HALOALKANE AND HALOARENE

 

Important Questions for CBSE Class 12 Chemistry – Haloalkanes and Haloarenes

PREVIOUS YEARS QUESTIONS

2015

Very Short Answer Type Questions [1 Mark]

 

Haloalkanes and Haloarenes Previous Year Question 1:

Answer:

Haloalkanes and Haloarenes Previous Year Question 2:

Answer:

Haloalkanes and Haloarenes Previous Year Question 3:

Answer:

Haloalkanes and Haloarenes Previous Year Question 4:

Answer:

Haloalkanes and Haloarenes Previous Year Question 5:

Answer:
CH3CH2Br will react faster due to less stearic hindrance.

Haloalkanes and Haloarenes Previous Year Question 6:

Answer:

Haloalkanes and Haloarenes Previous Year Question 7:

Answer:
CH3CH2 I will undergo SN2 reaction faster because C—I bond has lower bond dissociation enthalpy due to longer bond length than C—Br bond.

Short Answer Type Questions [II] [3 Marks]

Haloalkanes and Haloarenes Previous Year Question 8:
How do you convert the following:
(i) Prop-l-ene to propan-2-ol
(ii) Bromobenzene to 2-bromoacetophenone
(iii) 2-bromobutane to But-2-ene
Answer:

Haloalkanes and Haloarenes Previous Year Question 9:
How do you convert the following:
(i) ethyl chloride is treated with Nal in the presence of acetone,
(ii) chlorobenzene is treated with Na metal in the presence of dry ether,
(iii) methyl chloride is treated with KNO2?
Write chemical equations in support of your answer.
Answer:

Haloalkanes and Haloarenes Previous Year Question 10:
Give reasons for the following:
(i) Benzyl chloride is highly reactive towards the SN1 reaction.
(ii) 2-bromobutane is optically active but 1-bromobutane is optically inactive.
(iii) Electrophilic reactions in haloarenes occur slowly.
Answer:
(i) Benzyl carbonadon is stabilized by resonance.
(ii) 2-Bromobutane is chiral, therefore, optically active, whereas 1 -chlorobutane is not chiral, therefore optically inactive.
(iii) It is due to —I effect of halogens, it deactivates benzene ring towards electrophilic substitution reactions.

Haloalkanes and Haloarenes Previous Year Question 11:

Answer:
(i) 2-Bromobutane (ii) 1, 3-Dibromobenzene (iii) 3-Chloroprop-l-ene.

Haloalkanes and Haloarenes Previous Year Question 12:

Answer:

Haloalkanes and Haloarenes Previous Year Question 13:

Answer:

Haloalkanes and Haloarenes Previous Year Question 14:

Answer:

Haloalkanes and Haloarenes Previous Year Question 15:
How do you convert the following:
(i) Prop-l-ene to 1-fluoropropane
(ii) Chlorobenzene to 2-chlorotoluene
(iii) Ethanol to propanenitrile
Answer:

Haloalkanes and Haloarenes Previous Year Question 16:
Write the main products when
(i) n-butyl chloride is treated with alcoholic KOH.
(ii) 2, 4, 6-trinitrochlorobenzene is subjected to hydrolysis.
(iii) methyl chloride is treated with AgCN.
Answer:

Haloalkanes and Haloarenes Previous Year Question 17:

Answer:
(a) It is due to resonance, C—Cl bond is shorter due to sp2 hybridization and
(b) (CH3)2CHCl < CH3CH2Cl < CH3Cl < CH3Br
(c) l-chloro-6-methyl cyclohexene

Haloalkanes and Haloarenes Previous Year Question 18:
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 (-N02) at o/p positions increases the reactivity of haloarenes towards nucleophilic substitution reactions
Answer:
(a) It is because n-butyl bromide has more surface area than i-butyl bromide, therefore, more van der Waals’ forces of attraction, hence higher boiling point.
(b) Racemic mixture contains equal amount of dextro and laevo rotatory compounds. Half of molecules rotate plane polarised light towards left, remaining half towards right such that net optical rotation is zero. Therefore, Racemic mixture is optically inactive.
(c) -N02 stabilises the -ve charged ion formed during nucleophilic substitution reactions of haloarenes, therefore, increases reactivity.

Haloalkanes and Haloarenes Previous Year Question 19:
How can the following conversions be carried out:
(i) Aniline to bromobenzene
(ii) Chlorobenzene to 2-chloroacetophenone
(iii) Chloroethane to butane
Answer:

Haloalkanes and Haloarenes Previous Year Question 20:
What happens when
(i) Chlorobenzene is treated with Cl2/FeCl3
(ii) ethyl chloride is treated with AgN02,
(iii) 2-bromopentane is treated with alcoholic KOH?
Write the chemical equations in support of your answer.
Answer:

Haloalkanes and Haloarenes Previous Year Question 21:
(a) Why are alkyl halides insoluble in water?
(b) Why is Butan-l-ol optically inactive but Butan-2-ol is optically active?
(c) Although chlorine is an electron withdrawing group, yet it is ortho-, Para- directing in electrophilic aromatic substitution reaction. Why?
Answer:
(a) Alkyl halides are insoluble in water because they can neither form H-bonds with water nor they can break H-bonds between water molecules.
(b) Butan-l-ol is achiral, i.e. does not have chiral ‘C’ atom which is attached to four different groups, therefore, it is optically inactive.

2014

Very Short Answer Type Questions [1 Mark]

Question 22:

Answer:

Question 23:
Draw the structure of 2- Bromopentane
Answer:

Question 24:
Write structural formulae of the following compounds:
(i) 1 -Bromo-4-chlorobenzene (ii) l-chloro-4-ethyl cyclohexane.
Answer:

Short Answer Type Questions [I] [2 Marks]

Question 25:

Answer:

Question 26:
Draw the structure of major monohalo product in each of the following reactions:

Answer:

Question 27:
Write chemical equations when:
(i) ethyl chloride is treated with aqueous KOH.
(ii) chlorobenzene is treated with CH3COCl in presence of anhydrous AlCl3.
Answer:

Question 28:

Answer:

Question 29:
Write chemical equation when
methyl chloride is treated with AgNO2.
bromobenzene is treated with CH3Cl in the presence of anhydrous AlCl3.
Answer:

Question 30:
Write chemical equations when
(i) ethyl chloride is treated with alcoholic KOH.
(ii) chlorobenzene is treated with CH3Cl in the presence of anhydrous AlCl3.
Answer:

Question 31:

Answer:
(a) 3-Bromopropene
(b) Tris (trichloromethyl) chloromethane

Question 32:
Write structural formula of the following compounds
(i) 3-iodo-4-tert. butyl heptane
(ii) 4-bromo-3-methyl pent-2-ene
Answer:

Short Answer Type Questions [I] [3 Marks]

Question 33:

Answer:

2013

Very Short Answer Type Questions [1 Mark]

Question 34:

Answer:
4-Chloropent-l-ene

Question 35:
Write the IUPAC name of CH3CH=CH—C—CH3.
Answer:

Question 36:

Answer:
4-Bromo-4-methyl pent-2-ene

Question 37:
What happens when ethyl chloride is treated with aqueous KOH?
Answer:
Refer Ans. to Q.27 (i).

Question 38:
Write the IUPAC name of (CH3)2CHCH(Cl)CH3.
Answer:
2-Chloro-3-methylbutane.

Question 39:

Answer:

Question 40:

Answer:

Question 41:

Answer:
2-Bromo-4-chloropentane.

Question 42:

Answer:

Question 43:

Answer:
2-Chloro-3-methylbutane

Question 44:

Answer:

Question 45:

Answer:
l-Chloro-2, 4-dinitrobenzene

Short Answer Type Questions [I] [2 Marks]

Question 46:
Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. Give two reasons for the same.
Answer:

Question 47:

Answer:

Question 48:

Answer:

Short Answer Type Questions [II] [3 Marks]

Question 49:

Answer:
(i) It is because C-I bond is weaker than C-Br bond due to large size of I than Br.

Question 50:
Consider the three types of replacement of group X by group Y as shown here.

This can result in giving compound (A) or (B) or both. What is the process called if
(i) (A) is the only compound obtained?
(ii) (B) is the only compound obtained?
(iii) (A) and (B) are formed in equal proportions?
Answer:
(i) Retention of configuration, i.e. dextro rotatory substance remains dextro rotatory.
(ii) Inversion of configuration (Walden Inversion), i.e. dextro rotatory substance changes to laevo rotatory and vice versa.
(iii) Racemisation takes place, i.e. equal amount of dextro and laevo rotatory substances will be formed.

2012

Very Short Answer Type Questions [1 Mark]

Question 51:

Answer:
3-Bromo-2-methylprop-l-ene.

Question 52:

Answer:

Question 53:

Answer:
3-Bromo-2-methylprop-l-ene.

Short Answer Type Questions [I] [2 Marks]

Question 54:
What are ambident nucleophiles? Explain giving an example.
Answer:
Those nucleophile which can link through either of the two atoms are called ambident nucleophile, e.g. CN- and NC-, N02 and ONO-.

Question 55:
Explain as to why
(i) Alkyl halides, though polar, are immiscible with water.
(ii) Grignard’s reagent should be prepared under anhydrous conditions.
Answer:
(i) Alkyl halides cannot form H-bonds with water, therefore, immiscible with water.
(ii) It is because the Grignard reagent reacts with water to form alkane, therefore, should be prepared in absence of water, i.e. anhydrous conditions.

Question 56:
Explain why
(i) the dipole moment in chlorobenzene is lower than that of cyclohexyl chloride.
(ii) haloalkanes are only slightly soluble in water but dissolve easily in organic solvents.
Answer:
(i) It is due to +ve charge on ‘Cl’ in chlorobenzene due to resonance, its dipole moment is less than cyclohexyl chloride in which there is no resonance and +ve charge on halogen.
(ii) Halogens are very less polar, therefore, are less soluble in polar solvent like water but soluble in non-polar organic solvents due to attraction between them.

Short Answer Type Questions [II] [3 Marks]

Question 57:
Although chlorine is an electron withdrawing group, yet it is ortho-, para¬directing in electrophilic aromatic substitution reactions. Explain why it is SO?
Answer:
Chlorobenzene is resonance hybrid of following structures:

Since electron density is maximum at o and p-positions due to +R effect, therefore, electrophilic substitution will take place at o- and p-positions.

Question 58:
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 hydrolysed, CH3CHClCH2CH3 or CH3CH2CH2Cl?
(iii) Which one undergoes SN2 substitution reaction faster and why?

Answer:

2011

Very Short Answer Type Questions [1 Mark]

Question 59:
Write the IUPAC name of the following compound: (CH3)3CCH2Br
Answer:

Question 60:
Write the IUPAC name of the following compound:
CH2=CHCHgBr
Answer:
3-Bromoprop-1 -ene.

Question 61:
Which will react faster in SN2 displacement, 1-bromopentane or 2-bromopentane, and why?
Answer:
1- Bromopentane will react faster in SN2 because it is 1° (primary halide) and has less stearic hindrance.

Question 62:
Which will react faster in SN1 displacement, 1-Bromobutane or 2-bromobu- tane, and why?
Answer:
2- Bromobutane will react faster because it forms secondary carbocation which is more stable than primary carbocation.

Question 63:
Write the structure of the following compound:
2-(2-chlorophenyl)-1 -iodoethane.
Answer:

Question 64:
Write the structure of the following compound: 1 -Bromo-4-sec-butyl-2 -methyl-benzene.
Answer:

Question 65:
Write structure of the compound:
2 -chloro-3 -methylpentane
Answer:

Question 66:
Write the structure of the following compound:
1, 4-dibromobut-2-ene.
Answer:

Question 67:
Write the structure of the following compound: 2-(2-Bromophenyl) butane.
Answer:

Question 68:
Write the structure of the following compound: 3-(4-chlorophenyl)-2-methylpropane.
Answer:

Short Answer Type Questions [I] [2 Marks]

Question 69:

Answer:

Short Answer Type Questions [II] [3 Marks]

Question 70:
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?
Answer:
(i) It is because haloalkanes are non-polar in nature, therefore, they are soluble in organic solvents.
(ii) The mixture which contains equal amount of optically active d(+) dextro and levo rotatory l(-) substances is called racemic mixture, e.g. racemic mixture of glucose contains equal amount of dextrorotatory glucose and laevorotatory glucose.
(iii) C6H5CH(C6H5) Br will undergo SN1 substitution due to greater stability of carbocation formed.

Question 71:
Rearrange the compounds of each of the following sets in increasing order of reactivity towards SN2 displacement:
(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
(ii) l-Bromo-3-methylbutane, 2-Bromo-2-methyl butane, 3-Bromo-2- Methylbutane.
(iii) 1 -Bromobutane, l-Bromo-2,2 -dimethylpropane, 1 -Bromo-2 -methyl butane.
Answer:
(i) 2-Bromo-2-methylbutane < 2-Bromopentane < 1-Bromopentane
(ii) 2-Bromo-2-methylbutane < 3-Bromo-2-methylbutane < l-Bromo-3- methylbutane
(iii) 1 -Bromo-2,2-dimethylbutane < 1 -Bromo-2-Methylbutane < 1 -Bromobutane.

Question 72:

Answer:

Question 73:
(a) Write a chemical test to distinguish between:
(i) Chlorobenzene and Benzyl chloride.
(ii) Chloroform and Carbon tetrachloride.
(b) Why is methyl chloride hydrolysed more easily than chlorobenzene?
Answer:
(a) (i) Add aqueous NaOH or aqueous KOH. Then add silver nitrate solution. Benzyl chloride will give white precipitate due to formation of AgCl, t whereas chlorobenzene will not react.
(ii) Add aniline and ale. KOH. CHCl3 will give offensive smell due to phenyl isocyanide; whereas CCl4will not react.
(b) It is due to double bond character in chlorobenzene due to resonance which is difficult to break as compared to single bond (C—Cl) in CH.,Cl.

2010

Very Short Answer Type Questions [1 Mark]

Question 74:
A solution of KOH hydrolyses CH3CHClCH2CH3and CH3CH2CH2CH2Cl.
Which one of these is more easily hydrolysed?
Answer:
CH3—CH(Cl)CH2CH3 will be more easily hydrolysed because it will form secondary carbocation which is more stable than primary carbocation.

Question 75:

Answer:
4-Bromo-3-methylpent-2-ene.

Question 76:
Draw the structure of the following compound:
4- Bromo-3-methylpent-2-ene
Answer:

Short Answer Type Questions [II] [3 Marks]

Question 77:
(i) State one use each of DDT and iodoform.
(ii) Which compound in the following couples will react faster in SN2 displacement and why?
(a) 1-Bromopentane or 2-bromopentane
(b) 1 -Bromo-2-methylbutane or 2-bromo-2-methylbutane.
Answer:
(i) DDT is used as insecticide. Iodoform is used as antiseptic.
(ii) (a) 1 -Bromopentane because it is primary halide and less stearic hindrance, will react faster in SN2.
(b) 1 -Bromo-2-methyl pentane because it is primary halide and has less stearic hindrance, will react faster in SN2.

Question 78:
How would you differentiate between SN1 and SN2 mechanisms of substitution reactions ? Give one example of each.
Answer:
(i) Unimolecular nucleophilic substitution reaction (SN1): Those substitution reactions in which rate of reaction depends upon the concentration of only one of the reactants, i.e. alkyl halides are called SN1 reactions, e.g. hydrolysis of tertiary butyl chloride follow’s SN1 reaction. This reaction takes place in two steps. The first step involves formation of carbocation.

The slowest step is rate determining step w’hich involves one species only. Therefore, rate of reaction depends only on the concentration of tertiary butyl chloride. Tertiary halides follow SN1 mechanism. Polar protic solvents like water, alcohol favour SN1 because they stabilize carbocation by solvation.
Bimolecular nucleophilic substitution reaction (SN2): The reaction whose rate depends on the concentration of two species, alkyl halide and nucleophile. They involve one step mechanism. Back side attack of nucleophile and departing of leaving group take place simultaneously.

Question 79:
Answer the following:
(i) Identify chiral in CH3CHOHCH2CH3 and CH3CHOHCH3.
(ii) Among the following compounds, which one is more easily hydrolysed and why? CH3CHClCH2CH3 or CH3CH2CH2CH2Cl.
(iii) Which of these will react faster in SN2 displacement and why?
1-Bromopentane or 2-Bromopentane
Answer:

Question 80:
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 SN2 mechanism.
(iii) Ethers have low boiling points.
Answer:

2009

Short Answer Type Questions [I] [2 Marks]

Question 81:
Which one in the following pairs of substances undergoes SN2 substitution reaction faster and why?

Answer:

Question 82:
Draw the structure of major monohalo products in each of the following reaction:

Answer:

Question 83:
Which one in the following pairs undergoes SN1 substitution reaction faster and why?

Answer:

Question 84:

Answer:
(i) Refer Ans. to Q.72 (i).
(ii) Refer Ans. to Q.72 (iii).