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1Haloalkanes & Haloarenes
CHEMISTRY - XII OBJECTIVE
18. INTRODUCTION
Definition: Haloalkanes and haloarenes are
obtained by replacement of hydrogen atoms from alkane and arene (benzene) by halogen (F, CI,
Br, I) atoms,
Their general representation is
(1) Haloalkane (R-X) (2) Haloarene Ar-X (Ar is arene)
ISOMERISM IN ALKYL HALIDES
Alkyl halide shows structural as well as isomerism.
In structural isomerism they shows chain and
position isomerism while in stereoisomerism' they shows optical isomerism. Total number of isomeric 1halides can be calculated by formula, I = 2 n-2.
Monohalogen derivatives (alkyl halide):
They show chain, position and optical isomerism.
a) C
3H7X (2-isomers)
1) CH
3CH2CH2X I-halo propane
(2) CH
3CHXCH32-halo propane
1 and 2 are position isomers.
(b) C
4H9(4-isomers)
(1) CH
3CH2CH2CH2X I-halo butane
(2) (CH
3)2CHCH2X 2-methyl, I-halo
propane 1 (3) CH
3CH2CHXCH32-halo butane
4) (CH
3)3CX2-methyl, 2-halo propane
NATURE OF CoX BOND IN ALKYL
HALIDE (IIALOALKANE):
1. All alkyl halides are highly polar due to more
electronegativity difference between carbon and halogen.
The C-Xbond is formed by overlap of sp3-hybrid
Orbital of carbon and half filled p-orbital of halogen atom.
10Haloalkanes and Haloarenes
2. Thus, the carbon of C-X bond become a site for
the attack of nucleophile. Therefore most common reactions of alkyl halides are nucleophilic substitution reactions. In these reactions, the atom or group which looses its bond to carbon and takes an additional pair of electrons is known as leaving group.
Halides ions are good leaving species. Better the
leaving group, the faster is the nucleophilic substitution reaction. Among the alkyl halides, iodide ion is best leaving species and therefore iodoalkanes undergoes nucleophilic substitution reaction very fast rate. Thus the order of reactivi.ty of alkyl halides i.s due to polar nature of C-X bond. The polarity of alkyl halide is, R-Cl > R-Br > R-I
Therefore expected reactivity of alkyl halide is,
R-Cl > R-Br > R-I
But however, the actual order of reactivity of alkyl halide is, R-I > R-Br > R-CI > R-F This order is due to the bond dissociation energy.
Lesser the bond dissociation energy more the
reactivity. The bond dissociation energy of C-I is less due to larger atomic size of iodine atom.
Hence C-I bond undergoes easily breaking, while
C-Cl bond undergoes difficultly breaking.
3. From F to I atomic size increases, consequently
C-Xbond length also increases. Hence C-I bond
is weak and more reactive while C-F bond is strong and less reactive. Decreasing order of reactivity of C-Xbond is
C-I > C - Br > C - Cl > C - F
4. Inductive effect also play the important role in
the reactivity of 1°, 2° and 3° alkyl halides.
For example CH
3Cl is more reactive than CH4 as
inductive effect is present in CH
3Cl and no
inductive effectis present in methane. The electron donating groups (+ 1 effect)increase the reactivity of R-X. The reactivity order of alkyl halide is 3
0R- X > 20 R-X > 10R - X
ecSECTION - I HALOGEN DERIVATIVES OF ALKANES
2Haloalkanes & Haloarenes
CHEMISTRY - XII OBJECTIVE
This is due to three methyl groups on tertiary alkyl halide enhance the -I effect of the halogen atom by donating the electrons to 3° carbon atom, hence reactivity increases.
PREPARATION METHODS OF ALKYL
HALIDES
1. By halogenation of alkanes (paraffins): When
alkanes are halogenated in the presence of catalyst gives alkyl halides.
R - H + X
2 Calalyst
bbbr R-X + HX alkane alkyl halide
These are substitution reaction in which hydrogen
atoms fro alkanes are replaced by halogen atoms.
Reactivity halogen is F
2 > Cl2 > Br2 > 12,
In substitution reaction ease of replacement of
hydrogen is, 3
0 > 20 >10H
Monohalogenation of alkane is seldom (rare) used,
because several isomeric monosubstituted halides are formed because alkane have different type of hydrogen atoms. Their separation is difficult.
Thus, halogenation method is not used unless the
alkane contains equivalent hydrogen atoms, good yield are obtained in alkane such as methane, ethane, neo pentane etc. (a) Chlorination:
R - H + C1
2 U.V.light
bbbbr R - Cl + HCl excess (b) Bromination:
R-H+Br
2 3AlBr
bbbr R-Br + HBr (c) Iodination : Direct iodination ofalkane is not possible because of reducing nature of'Hl.
Ass uch iodination is carried out in the
presence of an oxidising agents such as HgO or HIO
3 or HN03. Oxidising agents oxidise
HI formed and liberate free iodine.
R - H + I
2
R - I + HI
5HI + HI0
3 r 3I2 + 3H2O
2R - H + 2I
2+ HgO r 2R - I + H2O + HgI2
5R - H + 212 + HIO
3 - 5R - 1+ 3H2O
BR - H+ 4I
2 + HNO3 r BR - I + 3H20 + NH3
(d) Fluorination: The fluorination of alkane is highly exothermic and explosive, results the breaking of C-C bond in higher alkanes.2. From alcohols and PX
3, PCl5, SOCl2, HX and
acidic KI or Nal :
Red phosphorous and halogen produces PX3
(PCI
3,PBr3, PI3) and PCl5 compounds. PBr5 and
PI
5 do not exist.
In case of preparation of R-Br and R-I, PBr3 or
PI3 required for the reaction is generally produced in situ by the action ofred phosphorous on bromine
2P + 3X
2 r 2PX3 2P + 3CI3 r 2PCI2
2P + 3Br
2 r 2PBr3 2P + 312 r 2PI3
(a) Action of PXs on alcohol :
When alcohols are heated with PKs to give alkyl
halide and phosphorous acid.
3R - OH + PX
3
bbr 3R - X alcohol alkyl halide (b) Action of PX5 on alcohols
2P + 5Cl
2 r 2PCl5
When aleohols are heated with PCl5 to give
R - OH + PCl
3 P Cl
bbbr R - Cl + POCl3 + HCl alcohol alkyl halide
Note: This method gives goodyield of primary
alkyl halides but poor yield of secondary and tertiary alkyl halides. This method is useful for preparing lower alkyl bromides and iodides in the laboratory. (c) From action of thionyl chloride on alcohols or sulphonyl chloride (Darzen method):
When alcohols are refluxed with thionyl chloride
in the presence of pyridine (C
5H5N) to give alkyl
chlorides.
R-OH + SOCl
2 P Pyridine
reflux bbbrR-Cl + HCl+SO2 alcohol alcohol halide
Note: It is convenient method for the preparation
of alkyl chlorides only as it has advantages over phosphorous chloride and HCI because the HCI, SO
2 being gas escape out from the reaction mixture
and alkyl chloride formed is pure.
Alkyl bromides and iodides are not prepared by
this method because thionyl bromide is unstable and thionyl iodide do not exist. (d) From action of dry or cone, HX [HCI,HBr, HI]:
R-OH + HX
catalyst bbbrR-X + H2O
3Haloalkanes & Haloarenes
CHEMISTRY - XII OBJECTIVE
Reaction involving cleavage of carbon - Oxygen
(C-OH)bond i.e. involving OHgroup as whole
Such reactions follow the order of reactivity
3° alco. > 20 alco. >1° alco. These reactions are
nucleophilic substitution reactions. (i) Reaction with cone. HCl (Grooves process):
Reaction with cone. HCI in the presence of
anhydrous ZnCl2 with alcohol is known as Lucas reagent.
R-OH + HCl
2AnhydrousZnCl
bbbbbrj R-CI + H2O Note : In alicyclic carbocation smaller ring releare strain and convert it into larger ring carbocation. (ii) Reaction with cone. HBr : When alcohols are refluxed with dry or cone. (48%) HBr to give alkyl bromide. HBI' is prepared i situ from NaBr and cone. H 2SO4,
R-OH + HBr
2 4NaBr Conc.H SO
bbbbbbr R-Br + HO
Note: In the case of HBr, a small amount of
concentrated H
2SO4 is also added for the reaction
with primary alcohols. But no catalyst is added with secondary and tertiary alcohols, which get dehydrated in the presence of concentrated H 2SO4, (iii) Reaction with cone. HI : When alcohols are refluxed with cone. HI gives alkyl iodide.
R - OH +
Conc.HIr R - I + H2O
Note: The R-I is best obtained by heating alcohol
with Nal or KI in 95% H 3PO4.
Hydroxy group from alcohol is replaced by X using
reagents PC1
5, PCI3, SOCI2,HCI, acidic Nal or
KI.
3. From addition of HX in alkenes
(Hydrohalogenation): These are electrophilic addition reaction. In which reactive intermediate is carbonium ion. Reactivity order of HX is
HI > HBr > HCI > HF.
+ HBr bbr Br
I-pheny lethene 1-pheny lbromomethane
Peroxide effect or Kharsch-Myo effect: When
unsymmetrical alkenes are reacted with HBr in the presence ofperoxide (organic peroxide orinorganic peroxide) gives anti Markownikov product. It is stated that "When unsymmetrical alkenes are reacted with HBr, then Br atom goes to that carbon atom, having more number of hydrogen atoms."
When propene is reacted with HBr, in the presence
of peroxide, gives n-propyl bromide. CH
3-CH = CH2 + HBr2 2R O
bbbr
Propene
CH
3-CH2-CH2-Br
n-propyl bromide
Mechanism of peroxide effect: Addition of HBr
in unsymmetrical alkenes in the presence of peroxide as follows : R
2O2 + 2RO
R
2O2 + HBr r ROH + Br
CH
3CH = CH2 + Br r CH3CH - CH2Br
2° free radical (more stable)
CH
3CH=CH2Br + HBr r CH3CH2CH2Br + Br
Br + Br r Br2
Out of H and HI' (obtained from II Br). The II
free radical is less stable than Br free radical, hence
H free radical combine first with RO free radical
which are obtained from peroxide. Then Br attack first on alkene and produces product. Therefore in free radical addition reaction, the first attacking species on alkene is Br free radical.
Note: Peroxide effect is applicable only for HBr
not for HCI, HI, H
2SO4, H2O, NH3, HOCI, etc.
Remember that HCI, HI, H
2SO4, H2O, NH3,
HOCI do not give anti Markownikoffs product in
the presence of peroxide because of following reasons, i) The HCI (103 K cal/mole), H
2SO4, H2O, HOCI,
NH
3 bonds are stronger than HBr bond
(87 K cal/mole). Hence peroxide not able to break bond in HCI, H
2SO4, H2O, HOCI, NH3.
(ii) The HI bond (71 K cal/mole) is weaker than HBr bond, broken by peroxide and produces iodine free radical. But the iodine free radical so formed readily combined with each other to yield iodine molecule rather than attack the double bond in alkene. I
0 + I0 r I2
These are free radical (n on ion ic ) addition
reactions.
4Haloalkanes & Haloarenes
CHEMISTRY - XII OBJECTIVE
4. Halogen exchange:
(a) Finkelstein reaction: Alkyl iodide is prepared from
R-CI and R-Br with Nal in dry acetone.
R - X + NaI
dryacetone bbbbrR - I + NaX (b) Swarts reaction: Only alkyl fluoride is prepared from R-CI and R-Br with metallic chloride i.e.
AgF, Hg2F
2, CoF2, SbF2.
R - X + AgF r R-F + AgCI
X = CI,Br
Nucleophilic Substitution Reactions
"The reaction in which halogen atom from alkyl halides are replaced by another nucleophile."
1. Alkaline hydrolysis: These are hydrolysed
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[PDF] Haloalkanes and Haloarenes - NCERT