Unit 4
Alkyl Halides: Nucleophilic
Substitution and Elimination
Reactions
Nomenclature of Alkyl Halides
 Apply the same rules you learned
for the alkanes.
CH3CH2Br
 IUPAC: Name the halogen as a
substituent. bromoethane
 Common name: The alkyl group
is named as the substituent.
Ethyl bromide
Nomenclature of Alkyl Halides
3-(iodomethyl)pentane
I
F
4-(2-fluoroethyl)heptane
Nomenclature of Alkyl Halides
 An alkyl halide has a halogen
bonded to an sp3 C.
 Methyl halides, CH3X: the sp3 C is
bonded only to H atoms. CH3Br
 X denotes a halogen: F, Cl, Br, I.
 Primary (1°) halides, RCH2X: the
sp3 C is bonded to one other C
atom.
CH3CH2Br
Nomenclature of Alkyl Halides
2° halide
methyl
halide
1° halide
3° halide
Nomenclature of Alkyl Halides
 A vicinyl dihalide has
two halogens bonded
to neighboring sp3 C
atoms.
 A geminal dihalide has
two halogens on the
same sp3 C.
Nomenclature of Alkyl Halides
 A allylic halogen is one that is
bonded to the sp3 C atom
immediately next to a double
bond.
 FYI: The allylic position can be
occupied by a substituent other
than a halogen.
IR Spectrum of an Alkene (for
comparison)
IR Spectrum of an Allylic
Chloride
The C-Cl str is 600-800 cm-1.
Nomenclature of Other Halides
 A vinyl halide has a halogen
bonded to an sp2 C of an alkene.
 An aryl halide has a
halogen bonded to one of
the sp2 C atoms of the
aromatic ring.
toluene
chlorobenzene
Nomenclature of Other Halides
 A benzylic halide has a halogen
bonded to an sp3 C substituent of
benzene.
Nomenclature: Nitro, Phenyl
 A benzene ring as a substituent is
called phenyl, -C6H5.
 Another substituent to know is
the nitro group, -NO2.
TNT
trans-9-phenyl-2-methylundec-3-ene
Uses of Alkyl Halides
 Solvents
 1,1,1-trichloroethane and methylene
chloride
 Starting materials for syntheses.
 Anesthetics
 Halothane
 Freons
 Pesticides
 DDT, lindane (lice), chlordane (termites)
C-X is a Polar Bond
 Polar bonds have dipole moments
which are proportional to both partial
charge and bond length.
 Partial charge increases with
electronegativity: I<Br<Cl<F.
 Bond length increases down the group.
 These two trends are in opposite
directions, and results in the C-Cl bond
being the most polar, with C-F and CBr close behind.
C-X is a Polar Bond
E.N.
bond
length
partial
charge
dipole
moment
F
4.0
1.38
0.23
1.51 D
Cl
3.2
1.78
0.18
1.56 D
Br
3.0
1.94
0.16
1.48 D
I
2.7
2.14
0.13
1.29 D
for comparison dipole moment
C-H
0.3 D
N-H
1.31 D
O-H
1.53 D
C=O
2.4 D
C≡N
3.6 D
Intermolecular Forces in Alkyl
Halides
 The strongest intermolecular force in
alkyl halides is the London force
(instantaneous dipole-induced dipole).
This force increases with molecular
weight and with surface area.
 Since the C-X bond is polar, there will
also be a contribution from the dipoledipole attraction.
Physical Properties of Alkyl
Halides - Boiling Points
 Here the surface area has more effect
than the C-X dipole. Since atomic size
increases down the halogen group,
boiling points will, too, if the alkyl part
of the compound is the same.
 For compounds with similar molecular
weights, branched compounds will
have lower boiling points than
compounds with straight chains.
Physical Properties of Alkyl
Halides - Densities
 Alkyl fluorides are comparable to
alkanes and are less dense than
water.
 Alkyl chlorides with one Cl are
less dense than water.
 Alkyl bromides and iodides are
denser than water.
Preparation of Alkyl Halides Allylic Bromination
 We will learn other ways to
prepare alkyl halides in later
units.
 Free-radical halogenations often
give a mixture of products.
 Allylic bromination, a free-radical
process, can be selective.
Allylic Bromination Mechanism
 Initiation
 Propagation,
step 1
hυ
Br2  2Br•
allylic free radical
Allylic Bromination Mechanism
 The allylic free radical is
stabilized by resonance and will
form two products.
Allylic Bromination Mechanism
Propagation, step 2.
+ Br•
+ Br•
Allylic Bromination - Overall
Reaction
 NBS, n-bromosuccinimide, is
often used as the source of
bromine. That way, the amount
of bromine is kept low…why?
NBS, hv
H2C=CH-CH2-CH3 
H2C=CH-CHBr-CH3 +
H2CBr-CH=CH2-CH3
Allylic Bromination - NBS
O
O
N
O
Br
+
HBr
Br2
+
NH
O
Br2 is present in trace amounts in NBS (for
initiation), and HBr is formed as soon as any
products are formed, and so is available to
produce more Br2.