Alcohols, Phenols And Ethers MCQ Quiz - Objective Question with Answer for Alcohols, Phenols And Ethers - Download Free PDF

CONCEPT:

Wolff–Kishner Reduction

• The Wolff–Kishner reduction is a chemical reaction that reduces carbonyl groups (such as aldehydes and ketones) to the corresponding alkanes.
• This reaction involves hydrazine (NH₂NH₂) in the presence of a strong base and high temperature.
• The reaction mechanism involves the formation of a hydrazone intermediate, which is then decomposed to yield the alkane product.

EXPLANATION:

• In the given reaction:

  C₆H₅COCH₃ → C₆H₅CH₂CH₃

  • C₆H₅COCH₃ is acetophenone (a ketone).
  • C₆H₅CH₂CH₃ is ethylbenzene (an alkane).
• To reduce the ketone (C=O group) to an alkane, the Wolff–Kishner reduction is an appropriate method.
• The reactant NH₂NH₂ (hydrazine) is used in this process to achieve the reduction.
• Other methods like Sn/HCl, Friedel–Crafts reaction, and LiAlH₄ are not suitable for this transformation:
  • Sn/HCl is used for the reduction of nitro groups.
  • Friedel–Crafts reaction is used for alkylation or acylation of aromatic compounds, not for reducing ketones.
  • LiAlH₄ reduces ketones to alcohols, not alkanes.

Therefore, the correct answer is option 1: NH₂NH₂.

CONCEPT:

Dehydration of Alcohols

• Dehydration of alcohols refers to the elimination of a water molecule (H₂O) from an alcohol to form an alkene.
• This reaction typically occurs in the presence of a strong acid catalyst such as concentrated sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄).
• The rate of dehydration of alcohols depends on the stability of the carbocation intermediate formed during the reaction.
• Carbocation stability increases in the order:

  3° (tertiary) > 2° (secondary) > 1° (primary)

EXPLANATION:

• During the dehydration process:
  • The alcohol first undergoes protonation by the acid to form an oxonium ion (R-OH₂⁺).
  • This is followed by the loss of water (H₂O), forming a carbocation intermediate.
• The stability of the carbocation intermediate is a key factor determining the rate of the dehydration reaction.
• The order of carbocation stability is:

  3° (tertiary carbocation) > 2° (secondary carbocation) > 1° (primary carbocation)

• As tertiary alcohols form the most stable carbocations, they dehydrate the fastest. Secondary alcohols are next, followed by primary alcohols, which form the least stable carbocations.

Therefore, the relative rate of dehydration of alcohols is: 3° > 2° > 1°.

CONCEPT:

Oxidation of Alcohols

• Alcohols can be oxidized to carbonyl compounds depending on their structure:
  • Primary alcohols are oxidized to aldehydes and can further oxidize to carboxylic acids.
  • Secondary alcohols are oxidized to ketones.
  • Tertiary alcohols cannot be oxidized to carbonyl compounds because they lack a hydrogen atom attached to the carbon bonded to the hydroxyl (-OH) group.

EXPLANATION:

• 1) O-butyl alcohol: This is a primary alcohol and can be oxidized to aldehyde and further to carboxylic acid.
• 2) Sec-butyl alcohol: This is a secondary alcohol and can be oxidized to a ketone.
• 3) Tet-butyl alcohol: This is a tertiary alcohol. It cannot be oxidized to a carbonyl compound because it lacks a hydrogen atom on the carbon bonded to the hydroxyl (-OH) group.
• 4) I-pentanol: This is a primary alcohol and can be oxidized to aldehyde and further to carboxylic acid.

Tet-butyl alcohol is the correct answer as it is a tertiary alcohol and cannot be oxidized to a carbonyl compound.

Therefore, the correct answer is option 3.

CONCEPT:

Acid-Catalyzed Dehydration of Alcohols

• In the presence of concentrated sulfuric acid (H₂SO₄) and heat, alcohols undergo dehydration to form alkenes.
• The mechanism proceeds through the formation of a carbocation intermediate, followed by a hydride or methyl shift to stabilize the intermediate.
• The reaction follows a Zaitsev's rule, where the more substituted alkene is the major product, as it is more thermodynamically stable.

EXPLANATION:

• The reaction starts with the dehydration of cyclohexanol in the presence of concentrated sulfuric acid and heat:

  C₆H₁₁OH (cyclohexanol) → C₆H₁₀ (cyclohexene) + H₂O

• The reaction follows an E1 mechanism, where the -OH group leaves, forming a carbocation intermediate.
• Once the carbocation is formed, a 1,2-methyl shift occurs, leading to a more stable carbocation.
• The more substituted alkene, which is thermodynamically favored, will be the major product.
• The resulting major product is cyclohexene, with a double bond between the second and third carbon atoms (which corresponds to the highest substituted position, forming the most stable product).

Therefore, the major product of this reaction is cyclohexene, formed via acid-catalyzed dehydration of cyclohexanol.

CONCEPT:

Oxidation and Cannizzaro Reaction

• The transformation involves a series of oxidative steps and a Cannizzaro reaction.
• Step 1: Oxidation of alkylbenzene to benzaldehyde involves:
  • Oxidation of n-propylbenzene using Cr₂O₃ at high temperature and pressure → forms toluene.
• Step 2: Controlled oxidation of toluene using CrO₂Cl₂ (Etard reaction) → benzaldehyde.
• Step 3: Cannizzaro reaction:
  • Benzaldehyde undergoes self-oxidation and reduction in presence of NaOH → gives benzoate (→ Q: benzoic acid) and benzyl alcohol (→ R).
• Step 4: Acidification of sodium benzoate to benzoic acid using H₃O⁺.

EXPLANATION:

• P → Toluene using Cr₂O₃, 770K, 20 atm
• Toluene → Benzaldehyde using CrO₂Cl₂, H₃O⁺
• Benzaldehyde → Benzoate + Benzyl Alcohol via Cannizzaro reaction in NaOH
• Benzoate → Benzoic acid (Q) after acidification with H₃O⁺

Therefore, the correct sequence is:

1. Cr₂O₃, 770 K, 20 atm
2. CrO₂Cl₂, H₃O⁺
3. NaOH
4. H₃O⁺

The correct answer is secondary alcohol.Key Points

• A hydroxy group (-OH) attached to a saturated carbon atom with two other carbon atoms attached to it is known as a secondary alcohol.
• There are two alkyl groups present; their structures can differ or be the same.
• Secondary alcohol examples include 2 – propanol and 2 – butanol.

Additional Information

• Primary alcohols have the hydroxy group attached to a carbon atom with only one other carbon atom attached to it.
• In the case of a tertiary alcohol, the hydroxy group is attached to a saturated carbon atom with three other carbon atoms attached to it.
• An aldehyde is a compound that contains a carbonyl group (-C=O) attached to at least one hydrogen atom.
  • This group is always located at the end of a carbon chain.

The correct answer is secondary alcohol.Key Points

• Butan-2-ol has a chemical formula of C₄H₁₀O and is a type of alcohol.
• It is a secondary alcohol because the hydroxyl (-OH) group is attached to a carbon atom that is bonded to two other carbon atoms.
• Butan-2-ol is commonly used as a solvent and is also used in the production of other chemicals such as butyl acetate.

Additional Information

• Tertiary alcohols have the hydroxyl group attached to a carbon atom that is bonded to three other carbon atoms, while primary alcohols have the hydroxyl group attached to a carbon atom that is bonded to only one other carbon atom.
  • Tertiary alcohols have a higher boiling point than primary and secondary alcohols because they have a more complex molecular structure.
• Ketones have a carbonyl group (C=O) in the middle of the carbon chain, which is not present in butan-2-ol.
  • Ketones are commonly used as solvents and are also used in the production of polymers, pharmaceuticals, and other chemicals.
• Primary alcohols can be oxidized to form aldehydes and then further oxidized to form carboxylic acids.
• Secondary alcohols can be oxidized to form ketones, but they cannot be further oxidized to form carboxylic acids.

Explanation:-

Acidic Strength:-

• Lewis acids are electron-pair acceptors while some acids are proton donors.
• HO^- is a conjugate base of water, where H₃O⁺ is the conjugate acid.
• Any molecule, atom or ion which stabilizes the conjugate base will always increase the acidity.
• The acidic character increases with the increase in formal charge in the molecule.
• Acidity is directly proportional to electronegativity.
• Resonance effect - delocalization of negative charge in a molecule increases acidity.
• Due to the increase in inductive effect acidic character increases.
• Due to higher s-character acidity increases. 

The correct answer is ether.Key Points

• Ether:-
  • It is a class of organic compounds that contains an oxygen atom between two alkyl groups.
  • The general formula for an ether is R-O-R', where .R and R' are alkyl groups
  • Ethers are commonly used as solvents in various industries, including pharmaceuticals, paints, and coatings.
  • Ethers are less reactive than alcohols, aldehydes, and ketones because the C-O bond is relatively stable.

Additional Information

• Alcohol:-
  • ​ A class of organic compounds that contains a hydroxyl (-OH) group attached to a carbon atom.
  • They are commonly used as solvents, disinfectants, and fuels.
• Aldehyde:-
  • ​ A class of organic compounds that contains a carbonyl group (-CHO) attached to a carbon atom at the end of the molecule.
  • They are commonly used in the production of plastics, resins, and dyes.
• Ketone:-
  • A class of organic compounds that contains a carbonyl group (-C=O) attached to a carbon atom in the middle of the molecule.
  • They are commonly used in the production of solvents, polymers, and pharmaceuticals.

Explanation:

• Alcohols react with sodium leading to the evolution of hydrogen.

e.g. 2Na + 2CH3CH2OH 🡪 2CH3CH2O–Na (Sodium ethoxide) + H2 (g)

• In the reaction of alcohols with sodium metal, bond cleavage of the O - H bond takes place.
• The ease of breakage of the O-H bond in alcohols is an indication of the acidity of alcohols.

We know that the ease of this bond breakage follows the order primary > secondary > tertiary.

So, the ease of reactivity of sodium follow the order

Additional InformationThe acidity of alcohols:

• Just as metals react with acids to liberate hydrogen, alcohols react with metals to liberate hydrogen acting as an acid.
• The conjugate base alkoxide ion formed after alcohol releases a proton is R -O -.
• The stability of the conjugate base depends on the +I of the alkyl group R attached because it is not resonance stabilized here.
• The strength of negative charge increases as we move on from primary to secondary to tertiary alcohols because of the number of alkyl groups increases.
• The increase in negative charge over the oxygen atom destabilizes the conjugate base.
• Thus, the acidity of alcohols follows the order Primary> Secondary > Tertiary.
• 

Concept:

Dehydration reaction of alcohols:

• Alcohols when heated at high temperatures in presence of conc acids, undergo dehydration.
• They lose a molecule of water and form alkenes.
• The reaction proceeds via a carbocationic intermediate.
• The ease of dehydration follows the order tertiary>secondary>primary.
• There is a formation of unexpected products because the rearrangement of carbocations takes place in the reaction.
• Carbocationic rearrangement takes place whenever there is a possibility of a more stable intermediate to be formed by rearrangement.
• More stable carbonation gives a more saturated alkene as the product.

Explanation:

• In the first step, the nucleophilic oxygen of the OH group of alcohol takes up a proton from the acid.
• In the next step elimination of water molecule takes place and gives the carbocationic intermediate. 
• In the third step Rearrangement of this intermediate takes place to give a more stable carbocation.
• Finally, elimination of proton occurs giving more substituted alkenes.
• The mechanism is:

Hence,  on dehydration is .

Methanol is an example of the Alcohol group.

• This liquid is acidic, flammable, colorless and has a distinctive fragrance close to that of ethanol (drinking alcohol).
• Methanol is a better proton donar than Water, so alcohols are weaker acids than water
• Methanol is slightly more acidic than water.
• The order of acidity in aqueous solution is as following:
• CH3OH > H2O > CH3CH2OH > (CH3)2 CHOH
• For an acid to be strong its conjugate base anion has to be very stable. Only then acid will disassociate faster to give hydronium ion.

Hence we can conclude that  CH3OH  is more acidic.

Concept:

Nucleophilic substitution reactions-

Substitution reactions are the types of reactions where a nucleophile is the attacking reagent.

• There are three types of substitution reactions depending on the nature of the substrate.
  • Nucleophilic substitution at saturated carbon. 
  • Nucleophilic acyl substitution
  • Nucleophilic aromatic substitution.
• SN1 - Unimolecular nucleophilic substitution:
  • Depends upon the concentration of the substrate.
  • Is independent of the concentration of the nucleophile.
  • Follows first-order kinetics.
• SN2- bimolecular nucleophilic substitution.
  • the rate depends on the concentration of both the reactant and the substrate.
  • It follows second-order kinetics

Elimination reactions:

• Benzene and other aromatic compounds show characteristic electrophilic substitution reactions.
• In this reaction, a hydrogen atom of the aromatic ring is substituted by an electrophile.
• The substitution takes place through an addition-elimination mechanism.

Explanation:

Dow's Process: 

Dow’s process is a process by which phenol is prepared by reacting it with molten NaOH at very high-temperature conditions. The phenol formation mechanism by this method is called the benzyne mechanism since it involves the formation of the reactive intermediate Benzyne. 

Chlorobenzene is fused with NaOH at 623K and 300 atmospheric pressure giving sodium phenoxide (C6H5O-Na+).

Phenol is obtained by the acidification of sodium phenoxide so produced.

The given reaction is well known as Dows process, for the preparation of phenols.

As we can see that the OH-ion is the attacking reagent in the process, so it is a nucleophilic substitution reaction.

Correct Answer: 3)

Concept:

The acidity of alcohols:

• Just as metals react with acids to liberate hydrogen, alcohols react with metals to liberate hydrogen acting as an acid.
• The conjugate base alkoxide ion formed after alcohol releases a proton is R -O ^-.
• The stability of the conjugate base depends on the +I of the alkyl group R attached because it is not resonance stabilized here.
• The strength of negative charge increases as we move on from primary to secondary to tertiary alcohols because of the number of alkyl groups increases.
• The increase in negative charge over the oxygen atom destabilizes the conjugate base.
• Thus, the acidity of alcohols follows the order Primary> Secondary > Tertiary.
• 

Explanation:

• In the reaction of alcohols with sodium metal, bond cleavage of the O - H bond takes place.
• The ease of breakage of the O-H bond in alcohols is an indication of the acidity of alcohols.

 We know that the ease of this bond breakage follows the order primary > secondary > tertiary.

So, the ease of reactivity of sodium follow the order 

Concept: 

• This is a type of Reduction Reaction. 
• The reduction is a chemical reaction that involves the gaining of electrons by one of the atoms involved in the reaction between two chemicals.
• Oxidation is the gain of oxygen and reduction in loss of oxygen.​

Explanation:

• When phenol is reacted with zinc dust it forms benzene with zinc oxide as a side product.
• It is because zinc dust is a strong reducing agent. 
• Zinc oxidized itself and become ZnO and reduces phenol to benzene.
• NaOH is just a base and H2SO4 is a strong oxidizing agent and acid.

​Additional Information

Phenol to C6H5COOH (Benzoic acid):

Phenol to C6H5CHO (Benzaldehyde)