HALOALKANES & HALOARENES – Gr 12
Give reasons
:
1.
For the preparation of an alkyl chloride from alcohol, thionyl chloride( SOCl
2
)is preferred over other reagents.
A.
Because the by-products, SO
2
& HCl are escapable gases which leave pure alkyl chloride as product.
2.
In preparation of R–I from R–OH, NaI or KI with H
3
PO
4
is used but not H
2
SO
4
.
A.
Because H
2
SO
4
converts KI or NaI to HI & then oxidise HI to I
2
.
3. The order of reactivity of alcohols to give haloalkanes with HX is 3 o
>2 o
>1 o
.
A. Due to e releasing inductive effect of alkyl group(+I effect), the polarity of C–O bond is in the order
3 o
>2 o
>1 o
.
CH
3
CH
3
C-OH > CH
3
CH CH > CH
3
CH
2
CH
2
OH
CH
3
OH
4. Aryl halides cannot be prepared from phenol, like the preparation alkyl halide from alcohol.
A. Due to resonance C–O in phenol has partial double bond character and is more difficult to break than
C–O single bond in alcohols.
5. Preparation of iodobenzene from benzene by electrophilic substitution requires the presence of an oxidizing agent – HNO
3 or HIO
3
.
A. The reaction is reversible. Backward reaction is prevented by oxidation of HI formed during iodination by HNO
3
or HIO
3
.
6. Haloalkanes are only slightly soluble in water.
A. A substance will dissolve readily in a solvent when the solute-solvent intermolecular forces are stronger than the solute-solute as well as solvent-solvent forces. In case of haloalkanes, the attractive force with water is weaker than the hydrogen bonds between water molecules. Therefore, solubility is low.
7. Boiling points of haloalkanes are considerably higher than those of hydrocarbons of comparable molecular mass.
A. Due to polarity of C–X bond, the intermolecular forces of attraction are stronger, i.e., dipole-dipole forces, whereas hydrocarbons are non-polar and the intermolecular forces are only weaker dispersion forces.
8. For the same alkyl group, the boiling point decreases in the order – RI > RBr > RCl > RF.
1

A. With increase in size and mass of halogen atom, the magnitude of intermolecular forces increases.
9. The order of boiling point for isomeric haloalkanes is in the order 1 o
> 2 o
> 3 o
,
CH
e.g.: CH
3
CH
2
CH
2
CH
2
Br > CH
3
CH
2
–CH–CH
3
> CH
3
–C–CH
3
Br Br
A. Boiling point decreases with increase in branching, as size of molecule decreases.
10.Melting point of p -dibromobenzene is higher than its o & m- isomers.
A. Due to the symmetry of its structure, p -isomer fits better in the crystal lattice.
11. Free radical halogenation of alkanes is not preferred for the preparation of haloalkanes.
A. This method gives a complex mixture of isomeric mono and polyhaloalkanes which is difficult to separate. Consequently the yield of any one compound is low.
12. Haloalkanes react with KCN to form alkyl cyanides RCN as main product while AgCN forms isocyanides as main product.
A. Answer on page 293.
13. The order of reactivity of alkyl halides by S
N
2 mechanism is 1 o
> 2 o
> 3 o
.
A. Bulky alkyl groups on or near the C with halogen atom cause steric hindrance to the approaching nucleophile. Of the pr. alkyl halides, methyl halide is the most reactive.
14. The order of reactivity of alkyl halides by S
N
1 mechanism is 3 o
> 2 o
> 1 o
.
A. Rate of reaction depends on the slowest step, which involves the formation of carbocation. Greater the stability of the carbocation, greater will be its ease of formation from alkyl halide and faster the reaction. A tertiary carbocation is the most stable of the three, then comes 2 o
and least stable is 1 o
. So the order is 3 o > 2 o > 1 o .
15. Allylic and benzylic halides show high reactivity towards S
N
1 reaction.
A. The carbocations formed here are stabilised by resonance.
H
2
C=CH–CH
2
+
H
2
C
+
CH=CH
2
(allyl carbocation)
+
+
CH
2
CH
2
CH
2
CH
2
+
+
16. For a given alkyl group, the reactivity of R–X in both the mechanisms is RI > RBr > RCl > RF.
A. As size of halogen atom increases the R–X bond weakens and can be broken more easily.
17. A racemic mixture is optically inactive.
2

A. Racemic mixture is a 50:50 mixture of enantiomers and the optical rotation of one enatiomer is opposite to the other.
18. An alkyl halide with aq. KOH undergoes nucleophilic substitution, while with alc. KOH undergoes elimination.
A. Answer on page 301.
19. Aryl halides X are extremely less reactive towards nucleophilic substitution reactions.
A. i) C–X bond in haloarenes is less polar than in haloalkanes because the C in haloarenes is sp
2 hybridised which is sp 3 hybridised and less electronegative.
ii) Due to resonance C–X bond in haloarenes acquires partial double bond character and is more difficult to break.
iii) Phenyl carbocation is not resonance stabilized so S
N
1 mechanism is ruled out.
iv) Because of possible repulsion e rich nucleophiles are less likely to approach e rich arenes.
20. The dipole moment of is lower than that of cyclohexyl chloride .
Cl Cl
A. C–Cl bond in haloarenes is less polar due to sp
2
hybridised C which is more electronegative than sp
3 hybridised C in
Cl
21. Reactivity towards nucleophilic substitution is in order
Cl Cl Cl Cl
O
2
N N
2
O NO
2
NO
2
NO
2
NO
2
A. Nucleophilic substitution in haloarenes is difficult (refer Q. 19). The presence of e withdrawing groups like –NO
2
at ortho- and para- positions increases the reactivity. –NO
2
groups withdraw the e density from the benzene ring and then facilitate the attack of the nucleophile. The carbanion thus formed is stabilised by resonance. More the no. of –NO
2
groups at o and p positions, greater the stability of the carbanion.
22. There is no difference in the reactivity of haloarenes if –NO
2
is in m - position.
A. In none of the resonating structures of the carbanion, negative charge is on the C bearing –NO
2 group.
Therefore –NO
2
at m- position does not stabilise the carbanion.
23. Electrophilic substitution in haloarenes X occurs at o - and p - positions.
A. Due to resonance e density is high in o - and p - positions.
3

24. Although Cl is an electron withdrawing group, yet it is o & p directing in electrophilic substitution reactions.
A. Answer on page 306.
25. Grignard reagent should be prepared under anhydrous conditions.
A. Grignard reagent is highly reactive and reacts with any source of proton to give hydrocarbon. They react with water, alcohol, amines.
26. Chloroform is stored in closed dark coloured bottles filled to the brim.
A. CHCl
3
is slowly oxidised by air in the presence of light to form extremely poisonous gas, phosgene
COCl
2
.
2CHCl
2
+ O
2 light 2COCl
2
+ 2HCl
********************************
Important conversions : Q. 10.19 (textbook) –
i) Propene to propan–1–ol
CH
3
–CH=CH
2
to CH
3
–CH
2
–CH
2
–OH
A. CH
3
–CH
2
=CH
2
+ HBr
org. peroxide
CH
3
–CH
2
–CH
2
Br
NaOH (aq)
CH
3
–CH
2
–CH
2
–OH ii) Ethanol to butyne–1
CH
3
–CH
2
OH CH
3
–CH
2
–C 
CH
A. CH
3
CH
2
OH
HBr
CH
3
CH
2
BR
CH

C–Na
CH
3
–CH
2
–C 
CH iii) 1–Bromopropane to 2–bromopropane
CH
3
CH
2
CH
2
Br CH
3
CH–CH
3
Br
A. CH
3
CH
2
CH
2
Br alc. KOH CH
3
–CH=CH
2
HBr CH
3
–CH–CH
3
Br iv) Toluene to benzyl alcohol
CH
3
CH
2
OH
A.
4

KMnO
4
/ KOH LiAlH
4
CH
3
H
3
O + COOH
KMnO
4
/KOH LiAlH
4
H
3
O
+ v) Benzene to 4–Bromonitrobenzene
NO
2
Br
Br Br
A.
Br
2
/ Fe HNO
3
/ H
2
SO
4
dark
NO
2
vi) Benzyl alcohol to 2–phenyl ethanoic acid
CH
2
OH
CH
2
OH CH
2
COOH
A
. CH
2
COOH
SOCl
2
CH
2
OH CH
2
Cl CH
2
CN
KCN H
2
O / H
+ vii) Ethanol to propane nitrile
CH
3
CH
2
OH to CH
3
CH
2
CN
A. CH
3
CH
2
OH CH
3
CH
2
Cl
KCN
CH
3
CH
2
CN viii) Aniline to chlorobenzene
NH
2
Cl
A. NH
2
N
2
+ Cl
-
Cl
NaNO
2
/ HCl Cu
2
Cl
2
0

5 o
C
5
HCl

0 – 5 o
C HCl ix) 2–Chlorobutane to 3,4–Dimethyl hexane
CH
3
–CH–CH
2
CH
3
to CH
3
–CH
2
–CH–CH–CH
2
–CH
3
Cl CH
3
CH
3
Na
A. 2 CH
3
CH
2
–CH–Cl dry ether
CH
3
–CH
2
–CH–CH–CH
2
–CH
3
CH
3
CH
3
CH
3 x) 2–methyl–1–propene to 2–chloro–2–methyl propane
Cl
CH
3
–C=CH
2
CH
3
–C–CH
3
CH
3
CH
3
Cl
A. CH
3
–C=CH
2
+ HCl CH
3
–C–CH
3
CH
3
CH
3 xi) Ethyl chloride to propanoic acid
CH
3
CH
2
Cl CH
3
CH
2
COOH
A. CH
3
CH
2
Cl
KCN
CH
3
CH
2
CN CH
3
CH
2
COOH xii) But–1–ene to n–butyl iodide
CH
3
–CH
2
–CH=CH
2
CH
3
CH
2
CH
2
CH
2
–I
A. CH
3
–CH
2
–CH=CH
2
HBr CH
3
–CH
2
–CH
2
–CH
2
Br NaI CH
3
CH
2
CH
2
CH
2
I xiii) 2–chloropropane to 1–propanol
CH
3
–CH–CH
3
CH
3
CH
2
CH
2
OH
Cl
A. CH
3
–CH–CH
3
alc. KOH
CH
3
–CH=CH
2
HBr
CH
3
–CH
2
–CH
2
Br
NaOH
CH
3
CH
2
CH
2
OH
Cl
xiv) Isopropyl alcohol to iodoform
CH
3
–CH–OH CHI
3
CH
3
A. CH
3
–CH–CH
3
OH
CHI
3
+ CH
3
COONa
6

xv) Chlorobenzene to p-nitrophenol
A. Cl Cl OH
HNO
3
/ H
2
SO
4
NaOH / 443K
H +
NO
2
NO
2 xvi) 2–Bromopropane to 1–bromopropane
A. CH
3
–CH–CH
3
alc. KOH CH
3
–CH=CH
2
HBr CH
3
–CH
2
–CH
2
Br
Br xvii) Chloroethane to butane
A. CH
3
–CH
2
Cl
Na
CH
3
–CH
2
–CH
2
–CH
3 xvii) Benzene to diphenyl
A. Cl
Cl
2
/ Fe Na
dark ether
(Fittig Rn.) xix) tert-butylbromide to isobutylbromide
A. CH
3
CH
3
–C–CH
3
alc.KOH CH
3
–C=CH
2
HBr CH
3
– CH – CH
2
Br
Br CH
3
org. peroxide CH
3 xx) Aniline to phenyl isocyanide
A. NH
2
NC
CHCl
3
& + KCl + H
2
O
alc. KOH
phenyl isocyanide
****************************
Q 10.11 (textbook-Pg 311) i) Ethanol to but–1–yne
A. CH
3
CH
2
OH CH ii) Propene to 1–nitropropane
3
CH
2
Cl + CH

C–Na CH
3
CH
2
–C 
CH
A. CH
3
–CH=CH
2
HBr
CH
3
CH
2
CH
2
Br CH
3
CH
2
–CH
2
–NO
2 iii) Toluene to benzyl alcohol
A. CH
3
CH
2
Cl CH
2
OH
Cl
2
aq. KOH
hυ or aq. NaOH
7

iv) Propene to propyne
A. CH
3
–CH=CH
2
+ Br
2
(aq.) CH
3
–CH–CH
2
Br alc. KOH CH
3
–CH=CHBr
CH
3
–C 
CH v) Ethanol to ethyl fluoride
A. CH
3
CH
2
OH CH
3
CH
2
Cl vi) Bromomethane to propanone
AgF CH
3
CH
2
F
A. CH
3
Mg
KCN
CH
3
CN CH
3
–C=NMgBr
CH
3
–C–CH
3 ether
CH
3
O vii) But–1–ene to but–2–ene
A. CH
3
–CH
2
–CH=CH
2
HBr
CH
3
–CH
2
–CH–CH
3
alc. KOH
CH
3
–CH=CH–CH
3
│
Br viii) 1–chlorobutane to n–octane
A. CH
3
CH
2
CH
2
–CH
2
Cl
Na
CH
3
CH
2
CH
2
–CH
2
–CH
2
–CH
2
–CH
2
–CH
3 ix) Benzene to biphenyl
A.
Br
2
Br Na
FeBr
3
dry ether x) Bromobenzene to benzene
A. Br
Br + Mg dry ether MgBr H
2
O + Mg
OH
**************************************
Practice the following conversions:
1) 2–bromo–2–methyl propane to 2–methyl propene
2) Benzene to 2–nitrophenol
3) Sodium ethoxide to dimethyl ether
4) Bromobenzene to benzene
5) Benzene to toluene
6) Bromoethane to ethane
7) Propene to 1–nitropropane
8) Propene to propyne
9) Ethanol to ethyl fluoride
10) But–1–ene to but–2–ene
11) 1–chlorobutane to n–octane
*******************************************
8