Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs) and Explanations

Here are some MCQs to test your understanding, along with detailed explanations.

Question 1: The reaction of an alcohol with SOCl2​ in the presence of pyridine is known as: (A) Finkelstein reaction (B) Swarts reaction (C) Darzen’s process (D) Williamson synthesis

Explanation 1:

  • Correct Answer: (C)
  • Reasoning: The reaction of an alcohol with thionyl chloride (SOCl2​) in the presence of pyridine to form an alkyl chloride is called Darzen’s process. It’s an excellent method because the byproducts (SO2​ and HCl) are gases and escape, leaving a pure alkyl halide.

Question 2: Which of the following alkyl halides will undergo SN​1 reaction most readily? (A) CH3​Cl (B) CH3​CH2​Cl (C) (CH3​)2​CHCl (D) (CH3​)3​CCl

Explanation 2:

  • Correct Answer: (D)
  • Reasoning: SN​1 reactions proceed via carbocation intermediates. The stability of carbocations follows the order: 3∘>2∘>1∘>CH3+​. Therefore, tertiary alkyl halides react most readily via SN​1 mechanism. (CH3​)3​CCl is a tertiary alkyl chloride.

Question 3: What is the major product when 1-bromopropane reacts with alcoholic KOH? (A) Propan-1-ol (B) Propan-2-ol (C) Propene (D) Propane

Explanation 3:

  • Correct Answer: (C)
  • Reasoning: Alcoholic KOH causes dehydrohalogenation (β-elimination) of alkyl halides, leading to the formation of alkenes. 1-bromopropane will lose HBr to form propene. Aqueous KOH would lead to substitution (formation of alcohol).

Question 4: The product formed when ethyl bromide reacts with NaI in the presence of dry acetone is: (A) Ethyl iodide (B) Ethyl fluoride (C) Ethane (D) Butane

Explanation 4:

  • Correct Answer: (A)
  • Reasoning: This is the Finkelstein reaction, a halogen exchange reaction used to prepare alkyl iodides. NaI in acetone helps to precipitate NaCl/NaBr, driving the reaction forward.

Question 5: Haloarenes are less reactive towards nucleophilic substitution reactions because of: (A) Partial double bond character of C-X bond due to resonance. (B) High electronegativity of halogen. (C) Steric hindrance from the aromatic ring. (D) All of the above.

Explanation 5:

  • Correct Answer: (A)
  • Reasoning: The primary reason for the lower reactivity of haloarenes towards nucleophilic substitution is the partial double bond character of the C-X bond, making it stronger and harder to break. While high electronegativity contributes to polarity, it’s the resonance stabilization that significantly alters reactivity. Steric hindrance is less of a factor compared to the electronic effects.

Question 6: Which of the following compounds has the highest boiling point? (A) Chloromethane (B) Bromomethane (C) Iodomethane (D) Fluoromethane

Explanation 6:

  • Correct Answer: (C)
  • Reasoning: For a given alkyl group, the boiling point of haloalkanes increases with increasing molecular mass (and thus increasing van der Waals forces) of the halogen. So, Iodomethane (CH3​I) has the highest boiling point among the given options.

Question 7: The reaction R−X+MgDry Ether​RMgX is used to prepare: (A) Alkane (B) Alcohol (C) Grignard reagent (D) Ether

Explanation 7:

  • Correct Answer: (C)
  • Reasoning: Alkyl halides react with magnesium in dry ether to form alkylmagnesium halides, commonly known as Grignard reagents.

Question 8: The conversion of chlorobenzene to phenol requires drastic conditions (high temperature and pressure). This reaction is an example of: (A) Electrophilic substitution (B) Nucleophilic aromatic substitution (C) Free radical substitution (D) Elimination reaction

Explanation 8:

  • Correct Answer: (B)
  • Reasoning: The replacement of a halogen directly attached to an aromatic ring by a nucleophile (OH− in this case) is a nucleophilic aromatic substitution reaction. Due to the inherent low reactivity of haloarenes, these reactions require harsh conditions.

Question 9: Identify the primary alkyl halide from the following: (A) 2-chloropropane (B) 2-methyl-2-chloropropane (C) 1-chlorobutane (D) 1-chloro-1-methylbenzene

Explanation 9:

  • Correct Answer: (C)
  • Reasoning: A primary alkyl halide has the halogen attached to a primary carbon atom (a carbon atom bonded to only one other carbon atom). 1-chlorobutane (CH3​CH2​CH2​CH2​Cl) fits this definition. 2-chloropropane is secondary, 2-methyl-2-chloropropane is tertiary, and 1-chloro-1-methylbenzene is a benzylic halide (not strictly an alkyl halide in this context, or if it means to imply a halogen directly on the benzene ring, it’s an aryl halide).

Question 10: The byproduct HCl formed during the preparation of alkyl chlorides from alcohols using thionyl chloride is removed by: (A) Distillation (B) Filtration (C) Absorption in water (D) Pyridine

Explanation 10:

  • Correct Answer: (D)
  • Reasoning: Pyridine is added in Darzen’s process to neutralize the HCl gas formed, preventing it from causing side reactions or being absorbed by the product. This helps in obtaining a pure alkyl chloride. The SO2​ gas also escapes.

Question 11: Which rule governs the formation of the major product in the dehydrohalogenation of a secondary alkyl halide? (A) Markovnikov’s rule (B) Anti-Markovnikov’s rule (C) Saytzeff’s rule (D) Hund’s rule

Explanation 11:

  • Correct Answer: (C)
  • Reasoning: Saytzeff’s rule (also spelled Zaitsev’s rule) states that in dehydrohalogenation, the more substituted alkene (the one with fewer hydrogen atoms on the double bond carbons) is the major product. Markovnikov’s rule relates to addition reactions.

Question 12: Which type of solvent favors SN​2 reactions? (A) Polar protic solvents (B) Non-polar solvents (C) Polar aprotic solvents (D) Both A and C

Explanation 12:

  • Correct Answer: (C)
  • Reasoning: SN​2 reactions are favored by polar aprotic solvents (e.g., acetone, DMSO, DMF) because they solvate cations well but do not strongly solvate anions, leaving the nucleophile free and highly reactive. Polar protic solvents solvate nucleophiles, reducing their reactivity.

Question 13: The reaction of an alkyl halide with AgCN gives alkyl isocyanide, while with KCN, it gives alkyl cyanide. This is due to: (A) KCN is ionic, AgCN is covalent. (B) KCN is covalent, AgCN is ionic. (C) KCN is a stronger nucleophile than AgCN. (D) AgCN is a stronger nucleophile than KCN.

Explanation 13:

  • Correct Answer: (A)
  • Reasoning: KCN is predominantly ionic, so the CN− ion with its carbon end is free to attack (C-C bond is more stable). AgCN is largely covalent, so nitrogen is the attacking atom (lone pair on N) leading to isocyanide. This is an example of an ambident nucleophile.

Question 14: The order of reactivity of hydrogen halides towards alcohols for the formation of haloalkanes is: (A) HCl > HBr > HI (B) HI > HBr > HCl (C) HBr > HCl > HI (D) HI = HBr = HCl

Explanation 14:

  • Correct Answer: (B)
  • Reasoning: The reactivity of HX is determined by the strength of the H-X bond, which decreases down the group. HI has the weakest H-I bond, making it the most reactive, followed by HBr and then HCl.

Question 15: Which of the following is a vicinal dihalide? (A) 1,1-dichloropropane (B) 1,2-dichloropropane (C) 2,2-dichloropropane (D) Trichloromethane

Explanation 15:

  • Correct Answer: (B)
  • Reasoning: Vicinal dihalides have halogen atoms on adjacent carbon atoms. 1,2-dichloropropane (CH3​CH(Cl)CH2​Cl) fits this. 1,1-dichloropropane and 2,2-dichloropropane are geminal dihalides. Trichloromethane (chloroform) is a trihalomethane.

Question 16: The Wurtz-Fittig reaction is used to prepare: (A) Symmetrical alkanes (B) Unsymmetrical alkanes (C) Alkylbenzenes (D) Biphenyls

Explanation 16:

  • Correct Answer: (C)
  • Reasoning: The Wurtz-Fittig reaction involves the coupling of an alkyl halide and an aryl halide with sodium in dry ether to form an alkylbenzene. Wurtz reaction forms symmetrical alkanes, and Fittig reaction forms biphenyls.

Question 17: Chloroform (CHCl3​) is stored in dark brown bottles because: (A) It is sensitive to moisture. (B) It reacts with light and air to form poisonous phosgene. (C) It is highly volatile. (D) It prevents evaporation.

Explanation 17:

  • Correct Answer: (B)
  • Reasoning: Chloroform oxidizes in the presence of light and air to form a highly poisonous gas, phosgene (COCl2​). Dark brown bottles prevent light exposure, and being filled completely minimizes air contact.

Question 18: Which of the following is a Freon? (A) CH2​Cl2​ (B) CHCl3​ (C) CCl4​ (D) CCl2​F2​

Explanation 18:

  • Correct Answer: (D)
  • Reasoning: Freons are chlorofluorocarbons (CFCs), derivatives of methane or ethane containing both chlorine and fluorine. CCl2​F2​ (Freon-12) is a common example. The others are haloalkanes but not Freons (CFCs).

Question 19: What is the specific name given to the anti-Markovnikov addition of HBr to an alkene in the presence of peroxides? (A) Wurtz reaction (B) Sandmeyer reaction (C) Peroxide effect (D) Finkelstein reaction

Explanation 19:

  • Correct Answer: (C)
  • Reasoning: The anti-Markovnikov addition of HBr to an alkene in the presence of peroxides is specifically known as the Peroxide effect or Kharasch effect. This mechanism proceeds via a free radical pathway.

Question 20: The conversion of primary alcohol to alkyl chloride is best carried out by: (A) HBr (B) HCl (C) SOCl2​ (D) PCl3​

Explanation 20:

  • Correct Answer: (C)
  • Reasoning: Darzen’s process using SOCl2​ (thionyl chloride) is considered the best method for preparing alkyl chlorides because the gaseous byproducts (SO2​ and HCl) escape, leaving a pure alkyl chloride.

Question 21: The dipole moment of chlorobenzene is less than that of cyclohexyl chloride. The reason is: (A) Chlorobenzene is aromatic. (B) sp2 hybridized carbon in chlorobenzene. (C) Resonance in chlorobenzene. (D) Both (B) and (C).

Explanation 21:

  • Correct Answer: (D)
  • Reasoning: In chlorobenzene, the carbon atom attached to chlorine is sp2 hybridized (more electronegative, holds electron pair closer), and there is partial double bond character due to resonance. Both factors reduce the polarity of the C-Cl bond compared to cyclohexyl chloride (where carbon is sp3 hybridized and no resonance).

Question 22: Which of the following will react with NaI in acetone most readily? (A) CH3​CH2​Cl (B) CH3​CH2​Br (C) CH3​CH2​I (D) CH3​CH2​F

Explanation 22:

  • Correct Answer: (B)
  • Reasoning: The Finkelstein reaction is a halogen exchange reaction where an alkyl chloride or bromide is converted to an alkyl iodide. As the leaving group ability is I−>Br−>Cl−>F−, ethyl bromide will react most readily to form ethyl iodide.

Question 23: The reaction of benzene diazonium chloride with Cu2​Cl2​/HCl is known as: (A) Gattermann reaction (B) Fittig reaction (C) Sandmeyer reaction (D) Balz-Schiemann reaction

Explanation 23:

  • Correct Answer: (C)
  • Reasoning: The reaction of benzene diazonium chloride with Cu2​Cl2​/HCl or Cu2​Br2​/HBr to form chloro- or bromo-benzene is called the Sandmeyer reaction. Gattermann reaction uses Cu powder/HX.

Question 24: Which of the following statements about SN​1 reaction is true? (A) It proceeds via a one-step mechanism. (B) Rate depends on the concentration of both alkyl halide and nucleophile. (C) It leads to racemization if the reactant is chiral. (D) It is favored by polar aprotic solvents.

Explanation 24:

  • Correct Answer: (C)
  • Reasoning: SN​1 is a two-step process involving a planar carbocation intermediate, leading to racemization for chiral reactants. Its rate depends only on the alkyl halide concentration, and it is favored by polar protic solvents.

Question 25: DDT is a powerful: (A) Antibiotic (B) Insecticide (C) Fertilizer (D) Refrigerant

Explanation 25:

  • Correct Answer: (B)
  • Reasoning: DDT (Dichlorodiphenyltrichloroethane) is a well-known synthetic insecticide, though its use is now largely restricted due to environmental persistence.

Question 26: The product of Wurtz reaction of methyl iodide is: (A) Methane (B) Ethane (C) Propane (D) Butane

Explanation 26:

  • Correct Answer: (B)
  • Reasoning: Wurtz reaction involves coupling of two alkyl halides. 2CH3​I+2Na→CH3​−CH3​+2NaI. So, ethane is formed.

Question 27: When ethyl alcohol is treated with conc. H2​SO4​ and KI, the product obtained is: (A) Ethyl iodide (B) Ethene (C) Diethyl ether (D) Iodoform

Explanation 27:

  • Correct Answer: (A)
  • Reasoning: This is a method for preparing alkyl halides from alcohols using HX (HI in this case, generated in situ from KI and H2​SO4​). CH3​CH2​OH+HI→CH3​CH2​I+H2​O.

Question 28: Which of the following is an example of an allylic halide? (A) CH2​=CH−Cl (B) CH3​−CH2​−CH2​−Cl (C) C6​H5​−CH2​−Cl (D) CH2​=CH−CH2​−Br

Explanation 28:

  • Correct Answer: (D)
  • Reasoning: An allylic halide has the halogen attached to an sp3 hybridized carbon atom that is adjacent to a carbon-carbon double bond. CH2​=CH−CH2​−Br (3-bromopropene) fits this definition. (A) is vinylic, (B) is primary alkyl, (C) is benzylic.

Question 29: In the preparation of chlorobenzene from benzene, the catalyst used is: (A) Anhydrous AlCl3​ (B) Cu2​Cl2​ (C) Ni (D) H2​SO4​

Explanation 29:

  • Correct Answer: (A)
  • Reasoning: Direct halogenation of benzene (electrophilic substitution) requires a Lewis acid catalyst like anhydrous AlCl3​ or FeCl3​ to generate the electrophile (Cl+). Cu2​Cl2​ is used in Sandmeyer reaction.

Question 30: Which of the following will undergo SN​2 reaction fastest? (A) CH3​CH2​Cl (B) (CH3​)2​CHCl (C) CH3​Cl (D) (CH3​)3​CCl

Explanation 30:

  • Correct Answer: (C)
  • Reasoning: SN​2 reactions are most favored by least steric hindrance around the carbon bearing the halogen. The reactivity order is CH3​X>1∘>2∘>3∘. Therefore, chloromethane (CH3​Cl) will react fastest.

Question 31: What is the nature of the C-X bond in haloalkanes? (A) Non-polar covalent (B) Polar covalent (C) Ionic (D) Metallic

Explanation 31:

  • Correct Answer: (B)
  • Reasoning: Due to the difference in electronegativity between carbon and halogen, the C-X bond is a polar covalent bond, with the halogen bearing a partial negative charge and carbon a partial positive charge.

Question 32: The presence of which group at ortho and para positions to the halogen in haloarenes increases their reactivity towards nucleophilic substitution? (A) Electron donating group (EDG) (B) Electron withdrawing group (EWG) (C) Alkyl group (D) Hydroxyl group

Explanation 32:

  • Correct Answer: (B)
  • Reasoning: Electron-withdrawing groups (like −NO2​) at ortho and para positions stabilize the carbanion intermediate formed during nucleophilic aromatic substitution, thereby activating the ring towards these reactions.

Question 33: The reaction used for the preparation of alkyl fluorides is: (A) Finkelstein reaction (B) Swarts reaction (C) Sandmeyer reaction (D) Wurtz reaction

Explanation 33:

  • Correct Answer: (B)
  • Reasoning: Swarts reaction is a halogen exchange method specifically used for the synthesis of alkyl fluorides using metallic fluorides like AgF, Hg2​F2​, etc.

Question 34: Which reagent is used to convert benzene diazonium chloride to iodobenzene? (A) Cu2​Cl2​/HCl (B) Cu2​Br2​/HBr (C) KI (D) HBF4​

Explanation 34:

  • Correct Answer: (C)
  • Reasoning: For the preparation of iodobenzene from benzene diazonium chloride, potassium iodide (KI) solution is used without the need for a copper catalyst.

Question 35: In the Friedel-Crafts alkylation of chlorobenzene, the major product(s) would be: (A) m-chlorotoluene (B) o-chlorotoluene (C) p-chlorotoluene (D) Both (B) and (C) with (C) as major

Explanation 35:

  • Correct Answer: (D)
  • Reasoning: Halogens are ortho-para directing groups. While they are deactivating, they direct incoming electrophiles to ortho and para positions. Due to steric hindrance, the para-isomer is usually the major product. So, both o-chlorotoluene and p-chlorotoluene are formed, with p-chlorotoluene being major.

Question 36: Grignard reagents react with water to form: (A) Alcohols (B) Alkenes (C) Alkanes (D) Ethers

Explanation 36:

  • Correct Answer: (C)
  • Reasoning: Grignard reagents are highly basic and react readily with any source of proton (like water, alcohols, amines) to form alkanes. RMgX+H2​O→R-H+Mg(OH)X.

Question 37: What type of reaction involves the formation of a higher alkane by the coupling of two alkyl halides? (A) Fittig reaction (B) Wurtz reaction (C) Sandmeyer reaction (D) Elimination reaction

Explanation 37:

  • Correct Answer: (B)
  • Reasoning: The Wurtz reaction specifically involves the coupling of two alkyl halides with sodium metal in dry ether to form a symmetrical alkane.

Question 38: The density of haloalkanes increases in the order: (A) R-I < R-Br < R-Cl < R-F (B) R-F < R-Cl < R-Br < R-I (C) R-Cl < R-Br < R-I < R-F (D) R-I < R-F < R-Br < R-Cl

Explanation 38:

  • Correct Answer: (B)
  • Reasoning: For a given alkyl group, the density of haloalkanes increases with increasing atomic mass of the halogen. Iodine is the heaviest, so iodoalkanes are densest, followed by bromoalkanes, chloroalkanes, and fluoroalkanes.

Question 39: Which of the following is true for an SN​2 reaction? (A) Carbocation is an intermediate. (B) Racemization occurs. (C) Inversion of configuration occurs. (D) Favored by tertiary alkyl halides.

Explanation 39:

  • Correct Answer: (C)
  • Reasoning: SN​2 is a concerted single-step reaction. The nucleophile attacks from the back, leading to inversion of configuration (Walden inversion). It does not involve a carbocation and is disfavored by steric hindrance, thus reacting fastest with methyl and primary alkyl halides.

Question 40: Which compound is considered responsible for ozone layer depletion? (A) Chloroform (B) Iodoform (C) Carbon tetrachloride (D) Freons (CFCs)

Explanation 40:

  • Correct Answer: (D)
  • Reasoning: Chlorofluorocarbons (Freons) are known to be highly stable in the lower atmosphere but migrate to the stratosphere where they release chlorine radicals upon UV radiation, leading to significant depletion of the ozone layer.

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