as the latter two contain electron-withdrawing groups that further stabilize the phenoxide anion, resulting in the ascending order (A) $<$ (C) $<$ (D) $<$ (B) $<$ (E).