Worksheets Chapter 10 Haloalkanes and Haloarenes Class 12 Chemistry

Students should refer to Worksheets Class 12 Chemistry Haloalkanes and Haloarenes Chapter 10 provided below with important questions and answers. These important questions with solutions for Chapter 10 Haloalkanes and Haloarenes have been prepared by expert teachers for Class 12 Chemistry based on the expected pattern of questions in the Class 12 exams. We have provided Worksheets for Class 12 Chemistry for all chapters on our website. You should carefully learn all the important examinations questions provided below as they will help you to get better marks in your class tests and exams.

Haloalkanes and Haloarenes Worksheets Class 12 Chemistry

Multiple Choice Questions (MCQs)

Question. Fitting reaction can be used to prepare
(a) Toluene
(b) Acetophenon
(c) Diphenyl
(d) Chlorobenzene

Question. Chloropicrin is formed by the reaction of
(a) steam on carbon tetrachloride.
(b) nitric acid on chlorobenzene.
(c) chlorine on picric acid.
(d) nitric acid on chloroform.

Question. S_N1 reaction of alkyl halides leads to
(a) Retention of configuration
(b) Racemisation
(c) Inversion of configuration
(d) None of the above

Question. Identify the end product (C) in the following sequence:

(a) C₂H₅CH₂NH₂
(b) C₂H₅CONH₂
(c) C₂H₅COOH
(d) C₂H₅NH₂ + HCOOH 

Question.

In the above reaction, the product D is
(a) Propane
(b) 2, 3-Dimethylbutane
(c) Hexane
(d) Allyl bromide

Question. Identify Z in the series:

(a) C₂H5I
(b) C₂H₅OH
(c) CHI₃
(d) CH₃CHO

Question. Identify X and Y in the following sequence
(a) X = KCN, Y = LiAlH₄
(b) X = KCN, Y = H₃O+
(c) X = CH₃Cl, Y = AlCl₃ HCl
(d) X = CH₃NH₂, Y = HNO₂

Question. p-dichlorobenzene has higher melting point than its o- and m- isomers because
(a) p-dichlorobenzene is more polar than o- and m- isomer.
(b) p-isomer has a symmetrical crystalline structure.
(c) boiling point of p-isomer is more than o- and m-isomer.
(d) All of these are correct reasons.

Question. In the following sequence of reactions:

(a) n-propylamine
(b) isopropylamine
(c) ethylamine
(d) ethylmethylamine 

Question.

Assertion-Reason Questions

DIRECTION: Mark the option which is most suitable:

(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.

Question. Assertion: Nucleophilic substitution reaction on an optically active alkyl halide gives a mixture of enantiomers.
Reason: The reaction occurs by S_N2 mechanism.

Question. Assertion: n-Butyl bromide has higher boiling point than isobutyl bromide.
Reason: The branching of the chain makes the molecule compact and therefore decreases the surface area. 

Question. Assertion: Tertiary haloakanes are more reactive than primary haloakanes towards elimination reactions.
Reason: The +I-effect of the alkyl groups weakens the C—X bond.

Question. Assertion: Vinyl chloride is less reactive than alkyl chloride.
Reason: Stability of alkyl halide decreases as the strength of C-X bond decreases. 

Question. Assertion: Boiling point of alkyl halides increases with increase in molecular weight.
Reason: Boiling point of alkyl halides is in the order Rl > RBr > RCl > RF. 

Question. Assertion: Boiling point of RCI is greater than RF.
Reason: R-CI is more stable than R-F. 

Question. Assertion: Lower members of alkyl halides are colourless gases.
Reason: Alkyl iodides in general turn black on exposure to air and light.

Question. Assertion: p-Dichlorchlorobenzene is less soluble in organic solvents than the corresponding o-isomer.
Reason: o-Dichlorobenzene is polar while p-dichlorobenzene is not.

Question. Assertion: p-Dichlorobenzene has higher melting point than o-dichlorobenzene.
Reason: Stronger the van der Waals’ forces of attraction, higher is the melting point. 

Question. Assertion: Benzyl chloride is more reactive than p-chlorotoluene towards aqueous NaOH.
Reason: The C—Cl bond in benzyl chloride is more polar than C—Cl bond in p-chlorotoluene. 

Case Based Questions

1. Nucleophilic substitution reactions are of two types; substitution nucleophilic bimolecular (S_N1) depending on molecules taking part in determining the rate of reaction. Reactivity of alkyl halide towards SN1 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 followed by secondary and tertiary. This order is reversed in case of S_N1 reactions. Answer the following questions by choosing the most appropriate options:

Question. Isopropyl chloride undergoes hydrolysis by
(a) S_N1 mechanism
(b) S_N2 mechanism
(c) S_N1 and S_N2 mechanism
(d) neither S_N1 nor S_N2 mechanism 

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

Question. The most reactive nucleophile among the following is
(a) CH₃O^–
(b) C₆H₅O^–
(c) (CH₃)₂CHO^–
(d) (CH₃)₂CO^–  

Question. Which of the following is the correct order of decreasing SN2 reactivity?
(a) RCH₂X > R₂CHX > R₃CX
(b) R₃CX > R₂CHX > RCH₂X
(c) R₂CHX > R₂CX > RCH₂X
(d) RCH₂X > R₃CX > RCHX

Question. Tertiary alkyl halides are practically inert to substitution by SN2 mechanism because of
(a) insolubility
(b) instability
(c) inductive effect
(d) steric hindrance

2. A chlorocompound (A) on reduction with Zn- Cu ethanol gives the hydrocarbon (B) with five carbon atoms. When (A) is dissolved in dry ether and treated with sodium metal it gave 2,2,5,5-tetramethylhexane. The treatment of (A) with alcoholic KCN gives compound (C).

Answer the following questions by choosing the most appropriate options:

Question. The reaction of (C) with Na, C₂H₅OH gives
(a) (CH₃)₃C CH₂CONH₂
(b) (CH₃)₃C NH₂
(c) (CH₃)₃C CH₂ CH₂NH₂
(d) (CH₃)₂CHCH₂NH₂  

Question. The compound (A) is
(a) 1-chloro-2,2-dimethylpropane
(b) 1-chloro-2,2-dimethyl butane
(c) 1-chloro-2 methyl butane
(d) 2-chloro-2-methyl butane.  

Question. The reaction of (C) with Na, C₂H₅OH is called
(a) Gilman reaction
(b) Mendius reaction
(c) Grooves process
(d) Swart’s reaction  

Question. Compound (B) is
(a) n-pentane
(b) 2, 2-dimethylpropane
(c) 2-methylbutane
(d) none of above

Question. The reaction of (A) with aq. KOH will preferably favour
(a) S_N1 mechanism
(b) S_N2 mechanism
(c) E₁ mechanism
(d) E₂ mechanism Ans

3. When haloalkanes with b-hydrogen atom are boiled with alocholic solution of KOH, they undergo elimination of hydrogen halide resulting in the formation of alkenes. These reactions are called b-elimination reactions or dehydro halo genation reactions. These reactions follow Saytzeff’s rule. Substitution and elimination reactions often compete with each other. Mostly bases behave as nucleophiles and therefore can engage in substitution or elimination reactions depending upon the alkyl halide and the reaction conditions.

Answer the following questions by choosing the most appropriate options:
                                            aq. OH
Question. The general reaction, R—X     →    ROH + X–, is expected to follow decreasing order of reactivity as in
(a) t-BuI > t-BuBr > t-BuCl > t-BuF
(b) t-BuF > t-BuCl > t-BuBr > t-BuI
(c) t-BuBr > t-BuCl > t-BuI > t-BuF
(d) t-BuF > t-BuCl > t-BuI > t-BuBr 

Question. The ease of dehydrohalogenation of alkyl halide with alcoholic KOH is
(a) 3º < 2º < 1º
(b) 3º > 2º > 1º
(c) 3º < 2º > 1º
(d) 3º > 2º < 1º 

Question. Reaction of t-butyl bromide with sodium methoxide produces
(a) sodium t-butoxide
(b) t-butyl methyl ether
(c) iso-butane
(d) iso-butylene

Question.Among the following the most reactive towards alcoholic KOH is
(a) CH₂ = CHBr
(b) CH₃COCH₂CH₂Br
(c) CH₃CH₂Br
(d) CH₃CH₂CH₂Br 

Question. In the elimination reactions the reactivity of alkyl halides follows the sequence
(a) R – F > R – Cl > R – Br > R – I
(b) R – I > R – Br > R – Cl > R – F
(c) R – I > R – F > R – Br > R – Cl
(d) R – F > R – I > R – Br > R – Cl 

4. Consider the given sequence of reactions:

Question. When X is treated Ni-Al/NaOH the product obtained is
(a) benzene
(b) phenol
(c) p-chlorophenol
(d) triphenyl 

Question. Compound Y is       

Question. When X reacts with CH₃COCl in the presence of anhy. AlCl₃, the reaction is known as
(a) Fitting reaction
(b) Ullmann reaction
(c) Wurtz-Fittig reaction
(d) Friedel-Crafts acylation reaction. 

Question. Identify W. 

Question. Compound Z is
(a) phenol
(b) p-chlorophenol
(c) p-nitrophenol
(d) nitrobenzene  

5. Haloarenes are less reactive than haloalkanes.The low reactivity of haloarenes can be attributed to
— resonance effect
— sp² hybridisation of C—X bond
— polarity of C—X bond
— instability of phenyl cation (formed by self-ionisation of haloarene)
— repulsion between the electron rich attacking nucleophiles and electron rich arenes.
Reactivity of haloarenes can be increased or decreased by the presence of certain groups at certain positions. For example, nitro (–NO₂) group at o/p positions increases the reactivity of haloarenes towards nucleophilic substitution reactions.

Answer the following questions by choosing the most appropriate options:

Question. Which of the following aryl halides is the most reactive towards nucleophilic substitution? 

Question. Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to
(a) the formation of less stable carbonium ion
(b) resonance stabilisation
(c) larger carbon-halogen bond
(d) inductive effect

Question. The reactivity of the compounds (i) MeBr,
(ii) PhCH₂Br, (iii) MeCl, (iv) p-MeOC₆H₄Br decreases as
(a) (i) > (ii) > (iii) > (iv)
(b) (iv) > (ii) > (i) > (iii)
(c) (iv) > (iii) > (i) > (ii)
(d) (ii) > (i) > (iii) > (iv) 

Question. Which one of the following will react fastest with aqueous NaOH? 

Question. Which chloro derivative of benzene among the following would undergo most readily with aqueous sodium hydroxide to furnish the corresponding hydroxy derivative?

6. Nucleophilic substitution reaction of haloalkane can be conducted according to both S_N1 and S_N2 mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent.
Influences of halogen: No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leaves the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom; that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I⁻ < Br⁻ < Cl⁻ < F⁻ and the order of their leaving tendency should be I⁻ > Br⁻ > Cl⁻ > F⁻ . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI> RBr > RCl > RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_N1 mechanism. If the leaving group is not easy to leave, the reaction is based on S_N2 mechanism.
Influences of solvent polarity: In S_N1 reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_N2 reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate ( S_N1) of tertiary chlorobutane in 25ºC water (dielectric constant 79) is 3,00,000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_N2) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both SN1 and S_N2 reactions, but with different results. Generally speaking, weak polar solvent is favourable for S_N2 reaction, while strong polar solvent is favourable for S_N1 reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favourable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_N1 mechanism in solvents with a strong polarity (for example, ethanol containing water).

(Source: Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.)

Answer the following questions by choosing the most appropriate options:

Question. Nucleophilic substitution will be fastest in case of:
(a) 1-Chloro-2,2-dimethyl propane
(b) 1-Iodo-2,2-dimethyl propane
(c) 1-Bromo-2,2-dimethyl propane
(d) 1-Fluoro-2,2-dimethyl propane 

Question. S_N1 mechanism is favoured in which of the following solvents:
(a) benzene
(b) carbon tetrachloride
(c) acetic acid
(d) carbon disulphide

Question. S_N1 reaction will be fastest in which of the following solvents?
(a) Acetone (dielectric constant 21)
(b) Ethanol (dielectric constant 24)
(c) Methanol (dielectric constant 32)
(d) Chloroform (dielectric constant 5) 

Question. S_N1 reaction will be fastest in case of:
(a) 1-Chloro-2-methyl propane
(b) 1-Iodo-2-methyl propane
(c) 1-Chlorobutane
(d) 1-Iodobutane 

Question. Polar solvents make the reaction faster as they:
(a) destabilize transition state and decrease the activation energy
(b) destabilize transition state and increase the activation energy
(c) stabilize transition state and increase the activation energy
(d) stabilize transition state and decrease the activation energy 

Nomenclature of Organic Compound

Question. Write the IUPAC name of 

Answer. IUPAC name : 4-chloropent-1-ene

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

IUPAC name : l-Bromo-prop-2-ene

Question. Draw the structure of 2-bromopentane. 
Answer.

Question. Write the IUPAC name of 

Answer. IUPAC name : 4-bromo-4-methylpent-2-ene

Question. Write the IUPAC name of the following compound : 

Answer. IUPAC name : 2, 5-dichlorotoluene

Question. Give the IUPAC name of the following compound. 

Answer. IUPAC name : 3-bromo-2-methyl propene

Question. Give the IUPAC name of the following compound : 

Answer.

Question. Write the IUPAC name of the following compound : (CH₃)₃ CCH₂Br 

Answer. IUPAC name : 1-bromo-2, 2-dimethyl propane

Question. Write the structure of 2, 4-dinitrochlorobenzene. 
Answer.

Question. Write the IUPAC name of the following compound: 

Answer. IUPAC name : 2-Chloro-3, 3-dimethylbutane

Very Short Answer Type Questions

Question. Write the IUPAC name of (CH₃)₂CH.CH(Cl)CH₃. 
Answer. IUPAC name : 2-chloro-3-methylbutane

Question. Give the IUPAC name of the following compound :

Answer.

Question.

which is an example of vinylic halide?
Answer.

Question.

which is an example of a benzylic halide?
Answer.

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

Question. Arrange the following in increasing order of boiling point :
(i) CH₃CH₂CH₂CH₂Br (ii) (CH₃)₃C.Br
(iii) (CH₃)₂.CH.CH₂.Br 
Answer. (CH₃)₃C.Br <(CH₃)₂.CHCH₂.Br < CH₃CH₂CH₂CH₂Br

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

Answer.

Question. What happens when CH₃ — Br is treated with KCN?
Answer.

Question. Predict the order of reactivity of four isomeric bromobutanes in S_N1 reaction.
Answer.

Question. Give a chemical test to distinguish between C₂H₅Br and C6H5Br. 
Answer. Both are heated with aqueous NaOH. C₂H₅Br gives ethanol and NaBr, which on reacting with AgNO₃, gives yellow precipitate of AgBr. C₆H₅Br does not respond to this test.

Question.

which is more reactive towards S_N1 reaction and why?
Answer.

Question. Write the IUPAC name of the following :

Answer.

Question. Identify the chiral molecule in the following pair : 

Answer.

Question. Write the IUPAC name of

Answer. IUPAC name : 4-chloropent-1-ene

Question. Write the IUPAC name of

Answer. IUPAC name : 4-bromo-4-methylpent-2-ene

Question. In the following pair of compounds, which will react faster by S_N1 mechanism and why?

Answer.

Question. What happens when ethyl chloride is treated with aqueous KOH? 
Answer.

Question. Write the structure of 2,4-dinitrochlorobenzene.
Answer.

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

Answer.

Question. Give the IUPAC name of the following compound.

Answer.IUPAC name : 3-bromo-2-methyl propene

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

Question. In the following pair of halogen compounds, which compound will react faster by S_N1 mechanism?

Answer.

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

Question. A hydrocarbon C₅H₁₂ gives only one mono-chlorination product. Identify the hydrocarbon. 
Answer. Since there is only one monochloroderivative, the compound contains 12 equivalent hydrogen in four equivalent CH₃. The compound is 2, 2–dimethyl propane

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

Answer.

Question. Which would undergo SN₂ reaction faster in the following pair and why?

Answer. CH₃CH₂Br reacts faster because it is a primary halide (1° halide).

Question. Write the IUPAC name of the following compound :

Answer. IUPAC name : 1-Bromo-2-methylprop-2-ene.

Question. Write the IUPAC name of the following :

Answer.IUPAC name : 3 Bromo-2-methyl propene

Question. Which would undergo SN1 reaction faster in the following pair :

Answer.

because the secondary carbocation formed is more stable than primary carbocation.

Question. Write the IUPAC name of the following compound :
(CH₃)₃ CCH₂Br
Answer.

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

(2-Bromo-2-methyl propane) or tert-butyl bromide is most reactive towards SN1 reaction as it can form 3° carbocation.

Question. Draw the structure of 2-bromopentane. 
Answer.

Question. Write the structure of 1-Bromo-4-chlorobut-2- ene.
Answer.

Question. Write the IUPAC name of the following compound :

Answer. IUPAC name : 2, 5-dichlorotoluene

Question.

which is an example of allylic halide?
Answer.

Question. Write the IUPAC name of the following compound:

Answer. IUPAC name : 2-Chloro-3, 3-dimethylbutane.

Question. Predict the order of reactivity of the following compounds in S_N1 reaction :
Answer.

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

Answer.

Question. Write the IUPAC name of the following compound :

Answer. IUPAC name : 2-Bromo-4-chloropentane

Short Answer Type Questions-I 

Question. Explain why :
(a) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(b) Alkyl halides, though polar, are immiscible with water. 
Answer. (a) Chlorobenzene has lower dipole moment than cyclohexyl chloride due to lower magnitude of –ve charge on the Cl atom and shorter C – Cl distance. Due to greater S-character, a sp²-hybrid carbon is more electronegative than a sp³-hybrid carbon. Therefore, the sp2-hybrid carbon of C – Cl bond in chlorobenzene has less tendency to release electrons to Cl than a sp³ hybrid carbon of cyclohexyl chloride.

(b) Alkyl halides and polar molecules are held together by dipole-dipole interaction. 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 (not soluble) with water.

Question. How will you carry out the following conversions :
(i) 2-Bromopropane to 1-bromopropane
(ii) Benzene to p-chloronitrobenzene
Answer.

Question. Complete the following reaction equations :

Answer.

Question. Haloalkanes undergo nucleophilic substitution whereas haloarenes undergo electrophilic substitution. Explain. 
Answer. Haloarenes (say chlorobenzene) is a resonance hybrid of the following five structures :

Resonance leads to lowering of energy and hence greater stability. On the other hand, no such resonance is possible in haloalkanes. Halogens directly attached to benzene ring are o, p-directing in electrophilic subsitution reactions. This is due to greater electron density at these positions in resonance.

Question. Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. Give two reasons for the same.
Answer. The reasons are:
(i) Due to resonance/diagrammatic repre-sentation, C – Cl bond acquires a partial double bond character. As a result, the C – Cl bond in chlorobenzene is shorter and hence stronger. Thus, cleavage of C – Cl bond in benzene becomes difficult which makes it less reactive towards nucleophilic substition.
(ii) Due to repulsion between nucleophile and electron rich arenes.

Question. (a) Why does p-dichlorobenzene have a higher m.p. than its o- and m-isomers?
(b) Why is (±)-Butan-2-ol optically inactive?
Answer. (a) p-isomers are comparetively more symmetrical and fit closely in the crystal lattice, thus require more heat to break these strong forces of attraction. Therefore higher melting point than o- and m-isomers.
(b) (±)-Butan-2-ol is optically inactive because in racemic mix one type of rotation is cancelled by other.

Question. Write the structures of the following organic halogen compounds :
(i) p-Bromochlorobenzene
(ii) 1-Chloro-4-ethylcyclohexane
Answer.

Question. Which alkyl halide from the following pair is
(i) Chiral and (ii) undergoes SN1 reaction faster?
(a) (CH₃)₃CBr (b) CH₃CH₂CHBrCH₃
Answer. (i) 2-Bromobutane is chiral as the central C atom has all 4-different groups.

(ii) 3° Alkyl bromide i.e. (CH₃)₃CBr undergoes S_N1 reaction faster due to more stability of 3° carbocation.

Question. What are ambident nucleophiles? Explain with an example.
Answer. Ambident nucleophile : A nucleophile that can form new bonds at two or more spots in its structure, usually due to resonance contributors.
Example : S = C = N– can act as a nucleophile with either the S or N attacking.

Question. Account for the following:
(i) The C – Cl bond length in chlorobenzene is shorter than that in CH₃ – Cl.
(ii) Chloroform is stored in closed dark brown bottles.
Answer. (i) In haloalkanes, the halogen atom is attached

in haloarenes it is attached to sp²-hybridized carbon whose size is smaller than sp³ orbital carbon.
Therefore C – Cl bond in chloro-benzene is shorter than alkyl chloride.
(ii) CHCl₃ is stored in dark coloured bottles to cut off light because CHCl₃ is slowly oxidised by air in presence of light to form an extremely poisonous gas, carbonyl chloride, popularly known as phosgene.

Question. Which one in the following pairs undergoes S_N1 substitution reaction faster and why?

Answer. 

Question. Give chemical tests to distinguish between the following pairs of compounds :
(a) Benzyl chloride and Chlorobenzene
(b) Chloroform and Carbon tetrachloride
Answer. (a) Chlorobenzene and Benzyl chloride :Benzyl chloride is more reactive than chlorobenzene towards nucleophillic substitution reactions, therefore, benzyl chloride on boiling with aqueous KOH produces benzyl alcohol and KCl.

The reaction mixture on acidification with dil. HNO₃ followed by treatment with AgNO₃ solution produces white ppt. of AgCl due to formation of KCl
KCl + AgNO₃   →  AgCl ↓    +   KNO₃
                          white ppt.
But chlorobenzene does not undergo hydrolysis under these mild conditions to give phenol and KCl.
(b) Chloroform and Carbon tetrachloride By Carbylamine test : CCl₄ does not give this reaction but chloroform gives this reaction and produces offensive smell of phenyl isocyanide.

Question. Which one in the following pairs of substances undergoes S_N2 substitution reaction faster and why?

Answer.

is a primary halide and therefore undergoes S_N2 reaction faster.

As iodine is a better leaving group because of its large size, therefore undergoes S_N2 reaction faster.

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.

(ii) (a) Inversion of configuration occurs with S_N2 reaction.
(b) Racemisation occurs with S_N1 reaction.

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

(ii) Methyl magnesium bromide to 2-methylpropan-2-ol

Question. Write the IUPAC names of the following compounds :
(i) CH₂ = CHCH₂Br
(ii) (CCl₃)₃ CCl
Answer. (i) CH₂ = CHCH₂ – Br
IUPAC name : 3-Bromopropene
(ii) (CCl₃)₃—C—Cl :
IUPAC name : 2-(Trichloromethyl) – 1, 1, 1, 2,3, 3, 3-heptachloropropane

Question. Write the structures of the following organic halogen compounds :
(i) 4-tert-Butyl-3-iodoheptane
(ii) 4-Bromo-3-methylpent-2-ene
Answer. 

Question. Write the equations for the preparation of 1-bromobutane from :
(i) 1-butanol (ii) but-1-ene
Answer. (i) Preparation of 1-bromobutane from 1-butanol

Question. Which compound in each of the following pairs will react faster in S_N2 reaction with —OH?
(i) CH₃Br or CH₃I (ii) (CH₃)₃ CCl or CH₃Cl
Answer. (i) CH₃I : Because Iodide is better leaving group than bromide.
(ii) CH₃Cl : Carbon atom leaving group is less hindered.

Question. Draw the structure of major monohalo product in each of the following reactions :

Answer.

Short Answer Type Questions-II 

Question. How would you differentiate between SN1 and S_N2 mechanisms of substitution reactions? Give one example of each.
Answer.

Question. Answer the following :
(i) Haloalkanes easily dissolve in organic solvents, why?
(ii) What is known as a racemic mixture? Give an example.
(iii) Of the two bromoderivatives, C₆H₅CH (CH₃)Br and C₆H₅CH(C₆H₅)Br, which one is more reactive in SN1 substitution reaction and why? (Delhi)
Answer. (i) Because the new forces of attraction set up between haloalkanes and solvent molecules are of the same strength as the forces of attraction being broken.
(ii) A mixture which contains the equal proportions of two enantiomers of a compound in equal proportions is called racemic mixture
Example : (±) butan-2-ol
(iii) Since the reactivity of S_N1 reactions increases as the stability of intermediate carbocation increases. Of the two 2° bromides, the carbocation intermediate derived from C₆H₅CH(C₆H₅)Br i.e. C₆H₅C+HC₆H₅ is more stable as compared to the carbocation C₆H₅C + H CH3 obtained from C₆H₅CH(CH₃)Br because it is stabilized by two phenyl groups due to resonance.

Question. Rearrange the compounds of each of the following sets in order of reactivity towards S_N2 displacement :
(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methyl-butane, 3-Bromo-2-methylbutane
(iii) 1-Bromobutane, 1-Bromo-2, 2-dimethyl-propane, 1-Bromo-2-methylbutane 
Answer. (i) 1-Bromopentane > 2-Bromopentane > 2-Bromo-2-methylbutane
( i i ) 1-Bromo-3-methylbutane > 3-Bromo-2 methylbutane > 2-Bromo-2-methylbutane
(iii) 1-Bromobutane > 1-Bromo-2-methylbutane > 1-Bromo-2, 2-dimethylpropane.

Question. Although chlorine is an electron withdrawing group, yet it is ortho-, para-directing in electrophilic aromatic substitution reactions. Explain why it is so? 
Answer. Chlorine withdraws electrons through inductive effect and releases through resonance. Although Cl shows– 1 effect but through resonance, Cl tends to stabilize the intermediate carbocation and the effect is more pronounced at ortho and para positions.
This can also be explained diagramatically as:

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 π 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 halidealkyl 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. Following compounds are given to you:
2-Bromopentane, 2-Bromo-2-methylbutane, 1-Bromopentane
(i) Write the compound which is most reactive towards S_N2 reaction.
(ii) Write the compound which is optically active.
(iii) Write the compound which is most reactive towards β-elimination reaction. 
Answer. (i) 1-Bromopentane is most reactive towards S_N2 reaction.
(ii) 2-Bromopentane is optically active.
(iii) 2-Bromo-2-methylbutane is most reactive towards β-elimination reaction.

Question. Give reasons for the following :
(i) Ethyl iodide undergoes S_N2 reaction faster than ethyl bromide.
(ii) (±) 2-Butanol is optically inactive.
(iii) C—X bond length in halobenzene is smaller than C—X bond length in CH₃—X. 
Answer. (i) I, is better leaving group/C—I bond is weaker than C—Br bond.
(iii) In halobenzene, halogen atom is attached to the sp² hybrid carbon atom while in
CH₃-X halogen atom is attached to sp³ hybrid carbon atom. Hence C-X bond length in halo benzene is smaller than CH₃-X.

Question. Name the following according to IUPAC system :

Answer.(i) 2-Bromotoluene (ii) 1 chloro, 2, 2 dimethyl propane

Question. (a) Which compound in each of the following pairs will react faster in S_N2 reaction with –OH group?
(i) CH₃Br or CH₃I (ii) (CH₃)₃CCl or CH₃Cl
(b) Write the product of the following reactions:
(i) CH₃ – Cl + KCN → ?

Answer. (a) (i) As I– ion is a good leaning group than Br^–, therefore reacts faster than CH₃Br in S_N2 reactions with–OH.
(ii) As 3° alkyl halides have more steric hindrance than 1° alkyl halide therefore CH₃Cl undergoes S_N2 reaction faster than 3° alkyl halide.
(b) (i) CH₃ – Cl + KCN ⎯⎯→ KCl + CH₃CN

Question. How can the following conversions be carried out :
(i) Aniline to bromobenzene (ii) Chlorobenzene to 2-chloroacetophenone
(iii) Chloroethane to butane 
Answer. 

Question. Write the structural formula of A, B, C and D in the following sequence of reaction :

Answer.

Question. What happens when
(i) Chlorobenzene is treated with Cl₂/FeCl₃, (ii) Ethyl chloride is treated with AgNO₂,
(iii) 2-bromopentane is treated with alcoholic KOH?
Write the chemical equations in support of your answer.
Answer.

Question. The following compounds are given to you:
2-Bromopentane, 2-Bromo-2-methylbutane, 1-Bromopentane
(i) Write the compound which is most reactive towards S_N2 reaction.
(ii) Write the compound which is optically active.
(iii) Write the compound which is most reactive towards β-elimination reaction. 
Answer.

Question. Give the IUPAC names of the following compounds :

Answer.

Question. Give reasons: 
(i) C—Cl bond length in chlorobenzene is shorter than C—Cl bond length in CH₃—Cl.
(ii) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(iii) S_N1 reactions are accompanied by racemization in optically active alkyl halides.
Answer. 
(ii) Dipole moment is the product of charge and inter nuclear distance. In case of cyclohexyl chloride carbon—chlorine band length is longer than C—Cl band length in chlorobenzene. Hence dipole moments of cyclohexyl chloride is more.
(iii) S_N1 reactions are accompanied by racemization in optically active alkyl halides because the nucleophile will have an equal opportunity to attack on sp² hybridised carbocation from either sides to give a racemic mixture.
Example:

Question. (i) State one use each of DDT and iodoform.
(ii) Which compound in the following couples will react faster in S_N2 displacement and why?
(a) 1-bromopentane or 2-bromopentane
(b) 1-bromo-2-methylbutane or 2-bromo-2-methylbutane.
Answer. (i) Use of DDT (Dichlorodiphenyl Trichloroethane) : As a powerful insecticide, it is widely used for sugarcane and fodder crops to kill mosquitoes, lice which carry pathogens.
Use of iodoform (CHI₃) : It is used as an antiseptic for dressing wounds. Its antiseptic action is due to liberation of iodine when iodoform comes in contact with skin but not due to iodoform itself.
(ii) In S_N2 reactions, reactivity depends upon steric hindrance
(a) 1-Bromopentane (1° halogen) having less steric hindrance therefore is more reactive than 2-Bromopentane hence undergoes S_N2 reactions faster.

(b) 1-Bromo-2-methylbutane having less steric hindrance, is thus more reactive towards S_N2 reaction than 2-bromo-2-methyl butane (more steric hindrance).

Question. Give reasons :
(a) n-Butyl bromide has higher boiling point than t-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 t-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) Draw the structures of major monohalo products in each of the following reactions :

(b) Which halogen compound in each of the following pairs will react faster in SN2 reaction :
(i) CH₃Br or CH₃I
(ii) (CH₃)₃ C – Cl or CH₃ – Cl
Answer.

(b) (i) CH₃Br will react faster in S_N2 reaction.
(ii) CH₃ – Cl will react faster in S_N2 reaction.

Question. In the following pairs of the halogen compounds which compound undergoes faster S_N1 reaction.

Answer.

Question. What happens when
(i) CH₃—Cl is treated with aqueous KOH? (ii) CH₃—Cl is treated with KCN?
(iii) CH₃—Br is treated with Mg in the presence of dry ether? 
Answer.

Question. Write the major product(s) in the following:

Answer.

Question. Write structures of compounds A, B and C in each of the following reactions:

Answer.

A = Phenylmagnesium bromide
B = Benzoic acid
C = Benzoyl chloride

Question.66. Answer the following questions : 
(i) What is meant by chirality of a compound? Give an example.
(ii) Which one of the following compounds is more easily hydrolyzed by KOH and why?
CH₃CHClCH₂CH₃ or CH₃CH₂CH₂Cl
(iii) Which one undergoes S_N2 substitution reaction faster and why?

Answer.