Download 11.Unit 10 Haloalkanes and Haloarenes.

UNIT-10
HALOALKANES AND HALOARENES
Important terms and concepts:
Alkyl halide or Haloalkane is the compound obtained by replacing one or more
hydrogen atoms of an alkane by same number of halogen atoms. Eg: CH3CI
,CH3Br
R-H+X2
R-X+H-X
For C4H9Br, four isomeric compounds are possible out of which one(sec-butyl
bromide) can show enantiomerism.
For C5H11Br, eight isomeric compounds are possible out of which three can
exhibit enantiomerism.
Alcohols can be converted into chlorides by reaction with (i)HCl/ZnCl2 (ii)PCl5
(iii)PCl3 (iv) SOCl2/Pyridine.The reaction with SOCl2/Pyridine is preferred
because in this case side products are gaseous and can be expelled during
distillation.
Halogenation of alkenes in the presence of peroxides takes place through free
radicals as intermediates and results in anti-Markownikoff’s product.
For same alkyl group boiling points of alkyl halide are in the order:RI˃ RBr˃
RCl ˃RF
Among isomeric halides boiling point decreases with increase in branching in
alkyl group.
Alkyl halides are insoluble in water due to their inability to form hydrogen
bonds with water.
Allylic halide :
CH2=CH-CH2X
Benzylic halide :
C6H5CH2Cl
Vinylic halide :
CH2=CH-Cl
Order of reactivity of Alkyl halide in SN1 reaction: 30 > 20 > 10
Order of reactivity of Alkyl halide in SN2 reaction: 10>
Finkelstein reaction: R−X + NaI ⟶ R−I + NaX
Swart
reaction : R−X + AgF ⟶ R−F +Ag
20˃
30
Saytzeff’sRule: It states that during dehydro halogenations, the
preferred product is that alkene which has greater number of alkyl
groups attached to the doubly bonded carbon atoms.
CH3-CH2–CH- CH3
Alc.KOH
Br
CH3–CH= CH- CH3
81% More highly substituted
Alkenes will be more stable
Some important name reaction:
1. Wurtz reaction
R–X+2Na + X –R
Dry ether
R-R+2NaX
2. Wurtz- fittig reaction
R–X+2Na +
R–Ar Dryether
R-Ar+2NaX
3.Sandmeyer’s reaction:
ArNH2 + HNO2+HCl
273 K
ArN2Cl Cu2X2+HX ArX +N2
CCl4 is used as fire extinguisher under the name Pyrene.
CONCEPTUAL QUESTIONS
Q1. Why are halo alkanes more reactive than haloarenes?.
Ans. In haloarenes, there is double bond character between carbon and
halogen due to resonance effect which makes it less reactive.
(ii)In benzene, carbon atom is sp2 hybridised which is shorter than sp3
present in halo alkanes. Hence C-Cl bond in aryl halides is shorter and
stronger.
Q2. Why do halo alkanes undergo nucleophillic substitution where as
haloarenes undergo electophillic substitution.
Ans. Due to more electro negative nature of halogen atom in halo alkanes
carbon atom becomes slightly positive and is easily attacked by nucleophillic
reagents. While in halo arenes due to resonance, carbon atom becomes
slightly negative and attacked by electrophillic reagents.
Q3. When an alkyl halide is treated with ethanolic solution of KCN, the
major product is alkylcyanide where as if alkyl halide is treated with
AgCN, the major product is alkyl isocyanide.
Ans. KCN is ionic they can attach through C or N but C-C bond is stronger than
C-N bond. So RCN is major product. AgCN is covalent and so more electro
negative N can attach to C and forms iso cyanides.
Q4. Aryl halides cannot be prepared by the action of sodium halide on
phenol in the presence H2SO4. Why?
Ans. Due to resonance the carbon-oxygen bond in phenol has partial double
bond character and it is stronger than carbon oxygen single bond.
Q5. Grignard reagent should be prepared under anhydrous conditions.
Why?
Ans. Grignard reagent reacts with H2O (moisture) to form alkanes, therefore
they are prepared under anhydrous condition.
Q6.Why is Sulphuric acid not used during the reaction of alcohols with
KI?
Ans. It is because HI formed will get oxidized to I2by concentrated Sulphuric
acid which is an oxidizing agent.
Q7. p-dichloro benzene has higher m.p.than those of ortho and misomers.?
Ans. p-dichloro benzene is symmetrical, fits into crystal lattice more readily
and has higher melting point.
Q8. Although chlorine is an electron-with drawing group, it is ortho
and para directing in electrophillic aromatic substitution reactions.
Why?
Ans. Chlorobenzene is resonance hybrid, there is –ve charge at ortho and
parapositions, electro phallic substitution reaction will takeplace at ortho and
para position due to +R effect which dominating over the –I effect
Q9.The treatment of alkyl chlorides with aqueous KOH lead to the
formation of alcohols but in presence of alcoholic KOH alkenes are
major products. Explain?
Ans. In aqueous KOH, (OH-) is nucleo phile which replaces another
nucleophile.
R-X+KOH
R-OH+KX
Where as in alcoholic KOH
C2H5OH+KOH
CH3CH2-Cl+alco.KOH
C2H5O- +K+
CH2=CH2+C2H5OH
Q10. Explain why vinylchloride is unreactive in nucleophillic
substitution reaction?
Ans. Vinylchloride is unreactive in nucleophillic substitution reaction
because of double bond character between C=C-Clbond which is difficult to
break.
Q11. Arrange the following compounds according to reactivity towards
nucleophilic substitution reaction with CH3ONa4-nitro chlorobenzene, 2, 4 di nitro chlorobenzene, 2, 4, 6, tri nitro
chlorobenzene
Ans- 2, 4, 6, trinitro chlorobenzene>2, 4 di nitro chlorobenzene>4-nitro
chlorobenzene
Q12. Which compound will react faster in SN2reaction with OH-?
Ans-(a)CH3Br and CH3I
CH3I will react faster than CH3Br since I is a easily leaving group
(b)(CH3)3C-Cl or CH3Cl
CH3Cl will react faster than 30halide since (CH3)3C has more steric
hindrance
Q13. Arrange in order of boiling points.
(a)Bromo benzene, Bromoform, chloro methane, Dibromo-methane
(b)1-chloro propane, Iso propyle chloride,1-Chlorobutane.
Ans. (a)chloro methane<Bromo benzene<Dibromo-methane<, Bromoform
(b), Iso propyl chloride<1-chloropropane<1-Chlorobutane
(As branching increases, boiling point decreases)
Q14. Predict the reactivity in SN1
(a) C6H5CH2Br, C6H5CH(C6H5)Br, C6H5CH(CH3)Br, C6H5C(CH3)(C6H5)Br
Ans. 30>20>10(SN1)
C6H5C(CH3)(C6H5)Br >C6H5(C6H5)Br > C6H5CH(CH3)Br >C6H5CH2Br
(30)
(20)
(20)
(10)
VERY SHORT ANSWER TYPE QUESTION[1MARKS]
1. Write the IUPAC name of : CH2=CHCH2Br ( All India 2011)
2. An alkyl halide having molecular formula C4H9Cl is optically active. What is
its structure?
3. Why is vinyl chloride less reactive than ethyl chloride?
4.Write the structural isomers of C3H6Cl2 which can exihibit enantiomerism?
5. Write down the structure of the following compounds;
(a)1-chloro-4-ethylcyclohexane
(b)1,4-dibronebut-2-ene
(c)4-terbutyl-3-iodoheptane
(d)1-bromo-4-secbutyl-2-methylbenzene
6. A hydro carbon C5H12sdoes not react with chlorine in dark but it gives a
single mono chloro compound in bright sunlight. Identify the compound.
7. Why is sulphuric acid not used during the reaction of alcohols with KI?
8. Out of C6H5CH2Cl &C6H5CH(Cl)C6H5which is more easily hydrolysed with
aq.KOH & why?
9. What is meant by the following terms:
(1)chirality (2)enantiomerism (3)Ambident nucleophiles.
10. Chloroform is stored in dark coloured & sealed bottle. Why?
Short answer type questions
1. Give the IUPAC names of the following compounds?
A) ClCH2C= CCH2Br
b) (CCl3)3CCl C)CH3CH(Cl)CH(Br)CH3
2. How would you carry out the following conversions?
( All India
2008)
(i) Ethyl magnesium chloride to propanol. (ii) Benzyl chloride to benzyl
alcohol.
3. What is significance of 12 in Freon 12?
4. Predict the product of the following reaction
CH2=CH-CH3 + HBr +Peroxide
5. Write the reaction involved in:
(A) Sandmeyers reaction (B) Swarts reaction (C) Finkelstein reaction (D)
Fittigs reaction
6. Rearrange the following in order of increasing ease of
dehydrohalogenations
CH3CH2CH2Cl, CH3CHClCH3, CH3–C-Cl(CH3)2
1. Account for the following:
( C.B.S.E. 2013)
(i)The C –Cl bond length in chlorobenzene is shorter than that in CH3−Cl?
(ii)Chloroform is stored in closed dark brown bottles.
8. Give the uses of
(a)CCl4
(b)iodoform
9. Propose the mechanism of the following reaction:
CH3-CH2-Br+CH3OH → CH3-CH2-OCH3+HBr
10.Suggest a possible mechanism for the following reaction: (C.B.S.E. DELHI
2009)
n-BuBr + KCN ⟶ n- BuCN
11. How will you bring the following conversion?
(a)Propene to Propyne
(b)Toluene to Benzyl Alcohol(c) Aniline to Phenyl iso cyanide
12. What happen when;
(a)n-butyl chloride is treated with alc.KOH.
(b) ethyl chloride is treated with aq.KOH.
(c)methyl chloride is treated with KCN.
13. How can we produce nitro benzene from phenol?
Ans.(I)First convert phenol to benzene by heating with Zn dust.
(II) Nitration of benzene with conc.nitric acid in presence of conc.sulphuric
acid.
14. Alcohols react with halogen acids to form haloalkanes but phenol does not
form halo benzene. Explain.
Ans. The C-O bond in phenol acquires partial double bond character due
to resonance. Hence it is not Cleaved by X- ions to form halobenzenes. But
in alcohols a pure single bond
C—O bond is maintained and can be
cleaved by X– ions.
LONG ANSWER TYPE QUESTIONS
Q1) How do you convert the following?
(i)Propenetopropan-1-ol
(ii)1-Bromopropaneto2-bromopropane
(iii)Toluenetobenzylalcohol
(iv)Benzeneto4-bromonitrobenzene
(v)Benzylalcoholto2-phenylethanoicacid
1.
2.
CH3-CH=CH2+HBr Peroxide
CH3CH2CH2Br
aq.KOH
CH3CH2CH2OH
3.
4.
.
5.
Q2) How are the following conversions carried out?
(i)Ethanol to propane nitrile
(ii)Aniline to chloro benzene
(iii)2-Chloro butane to 3, 4-dimethyl hexane
(iv)2-Methyl-1-propene to 2-chloro-2-methylpropane
(v)Ethyl chloride to propanoic acid
1.
2.
3.
4.
5.
Q3) How are the following conversions carried out?
(i)
But-1-ene to n-butyliodide
(ii)
2-Chloropropane to1-propanol
(iii) Iso propyl alcohol to iodoform
(iv) Chloro benzene to p-nitrophenol
(v)
2-Bromopropane to 1-bromopropane
1.
2.
3.
4.
5.
.
Q4.How will you carry out the followingconversions?
(i)Chloro ethane to butane
(ii)Benzene to diphenyl
1.
2.