Aromatic Hydrocarbon MCQ Quiz in मराठी - Objective Question with Answer for Aromatic Hydrocarbon - मोफत PDF डाउनलोड करा

Concept:

• 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 by addition- elimination mechanism.

Mechanism:

• In the first step, the benzene ring donates pi electrons to the electrophile.
• One of the carbon atoms forms a bond with the electrophile.
• In the second step, the complex formed then loses a proton from the saturated carbon atom with the help of a base.
• The aromatic ring is finally regenerated in the last step.
• Nitric acid and sulfuric acid used in nitration generate nitronium NO2+ ion as electrophiles.
• The electrophile then attacks the benzene ring forming a sigma complex.
• The σ complex is resonance stabilized.
• The complex then loses a proton to form nitrobenzene.
• The mechanism is shown as follows:

Explanation:

• Anthracene is nitrated in presence of nitric acid, and acetic anhydride or NaOH at temperatures about 15 - 20⁰C.
• The reaction is:

​

• The reaction follows an electrophilic substitution reaction whose mechanism is given above.
• A mixture of 9 - nitroanthracene and 9, 10 - dinitroanthracene is obtained.
• This is because 9 and 10 position of anthracene is the more reactive position.

Hence, product A is .

Mistake Points

• Only 9 and 10 positions will be reactive towards the electrophilic substitution and not any other.
• Same position is reactive towards sulphonation as well as chlorination or halogenation of anthracene. 

The correct answer is None of the above

Concept:

Huckel's rule of aromaticity:

• The compounds are said to be aromatic if they satisfy the following conditions:-
  • The molecule should be cyclic.
  • The molecule must be conjugated, i.e. every atom in the molecule should be sp2 hybridized.
  • The number of π electrons in the system is (4n + 2), where values of n can be 0, 1, 2...
  • The molecule has to be planar so that all π electrons in the system are delocalized.
  • Aromatic compounds are exceptionally stable.
• Conditions for compounds to be anti-aromatic:-
  • The compounds should possess a 4n number of π electrons, where n = 0, 1, 2....
  • The molecule must be cyclic.
  • The molecule must be conjugated, i.e. every atom in the molecule should be sp2 hybridized.
  • The molecule has to be planar.
  • Anti-aromatic compounds are unstable.
• Conditions for compounds to be Non-aromatic:-
  • Non-aromatic compounds have 4n + 2 or 4n number of π electrons
  • There is no delocalized electron system in that molecule.
  • The system must be non-planar and unconjugated.

​Explanation:

• Naphthalene: This compound has two fused aromatic rings, each of which contains 6 π electrons (2 π electrons from each double bond) and 14 π electrons. Therefore, both rings have 4n+2 π electrons (with n=3), which satisfies Hückel's rule.
• Anthracene: This compound consists of three linearly fused rings with 14 π electrons in total. The two outer rings each contain six π electrons which, like naphthalene, follow the 4n+2 rule (with n=3). The central ring, while forming part of the conjugated system, does not significantly contribute to the aromaticity. Thus, each of the rings in anthracene is separately aromatic.
• Phenanthrene: Similar to anthracene, phenanthrene has 14 π electrons. However, the arrangement is different. One way to consider the aromaticity of phenanthrene is to assign the 10 electrons from the outer rings to make them aromatic according to Hückel's rule (n=3), with the remaining 4 electrons as the central double bond. Thus, phenanthrene is considered aromatic.
• 

So, all three compounds (Naphthalene, Anthracene, and Phenanthrene) are aromatic according to Hückel's rule.

Thus, the correct answer is:

5) None of the above

Correct answer: 2)

Concept:

• Friedel-Craft reactions are mainly of two types, Friedel-Crafts alkylation reaction, and Friedel-Crafts acylation reaction.
• Both the alkylation and acylation reactions follow the electrophilic aromatic substitution mechanism.
• An alkyl group can be added to a benzene molecule by an electrophile aromatic substitution reaction called the Friedel‐Crafts alkylation reaction.
• One example is the addition of a methyl group to a benzene ring.
• The mechanism for this reaction begins with the generation of a methyl carbocation from methyl bromide.
• The Friedel–Crafts acylation is the reaction of an arene with acyl chlorides or anhydrides using a strong Lewis acid catalyst.
• This reaction proceeds via electrophilic aromatic substitution to form monoacetylated products.

Explanation:

• Benzene reacts with n-propyl chloride in the presence of anhydrous aluminum chloride to give predominantly isopropyl benzene also known as cumene.
• In this reaction, n propyl group rearranges to isopropyl group.
• 1o RX' are less reactive than 2^o and 3^o halides.
• High temperature is required for 1^o RX.

The mechanism of the reaction is as:

Conclusion:

Thus, Benzene reacts with n-propyl chloride in the presence of anhydrous AlCl3 to give predominantly Isopropyl benzene.

Correct answer: 4)

Concept:

• Cumene is oxidized with oxygen or air into cumene hydroperoxide in presence of a catalyst.
• This is decomposed by dilute sulphuric acid into phenol and acetone. 
• Phenol is a colourless crystalline, deliquescent solid.
• It attains pink colour on exposure to air and light. 

Explanation:

Formation of cumeme hydroperoxide:

Acidic hydrolysis of cumene hydroperoxide to form B and C.

Conclusion:

Thus, the correct option is 4

Concept:

The reaction involves hydrolysis or nucleophilic substitution in first step followed by oxidation and dehydration in last step. The most important fact is that, the Br group attached directly to aromatic ring will not undergo substitution.

Hydrolysis is defined as (hydro-, meaning 'water', and lysis, meaning 'to unbind') is any chemical reaction in which a molecule of water ruptures one or more chemical bonds.

The term is used broadly for substitution, elimination, and fragmentation reactions in which water is the nucleophile.

Nucleophilic substitutions a fundamental class of reactions in which an electron rich nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group the positive or partially positive atom is referred to as an electrophile.

The whole molecular entity of which the electrophile and the leaving group are part is usually called the substrate. The nucleophile essentially attempts to replace the leaving group as the primary substituent in the reaction itself, as a part of another molecule

From the given reaction the aryl halide remains unaffected and the directly attached Br group undergo substitution process then it react in the presence of chromium trioxide and hydrogen the nucleophilic substitution takes place.

CONCEPT:

Percentage Composition

• The percentage composition of a compound is the percentage by mass of each element in the compound.
• It can be calculated using the formula:

  Percentage of element = (Mass of element in the compound / Molar mass of the compound) × 100

EXPLANATION:

• To find the percentage of carbon in the product 'X' formed in the given reaction, we need to know the molecular formula of the product 'X'.
• Assuming 'X' is a compound containing carbon, we will denote its molecular formula as C_aH_bO_c (where a, b, and c are the number of carbon, hydrogen, and oxygen atoms respectively).
• Let's assume the molar mass of the product 'X' is M.
• The mass of carbon in one mole of 'X' is given by:
  • a × 12.01 g/mol (where 12.01 g/mol is the atomic mass of carbon)
• Therefore, the percentage of carbon in 'X' is:
  • (a × 12.01 / M) × 100

Given that the correct answer is option 3, we can conclude that the percentage of carbon in the product 'X' is 90.6%.

CONCEPT:

Effect of Nitro Group on Benzene Ring

• The presence of a nitro group (-NO₂) on a benzene ring has a significant effect on the reactivity of the ring.
• The nitro group is an electron-withdrawing group due to its strong electronegativity and resonance effects.

EXPLANATION:

• The nitro group withdraws electron density from the benzene ring through both inductive and resonance effects.
  • Inductive effect: The electronegative nitrogen atom pulls electron density away from the ring through the sigma bonds.
  • Resonance effect: The nitro group can participate in resonance, further delocalizing the electron density away from the ring.
• Due to the electron-withdrawing nature of the nitro group, the electron density on the benzene ring decreases.
• This decrease in electron density makes the benzene ring less reactive towards electrophilic substitution reactions because the ring becomes less nucleophilic and less able to donate electrons to electrophiles.

Therefore, the presence of a nitro group in a benzene ring deactivates the ring towards electrophilic substitution.

Concept:

Electrophilic Aromatic Substitution:

• Electrophilic aromatic substitution is a reaction where an atom attached to an aromatic system is replaced by an electrophile.
• Typical reactions include nitration, halogenation, sulfonation, Friedel-Crafts alkylation, and Friedel-Crafts acylation.

Effect of Electron-Withdrawing Groups (EWG):

• EWGs (e.g., -NO₂, -CN) decrease the electron density on the aromatic ring, making it less reactive towards electrophiles.
• EWGs typically direct incoming electrophiles to the meta position relative to themselves.

Effect of Electron-Donating Groups (EDG):

• EDGs (e.g., -OH, -NH₂) increase the electron density on the aromatic ring, making it more reactive towards electrophiles.
• EDGs typically direct incoming electrophiles to the ortho and para positions relative to themselves.

Explanation:

Step 1: Nitration: Bromobenzene reacts with concentrated nitric acid in the presence of a catalyst (usually sulfuric acid) to undergo electrophilic aromatic substitution. The nitro group (-NO₂) is an electrophile and substitutes one of the hydrogen atoms on the benzene ring, forming a nitrobenzene derivative. The bromine atom directs the nitro group to the 2 and 4 positions due to its ortho-para directing nature.

Step 2: Hydrolysis: The nitro groups on the phenol ring are electron-withdrawing groups, making the phenol more acidic. Sodium hydroxide acts as a base and deprotonates the phenol group, forming the corresponding phenoxide ion.

Step 3: Acidification: In final step, hydrochloric acid is added to the reaction mixture to protonate the phenoxide ion, regenerating the phenol group. This step is necessary to isolate the final product, 2,4-dinitrophenol.

Conclusion:

 A and B in the given reaction sequence is correctly represented by Option 1.

CONCEPT:

Conversion of Chlorobenzene to Phenol and Further Oxidation

• The reaction involves two main steps:
  • Step 1 (Conversion to Phenol):
    • Chlorobenzene reacts with NaOH at 623 K and 300 atm to form phenol via nucleophilic aromatic substitution.
    • The -Cl group is replaced by -OH under these severe conditions.

• • Step 2 (Oxidation to Para-Benzoquinone):
    • Phenol is oxidized using Na₂Cr₂O₇ and H₂SO₄, forming para-benzoquinone.
    • During oxidation, the hydroxyl group (-OH) is converted to a carbonyl group (=O) at para positions, resulting in para-benzoquinone.

EXPLANATION:

• Step-by-step Analysis:
  • Initial Compound: Chlorobenzene (C₆H₅Cl).
  • Product [A]: Phenol (C₆H₅OH) after reaction with NaOH.
  • Product [B]: Para-benzoquinone (C₆H₄O₂) after oxidation with Na₂Cr₂O₇/H₂SO₄.
• The correct set of products matches option (3), where [A] is phenol and [B] is para-benzoquinone.

Hence, the correct answer is: Option 3 (Phenol and Para-benzoquinone).

CONCEPT:

Ozonolysis of Alkenes and Alkynes

• Ozonolysis is an oxidative cleavage reaction where ozone (O₃) reacts with unsaturated hydrocarbons (alkenes or alkynes) to form carbonyl compounds (aldehydes, ketones, or carboxylic acids).
• The position of the double bonds in the starting material determines the nature and position of the resulting carbonyl compounds.
• For cyclic compounds, ozonolysis breaks the double bonds, opening the ring or cleaving substituents, and forms corresponding carbonyl compounds.

EXPLANATION:

• Matching each isomer of C₁₀H₁₄ with its ozonolysis product:
  • Compound A: Ozonolysis produces a linear dicarbonyl compound corresponding to structure (III).
  • 
  • Compound B: Produces a carbonyl compound with a four-carbon chain and ketone groups corresponding to structure (IV).
  • 
  • Compound C: Yields a symmetrical dicarbonyl compound corresponding to structure (I).
  • 
  • Compound D: Produces a compound corresponding to structure (II) with two distinct carbonyl groups.
  • 
• Correct Matching:

  A → (III), B → (IV), C → (I), D → (II)

Hence, the correct answer is: Option 2 (A-(III), B-(IV), C-(I), D-(II)).