CBSE Class 12 Chemistry Halo Alkanes and Halo arenes Questions

SHORT ANSWER TYPE QUESTIONS

Question. Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanides as the chief product. Explain.
Answer. KCN is predominantly ionic and provides cyanide ions in solution. Both carbon and nitrogen atoms are in a position to donate electron pairs. The attack takes place mainly through carbon atom since C—C bond is more stable than C—N bond.
However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanides as the main product.

Question. Grignard reagents should be prepared under anhydrous conditions. Why?
Answer. Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons.
RMgX + H₂O → RH + Mg(OH)X It is therefore necessary to avoid even traces of moisture from a Grignard reagent

Question. What are chiral and achiral molecules
Answer. The objects which are non- superimposable on their mirror image are said to be chiral and this property is known as chirality.
The objects, which are, superimposable on their mirror images are called achiral

Question. What do you mean by racemic mixture and racemization?
Answer. A mixture containing two enantiomers in equal proportions is known as racemic mixture.
It will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer.
The process of conversion of enantiomers into a racemic mixture is known as racemisation

Question. Cl is an electron withdrawing group but it is ortho, para- directing in electrophilic aromatic substitution reactions. Why?
Answer. • Chlorine withdraws electrons through inductive effect and releases electrons through resonance.
• Through inductive effect, chlorine destabilizes the intermediate carbocation and through resonance, halogen tends to stabilize the carbocation and the effect is more pronounced at ortho- and para- positions.
• The resonance effect tends to oppose the inductive effect for the attack at ortho- and para- positions and hence makes the deactivation less for ortho- and para- attack.
• Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect.

CASE BASED QUESTIONS

Read the following passage and answer the questions given below.

S_N¹ reactions are generally carried out in presence of protic solvents. Like acetic acid. The reaction between a tertiary alkyl halide and nucleophile follows the first order kinetics. The rate of reaction depends on the concentration of alkyl halide .it occurs in two steps. In first step step C-X bond undergoes cleavage to form carbocation, it is then attacked by nucleophile in second step which is very fast. The energy needed for breaking C-X bond is obtained through solvation of halide ion with proton of protic solvent. Greater the stability of carbocation, Greater will be its ease of formation and faster will be its rate of reaction. In the case of optically active alkyl halide S_N¹ reaction is accompanied by Racemisation. The carbocation formed in step one is sp² hybridised. Which is triagonal planar. The attack of nucleophile may be accomplished from either side to give a mixture of products.

Question. Assertion: S_N² reaction takes place in presence of protic solvent.
Reason: The energy is released by the solvation of halide ion with proton.
Answer. D

Question. Assertion. The rate of SN1 reaction depends only on the concentration of alkyl halide.
Reason: Formation of carbocation is slow step and slow step determines the rate of reaction.
Answer. A

Question. Assertion: Tertiary alkyl halides reacts faster in SN1 reaction
Reason. Tertiary carbocations are more stable and formed at faster rate.
Answer. A

Question. Assertion: During S_N¹ reaction of an optically active alkyl halide Racemisation takes place
Reason. Racemic mixture consists of equal amount of dextro and leavo isomer.
Answer. B

Question. Assertion: Attack of nucleophile to carbon of carbocation takes place through either side forming two isomers with opposite configuration.
Reason: Carbocation is sp³ hybridised and have triogonal planar geometry
Answer. C

Question. Assertion: - In S_N¹ reaction second step is very fast and reversible.
Reason: - The carbocation formed is highly unstable and highly reactive.
Answer. D

OR
Read the passage given below and answer the following questions: Nucleophilic substitution reactions are of two types; substitution nucleophilic bimolecular (S_N²) and substitution nucleophilic unimolecular (S_N¹) depending on molecules taking part in determining the rate of reaction. Reactivity of alkyl halide towards S_N¹ and S_N² reactions depends on various factors such as steric hindrance, stability of intermediate or transition state and polarity of solvent. S_N² reaction mechanism is favoured mostly by primary alkyl halide then secondary and then tertiary. This order is reversed in case of S_N¹ reactions.
The following questions are multiple choice questions. Choose the most appropriate Answer.

Question. Which of the following is most reactive towards nucleophilic substitution reaction?
(a) C₆H₅Cl
(b) CH₂=CHCl
(c) ClCH₂CH=CH₂
(d) CH₃CH=CHCl
Answer. C

Question. Isopropyl chloride undergoes hydrolysis by
(a) S_N¹ mechanism
(b) S_N²mechanism
(c) S_N¹ and SN2mechanism
(d) neither S_N¹ and S_N² mechanism
Answer. C

Question. The most reactive nucleophile among the following is
(a) CH₃O-
(b) C₆H₅O-
(c) (CH₃)₂CHO-
(d) (CH3)₃CO-
Answer. A

Question. Tertiary alkyl halides are practically inert to substitution by SN2mechanism because of
(a) insolubility
(b) instability
(c) inductive effect
(d) stearic hindrance.
Answer. D

Question. Account for the following :
(a) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(b) Alkyl halides, though polar, are immiscible with water.
(c) Grignard’s reagents should be prepared under anhydrous conditions.
Answer. (a) Electron pairs of Cl atom are in conjugation with n electrons of the benzene ring so C-Cl bond in chlorobenzene acquires some double bond character while C-Cl bond in cyclohexyl chloride is a pure single bond. C – Cl bond in chlorobenzene is shorter than in cyclohexyl chloride. Since dipole moment is a product of charge and distance, so chlorobenzene has lower dipole moment than cyclohexyl chloride.
(b) Alkyl halides are polar molecules, therefore, their molecules are held together by dipole-dipole attraction. The molecules of H₂O are held together by H-bonds. Since the new forces of attraction between water and alkyl halide molecules are weaker than the forces of attraction already existing between alkyl halide- alkyl halide molecules and water- water molecules, therefore, alkyl halides are immiscible with water.
(c) Grignard’s reagents are very reactive. They react with alcohol, water, amines etc. to form corresponding hydrocarbon. R-MgX + HOH → RH + Mg(OH)X Therefore, Grignard’s reagents must be prepared under anhydrous conditions

Question. Give reasons : (a) n-Butyl bromide has higher boiling point than f-butyl bromide
(b) Racemic mixture is optically inactive.
(c) The presence of nitro group (-NO₂) at o/p positions increases the reactivity of haloarenes towards nucleophilic substitution reactions.
Answer.
(a) n-Butyl bromide has higher boiling point than f-butyl bromide because it has larger surface area hence have more Van der Waals’ forces.
(b) Rotation due to one enantiomer is cancelled by another enantiomer.
(c) The presence of nitro group (-NO₂) at ortho and para positions withdraws the electron density’ from benzene ring and thus facilitating the attack of nucleophile.

Question. a) Account for the following : (i) Electrophilic substitution reactions in haloarenes occur slowly.
(ii) Haloalkanes, though polar, are insoluble in water.
(b) Arrange the following compounds in increasing order of reactivity towards S_N² displacement: 2-Bromo-2-Methylbutane, 1-Bromopentane, 2-Bromopentane 
Answer. (a) (i) Due to -I effect of halogen atom, it withdraws electrons from the benzene ring and thus ring gets deactivated.
(ii) They fail to form hydrogen bonds with water. More energy is required to break hydrogen bonds in water and less energy is released when new attractions are set up.
(b) 2-Bromo-2-Methylbutane < 2-Bromopentane < 1-Bromopentane

(ix) (CCl₃)₃ CCl
2. Write the structure of following halogen compounds
(i) 2-chloro-3-methylpentane
(ii) 2-(2-chlorophenyl)-1-iodooctane
(iii) 1-bromo-4-sec-butyl-2–methylebenzene.
(iv) p-bromotoluene.
(v) chlorophenylmethane

3. Arrange the following in the increasing order of properly indicated :
(i) bromomethane, chloromethane, dichloromethane. (Increasing order of boiling points).
(ii) 1-chloropropane, isopropyl chloride, 1-chlorobutane (Increasing order of boiling point)
(iii) dichloromethane, chloroform, carbon terachloride. (Increasing order of dipole moment.
(iv) CH₃F, CH₃Cl, CH₃Br, CH₃l (Increasing reactivity towards nucleophilic substitution and increasing order of dipole moment)
(v) o,m.p-dichlorobenzenes (Increasing order of melting points).

4. Complete the following reactions

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