Alcohols, Phenols and Ethers
Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by -OH group. These classes of compounds find wide applications in industry as well as in day-to-day life. For instance, have you ever noticed that ordinary spirit used for polishing wooden furniture is chiefly a compound containing hydroxyl group, ethanol. The sugar we eat, the cotton used for fabrics, the paper we use for writing, are all made up of compounds containing -OH groups. The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. For example, \(\text{CH}_3\text{OH}\) is methyl alcohol. The simplest hydroxy derivative of benzene is phenol. It is its common name and also an accepted IUPAC name. As structure of phenol involves a benzene ring, in its substituted compounds the terms ortho (1,2-disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted) are often used in the common names.
In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (-R) groups.
Question: 1
What is the common name for the following structure given below?
Concept:
Benzenediols are aromatic compounds containing two hydroxyl groups attached directly to a benzene ring.
The three positional isomers of dihydroxybenzene have established common names accepted alongside systematic IUPAC nomenclature. Step 1: Identifying the Substitution Pattern:
The structure displayed in the problem shows a benzene ring substituted with two phenolic hydroxyl groups at positions 1 and 4 relative to each other:
\[
\text{Benzene-1,4-diol} \quad (\text{1,4-dihydroxybenzene})
\]
This represents the para-disubstituted isomer. Step 2: Analysis of Common Names of Benzenediols:
1. 1,2-Dihydroxybenzene (ortho isomer) is commonly called Catechol.
2. 1,3-Dihydroxybenzene (meta isomer) is commonly called Resorcinol.
3. 1,4-Dihydroxybenzene (para isomer) is commonly called Hydroquinone or Quinol.
4. m-Cresol is a monohydric phenol bearing a methyl group at the 3-position (3-methylphenol). Step 3: Matching Structure with Common Name:
Because the two \(-\text{OH}\) groups are positioned para (1,4) to each other across the benzene ring, the common name of the structure is hydroquinone. Final Answer:
The common name for the given structure is Hydroquinone, corresponding to option (C).
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Question: 2
The catalyst involved in the production of methanol through hydrogenation of carbon monoxide is
Concept:
Methanol (\(\text{CH}_3\text{OH}\)), historically known as wood spirit, is industrially manufactured on a large scale by the catalytic hydrogenation of water gas (a mixture of carbon monoxide and hydrogen).
This heterogeneous catalytic reaction requires specific mixed metal oxide catalysts operating at elevated temperatures and pressures. Step 1: The Industrial Synthesis Reaction:
Carbon monoxide reacts with gaseous hydrogen according to the balanced chemical equation:
\[
\text{CO}(\text{g}) + 2\text{H}_2(\text{g}) \xrightarrow{\text{catalyst}, \, 573\text{--}673\text{ K}, \, 200\text{--}300\text{ atm}} \text{CH}_3\text{OH}(\text{l})
\]
Step 2: Identifying the Heterogeneous Catalyst:
According to NCERT Alcohols, Phenols and Ethers:
"Methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of \(\text{ZnO}-\text{Cr}_2\text{O}_3\) catalyst."
The mixed catalyst composed of zinc oxide and chromium(III) oxide (\(\text{ZnO}-\text{Cr}_2\text{O}_3\)) provides active catalytic sites that selectively adsorb \(\text{CO}\) and \(\text{H}_2\), facilitating hydride and proton transfer to yield methanol with high selectivity. Step 3: Evaluating Alternative Options:
- \(\text{ZnO/Pt}\) is not used commercially for this transformation.
- Anhydrous \(\text{AlCl}_3\) is a strong Lewis acid used in Friedel-Crafts alkylation and acylation reactions.
- Anhydrous \(\text{FeCl}_3\) is a Lewis acid catalyst used in the electrophilic halogenation of aromatic rings. Final Answer:
The catalyst involved is \(\text{ZnO-Cr}_2\text{O}_3\), corresponding to option (A).
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Question: 3
Consider the following chemical reaction and identify the intermediate involved in the reaction.
Concept:
The conversion of phenol to salicylaldehyde by heating with chloroform in the presence of aqueous sodium hydroxide is known as the Reimer-Tiemann reaction.
The reaction proceeds through an electrophilic aromatic substitution pathway involving dichlorocarbene, generating a substituted benzal chloride intermediate. Step 1: Generation of the Electrophilic Carbene:
In the initial step, hydroxide ion abstracts an acidic proton from chloroform (\(\text{CHCl}_3\)), which subsequently loses a chloride ion by \(\alpha\)-elimination to form neutral dichlorocarbene (:\(\text{CCl}_2\)):
\[
\text{CHCl}_3 + \text{OH}^- \rightleftharpoons :\text{CCl}_3^- + \text{H}_2\text{O} \rightarrow :\text{CCl}_2 + \text{Cl}^-
\]
Dichlorocarbene has a sextet of valence electrons, making it an electrophile. Step 2: Electrophilic Attack on Sodium Phenoxide:
Simultaneously, sodium hydroxide deprotonates phenol to generate the nucleophilic phenoxide ion (\(\text{C}_6\text{H}_5\text{O}^-\text{Na}^+\)).
The phenoxide ion attacks the electrophilic dichlorocarbene primarily at the electron-rich ortho-position, generating an intermediate carrying a dichloromethyl group:
\[
\text{C}_6\text{H}_4(\text{O}^-\text{Na}^+)(\text{CHCl}_2)
\]
This species is sodium 2-(dichloromethyl)phenoxide (intermediate B). Step 3: Hydrolysis to the Aldehyde:
In the subsequent step, the two chlorine atoms of the \(-\text{CHCl}_2\) group are nucleophilically displaced by hydroxide ions to form an unstable gem-diol:
\[
-\text{CH}(\text{OH})_2 \xrightarrow{-\text{H}_2\text{O}} -\text{CH}=\text{O}
\]
Subsequent acidification yields salicylaldehyde.
Thus, the isolated intermediate before hydrolysis is the sodium phenoxide bearing an ortho-\(\text{CHCl}_2\) group. Final Answer:
The intermediate involved in the reaction is the ortho-dichloromethyl phenoxide intermediate, corresponding to option (B).
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Question: 4
Consider the following chemical reaction and identify the final product(s)
Show Hint
Nitration of Phenol:
- With dilute \(\text{HNO}_3\) at \(298\text{ K} \rightarrow\) ortho-nitrophenol + para-nitrophenol (separable by steam distillation).
- With concentrated \(\text{HNO}_3 \rightarrow\) 2,4,6-trinitrophenol (picric acid).
Concept:
The hydroxyl group (\(-\text{OH}\)) attached directly to a benzene ring activates the ring strongly towards electrophilic aromatic substitution through \(+M\) resonance, directing incoming electrophiles to the ortho- and para-positions. Step 1: Reaction Conditions with Dilute Nitric Acid:
When phenol is treated with dilute nitric acid (\(\text{dil. HNO}_3\)) at low temperature (\(298\text{ K}\)), the mild nitrating conditions prevent destructive over-oxidation of the activated phenolic ring.
The electrophilic nitronium ion (\(\text{NO}_2^+\)) attacks the activated ortho- and para-positions, yielding a mixture of two mononitrophenols:
\[
\text{C}_6\text{H}_5\text{OH} + \text{dil. HNO}_3 \xrightarrow{298\text{ K}} \text{\textit{o}-nitrophenol} + \text{\textit{p}-nitrophenol}
\]
Step 2: Analysis of the Products Formed:
- ortho-Nitrophenol (2-nitrophenol) contains an intramolecular hydrogen bond between the phenolic \(-\text{OH}\) and the adjacent nitro group, making it steam-volatile.
- para-Nitrophenol (4-nitrophenol) forms intermolecular hydrogen bonds with surrounding molecules, resulting in higher boiling points and lower volatility.
These two structural isomers are separated in the laboratory by steam distillation. Step 3: Comparison with Concentrated Nitric Acid:
If concentrated nitric acid in the presence of concentrated sulfuric acid were used, phenol would undergo extensive nitration at all available ortho- and para-positions to produce 2,4,6-trinitrophenol (picric acid).
Because dilute nitric acid is specified, only mononitration occurs, yielding a mixture of ortho- and para-nitrophenols. Final Answer:
The reaction yields a mixture of ortho-nitrophenol and para-nitrophenol, which corresponds to option (C).
Concept:
Passing alcohol vapors over heated copper metal at \(573\text{ K}\) is an industrial and laboratory method for the selective catalytic dehydrogenation of primary and secondary alcohols.
This oxidation method avoids the use of strong aqueous oxidizing agents and prevents over-oxidation to carboxylic acids. Step 1: Mechanism of Catalytic Dehydrogenation:
Ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)) is a primary (\(1^\circ\)) alcohol.
When ethanol vapors are passed over finely divided metallic copper heated to \(573\text{ K}\) (\(300^\circ\text{C}\)), it undergoes dehydrogenation (loss of one molecule of hydrogen gas, \(\text{H}_2\)).
The two hydrogen atoms—one from the hydroxyl group and one from the \(\alpha\)-carbon—are removed:
\[
\text{CH}_3-\text{CH}_2\text{OH} \xrightarrow{\text{Cu}, \, 573\text{ K}} \text{CH}_3-\overset{\text{O}}{\overset{\parallel}{\text{C}}}-\text{H} + \text{H}_2\uparrow
\]
The product is ethanal (acetaldehyde). Step 2: Behavior of Other Classes of Alcohols over Heated Copper:
- Secondary (\(2^\circ\)) alcohols undergo dehydrogenation to produce ketones (e.g., propan-2-ol forms acetone).
- Tertiary (\(3^\circ\)) alcohols lack an \(\alpha\)-hydrogen atom; instead of dehydrogenating, they undergo dehydration in the presence of heated copper at \(573\text{ K}\) to yield alkenes.
Since ethanol is a primary alcohol, it dehydrogenates cleanly to yield ethanal. Step 3: Matching Product Structure:
Option (A) displays the structural formula of ethanal:
\[
\text{H}_3\text{C}-\text{CH}=\text{O}
\]
Final Answer:
The final product is ethanal, corresponding to option (A).
