Unit 4 – Alkyl Halides, Nucleophilic
Substitution, and Elimination Reactions
 Nomenclature and Properties of Alkyl Halides
 Synthesis of Alkyl Halides
 Reactions of Alkyl Halides
 Mechanisms of SN1, SN2, E1, and E2 Reactions
 Nucleophilicity, Substrate, and Leaving Group
Effects
Alkyl Halides
 Alkyl halide:
 a compound with a halogen atom bonded to
one of the sp3 hybridized carbon atoms of an
alkyl group
 Two types of names:
 IUPAC system
 Common names
Nomenclature
 IUPAC System:
 Alkyl halides are named as an alkane with a
Br
halo-substituent:
 Review the rules for naming alkanes
covered in Unit 2
CH3CH2CH2Cl
1-chloropropane
Br
bromocyclohexane
Br
Nomenclature
 Common Names:
 alkyl group name + halide
CH3CH2CH2Cl
n-propyl chloride
Br
Cyclohexyl bromide
CH3CH2CH2Cl
Nomenclature
 Special common names:
 CH2X2 = methylene halide
 CHX3 = haloform
 CX4 = carbon tetrahalide
CH2Cl2
Methylene chloride
dichloromethane
CHCl3
chloroform
trichloromethane
CCl4
Carbon tetrachloride
tetrachloromethane
Types of Alkyl Halides
 Alkyl halides can be classified by the type of
carbon atom the halogen is bonded to:
 primary halide (1o):
 halogen attached
CH3CH2Cl
o
to a 1 carbon
 secondary halide (2o):
 halogen attached to a 2o carbon
 tertiary halide (3o):
 halogen attached to a 3o carbon
I
CH3CHCH3
CH3
CH3CBr
CH3
Types of Alkyl Halides
 Geminal dihalide:
 2 halogens bonded to the same carbon atom
H
H C
Cl
Cl
 Vicinal dihalide:
 2 halogens bonded to
adjacent carbon atoms
Br
Br
H
H
Cl
C C
H
Other Organic Halides
F
F
C C
 Aryl halide:
F
F
 halogen is attached directly to an aromatic
ring
I
I
HO
NH2
CH2 CH CO2H
I
I
thyroxine
 Benzylic halide
 halogen is attached to a carbon that is
attached to a benzene ring
benzylic carbon
CH2Cl
benzylic chloride
Other Organic Halides
 Allylic halide:
 halogen is attached to a carbon that is
attached to a C=C
Allylic carbon
H
H
C
C
CH2Cl
H
Allylic
chloride
Other Organic Halides
H
Cl
 Vinyl Halide:
C C
 halogen attached to a carbon that is part of
H
a C=C
H
H
C C
Cl
F
H
F
Monomer for PVC
F
F
H
C C
F
I Monomer
I for teflon NH
F
F
C C
F
2
HO
CH2 CH CO
I
I
Uses of Alkyl Halides
 Anesthetics:
 Chloroform (CHCl3)
 toxic
 carcinogenic (causes cancer)
 Solvents:
 CCl4
 formerly used in dry cleaning
 CH2Cl2
 formerly used to decaffeinate coffee
 liquid CO2 used now
Uses of Alkyl Halides

Cl H
Freons:
Cl C C
 Freon-12:
 Freon-22:
Cl
 Freon-134a:
 Pesticides:
CF2Cl2Cl
CHClF2
Cl
Cl H
Cl C C
Cl Cl
Cl
Cl
Chlordane
(termites)
F F
F C C H
F H
Cl
Cl
Cl
DDT
Cl
Cl
Cl
Cl
Physical Properties
 Boiling Point:
 Compounds with higher MW’s and greater
surface area (more linear) tend to have
higher BP.
 BP increases as size of halogen increases
 F < Cl < Br < I
 BP decreases as branching increases
Physical Properties
 Density:
 Alkyl chlorides are common solvents for
organic reactions.
 CH2Cl2
 CHCl3
 CCl4
More dense than water
Preparation of Alkyl Halides
 Alkyl halides can be prepared from a variety of
starting materials including alkanes, alkenes,
alkynes, alcohols, and other alkyl halides.
 You are responsible for knowing and applying the
synthesis of R-X by:
 free radical halogenation reactions
 free radical allylic bromination reactions
Preparation of Alkyl Halides
 Free Radical Halogenation of Alkanes
alkane + X2
hu
or D
alkyl halide(s) + HX
 Poor selectivity and moderate yields often limit
usefulness.
 Bromination is more selective and gives the
product formed from the most stable free
radical.
 Chlorination is useful when only one type of
reactive hydrogen is present
3
3
2
CH3
CH3
Preparation of Alkyl Halides
 Useful Examples:
+ Cl2
Cl
hu
+ HCl
50 %
CH3CHCH3
CH3
+ Br2
h
Br
CH3CCH3
CH3
90%
+ HBr
Preparation of Alkyl Halides
 The following free radical halogenation is
doomed to failure!
Br2
hu
Br
 The following addition reaction occurs instead:
Br2
Br
hu
Br
Preparation of Alkyl Halides
 Free Radical Allylic Bromination:
C C
C
+ NBS
h
C C
C
H
Br
 where NBS = N-bromosuccinimide
O
O
N Br
O
NBS
N H + Br2
+ HBr
O
Preparation of Alkyl Halides
 NBS is used to generate low levels of Br2 in
situ.
 Minimizes addition of bromine across the C=C
 Allylic bromination is highly selective and occurs
in the allylic position due to resonance
stabilization of the resulting free radical.
HH
HH
HH
HH
HH
HH
Br
+ NBS
Preparation of Alkyl Halides
Br
 Examples:
Br
+ NBS
+ NBS
+ NBS
hu
hu
Br
Reactions of RX
 Most reactions of alkyl halides involve breaking
the C-X bond.
 Nucleophilic substitution
 Elimination
C
d+
X
d-
 The halogen serves as a leaving group in these
reactions.
 the halogen leaves as X-, taking the bonding
electrons with it
Reactions of RX
 Nucleophilic substitution:
 reaction in which a nucleophile replaces a
leaving group
 Nucleophile:
 electron pair donor
 Leaving group:
 an atom or group of atoms that are lost
during a substitution or elimination reaction
 retains both electrons from the original
bond
Reactions of RX
 General Equation for Nucleophilic Substitution
C C X
Nuc
+
-
C C Nuc + X
 The nucleophile can be neutral or negatively
charged, but it must have at least one lone pair
of electrons.
 Example:
Br
OCH3
+
CH3O
+
Br
Reactions of RX
 Elimination Reaction:
 two substituents are lost from adjacent
(usually) carbons, forming a new p bond
 Dehydrohalogenation:
 an elimination reaction in which H+ and X- are
lost, forming an alkene
H
H H
C
H
CH3CH3
CH
O
3
C
CHCH3
C C
H H
Br Br
3
H H
CH3CH3
C C
C C
CH
CH
H
3
H
3
Reactions of RX
 There are two common types of nucleophilic
substitution reactions:
 SN1 reactions
 substitution, nucleophilic, unimolecular
 3o, allylic, benzylic halides
 weak nucleophiles
 SN2 reactions
 substitution, nucleophilic, bimolecular
 methyl and 1o halides
 strong nucleophiles
Reactions of RX
Reactions of RX
 Common strong nucleophiles:
 hydroxide ion
 alkoxide ions
 many amines
 iodide and bromide ions
 cyanide ion
 Common weak nucleophiles:
 water
 alcohols
 fluoride ion
SN2 Reactions
 The reaction between methyl iodide and
hydroxide ion is a concerted reaction that takes
places via an SN2 mechanism
H
H C
H
I
H
+
OH
nucleophile
substrate
HO C H
H
product
 Substrate:
 the compound attacked by a reagent
(nucleophile)
+
I
Leaving
group
SN2 Reactions
 Concerted reaction:
 a reaction that takes place in a single step
with bonds breaking and forming
simultaneously
 SN2:
 substitution, nucleophilic, bimolecular
 transition state of rate-determining step
involves collision of 2 molecules
 2nd order overall rate law
 Rate = k[RX][Nuc]
SN2 Reactions
 SN2 Mechanism:
 Nucleophile attacks the back side of the
electrophilic carbon, donating an e- pair to
form a new bond
X
X
H
H
C
+
H
Nuc
H
H
C
Nuc
H
H
H
C
H
+
X
Nuc
 Since carbon can only have 8 valence
electrons, the C-X bond begins to break as
the C-Nuc bond begins to form
SN2 Reactions
 SN2 Mechanism for the reaction of methyl
iodide and hydroxide ion:
I
I
H
H
C
+
H
OH
H
H
H
C
H
OH
H
C
OH
H
+
I
SN2 Reactions
 Reaction Energy Diagram:
 large Ea due to 5-coordinate carbon atom in
transition state
 no intermediates
 exothermic
SN2 Reactions
 SN2 reactions occur with inversion of
configuration at the electrophilic carbon.
 The nucleophile attacks from the back side
(the side opposite the leaving group).
 Back-side attack turns the tetrahedron of
the carbon atom inside out.
SN2 Reactions
 Inversion of configuration:
 a process in which the groups bonded to a
chiral carbon are changed to the opposite
spatial configuration:
R
S
or S
R
SN2 Reactions
Example: Predict the product formed by the SN2
reaction between (S)-2-bromobutane and sodium
cyanide. Draw the mechanism for the reaction.
H
H3C
SN2 Reactions
 The SN2 displacement reaction is a
stereospecific reaction
 a reaction in which a specific stereoisomer
reacts to give a specific diastereomer of the
product
H
Br
+
-
OH
H
H3C
OH
H
Br
OH
H
H3C
H
+
-
Br
SN2 Reactions
 SN2 reactions occur under the following
conditions
 Nucleophile:
 strong, unhindered nucleophile
 OH- not H2O
 CH3O- not CH3OH
 CH3CH2O- not (CH3)3CO-
 Substrate:
 1o or methyl alkyl halide (most favored)
 2o alkyl halide (sometimes)
 3o alkyl halides NEVER react via SN2
SN2 Reactions
 The relative rate of reactivity of simple alkyl
halides in SN2 reactions is:
methyl > 1o > 2o >>>3o
 3o alkyl halides do not react at all via an SN2
mechanism due to steric hinderance.
 The back side of the electrophilic carbon
becomes increasingly hindered as the number
or size of its substituents increases
SN2 Reactions
 Steric hinderance at the electrophilic carbon:
SN2 Reactions
 SN2 reactions can be used to convert alkyl
halides to other functional groups:
 RX + I R-I
 RX + OHR-OH
 RX + R’OR-OR’
 RX + NH3
R-NH3+ X RX + xs NH3
R-NH2
 RX + CNR-CN
 RX + HSR-SH
 RX + R’SR-SR’
 RX + R’COOR’CO2R
KNOW
THESE!
Be able
to apply
these!
SN2 Reactions
Example: Predict the product of the following
reactions:
NaOH
Br
DMF
I
NH3 (xs)
(CH
)
CHCH
CH
Cl
+
NH
(xs)
3
2
2
2
3
SN2 Reactions
Example: What reagent would you use to do the
CH3CHreactions:
CH2CH2Cl + NaCN
2
following
CH3CH2CH2Br + ?
CH3CH2I +
?
CH3CH2CH2OCH2CH3
CH3CH2C CH