Important MCQs on Alcohols, Phenols and Ethers with Explanation

Ans. (d) P-hydroxyazobenzene

Explanation: Under weakly basic conditions, phenol reacts with Benzene diazonium chloride to give p-hydroxyazobenzene as the product. 

The reaction involved is:

Ans. (a) 1°>2°>3°

Explanation: Alcohols are acidic in nature due to the presence of polarity in the O-H bond. The electron density on oxygen increases due to the presence of an electron-donating alkyl group which reduces the polar nature of the O-H bond which in turn result in the decrease of acidic strength. 

Therefore, the acidic strength in the alcohols follows the order: 

Ans. (c) 2-hydroxybenzoic acid

Explanation: Salicylic Acid, chemically known as 2-Hydroxybenzoic acid is an ortho-substituted hydroxybenzoic acid that is formed when phenol reacts with sodium hydroxide to generate a phenoxide ion. This phenoxide ion undergoes electrophilic substitution when reacted with carbon dioxide i.e. a weak electrophile and results in the formation of salicylic acid.

This reaction is also known as Kolbe’s reaction.

Ans. (b) Tert-butyl alcohol

Explanation: Tert-butyl alcohol is tertiary alcohol and therefore reacts with conc.HCl and ZnCl₂ (Lucas reagent) form alkyl halide. To distinguish between the reactivities of the three classes of alcohols, the Lucas test is carried out.

Alcohols react with Lucas reagent (Conc. HCl and ZnCl₂) which results in turbidity in the solution due to the formation of alkyl halides.

• Primary alcohols do not produce any turbidity.
• Secondary alcohols turn the solution cloudy after gentle heating for a few minutes.
• Tertiary alcohols produce turbidity immediately after reaction since they form halides easily.

Ans. (a) (CH₃)₃COH

Explanation: (CH₃)₃COH is a type of tertiary alcohol and hence forms the most stable carbocation. Due to the electron-releasing nature of the alkyl group present in alcohols, the positive charge gets stabilized. 

In the case of tertiary alcohols, the bulkier alkyl groups donate more electron density and stabilize the carbocation more easily as compared to that in the case of secondary and primary alcohols. Therefore, tertiary alcohols undergo dehydration at a faster rate.

Thus, the order of dehydration in alcohols is as follows:

Tertiary > Secondary > Primary 

Ans. (d) 2,4,6-tribromophenol

Explanation: Phenol forms 2,4,6-tribromophenol on treatment with excess bromine water undergoing a halogenation reaction.

The reaction involved is as follows:

Ans. (c) Acidified potassium permanganate

Explanation: Acidified potassium permanganate is a strong oxidizing agent used for the conversion of alcohol to carboxylic acid directly. 

The reaction involved is 

PCC and Chromic anhydride is used to convert primary alcohols to aldehydes. 

Ans. (c) Markovnikov’s rule

Explanation: Alcohols can be prepared by acid-catalyzed hydration of alkenes in which alkenes react with water in presence of acid as a catalyst and give alcohol as a product. The mechanism of addition reaction in the case of unsymmetrical alkenes follows Markovnikov’s principle.

Markovnikov’s rule: It states that in addition reaction of alkenes, the proton attacks the carbon atom having the greatest number of hydrogen atoms attached to it or the nucleophile attacks the carbon having the least number of hydrogen atoms attached to it. 

The reaction takes place as follows:

Ans. (d) Methyl bromide and magnesium (Grignard reagent)

Explanation: One method of preparation of secondary alcohols is by reacting aldehydes with the Grignard reagent.

1-Phenylethanol is a type of secondary alcohol formed by the reaction of benzaldehyde with Methyl bromide and magnesium (Grignard Reagent).

The reaction involved is:

Ans. (b) Methoxymethane

Explanation: Methoxymethane forms when 2 moles of methyl alcohol react with sulphuric acid at 413K undergoing dehydration. 

The resultant product depends on the reaction conditions i.e. the product can be either ether or an alkene. An alkene is formed when the alcohol reacts with sulphuric acid at 443K.

Only the ethers that have primary alkyl groups can be formed by using this method. 

The mechanism followed by this reaction is the Nucleophilic biomolecular substitution reaction (SN₂).

Reactions involved are:

Ans. (a) Propan-1-ol

Explanation: Propan-1-ol belongs to the class of primary alcohol. Due to the presence of intermolecular hydrogen bonding in alcohols, they have a high boiling point.

Ans. (b) III > I > II > IV

Explanation: 

• p-nitrophenol shows both -M and -I effect. 
• Due to the delocalization of conjugated electrons, -M comes into effect.
• The nitro group (-NO₂) present in p-nitrophenol has an electron-withdrawing nature and therefore, the carbanion formed after the loss of proton gets stabilized easily.
• Thus, p-nitrophenol is highly acidic in nature. 
• p-chlorophenol shows only -I effect.
• p-methylphenol shows +I effect due to the electron-donating nature of the methyl group and hence, the carbanion form gets highly destabilized.
• P-methoxyphenol shows +M effect.

Ans. (c) Benzyl iodide and ethanol

Explanation: Benzylethyl ether contains two different alkyl groups and therefore, the cleavage of C-O bond takes place at one of the alkyl-oxygen bonds. 

The reaction involved is as follows:

Ans. (a) p-Bromoanisole

Explanation: Anisole being a phenyl alkyl ether undergoes bromination when it reacts with bromine in ethanoic acid as the aromatic ring is activated by the presence of the methoxy group. The major product formed as result is p-bromoanisole.

The reaction involved is:

Ans. (d) Primary alcohol

Explanation: Formaldehyde reacts with Grignard reagent following a nucleophilic addition mechanism to form primary alcohol.