Alcohols MCQ Quiz in தமிழ் - Objective Question with Answer for Alcohols - இலவச PDF ஐப் பதிவிறக்கவும்

Concept:

There are certain chemical reactions in which water is eliminated. Those reactions are called dehydration reactions.

Explanation:

The given reactant is tert-butylalcohol.

On heating with phosphoric acid, it loses water molecules and forms 2-methylpropene.

The product formation follows the Zaitsev rule.

The major substituted alkene is formed as the product.

The carbocation intermediate is formed as the intermediate during the reaction.

The chemical reaction is shown below:

Conclusion:

The correct answer is option 4) 2-methylpropene.

Concept:

Lucas Test - 

• Lucas test is used to distinguish between primary, secondary and tertiary alcohol.
• Alcohol reacts with Lucas reagent(conc. HCl + anhy. ZnCl₂) to give alkyl halide.
• The rate of reaction is different for different alcohols.
• If turbidity appears immediately, the alcohol is tertiary. 

Explanation:

→ The reaction of alcohols with Lucas reagent involves the formation of a carbocation intermediate.

• Thus, the stability of the carbocation determines the rate of reaction.
• The reactant alcohol which gives a 3° carbocation shows immediate turbidity when reacts with Luca's reagent.
• The stability order of carbocation is tertiary butyl > benzylic carbocation > allylic > 3°> 2°> 1°. 

Thus, the alcohol which give the most stable carbocation intermediate shows immediate turbidity with Luca's reagent.

The above-given alcohols give the following carbocation - 

As we know, primary carbocation is not stable, so n-butanol cannot show turbidity with Lucas reagent. 

Conclusion:

Therefore, only (ii) tertiary-Butanol (iii) Benzyl alcohol (iv) Allylic alcohol gives white turbidity almost immediately with the Lucas reagent at room temperature.

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°.

Correct answer: Option 2

Concept:

• A tertiary allylic alcohol is an alcohol where the hydroxyl group (-OH) is attached to a carbon atom that is bonded to three other carbon atoms and is adjacent to a double bond (allylic position).
• 

Explanation:

•  Prop-2-en-1-ol: The -OH group is attached to the first carbon, which is a primary carbon (not tertiary).
•  But-3-en-2-ol: The -OH group is attached to the second carbon, which is a secondary carbon (not tertiary).
•  2-Methylprop-2-en-1-ol: The -OH group is attached to the first carbon, which is a primary carbon (not tertiary).
•  2-Methylbut-3-en-2-ol: The -OH group is attached to the second carbon, and this carbon is connected to three other carbon atoms (tertiary) and is adjacent to a double bond, making it a tertiary allylic alcohol.

Conclusion:

Therefore, 2-Methylbut-3-en-2-ol is the tertiary allylic alcohol among the given options.

Concept:

• The solvent plays a critical role in the bromination reaction of phenol, influencing the position and extent of bromination.

Explanation:

• Phenol reacts with bromine in the presence of different solvents to yield various products.
• For obtaining 2,4,6-tribromophenol, water is typically used as the solvent. This is because water helps in the rapid ionization of bromine (Br2) and enhances the reaction, leading to complete substitution of hydrogen atoms at ortho and para positions with bromine.
• 

Conclusion:

Therefore,  Water is the solvent used in the bromination of phenol to obtain 2,4,6-tribromophenol.

Concept:

• A tertiary allylic alcohol is an alcohol where the hydroxyl group (-OH) is attached to a carbon atom that is bonded to three other carbon atoms and is adjacent to a double bond (allylic position).
• 

Explanation:

•  Prop-2-en-1-ol: The -OH group is attached to the first carbon, which is a primary carbon (not tertiary).
•  But-3-en-2-ol: The -OH group is attached to the second carbon, which is a secondary carbon (not tertiary).
•  2-Methylprop-2-en-1-ol: The -OH group is attached to the first carbon, which is a primary carbon (not tertiary).
•  2-Methylbut-3-en-2-ol: The -OH group is attached to the second carbon, and this carbon is connected to three other carbon atoms (tertiary) and is adjacent to a double bond, making it a tertiary allylic alcohol.

Conclusion:

Therefore, 2-Methylbut-3-en-2-ol is the tertiary allylic alcohol among the given options.

Concept:

• The solvent plays a critical role in the bromination reaction of phenol, influencing the position and extent of bromination.

Explanation:

• Phenol reacts with bromine in the presence of different solvents to yield various products.
• For obtaining 2,4,6-tribromophenol, water is typically used as the solvent. This is because water helps in the rapid ionization of bromine (Br2) and enhances the reaction, leading to complete substitution of hydrogen atoms at ortho and para positions with bromine.
• 

Conclusion:

Therefore,  Water is the solvent used in the bromination of phenol to obtain 2,4,6-tribromophenol.

CONCEPT:

Reaction Sequence of Alcohols with Phosphorus Triiodide and Grignard Reagents

• The given sequence involves the conversion of ethanol (CH₃CH₂OH) to various intermediates through a series of reactions.
• In the first step, ethanol reacts with phosphorus triiodide (P + I₂) to form an alkyl iodide (ethyl iodide, A).
• The alkyl iodide then reacts with magnesium in ether to form a Grignard reagent (ethylmagnesium iodide, B).
• Grignard reagents react with formaldehyde (HCHO) to form primary alcohols with one additional carbon atom after hydrolysis.

CALCULATION:

• Step 1: Ethanol (CH₃CH₂OH) reacts with phosphorus triiodide (P + I₂) to form ethyl iodide (CH₃CH₂I), which is compound A.
• Step 2: Ethyl iodide (CH₃CH₂I) reacts with magnesium in ether to form ethylmagnesium iodide (CH₃CH₂MgI), which is compound B.
• Step 3: Ethylmagnesium iodide (CH₃CH₂MgI) reacts with formaldehyde (HCHO) to form a three-carbon compound after hydrolysis, resulting in n-propyl alcohol (CH₃CH₂CH₂OH), which is compound C.
• Step 4: Oxidation of n-propyl alcohol (C) would yield propanal (CH₃CH₂CHO), which is compound D.
• Reaction: 

CONCLUSION:

The correct option is: Option 4

CONCEPT:

Hydroboration-Oxidation and Oxymercuration-Demercuration

• Hydroboration-Oxidation:
  • The reaction follows anti-Markovnikov's rule, where the hydroxyl group (-OH) is added to the carbon with more hydrogen atoms.
  • This reaction forms a primary alcohol as the major product.
• Oxymercuration-Demercuration:
  • This reaction follows Markovnikov's rule, where the hydroxyl group (-OH) is added to the carbon with fewer hydrogen atoms.
  • This reaction forms a secondary alcohol as the major product.

EXPLANATION:

• In the hydroboration-oxidation reaction, water is added to the alkene in an anti-Markovnikov fashion, forming the primary alcohol.

Where the negative part of the reagent attached to that carbon where number of hydrogen atom are more and forms primary alcohol.

In the above reaction addition of water takes place according to Markonikov’s Rule where the negative part of the reagent attached to that carbon where number of hydrogen atom are less and forms secondary alcohol.

• In the oxymercuration-demercuration reaction, water is added following Markovnikov's rule, forming the secondary alcohol.

CONCLUSION:

The correct major products are given by Option 2, where A is a primary alcohol and B is a secondary alcohol.

CONCEPT:

Boiling Point Order Based on Molecular Interactions

• The boiling point of a compound depends on the type and strength of intermolecular forces present.
• Hydrogen bonding increases the boiling point significantly compared to compounds with only dipole-dipole interactions or Van der Waals forces.
• Among compounds with similar molecular weights, the order of boiling points typically follows: hydrogen bonding > dipole-dipole interactions > Van der Waals (dispersion) forces.

EXPLANATION:

• Compound A (Ether): It has no hydrogen bonding, only weak dipole-dipole interactions, leading to a relatively low boiling point.
• Compound B (Alkane): It only exhibits Van der Waals forces, resulting in the lowest boiling point among the compounds.
• Compound C (Ketone): It has dipole-dipole interactions, which provide moderate boiling points.
• Compound D (Alcohol): It has strong hydrogen bonding, resulting in the highest boiling point.

Compounds having same number of carbon atoms follow the boiling point order as: 

(Boiling point)Hydrogen bonding > (Boiling point)high polarity > (Boiling point)low polarity > (Boiling point)non polar 

Conclusion:-

The correct increasing order of boiling points is: (B) < (A) < (C) < (D).