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

CONCEPT:

Hemiacetal Formation

• Hemiacetals are formed when an aldehyde or ketone reacts with an alcohol.
• A hemiacetal contains both an ether (R-O-R') and an alcohol (R-OH) functional group attached to the same carbon atom.
  

EXPLANATION:

• Option 1: This compound does not have both functional groups attached to the same carbon.
• Option 2: This compound has both an ether and an alcohol functional group attached to the same carbon, indicating it is a hemiacetal.
• Option 3: This compound does not have both functional groups attached to the same carbon.
• Option 4: This compound does not have both functional groups attached to the same carbon.

Therefore, the correct answer is option 2.

CONCEPT:

Reactions and Functional Group Transformations

• The reaction starts with the cleavage of an ether bond using HI, yielding an aromatic hydroxyl compound (phenol).
• The phenol then undergoes the Kolbe-Schmitt reaction, where it reacts with CO₂ in the presence of a base (NaOH) and acidic workup (H₃O⁺) to form a carboxyl-substituted aromatic compound (salicylic acid).
• Further, salicylic acid reacts with SOCl₂ (thionyl chloride) to form an acyl chloride, which then reacts with CH₃OH (methanol) to form a methyl ester, producing methyl salicylate (wintergreen oil).

EXPLANATION:

• Step 1: Ether bond cleavage using HI results in phenol as Y.
• Step 2: The phenol reacts with CO₂ in the presence of NaOH and H₃O⁺, forming salicylic acid as Z.
• Step 3: Salicylic acid reacts with SOCl₂, forming an acyl chloride. This is followed by reaction with CH₃OH, yielding methyl salicylate as W.

• The reaction proceeds through known organic transformations: ether cleavage, Kolbe-Schmitt reaction, and esterification.
• W is methyl salicylate, which contains a methyl ester functional group and is used as a muscle relaxant in topical analgesic products like Bengay.
• This excludes options related to prostaglandin inhibition, narcotic effects, and salicylic acid (which is Z).

The correct answer is A: It contains an ester functional group and is used as a muscle relaxant.

CONCEPT:

Reactions and Their Products

• Aldol condensation: Involves the formation of an α,β-unsaturated carbonyl compound through the reaction of aldehydes or ketones.
• Cannizzaro reaction: A reaction in which an aldehyde undergoes disproportionation to give an alcohol and a carboxylate.
• Wolff-Kishner reduction: A method of reducing carbonyl compounds to alkanes using hydrazine and a strong base.
• Clemmensen reduction: A reduction of aldehydes or ketones to alkanes using zinc amalgam and hydrochloric acid.

MATCHING EXAMPLES:

• Aldol condensation: α,β-unsaturated carbonyl compound
• Cannizzaro reaction: Alcohol and carboxylate
• Wolff-Kishner reduction: Alkane
• Clemmensen reduction: Alkane

Conclusion:-

The correct match is (i) - a, (ii) - b, (iii) - c, (iv) - d

Explanation:

→ Methanal (formaldehyde) cannot undergo Aldol condensation because it has only one carbon atom and does not have an alpha hydrogen atom.

→ Benzaldehyde has an alpha hydrogen atom and can undergo Aldol condensation, but it forms a mixture of products due to the presence of two carbonyl groups that can react.

2,2-Dimethylbutanal does not have an alpha hydrogen atom and cannot undergo Aldol condensation.

→ Phenylacetaldehyde has an alpha hydrogen atom and can undergo Aldol condensation to form an alpha-beta unsaturated aldehyde.

→ Aldol condensation is a reaction between two molecules of aldehydes or ketones that contain alpha hydrogen atoms. The alpha hydrogen atom undergoes deprotonation to form an enolate ion, which then reacts with another molecule of aldehyde or ketone to form a beta-hydroxy aldehyde or ketone. The product undergoes dehydration to form an alpha-beta unsaturated aldehyde or ketone.

→ Therefore, only Phenylacetaldehyde can undergo Aldol condensation among the given options.

Aldehydes which contain at least 2α hydrogen atoms, undergo Aldol condensation reaction Phenylacetaldehyde contains 2α hydrogen atoms and undergoes Aldol condensation reaction in presence of dilute alkali. 

The correct answer is  β-hydroxy esters.

Concept:

• Reformatsky Reaction: The Reformatsky reaction is an organozinc reaction that converts aldehydes or ketones to β-hydroxy esters by reacting with α-bromo esters in the presence of zinc and an acid workup. This reaction is typically performed in an inert solvent such as diethyl ether or tetrahydrofuran (THF).

Explanation:

In the Reformatsky reaction, an α-bromo ester reacts with zinc to form an organozinc intermediate (a Reformatsky reagent). This organozinc intermediate then reacts with an aldehyde or ketone to form a β-hydroxy ester after an acid workup.

The general reaction mechanism can be illustrated as follows:

• The α-bromo ester reacts with zinc to form the organozinc intermediate.
• The organozinc intermediate undergoes nucleophilic addition to the carbonyl group of the aldehyde or ketone.
• After acid workup, a β-hydroxy ester is formed.

Reformatsky reaction is defined as an organion reaction that is used to convert ketone or an aldehydes and α-haloester to a β-hyroxyester by using metallic zinc and acid workup. Here, inert solvent like diethyl ether or THF (Tetra hydro furan) is often used as a solvent for a reaction.

Thus, the reaction between aldehydes or ketones and α-bromo esters in the presence of zinc yields β-hydroxy esters.

Explanation:

Ethylene undergoes oxidation in the presence of Pd(II) catalyst and forms acetaldehyde. This process is Wacker's process.

The primary alcohol is converted into carboxylic acid in the presence of a strong oxidizing agent as CrO₃-H₂SO₄

The cyanide group is converted into an aldehyde group with mild reducing reagent DIBAL-H.

Ethyl alcohol is converted into acetaldehyde in the presence of Cu metal under high temperatures.

Hence, option 2 is correct.

CONCEPT:

Formation of Aldehyde from Alcohol

• The conversion of alcohol to aldehyde is an oxidation process.
• Primary alcohols are oxidized to aldehydes using mild oxidizing agents.
• Strong oxidizing agents may further oxidize aldehydes to carboxylic acids, so a selective reagent is required for controlled oxidation.

EXPLANATION:

• Among the options, PCC in CH₂Cl₂ (dichloromethane) is the correct reagent for the formation of aldehyde from alcohol.

Therefore, the correct answer is Option 3: PCC/CH₂Cl₂.

CONCEPT:

Crossed Cannizzaro Reaction & Polymerization of Formaldehyde

• This reaction is a well-known example of a crossed aldol condensation under strong basic conditions (Conc. NaOH), involving formaldehyde and acetaldehyde (CH₃CHO).
• Formaldehyde acts as a chain-extending agent through aldol-type reactions and successive additions.
• The product formed in the image is a sugar-like molecule: a tetrahydroxy butane derivative, which shows 4 carbon atoms with hydroxyl groups.

EXPLANATION:

• Initial molecule: Acetaldehyde (CH₃CHO) – 2 carbon atoms
• Final product: A 6-carbon polyhydroxy compound
• This means 4 additional carbon atoms have been added from formaldehyde (HCHO) units.
• Hence, the number of HCHO molecules added = 4

Therefore, the value of x = 4 (i.e., 4 moles of formaldehyde are consumed)

CONCEPT:

Grignard Reaction

• Grignard reagents (RMgX) are nucleophilic and react with carbonyl compounds to form alcohols after hydrolysis.
• The general mechanism involves:
  • Formation of the Grignard reagent by reacting an alkyl or aryl halide with magnesium in dry ether.
  • Reaction of the Grignard reagent with a carbonyl compound (aldehyde, ketone, etc.).
  • Hydrolysis of the intermediate to produce an alcohol.

EXPLANATION:

• Step 1: Formation of Grignard reagent
  • The given compound (bromobenzene) reacts with magnesium in dry ether to form the Grignard reagent  Ph-MgBr .
• Step 2: Nucleophilic addition
  • The Grignard reagent reacts with 4-chlorobenzaldehyde, where the nucleophilic carbon of the Grignard reagent attacks the electrophilic carbonyl carbon of the aldehyde.
  • This forms a tetrahedral alkoxide intermediate.
• Step 3: Hydrolysis
  • The alkoxide intermediate is hydrolyzed using aqueous ammonium chloride (NH₄Cl) to give the final product, a secondary alcohol.
• The product has two phenyl rings, one with a hydroxyl group and one with a chloro substituent.

Therefore, the product of the reaction is the compound shown in Option 1.

CONCEPT:

Aldol Condensation Reaction

• Aldol condensation is a reaction between aldehydes or ketones with at least one alpha-hydrogen in the presence of a base, resulting in the formation of a β-hydroxy aldehyde (aldol) or β-hydroxy ketone, which further undergoes dehydration to yield an α,β-unsaturated aldehyde or ketone.
• When benzaldehyde (C₆H₅CHO), which lacks α-hydrogens, reacts with ethanal (CH₃CHO) in the presence of dilute NaOH, the enolate ion formed from ethanal attacks the carbonyl carbon of benzaldehyde, leading to the formation of the aldol product.

EXPLANATION:

• The steps involved when benzaldehyde and ethanal react in the presence of dilute NaOH:
  1. Addition Step: Ethanal forms an enolate ion (CH₃CH^-O) in the presence of NaOH, which then attacks the carbonyl carbon of benzaldehyde to form β-hydroxy-3-phenylpropanal as the β-hydroxy aldehyde intermediate.
  2. Dehydration Step: Upon heating, this intermediate undergoes dehydration (loss of water molecule) to form an α,β-unsaturated aldehyde, specifically 3-Phenylprop-2-enal (cinnamaldehyde).

The balanced chemical equation for the reaction is:

C₆H₅CHO + CH₃CHO →(NaOH) C₆H₅–CH=CH–CHO + H₂O

The product formed in this reaction is 3-Phenylprop-2-enal (cinnamaldehyde).

Therefore, option D is incorrect.
The correct product formed is 3-Phenylprop-2-enal.