Chapter Seven
Haloalkanes
Key words
 SN1
 SN2
 E1
 E2
 Racemic mixture and racemization
Haloalkanes
X = Cl, Br, I


C X

Halogens are more electronegative than carbon so dipole moments
would point towards the halogen and the bond is longer….easily
breaks!!!
Haloalkanes
sp3
C
Attached to
1 carbon atom
C
a
1o
Cl
chloride
X
Attached to
Attached to
2 carbon atoms 3 carbon atoms
C
C
C
C
Br
a 2o bromide
C
I
a 3o iodide
Nomenclature - IUPAC
• Locate the parent alkane.
• Number the parent chain to give the substituent encountered
first the lower number.
• Show halogen substituents by the prefixes fluoro-, chloro-,
bromo-, and iodo- and list them in alphabetical order with other
substituents.
• Locate each halogen on the parent chain.
2
5
3
4
3
4
Br
1
2
Br
3-Bromo-2-methylp entan e
6
OH
5
1
1
5
6
4-Bromocyclohexene
4
3 2
Cl
t rans-2-Chlorocyclohexan ol
Nomenclature - IUPAC
• Several polyhaloalkanes are common solvents and are generally
referred to by their common or trivial names.
CH 2 Cl 2
Dichloromethane
(Methylene chloride)
CHCl 3
Trichloromethane
(Chloroform)
CH 3 CCl 3
1,1,1-Trichloroethane
(Methyl chloroform)
CCl 2 =CHCl
Trichloroethylene
(Trichlor)
Freons and Their Alternatives
• The Freons are chlorofluorocarbons (CFCs)
• Among the most widely used are/were
CCl3 F
Trichlorofluorometh ane
(Freon-11)
CCl2 F2
D ich lorodiflu orometh ane
(Freon-12)
• Much lower ozone-depleting alternatives are the
hydrofluorocarbons (HFCs) and the hydrochlorofluorocarbons
(HCFCs), including
CH2 FCF3
HFC-134a
CH3 CCl2 F
HCFC-141b
Substitution & Elimination
• In this chapter we, concentrate on two types of reactions:
• nucleophilic substitution
• b-elimination
NUCLEOPHILIC SUBSTITUTION
Nu +
(nucleophile)
The Nu⊖
donates
an e⊖ pair
to the
substrate
Ch. 6 - 8



C X
(substrate)
The bond
between
C and LG
breaks,
giving both
e⊖ from the
bond to LG
Nu
C
(product)
The Nu⊖ uses
its e⊖ pair to
form a new
covalent bond
with the
substrate C
+ X
(leaving
group)
The LG
gains the
pair of e⊖
originally
bonded
in the
substrate
NUCLEOPHILIC SUBSTITUTION
• In the following general reaction, substitution takes place on an
sp3 hybridized (tetrahedral) carbon.
leaving
group
-
+
Nu
N ucleoph ile
C X
nucleoph ilic
sub stitution
C Nu +
-
X
Slow
NUCLEOPHILIC SUBSTITUTION
• Table 7.1 Some Nucleophilic Substitution
Reactions
Mechanism
• Chemists propose two limiting mechanisms for nucleophilic
substitutions.
• A fundamental difference between them is the timing of bond
breaking and bond forming steps.
• At one extreme, the two processes take place simultaneously;
designated SN2.
• S = substitution
• N = nucleophilic
• 2 = bimolecular (two species are involved in the ratedetermining step)
• rate = k[haloalkane][nucleophile]
SN2 Reaction
• Both reactants are involved in the transition state of the ratedetermining step.
• The nucleophile attacks the reactive center from the side
opposite the leaving group.
SN2 Reaction
• Table 7.1 An energy diagram for an SN 2 reaction.
• There is one transition state and no reactive intermediate.
SN2 - Order of Reactivity of ALKYL HALIDES
SN2 – RESULTS IN ENATIOMERS OF ASSYMETRIC
CHIRAL COMPOUNDS
SN1 Reaction
• In the other limiting mechanism, bond breaking between carbon
and the leaving group is entirely completed before bond forming
with the nucleophile begins.
• This mechanism is designated SN1 where
• S = substitution
• N = nucleophilic
• 1 = unimolecular (only one species is involved in the ratedetermining step)
• rate = k[haloalkane]
SN1 Reaction
• SN1 is illustrated by the solvolysis of tert-butyl halide.
Step 1: Ionization of the C-X bond gives a carbocation
intermediate.
SN1 Reaction
RACEMIZATION:
SN1 can result to two products being formed in EQUAL
amounts. Such products make a racemic mixture with
50:50 percentage composition. This is because the
Nucleophile in the second FAST step may attack the sp2
Trigonal PLANAR hybridized carbon from the top or
bottom face with EQUAL chance.
SN1 Reaction
SN1 Reaction
• SN1 is illustrated by the solvolysis of tert-butyl bromide.
• Step 1: Ionization of the C-X bond gives a carbocation
intermediate.
H 3C
C
Br
slow , rate
d etermining
CH3
C+
+
Br
H3 C
H3 C
H 3 C CH3
A carbocation intermediate;
carbon is trigonal p lanar
SN1 Reaction
• Step 2: Reaction of the carbocation (an electrophile) with
methanol (a nucleophile) gives an oxonium ion.
CH3
+
CH3 O
H
C+
H3 C
+
OCH3
fast
O
H
H
H3 C CH3
CH3
H3 C
+
C
CH3
CH3
CH3
C O
H3 C
H3 C
H
• Step 3: Proton transfer to methanol completes the reaction.
H3 C
H3 C
H3 C
C
+
O
CH3
H
H
+
O
CH3
H3 C
fas t
H3 C
H3 C
CH3
C O
+
+ H O
H
CH3
SN1 Reactions and Rearrangements
H
Cl
H
Br
+
OH
OH
+
Evidence for Substitution Reactions
• Let us examine some of the experimental evidence on which
these two mechanisms are based and, as we do, consider the
following questions.
• What affect does the structure of the nucleophile have on the
rate of reaction?
• What effect does the structure of the haloalkane have on the
rate of reaction?
• What effect does the structure of the leaving group have on the
rate of reaction?
• What is the role of the solvent?
1. Nucleophilicity
• Nucleophilicity: a kinetic property measured by the rate at which
a Nu: attacks a reference compound under a standard set of
experimental conditions.
• For example, the rate at which a set of nucleophiles displaces
bromide ion from bromoethane in ethanol at 25°C.
CH3 CH2 Br + NH3
CH3 CH2 NH3
+
+ Br-
• Table 7.2 shows common nucleophiles and their relative
nucleophilicities
Relative Nucleophilicity
• Some Common Nucleophiles and their Relative Effectiveness
2. Structure of the Haloalkane
• SN1 reactions
• Governed by electronic factors, namely the relative stabilities
of carbocation intermediates.
• Relative rates: 3° > 2° > 1° > methyl
• SN2 reactions
• Governed by steric factors, namely the relative ease of
approach of the nucleophile to the site of reaction.
• Relative rates: methyl > 1° > 2° > 3°
2. Structure of the Haloalkane
• Steric factors
• Compare access to the reaction center in bromoethane and
2-bromo-2-methylpropane (tert-butyl chloride).
2. Structure of the Haloalkane
• Effect of electronic and steric factors in competition between SN1
and SN2 reactions of haloalkanes.
The Leaving Group
• The best leaving groups in this series are the halogens I-, Br-, and
Cl-.
• OH-, RO-, and NH2- are such poor leaving groups that they are
rarely if ever displaced in nucleophilic substitution reactions.
Rarely act as leavi ng gro ups
in nucleo phil ic substi tutio n
G reater abi lity as leavi ng gro up and -el imin atio n reaction s
O
-
I > Br
-
-
-
-
-
-
> Cl >> F > CH3 CO > HO > CH3 O > NH2
-
Greater stabi li ty o f ani on; greater stren gth o f co njug ate acid
The Solvent
• Protic solvent: a solvent that contains an -OH group.
• Table 7.3 Common Protic Solvents
The Solvent
• Aprotic solvent: does not contain an -OH group.
• Aprotic solvents favor SN2 reactions. Although the solvents at
the top of the table are polar, formation of carbocations in them
is more difficult than in protic solvents.
Table 7.5 Summary of SN1 and SN2
Predict the substitution products?
Which of these will be more reactive towards SN2 reaction?
Br
Br
Br
I