Alkyl Halides
(haloalkanes)
RX
Classification
Alkyl Halide
Primary
Alkyl Halides
R
H

C
H
X
Secondary
Alkyl Halide
R
H

C
R'
X
Tertiary
Alkyl Halide
R
R"

C
X
R'
2
Nomenclature
A- Monohalogen Derivatives :
Common Name
CH3 CH2 Cl
Ethyl chloride
CH3CH2CH2Cl
N-Propyl chloride
CH2Cl
Benzyl chloride
3
Nomenclature (Contd)
IUPAC Name
CH3CH2CH2Cl
1-chloropropane
Br

CH3CH2CHCH2CH3
3-bromopentane
Br
3 2
1
CH3CH2CHCH= CH2
3-bromo-1-pentene
4
Nomenclature (Contd)
B- Dihalogen Derivatives :
Dihalogen
Derivatives
Geminal
Dihalides
e.g.
Br
CH3CH
Br
Vicinal
Dihalides
e.g.
H2C
Cl
CH2
Cl
5
Nomenclature (Contd)
•
Geminal Dihalides:
IUPAC
Br
CH3CH
1,1-Dibromoethane
Br
Cl
CH3CCH3
Cl
2,2-Dichloropropane
6
Nomenclature (Contd)
•
Vicinal dihalides:
IUPAC
H2C
Cl
CH2
1,2-Dichloroethane
Cl
CH3
CH3CCH2Br
1,2-Dibromo-2-methylpropane
Br
7
Synthesis
I- From Hydrocarbons:
e.g.
CH4
h 
Cl2
H3C Cl
(Free radical mechanism)
II- From alkenes:
e.g.
H3C C CH2
H
HCl / H2O2
AntiMarkonikov
addition
CH3CH2CH2Cl
HCl
Markonikov
addition
H
H3C C CH3
Cl
(Allyl chloride)
8
X
III- From alkynes:
eg. :
CH3
C
HX
Markonikov. addition
CH
1-propyne
HX, H2O2
Antimark.
addition
CH3
CH2
C
HX
X
CH3
C
CH3
H
X
CH3
CH - X
C
HX, H2O2
X
CH3
CH2
CH
X = I, Br, Cl
X
IV- From alcohols:
ROH + SOCl2
RCl + SO2 + HCl
9
Reactions
I- Preparation of Grignard reagent:
 
RX
Mg / Dry ether


RMgX
10
Reactions (Contd)
II- Nucleophilic substitution reaction:
RX + Nu
e.g. RBr + NaOH
RNu + XROH + NaBr
can proceed through
SN1 OR SN2
11
• Bimolecular nucleophilic substitution reaction
(SN2)
Nu
 
R-X
Slow


[Nu....C....X]
highly unstable T.S.
Fast
Nu-R + X
12
• Bimolecular nucleophilic substitution reaction
(SN2) (Contd)
e.g. CH3Br
NaOH
NaBr
CH3OH
Mech.
H
H
Nu


C
Br
H
Slow step



C
Nu
Br
H
H
unstable transition
state
H
Nucleophile attacks the
back side of C
Fast
H
Nu
Br
C
H
H
13
Aq. NaOH
e.g.2 CH3CH2CHCH3
CH3CH2CHCH3
Cl
OH
(R)-2-Chlorobutane
(S)-2-butanol
Mech.
CH3
H
H
Cl
H
C

Et
Et
Cl

OH

C
HO
Cl
Et
H3C

CH3
(R)
unstable transition
state
CH3
H
H
OH
HO
C
Et
Et
(S)
CH3
inversion of configuration
14
Unimolecular nucleophilic substitution reaction
(SN1)
RX
Slow
R
+
X
Fast
Nu
Nu-R
15
Unimolecular nucleophilic substitution reaction
(SN1) (Contd)
CH3
CH3
e.g. H3C C Br
H 3C
H 2O
C OH
CH3
CH3
Mech.
b
H3C
C
sp3c
H3C
complete
Br
ionization
H3 C
CH3
a
C
H3 C
sp2
CH3
planar
Nu
attack from both sides
CH3
Nu
H3 C
C
C
CH3
CH3
50 %
Nu
H3 C
H3 C
50 %
16
Br
OH
H2O
e.g. H3CH2CH2C C CH2CH3
C CH2CH3
H3CH2CH2C
CH3
CH3
50 % (R)-3-hydroxy-3-methylhexane
50 % (S)-3-hydroxy-3-methylhexane
(R)-3-bromo-3-methylhexane
Mech.
Et
Br
Pr
b
H3C
CH3
CH3
C
a
C
Br
sp3c
H3CH 2CH 2C
H3CH2C
H3CH2CH2C
sp2
CH2CH3
planar
H2O
attack from
both sides
H3C
CH3
HO
C
C
CH2CH2CH3
H3CH2CH2C
CH2CH3
CH2CH3
50 % (S)
OH
50 % (R)
Racemic modification
17
Factors affecting nucleophilic
substitution reaction (SN2 &SN1)
1- Structure of alkyl halide:
H
Nu


C
X
SN2 mechanism
H
H
Thus
R
R
CH3
> RCH X >
CH X
2
R
>
R C X
R
18
1- Structure of alkyl halide (Contd):
b
H3C
C
sp3c
H3C
complete
Br
ionization
H3C
CH3
a
C
H3C
sp2c CH3
planar
SN1 mechanism
Thus
R
R
R
R
C X
>
> RCH X
2
CH X
>
CH3
R
19
2- Solvent
Solvent
Non polar
e.g. benzene
Polar
Polar protic
Polar aprotic
e.g. H2O, ROH,
CH3COOH
e.g. DMF HCON(CH3)2
DMSO (CH3)2SO
Polar protic solvents (which release H+ by ionization, e.g. H2O,
alcohol) favour SN1.
Polar aprotic solvents (do not release H+ by ionization) as DMF,
DMSO favour SN2.
20
3- Nature of nucleophile.
CN - > I- > OR- > OH- > Br- > Cl- > ROH > H2O
nucleophilicity increase
_
_
OR , OH strong Nu
ROH , H2O weak Nu
strong Nu favour SN2 reaction
weak Nu favour SN1 reaction which is not dependent on conc.
of Nu
+
H 2C
CH
CH2Cl
does not undergo SN2 reaction but
undergo SN1 reaction due to
stabilization of the formed
carbocation by resonance
H 2C
+
H 2C
CH
CH2
H
C
CH2
N.B.
CH2=CHCH2Cl
allyl chloride
CH2=CHCH2
CH2-CH=CH2
CH2Cl
CH2
CH2
benzyl chloride
CH2=CH X
CH2-CH-X
vinyl halide
23
N.B. (Contd):
 Neopentyl carbocation rearrangement for neo-carbon
CH3
H3C
C CH2Cl
CH3
weak Nu
H2O
SN1
CH3
H3C
C CH2
CH3
neopentyl chloride
methide shift
CH3
H3C
C CH2CH3
CH3
H2O
H3C
C CH2CH3
OH
2-hydroxy-2-methylbutane
24
Applications
e.g.1
CH3 Cl
H3C
C
C CH3
H
H2 O weakNu
SN1
CH3
H3C
C CH CH3
CH3
CH3
2-chloro-3,3-dimethyl
butane
CH3 CH3
H3C
C
methide shift
CH3 CH3
H2O
CHCH3
C
H3C
CHCH3
OH
I
e.g.2
HC
H2O
CHCH2I
1,3-diiodide-1-propene
I
HC
CHCH2OH
SN1
25
Applications (Contd)
e.g.3
Cl
H 2C
C
H
CHCH3
H2O or EtOH
SN1
H2C
C
H
CHCH3
H2 C
OH
H 2C
C
H
CHCH3
OEt
C
H
CHCH3
OH
H2C
OR
H2C
C
H
CH CHCH3
OR
CHCH3
OEt
H2C
CH CHCH3
Major
26
Reactions (Contd)
III- Elimination reaction:
Dehydrohalogenation of alkyl halide
H


C C
+ B:
X
alc. KOH / 
_
+
BH
+
X
C C
can proceed through
E1 OR E2
Dehalogenation of vicinal dihalide
Br
C C C
H
Br
Zn / HOAc
NaI / acetone
C HC C
27
• Bimolecular Elimination Reaction (E2)
Br
H3C C CH3
H
+ NaOEt
EtOH
CH2=CHCH3
propylene
Mechanism
H H
EtO
C
H
C
H Br

CH3
slow
CH3
EtO H H
H
H
fast
C C
C C
H
H
CH3
H Br

Rate-determining step
+ EtOH
+ Br-
28
• Unimolecular Elimination Reaction (E1)
CH3
H2O
H3C C Br
H3C C CH2 +
CH3
H3O+
CH3
Mechanism
H3C
H3C
H3C
CH3
 
C Br
H3C
CH3
T.S.1
H3C
H
+
C Br

C C H
H
H3C
Slow
H2O
H3C

C
Fast
C
H
H3C
H3C
H3O+ +
H
H OH2
H
T.S.2
C C
H3C
H
29