Alcohols Phenols And Ethers – NEET UG ChemistryPractice Questions & PYQs

Large-scale commercial synthesis of methanol relies on the catalytic hydrogenation of carbon monoxide gas under high pressure and temperature. The chemical transformation is represented by the equation $\mathrm{CO} + 2\mathrm{H}_2 \xrightarrow{\mathrm{ZnO-Cr_2O_3}} \mathrm{CH}_3\mathrm{OH}$, where a specialized mixture of zinc oxide and chromium sesquioxide acts as the heterogeneous catalyst. This specific metallic oxide combination facilitates the efficient addition of hydrogen across the carbon-oxygen multiple bond of carbon monoxide to yield wood alcohol.

Symmetrical and unsymmetrical ethers are successfully prepared through a bimolecular nucleophilic substitution pathway where a metal alkoxide reacts with an alkyl halide like $\mathrm{C}_2\mathrm{H}_5\mathrm{Cl}$. In this specific transformation, sodium ethoxide acts as a strong nucleophile that displaces the chloride ion to form $\mathrm{C}_2\mathrm{H}_5-\mathrm{O}-\mathrm{C}_2\mathrm{H}_5$. This etherification process is universally recognized in organic chemistry as Williamson's synthesis.

The rate of reaction between alcohols and hydrogen halides via a substitution mechanism is directly determined by the stability of the carbocation intermediate formed after the protonated hydroxyl group departs as water. Tertiary alcohols readily yield tertiary carbocations, which benefit from maximum hyperconjugation and inductive stabilization by three surrounding alkyl groups. Since this tertiary intermediate forms much more rapidly than primary or secondary alternatives, tertiary substrates undergo substitution with hydrogen bromide significantly faster than any other alcohol class.

Reaction of ethyl bromide (\text{CH}_3\text{CH}_2\text{Br}$) with potassium cyanide yields propionitrile (\text{CH}_3\text{CH}_2\text{CN}$), which upon acid hydrolysis forms propionic acid (\text{CH}_3\text{CH}_2\text{COOH}$). Subsequent reduction of propionic acid using lithium aluminium hydride (\text{LiAlH}_4$) converts the carboxylic acid group into a primary alcohol, producing n-propyl alcohol (\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$) as the final product D.

The reduction of carbonyl compounds and their derivatives using strong reducing agents like lithium aluminum hydride ($\mathrm{LiAlH}_4$) selectively yields primary alcohols. Throughout the given reaction sequence, the sequential functional group transformations convert the initial reactant into an ethyl derivative capable of yielding a hydroxyl group upon final reduction or hydrolysis. Therefore, the culmination of these steps successfully produces ethanol ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}$) as the major product $D$.

The rate of reaction with Lucas reagent depends entirely on the stability of the carbocation intermediate formed via an $S_N1$ mechanism. Tertiary alcohols generate highly stable tertiary carbocations due to maximum hyperconjugation and inductive effects from surrounding alkyl groups. Because of this high stability, the reaction proceeds rapidly at room temperature, causing an immediate phase separation and turbidity. Primary and secondary alcohols form less stable carbocations and therefore require heat or prolonged reaction times to show any visible change.

Cumene (\text{C}9\text{H}{12}$) undergoes catalytic oxidation with atmospheric oxygen to form a hydroperoxide intermediate. Subsequent acid-catalyzed cleavage of this intermediate yields phenol as product A and acetone as product B through the migration of a phenyl group to an oxygen atom.

In this multi-step organic synthesis, the initial reactant undergoes a selective functional group transformation under the first set of reaction conditions to yield intermediate A. This intermediate then participates in a subsequent substitution or elimination pathway driven by the second reagent to afford the final stable product B. The stereochemical and regiochemical outcomes are dictated by the stability of the intermediate species and the electronic effects of the substituents involved.

Acid-catalyzed dehydration of alcohols proceeds via an $E1$ mechanism where the rate-determining step involves the loss of a water molecule to form a carbocation intermediate. The alcohol in option A readily undergoes this process because it forms the most stable carbocation among the choices, significantly lowering the activation energy required for the reaction. This superior carbocation stability makes it the most reactive substrate toward dehydration under acidic conditions.

The Lucas test exploits differences in reaction kinetics to differentiate primary, secondary, and tertiary alcohols through zinc chloride catalyzed substitution with hydrochloric acid. Tertiary alcohols react instantaneously at room temperature because they form the most stable carbocation intermediate, resulting in immediate cloudiness. Conversely, primary alcohols yield the least stable carbocations and fail to produce turbidity at room temperature even after extended periods. Therefore, claiming that primary alcohols are the most reactive and immediately produce turbidity contradicts established reaction mechanisms.

Phenol undergoes Kolbe's reaction when treated with $\mathrm{NaOH}$ followed by $\mathrm{CO}_2$ and $\mathrm{H}^{+}$, yielding salicylic acid. The reaction of phenol with aqueous $\mathrm{NaOH}$ and $\mathrm{CHCl}_3$ followed by acidification corresponds to the Reimer-Tiemann reaction, which forms salicyl aldehyde. Heating phenol with zinc dust results in the reduction of the hydroxyl group to yield benzene, while oxidation using $\mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7$ and $\mathrm{H}_2\mathrm{SO}_4$ converts phenol into benzoquinone.

Grignard reagents possess a highly polarized carbon-metal bond where the alkyl group acts as a potent nucleophile. During the carboxylation step, this nucleophilic carbon attacks the electrophilic carbon of carbon dioxide to form a magnesium salt of a carboxylic acid. This intermediate compound maintains an ionic bond between the carboxylate oxygen and the magnesium halide moiety. Subsequent aqueous acid workup protonates this intermediate to yield the final carboxylic acid product.

Sequential organic transformations involve reagents that selectively alter specific functional groups in a predetermined multi-step pathway, where the initial reagent modifies the reactant's oxidation state or carbon skeleton to yield intermediate A. Subsequent treatment with the second reagent induces a characteristic substitution or addition reaction on intermediate A to yield the final product B. Careful tracking of reaction conditions and regiochemistry ensures the precise identification of each successive product in the pathway.

Standard organic name reactions yield distinct functional groups through characteristic synthetic pathways. Gabriel synthesis converts phthalimide derivatives specifically into primary amines, whereas the Kolbe reaction carboxylates sodium phenoxide to produce salicylic acid. Williamson synthesis couples alkyl halides and alkoxides to form ethers according to the general pathway $\mathrm{R}-\mathrm{X} + \mathrm{R}'\mathrm{ONa} \rightarrow \mathrm{R}'\mathrm{OR} + \mathrm{NaX}$. Finally, the Etard reaction uses chromyl chloride to selectively oxidize toluene into benzaldehyde.

Nucleophilic addition of a Grignard reagent to the carbonyl carbon forms an alkoxide intermediate, which upon aqueous workup yields a substituted alcohol. Subsequent acid-catalyzed dehydration of this intermediate removes a water molecule to yield the thermodynamically favored Zaitsev alkene as the major product.

The conversion of chlorobenzene to phenol requires drastic conditions such as high temperature and high pressure because the $\text{C}-\text{Cl}$ bond acquires partial double bond character due to resonance with the benzene ring. Any reaction sequence attempting to perform this hydrolysis under mild or room temperature conditions fails because the aromatic carbon-chlorine bond is too stable to undergo nucleophilic substitution.

Sodium borohydride ($NaBH_4$) functions as a selective reducing agent in organic synthesis. It efficiently transforms active carbonyl groups into alcohols, but lacks the strength to affect less reactive carboxylic acid derivatives like esters. Consequently, any ester functionality present in the reactant remains entirely intact while the carbonyl site undergoes reduction to yield the final structure.

Metamerism is a specialized form of structural isomerism exhibited by compounds containing a polyvalent functional group where different alkyl groups are attached to the same heteroatom. Among the given molecular formulas, $\mathrm{C}_4\mathrm{H}_{10}\mathrm{O}$ corresponds to ethers, which can possess varying alkyl chain lengths on either side of the central oxygen atom. For instance, ethoxyethane features symmetrical ethyl groups, whereas methoxypropane contains methyl and propyl groups, exemplifying this distinct isomeric phenomenon.

Saytzeff's rule governs elimination reactions by predicting that the more highly substituted alkene will be the favored product due to increased hyperconjugative stability. When 2-bromopentane undergoes dehydrohalogenation in the presence of a base, a proton is eliminated from the adjacent carbon atom to yield an alkene. Between the possible products, pent-2-ene contains more alkyl substituents on the carbon-carbon double bond than pent-1-ene, rendering it the thermodynamically more stable major product.

The cleavage of aromatic ethers by hydrogen iodide proceeds via protonation of the ether oxygen followed by a nucleophilic substitution reaction. In anisole ($C_6H_5OCH_3$), the bond between oxygen and the phenyl ring acquires partial double-bond character due to resonance, making this carbon-oxygen bond extremely resistant to cleavage. Consequently, the iodide ion selectively attacks the methyl carbon to yield iodomethane and phenol.