Download Haloalkanes-haloarenes

HALOALKANES & HALOARENES – Gr 12
Give reasons:
1. For the preparation of an alkyl chloride from alcohol, thionyl chloride( SOCl2 )is preferred over other
reagents.
A. Because the by-products, SO2 & HCl are escapable gases which leave pure alkyl chloride as
product.
2. In preparation of R–I from R–OH, NaI or KI with H3PO4 is used but not H2SO4.
A. Because H2SO4 converts KI or NaI to HI & then oxidise HI to I2.
3. The order of reactivity of alcohols to give haloalkanes with HX is 3o>2o>1o.
A. Due to e releasing inductive effect of alkyl group(+I effect), the polarity of C–O bond is in the order
3o>2o>1o.
CH3
CH3
C-OH > CH3
CH3
CH
CH
>
CH3
CH2
CH2
OH
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 – HNO3 or HIO3.
A. The reaction is reversible. Backward reaction is prevented by oxidation of HI formed during
iodination by HNO3 or HIO3.
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.
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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 1o > 2o > 3o,
CH
e.g.: CH3CH2CH2CH2Br > CH3CH2–CH–CH3 > CH3–C–CH3
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 SN2 mechanism is 1o > 2o > 3o.
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 SN1 mechanism is 3o > 2o > 1o.
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 2o and least stable is 1o. So
the order is 3o > 2o > 1o.
15. Allylic and benzylic halides show high reactivity towards SN1 reaction.
A. The carbocations formed here are stabilised by resonance.
H2C=CH–CH2+
H2C+ CH=CH2 (allyl carbocation)
+
+
CH2
CH2
CH2
CH2
+
+
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.
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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 sp2
hybridised which is sp3 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 SN1 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 sp2 hybridised C which is more electronegative than sp3
hybridised C in
Cl
21. Reactivity towards nucleophilic substitution is in order
Cl
Cl
O2N
N2O
Cl
Cl
NO2
NO2
NO2
NO2
A. Nucleophilic substitution in haloarenes is difficult (refer Q. 19). The presence of e withdrawing groups
like –NO2 at ortho- and para- positions increases the reactivity. –NO2 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 –NO2 groups at o- and p- positions, greater the stability of the
carbanion.
22. There is no difference in the reactivity of haloarenes if –NO2 is in m- position.
A. In none of the resonating structures of the carbanion, negative charge is on the C bearing –NO2 group.
Therefore –NO2 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.
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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. CHCl3 is slowly oxidised by air in the presence of light to form extremely poisonous gas, phosgene
COCl2.
2CHCl2 + O2 light 2COCl2 + 2HCl
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Important conversions: Q. 10.19 (textbook) –
i) Propene to propan–1–ol
CH3–CH=CH2 to CH3–CH2–CH2–OH
A. CH3–CH2=CH2 + HBr org. peroxide CH3–CH2–CH2Br
ii) Ethanol to butyne–1
CH3–CH2OH
A. CH3CH2OH HBr
CH3–CH2 –CCH
CH3CH2BR CHC–Na
NaOH (aq)
CH3–CH2–CH2–OH
CH3–CH2–CCH
iii) 1–Bromopropane to 2–bromopropane
CH3CH2CH2Br
CH3CH–CH3
A. CH3CH2CH2Br
alc. KOH
iv) Toluene to benzyl alcohol
CH3
Br
CH3–CH=CH2
HBr
CH2OH
A.
4
CH3–CH–CH3
Br
KMnO4 / KOH
LiAlH4
+
CH3
H3O COOH
KMnO4/KOH
CH2OH
LiAlH4
H3O+
v) Benzene to 4–Bromonitrobenzene
NO2
Br
Br
Br
A.
Br2 / Fe
dark
HNO3 / H2SO4
NO2
vi) Benzyl alcohol to 2–phenyl ethanoic acid
CH2OH
A
.
CH2COOH
CH2COOH
SOCl2
CH2OH
KCN
CH2Cl
H2O/H+
vii) Ethanol to propane nitrile
CH3CH2OH to CH3CH2CN
A. CH3CH2OH
CH3CH2Cl
viii) Aniline to chlorobenzene
NH2
A.
KCN
CH3CH2CN
Cl
N2+ Cl-
NH2
NaNO2/HCl
0  5o C
Cl
Cu2Cl2
HCl
5
CH2CN
0 – 5oC
HCl
ix) 2–Chlorobutane to 3,4–Dimethyl hexane
CH3–CH–CH2CH3 to CH3–CH2–CH–CH–CH2–CH3
Cl
CH3 CH3
Na
A. 2 CH3CH2–CH–Cl
CH3–CH2–CH–CH–CH2–CH3
dry ether
CH3
CH3 CH3
x) 2–methyl–1–propene to 2–chloro–2–methyl propane
Cl
CH3–C=CH2
CH3–C–CH3
CH3
CH3
Cl
A. CH3–C=CH2 + HCl
CH3–C–CH3
CH3
CH3
xi) Ethyl chloride to propanoic acid
CH3CH2Cl
CH3CH2COOH
A. CH3CH2Cl
KCN
CH3CH2CN
CH3CH2COOH
xii) But–1–ene to n–butyl iodide
CH3–CH2–CH=CH2
CH3CH2CH2CH2–I
HBr
A. CH3–CH2–CH=CH2
CH3–CH2–CH2–CH2Br
xiii) 2–chloropropane to 1–propanol
CH3–CH–CH3
CH3CH2CH2OH
Cl
A. CH3–CH–CH3 alc. KOH CH3–CH=CH2 HBr
Cl
xiv) Isopropyl alcohol to iodoform
CH3–CH–OH
CHI3
CH3
A. CH3–CH–CH3
OH
CHI3 + CH3COONa
6
NaI
CH3CH2CH2CH2I
CH3–CH2–CH2Br
NaOH
CH3CH2CH2OH
xv) Chlorobenzene to p-nitrophenol
A.
Cl
Cl
OH
HNO3/H2SO4
NaOH/443K
H+
NO2
NO2
xvi) 2–Bromopropane to 1–bromopropane
A. CH3–CH–CH3 alc. KOH
CH3–CH=CH2
Br
xvii) Chloroethane to butane
A. CH3–CH2Cl Na
CH3–CH2–CH2–CH3
xvii) Benzene to diphenyl
A.
HBr
CH3–CH2–CH2Br
Cl
Cl2/Fe
dark
Na
ether
(Fittig Rn.)
xix) tert-butylbromide to isobutylbromide
A.
CH3
CH3–C–CH3 alc.KOH CH3–C=CH2 HBr
CH3 – CH – CH2Br
Br
CH3
org. peroxide
CH3
xx) Aniline to phenyl isocyanide
A.
NH2
NC
CHCl3 &
+ KCl + H2O
alc. KOH
phenyl isocyanide
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Q 10.11 (textbook-Pg 311)
i) Ethanol to but–1–yne
A. CH3CH2OH
CH3CH2Cl + CHC–Na
CH3CH2–CCH
ii) Propene to 1–nitropropane
A. CH3–CH=CH2
HBr
CH3CH2–CH2–NO2
CH3CH2CH2Br
iii) Toluene to benzyl alcohol
A.
CH3
Cl2
hυ
CH2Cl
aq. KOH
or aq. NaOH
7
CH2OH
iv) Propene to propyne
A. CH3–CH=CH2 + Br2(aq.)
CH3–CCH
CH3–CH–CH2Br
alc. KOH
CH3–CH=CHBr
v) Ethanol to ethyl fluoride
A. CH3CH2OH
CH3CH2Cl
AgF
CH3CH2F
vi) Bromomethane to propanone
A. CH3Mg
KCN
CH3–C=NMgBr
CH3
CH3CN
ether
CH3–C–CH3
O
vii) But–1–ene to but–2–ene
alc. KOH
A. CH3–CH2–CH=CH2 HBr CH3–CH2–CH–CH3
CH3–CH=CH–CH3
│
Br
viii) 1–chlorobutane to n–octane
A. CH3CH2CH2–CH2Cl Na
CH3CH2CH2–CH2–CH2–CH2–CH2–CH3
ix) Benzene to biphenyl
A.
Br2
FeBr3
Br
Na
dry ether
x) Bromobenzene to benzene
A.
Br + Mg dry ether
MgBr
H2 O
**************************************
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
Br
+ Mg
OH