Haloalkanes and Haloarenes Class 12 Chemistry Important Questions

Students can read the important questions given below for Haloalkanes and Haloarenes Class 12 Chemistry. All Haloalkanes and Haloarenes Class 12 Notes and questions with solutions have been prepared based on the latest syllabus and examination guidelines issued by CBSE, NCERT and KVS. You should read all notes provided by us and Class 12 Chemistry Important Questions provid ed for all chapters to get better marks in examinations. Chemistry Question Bank Class 12 is available on our website for free download in PDF.

Important Questions of Haloalkanes and Haloarenes Class 12

Very Short Answer Questions

Question. Write the structure of an isomer of compound C₄H₉Br which is most reactive towards S_N1 reaction.
Answer. 

Question. Which would undergo S_N2 reaction faster in the following pair and why?

Answer. CH₃ — CH₂ Br would undergo S_N2 reaction faster due to formating of less steric hindrance.

Question. What happens when bromine attacks
CH₂ = CH — CH₂ — C ≡ CH?
Answer. 

Question. Complete the following chemical equation :

Answer. 

Question. Write the structure of the compound 1-chloro-4-ethylcyclohexane.
Answer. 

Question. Write the IUPAC name of the following compound :

Answer. 

Question. Write the IUPAC name of the following compound :
CH₂=CHCH₂Br
Answer. 

Question. Which halogen compound in each of the following pairs will react faster in S_N2 reaction :
(i) CH₃Br or CH₃I
(ii) (CH₃)₃CCl or CH₃Cl
Answer. (i) CH₃I will give faster S_N2 reaction.
(ii) CH₃Cl will give faster S_N2 reaction.

Question. What happens when CH₃ Br is treated with KCN?
Answer. CH₃CN is formed by nucleophilic substitution reaction.
CH₃Br + KCN → CH₃CN + KBr

Question. 

Answer. The S_Nl reaction proceeds through carbocation formation thus, the compound which forms more stable carbocation will be more reactive.

As, 2° carbocation is more stable than 1° carbocation thus, 2-chlorobutane is more reactive towards S_Nl reaction

Question. Identify the chiral molecule in the following pair :

Answer. 

Question. Which compound in the following pair undergoes faster S_N1 reaction?

Answer. Tertiary halide

reacts faster than the secondary halide because of the greater stability of tert-carbocation.

Question. How may methyl bromide be preferentially converted to methyl isocyanide? 
Answer. KCN is predominantly ionic and provides cyanide ions in solution
CH₃Br + KCN              →         CH₃C ≡ N + KBr
Methyl Methyl                             cyanide
bromide
AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as main product.
CH₃Br + AgCN          →          CH₃N ≡ C + AgBr
Methyl Methyl                            isocyanide
bromide

Question. Write the structure of the following compound :
2-(2-chlorophenyl)-1-iodooctane 
Answer. 

Question. Write the structure of the following compound :
1-bromo-4-sec-butyl-2-methylbenzene
Answer. 

Question. Draw the structure of major monohalogen product in the following reaction :

Answer. 

Question. Write the structure of the following compound :
1, 4-dibromobut-2-ene
Answer. 

Question. Write the structure of the following compound :
2-(2-Bromophenyl)butane
Answer. 

Question. What happens when ethyl chloride is treated with aqueous KOH?
Answer. When ethyl chloride is treated with aqueous KOH, ethanol is formed,
CH₃CH₂Cl + KOH_(aq) → CH₃CH₂OH + KCl

Question. Why is (±)-butan-2-ol is optically inactive?
Answer. The (±)-Butan-2-ol is optically inactive because it exist in two enantiomeric forms which are non-superimposable mirror images of each other. Both the isomers are present in equal amounts therefore, it does not rotate the plane of polarized light and is optically inactive.

Question. Draw the structure of major monohalogen product formed in the following reaction :

Answer. 

Question. State the IUPAC name of the following compound :

Answer. 1-Bromobut-2-ene

Question. Write the IUPAC name of ClCH₂C ≡ CCH₂Br.
Answer. 

Question. How do you convert :
Propene to 1-iodopropane ?
Answer. 

Question. Give reason :
n-Butyl bromide has higher boiling point than t-butyl bromide.
Answer. n-Butyl bromide, being a straight chain molecule have strong intermolecular forces whereas t-butyl bromide being a branched chain molecule have weaker intermolecular forces due to smaller surface area. Hence, boiling point of n-butyl bromide is higher than that of t-butyl bromide.

Question. Why are alkyl halides insoluble in water?
Answer. Alkyl halides are polar but are insoluble in water because energy required to break the intermolecular H – bond among water molecules is much higher than energy released by water halide interaction.

Question. Which would undergo S_N1 reaction faster in the following pair :

Answer. 

Question. Which would undergo S_N2 reaction faster in the following pair and why?
CH₃ CH₂ — Br and CH₃ — CH₂ — I
Answer. Since I^– is a better leaving group than Br–, thus, CH₃CH₂I undergoes S_N2 reaction faster than CH₃CH₂Br.

Short Answer Questions

Question. Write the mechanism of the following reaction :

Answer. 

Question. Complete the following reaction equation :
(i) C₆H₅N₂Cl + KI →

Answer. 

Question. (i) Which alkyl halide from the following pair is chiral and undergoes faster S_N2 reaction?

(ii) Out of S_N1 and S_N2, which reaction occurs with
(a) inversion of configuration
(b) racemisation?
Answer. 

Question. Suggest a possible reason for the following observations :
(i) The order of reactivity of haloalkanes is RI > RCl > RBr.
(ii) Neopentyl chloride (CH₃)₃CCH₂Cl does not follow S_N2 mechanism.
Answer. (i) Among the various halides with same alkyl group the order of reactivity is RI > RBr > RCl. Due to increasing bond strength of C I, C Br and C Cl the reactivity decreases.
(ii) Neopentyl chloride being a primary halide reacts slowly through S_N1 and the carbon carrying halogen is sterically more hindered. Hence it does not follow S_N2 mechanism.

Question. A solution of KOH hydrolyses
CH₃CHClCH₂CH₃ and CH₃CH₂CH₂CH₂Cl.
Which one of these is more easily hydrolysed?
Answer. 

Question. Discuss the mechanism of SN1 reaction of haloalkanes.
Answer. In S_N1 mechanism of substitution reaction, the rate of reaction depends upon the concentration of only one reactant. It is two steps reactants.

Question. Give the IUPAC names of the following compounds :

Answer. (i) 2-Bromobutane
(ii) 1,3-Dibromobenzene
(iii) 3-Chloropropene

Question. Write the mechanism of the following reaction :

Answer. Normal butyl bromide will give S_N2 reaction :

Question. Which compound in the following couple will react faster in S_N2 displacement and why?
(i) 1-Bromopentane or 2-bromopentane
(ii) 1-Bromo-2-methylbutane or 2-bromo-2-methylbutane.
Answer. (i) 1-Bromopentane, as it is a primary alkyl halide.
(ii) 1-Bromo-2-methylbutane, as it is a primary alkyl halide.

Question. (i) Which alkyl halide from the following pairs would you expect to react more rapidly by an S_N2 mechanism and why?

(ii) Racemisation occurs in S_N1 reactions. Why?
Answer. (i) 1-Bromobutane is 1° alkyl halide while 2-bromobutane is 2° alkyl halide. Due to steric hindrance in 2° alkyl halides, 1° alkyl halide will react faster than 2° alkyl halide in S_N2 reaction.
(ii) Carbocations are formed in S_N1 reaction which are planar species, thus, racemisation occurs.

Question. Write chemical equations when
(i) methyl chloride is treated with AgNO₂.
(ii) bromobenzene is treated with CH₃Cl in the presence of anhydrous AlCl₃.
Answer. 

Question. Give reasons :
(i) C — Cl bond length in chlorobenzene is shorter than C — Cl bond length in CH₃ — Cl.
(ii) S_N1 reactions are accompanied by racemization in optically active alkyl halides.
Answer. (i) In halobenzene C — X bond has partial double bond character due to resonance while CH3 — X bond is single bond.
Thus bond length of C — X bond in halobenzene is smaller than that in CH3 — X.

(iii) In S_N1 reaction carbocation intermediate is formed which is a planar molecule so,an incoming nucleophile can attack from either side and a equilmolar mixture of two components are formed and resulting mixture is optically inactive.

Question. How do you convert?
(i) Chlorobenzene to biphenyl
(ii) 2-bromobutane to but-2-ene
Answer. 

Long Answer Questions

Question. Differentiate between S_N1 and S_N2 mechanisms and give examples.
Answer. 

Question. Explain the following reactions with an example :
Friedel-Crafts reaction.
Answer. Haloarenes can undergo both freidal craft alkylation (with alkyl halide) or freidal craft acylation (with acid halide) in presence of Lewis acid catalyst to give a mixture of o- and p-haloalkyl benzene or o- and p-haloacylbenzene.

Question. How are the following conversions carried out?
(i) Benzyl chloride to benzyl alcohol,
(ii) Methyl magnesium bromide to methylpropan-2-ol.
Answer.