Unit 4 - Alkyl Halides

Report
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.

similar documents