Halogeno-compounds
1. Classification:
(a) Primary halide (1 halide)
H
H
H
H
H
H
H
H
H
Cl
(b) Secondary halide (2 halide)
Cl
H
H
H
H
H
H
H
H
H
(c) Tertiary halide (3 halide)
H
H
H
H
Cl H
H
H
H
H
2. Preparation
(a) Alcohol reacts with hydrogen halide
R  OH  HCl ZnCl
2  R  Cl  H 2 O
(Lucas test)
The above reaction is a SN1 reaction, and ZnCl2 is used as a catalyst. The rate follows the
following order.
3 > 2 > 1
For bromide and iodide, they are more reactive, and catalyst is not required.
NaBr  H 3 PO4 
 NaH 2 PO4  HBr
Phosphoric acid is a non-volatile acid.
R  OH  HBr 
 R  Br  H 2 O
NaI  H 3 PO4 
 NaH 2 PO4  HI
R  OH  HI 
 R  I  H 2 O
Iodide > bromide > chloride
(b) Alcohol reacts with phosphorus trichloride
3ROH  PX 3 
 3RX  H 3 PO3
(c) Alcohol reacts with phosphorus pentachloride
R  O  H  PX 3 
 R  X  POX 3  HX
(white fumes, a test for –OH group)
(d) Alcohol reacts with thionyl chloride
R  O  H  SOCl 2 
 R  Cl  SO2  HCl
3. Properties
2 types of nucleophilic substitution
(a) Unimolecular nucleophilic substitution / SN1:
Halide / page 1
Br
OH
H
+
O
H
+
H
Br
Reaction mechanism:
Step I
H
H
Br H
H
H
H
H
H
rds
H
H
H
H
H
H
+
H
H
H
+
Br
-
H
* carbonium ion is trigonal planar (sp2)
Step II
H
+
H
OH
O
+
+
+
fast
fast
H
+
H
H
O
* racemic mixture if the halide is optically active
rate = k[halide]
rate order = 1
Enthalpy
carbonium ion
alcohol
halide + water
Reaction Profile
(b) Bimolecular nucleophilic substitution / SN2:
Br
+
OH
-
NaOH(aq)
OH
+
Br
-
Reaction mechanism:
Rate determining step
Halide / page 2
H
H
H
+
Br
H
OH
H
-
H
H
H
Br
Br
-
+
OH
H
OH
H
* The transition state is a trigonal bipyramid.
d-isomer
l-isomer
The reaction causes optical inversion.
rate = k[halide][nucleophile]
rate order = 2
Transition state
Enthalpy
alcohol
halide + OH
Reaction Profile
(c) Competition between SN1 and SN2:
(i)
Structure of the halide:
(I)
Electronic effect
Inductive effect: Highly branched halide can form stable carbonium ion, so it
prefers SN1.
Resonance effect: Conjugated  bond can form stable carbonium ion, so it
prefers SN1.
(II)
Steric hindrance
Highly branched halide prefers SN1, because the intermediate becomes less
bulky.
Primary halide prefers SN2.
(ii) Nucleophilic strength: Strong nucleophile makes SN2 easily.
(iii) Polarity of solvent: Polar solvent can stabilize the carbonium ion.
Rate of SN1:
3 > 2 > 1
Rate of SN2: 1 > 2 > 3
Overall rate: 3 > 1 > 2
(halide)
(halide)
Halide / page 3
rate
1
2
3
(d) Other examples of nucleophilic substitution:
(i) Halides reacts with cyanide ion (KCN, NaCN in alcohol solution)
N
Cl
O
+
H / H O
2
NaCN(alcohol)
OH
(ii) Amine formation
H
H
NH
H
3
H
NH2
H
H
Br
CH3
N
CH Br
3
H
CH Br
3
CH3
1 amine
CH3
N
CH Br
3
Br
CH3
CH3
+N
CH3
CHCH
3 3
CH3
2 amine
-
3 amine
tetramethylammonium bromide
(e) Elimination
Cl
CH3
CH3
CH3
H
CH3
+
OH
CH3
CH3
-
+
CH3
H
O
H
+
Cl
-
CH3
The driving force of elimination if steric hindrance.
Factors affecting elimination:
(i) highly branched halide: The product becomes less bulky.
(ii) non-aqueous solution (non-polar solvent): The equilibrium shifts to the RHS.
(iii)high temperature: The equilibrium shifts to the RHS, because alkene has a lower bp.
(iv) strong base: It removes hydrogen ion, and forces the equilibrium to RHS.
Conclusion:
Halide
SN1
SN2
E
1


strong nucleophile
room temperature

halide with bulky group
strong base
high temperature
non-aqueous solution
2

low temperature
aqueous solution
weak base

low temperature
strong nucleophile

high temperature
non-aqueous solution
strong base
3

low temperature
aqueous solution
weak base


high temperature
non-aqueous solution
strong base
Halide / page 4