Transcript No Slide Title

Structure
Vinylic and Aryl Halides
u Alkyl
halide: a compound containing a halogen
atom covalently bonded to an sp 3 hybridized
carbon atom
• given the symbol RX
R
the halogen is bonded to an sp2 hybridized
carbon, it is a called a vinylic halide
u If it is bonded to a benzene ring, it is called an
aryl halide, given the symbol Ar-X
u If
R
X
X
C
C
X
R
R
A haloalkene
(a vinylic halide)
A haloalkane
(an alkyl halide)
A haloarene
(an aryl halide)
• we do not study vinylic and aryl halides in this chapter
7-1
Nomenclature
Nomenclature
• locate the parent alkane
• number the parent chain to give the substituent
encountered first, be it halogen or an alkyl group, the
lower number
• halogen substituents are indicated by the prefixes
fluoro-, chloro-, bromo-, and iodo- and listed in
alphabetical order with other substituents
Cl Cl
Br CH 3
CH 3 CH 2 CHCHCH 3
5
4
3
2
1
3-Bromo-2-methylpentane
7-2
u For
haloalkenes, numbering is determined by the
location of the C-C double bond
4
3
CH3
2
3
1
CH3 CCH=CH 2
Cl
3-Chloro-3methyl-1-butene
4
2
1
Br
5
6
4-Bromocyclohexene
CH 3 CHCHCH 3
1
2
3
4
2,3-Dichlorobutane
7-3
7-4
Nomenclature
Nomenclature
u Common
names - name the alkyl group followed
by the name of the halide
CH 3 CH 2 Cl
Chloroethane
(Ethyl chloride)
CH 2 =CHCl
Chloroethene
(Vinyl chloride)
u Several
polyhaloalkanes are common solvents
and are generally referred to by their common or
trivial names
CH 2 =CHCH 2 Cl
3-Chloropropene
(Allyl chloride)
CH2 Cl 2
Dichloromethane
(Methylene chloride)
CHCl 3
Trichloromethane
(Chloroform)
CH3 CCl 3
1,1,1-Trichloroethane
(Methyl chloroform)
CCl 2 =CHCl
Trichloroethylene
(Trichlor)
7-5
Nomenclature
Dipole Moments
u Hydrocarbons
in which all hydrogens are
replaced by halogens are commonly named as
perhaloalkanes or perhaloalkenes
F F F
Cl
F-C-C-C-F
F F F
Perfluoropropane
7-6
moment of RX depends on:
• the sizes of the partial charges,
• the distance between them, and
• the polarizability of the unshared electrons on halogen
Cl
Electronegativity of X
CH3 F
4.0
CH3 Cl
3.0
C C
Cl
u Dipole
Cl
Perchloroethylene
7-7
C-H Bond
Length (Å)
1.39
1.78
Dipole
Moment (D)
1.85
1.87
CH3 Br
2.8
1.93
1.81
CH3 I
2.5
2.14
1.62
7-8
van der Waals forces
van der Waals radii
u van
der Waals forces - a group of intermolecular
forces, including
• dipole-dipole
• dipole-induced dipole
• induced dipole - induced dipole (dispersion forces)
atoms or molecules are brought closer
together, van der Waals attractive forces are
overcome by repulsive forces between electron
clouds of adjacent atoms
u Energy
minimum is where net attractive forces
are the strongest
u Nonbonded
interatomic and intermolecular
distances at these minima can be measured by
x-ray crystallography
u As
u Each
atom or group of atoms can be assigned an
atomic or molecular radius called a van der Waals
radius
7-9
van der Waals radii
H
F
Cl
Br
CH 2
1.2
1.35
1.80
1.95
2.0
Bond Lengths, Strengths
CH 3
2.0
u C-F
bonds are stronger than C-H bonds; C-Cl,
C-Br, and C-I bonds are weaker
I
2.15
Increasing van der Waals radius (Å)
u Notice
7-10
Bond
C H
C F
C Cl
C Br
C I
that
• F is only slightly larger than H
• among the halogens, only I is larger than CH3
7-11
BDE
Bond Length
(kcal/mol)
(Å)
1.09
90-100
1.42
105
1.78
80
1.93
65
2.14
50
7-12
Halogenation of Alkanes
Halogenation of Alkanes
u If
a mixture of methane and chlorine is kept in the
dark at room temperature, no change occurs
the mixture is heated, or exposed to visible or
ultraviolet light, reaction begins at once with the
evolution of heat
u What
occurs is a substitution reaction, in this
case, substitution of a chlorine atom for a
hydrogen atom in methane
u If
heat
CH 4 + Cl 2
Methane
CH 3 Cl
+
HCl
u Substitution:
a reaction in which an atom or
group of atoms is replaced by another atom or
group of atoms
CH3 CH3 + Br 2 heat
CH3 CH2 Br +
Ethane
Bromoethane
(Ethyl bromide)
Chloromethane
(Methyl chloride)
HBr
7-13
7-14
Regioselectivity
Regioselectivity
u Regioselectivity
u but
of 2° hydrogen over a 1°
hydrogen is high for bromination
CH 3 CH 2 CH 3
Propane
Br 2
heat or
light
not as high for chlorination
CH3 CH2 CH3
Propane
Br
CH 3 CHCH 3 +
2-Bromopropane
(92%)
Cl 2
heat of
light
Cl
CH3 CHCH3
CH 3 CH 2 CH 2 Br
2-Chloropropane
(57%)
1-Bromopropane
(8%)
7-15
+ CH3 CH2 CH2 Cl
1-Chloropropane
43%)
7-16
Regioselectivity
u Regioselectivity
Energetics
is 3° > 2° > 1°
u Using
BDE (Appendix 3), we can calculate the
heat of reaction, DH°, for the halogenation of an
alkane
• for bromination - approximately 1600:80:1
• for chlorination - approximately 5:4:1
u Example:
draw all monobromination products
and predict the % of each for this reaction
CH 3
CH 3 CH
+
Br 2
7-17
Mechanism
7-18
Formation of Radicals
uA
radical chain mechanism
u Radical: any chemical species that contains one
or more unpaired electrons
u Radicals
CH3 CH2 O OCH2 CH3
Diethyl peroxide
are formed by homolytic cleavage of a
CH 3 CH 3
Ethane
bond
Cl
Cl
Chlorine
light
CH 3 Cl + HCl
-85
-103
DH° = +105 + 59 + (-85) + (-103) = -24 kcal/mol
heat
CH 3
2-Methylpropane
CH 4 + Cl 2
BDE
+105
+59
(kcal/mol)
+
Cl •
• Cl
Chlorine atoms
• a barbed curved (fishhook) arrow is used to show the
change in position of a single electron
7-19
heat
80
o
CH3 CH2 O• + • OCH2 CH3
Ethoxy radicals
CH 3 •
+ • CH 3
Methyl radicals
u The
order of stability of alkyl radicals is 3° > 2° >
1° > methyl
7-20
Mechanism
Mechanism
u Chain
initiation: a step in a radical chain reaction
characterized by formation of radicals from
nonradical compounds
heat
or light
Cl •
+
propagation: a step in a radical chain
reaction characterized by reaction of a radical
and a molecule to form a new radical
Chain propagation
2. CH 3 CH 3 +
Chain initiation
1. Cl 2
u Chain
• Cl
3. CH 3 CH 2 • +
Cl •
CH 3 CH 2 •
+ HCl
Cl 2
CH 3 CH 2 Cl +
Cl •
u Chain
7-21
Mechanism
Chain Propagation Steps
u Chain
termination - a step in a radical chain
reaction that involves destruction of radicals
Chain termination
4. 2 CH3 CH2 •
5.
6.
CH3 CH2 •
Cl •
+
length, n: the number of times the cycle of
chain propagation steps repeats in a chain
reaction
7-22
u For
any set of chain propagation steps, their
• equations add to the observed stoichiometry
• energies of add to the observed DH°
CH3 CH2 CH2 CH3
•
+
Cl
•
Cl
CH3 CH3 + Cl •
+100
CH3 CH2 Cl
Cl 2
CH3 CH2 •
CH3 CH3 +
7-23
+ Cl 2
+59
Cl 2
Step 2:
CH3 CH2 • + HCl
-103
Step 3:
DH°
(kcal/mol)
-3
CH3 CH2 Cl + Cl •
-80
-21
CH3 CH2 Cl + HCl
-24
7-24
Hammond’s Postulate
Regioselectivity?
u The
regioselectivity of chlorination and
bromination can be accounted for in terms of the
relative stabilities of alkyl radicals (3° > 2° > 1° >
methyl)
u But
how do we account for the greater
regioselectivity of bromination (1600:80:1)
compared with chlorination (5:4:1)?
u Hammond’s
Postulate: the structure of the
transition state
• for an exothermic reaction looks more like the
reactants of that step
• for an endothermic reaction looks more like the
products of that step
u This
postulate applies equally well to the
transition state for a one-step reaction and to
each transition state in a multi-step reaction
7-25
Hammond’s Postulate
Hammond’s Postulate
u In
halogenation of an alkane, the rate-limiting
step is hydrogen abstraction
u Because
hydrogen abstraction for chlorination is
exothermic,
• this step is exothermic for chlorination and
endothermic for bromination
• the transition state resembles the alkane and a chlorine
atom,
• there is little radical character on carbon in the
transition state, and
• regioselectivity is only slightly influenced by radical
stability
DH°
(kcal/mol)
CH3 CH3 +
+98
•
CH3 CH3 +
+98
•
Cl
Br
7-26
CH3 CH2 • + HCl
-103
-5.0
CH3 CH2 • + HBr
-88
+10.0
7-27
7-28
Hammond’s Postulate
Stereochemistry
u Because
hydrogen abstraction for bromination is
endothermic,
• the transition state resembles an alkyl radical and HBr,
• there is significant radical character on carbon in the
transition state, and
• regioselectivity is greatly influenced by radical
stability.
• Radical stability is 3° > 2° > 1° > methyl, and
regioselectivity is in the same order
u When
radical halogenation produces a
stereocenter or takes place at a hydrogen on an
existing stereocenter, the product is an R,S
mixture
CH3 CH2 CH2 CH3 + Br 2
Butane
heat
or light
Br
CH3 CH2 CHCH 3 + HBr
(R,S)-2-Bromobutane
7-29
Stereochemistry
Allylic Halogenation
u For
simple alkyl radicals, the carbon bearing the
radical is sp2 hybridized and the unpaired
electron occupies the unhybridized 2p orbital
the unhybridized
2p orbital
C 2 H5
C
H
CH 3
Radical intermediate
Br
u Allylic
C
H
CH 3
Br
(R)-2-Bromobutane
carbon: a carbon adjacent to a C-C double
bond
u Allylic
C 2 H5
2
7-30
hydrogen: a hydrogen on an allylic carbon
Br
+
C
H
CH 3
(S)-2-Bromobutane
C 2 H5
7-31
CH 2 =CHCH 3 + Cl 2
Propene
350°C
CH 2 =CHCH 2 Cl + HCl
3-Chloropropene
(Allyl chloride)
7-32
Allylic Halogenation
Allylic Bromination
u An
allylic C-H bond is weaker than a vinylic C-H
bond
u Allylic
bromination using NBS
O
86 kcal/mol
H
+
H
C
C
106 kcal/mol
H
C
H
H
Cyclohexene
H
NBr
Br2 necessary for radical halogenation is
provided by reaction of NBS with HBr
NBr
O
NH
O
+
3-Bromocyclohexene
uA
NH
O
Succinimide
.
7-34
radical chain mechanism
Chain propagation:
O
+ HBr
Br
Allylic Bromination
u The
O
CCl 4 , heat
O
N-Bromosuccinimide
(NBS)
7-33
The Function of NBS
peroxide
+
CH 2 =CHCH 3 + • Br
CH 2 =CHCH 2 • + Br 2
Br 2
O
CH 2 =CHCH 3 + Br 2
7-35
CH 2 =CHCH 2 • + HBr
CH 2 =CHCH 2 Br + • Br
CH 2 =CHCH 2 Br +
HBr
7-36
The Allyl Radical
uA
Organometallic Cmpds
hybrid of two equivalent contributing structures
CH
CH 2
•
•
CH 2
CH 2
CH
u Organometallic
compound: a compound that
contains a metal bonded to a carbon atom
RMgX
An organomagnesium
compound
(a Grignard reagent)
CH 2
(Equivalent contributing structures)
R2 CuLi
A lithium
diorganocopper compound
(a Gilman reagent)
RLi
An organolithium
compound
7-37
Organometallic Cmpds
Organometallic Cmpds
u Grignard
reagents are formed by reaction of an
alkyl halide with magnesium metal in diethyl ether
or tetrahydrofuran (THF)
CH3 CH2 CH2 CH2 Cl + Mg
1-Chlorobutane
7-38
ether
CH3 CH2 CH2 CH2 MgCl
Butylmagnesium chloride
7-39
u Organolithium
reagents are formed by reaction of
an alkyl halide with lithium metal in a
hydrocarbon solvent such as pentane
pentane
CH3 CH2 CH2 CH2 Cl + 2 Li
1-Chlorobutane
CH3 CH2 CH2 CH2 Li + LiCl
Butyllithium
7-40
Organometallic Cmpds
u Carbon-metal
Organometallic Cmpds
bonds are polar covalent
C-M
Bond
Difference in
Percent Ionic
Electronegativity character*
C-Li
C-Mg
2.5 - 1.0 = 1.5
2.5 - 1.2 = 1.3
60
52
C-Zn
C-Cu
C-Hg
2.5 - 1.6 = 0.9
2.5 - 1.9 = 0.6
2.5 - 1.9 = 0.6
36
24
24
*P ercent ionic character =
are strong bases and react with
any proton donor stronger than the alkane from
which they are derived
dd+
CH3 CH2 - MgBr + H-OH
pK a 15.7
Stronger
acid
EC - EM
EC
u Organometallics
CH3 CH2 -H + Mg(OH)Br
pK a 51
Weaker
acid
x 100
7-41
Organometallic Cmpds
Gilman Reagents
u Classes
of proton donors that react with Grignard
and organolithium reagents are
R 2 NH
pK a 38-40
RC CH
pK a 25
ROH
pK a 16-18
Amines
Alkynes
Alcohols
HOH
pK a 15.7
ArOH
pK a 9-10
RSH
pK a 8-9
RCO 2 H
pK a 4-5
Water
Phenols
Thiols
Carboxylic
acids
7-42
u Prepared
from an organolithium reagent and
copper(I) iodide
2 CH 3 CH 2 CH 2 CH 2 Li + CuI
Copper(I)
Butyllithium
iodide
diethyl ether
or THF
(CH 3 CH 2 CH 2 CH 2 ) 2 Cu - Li
+
+ LiI
Lithium dibutylcopper
(a Gilman reagent)
7-43
7-44
Gilman Reagents
Gilman Reagents
u Gilman
reagents can be used to form new carboncarbon bonds by cross-coupling with alkyl or
vinylic halides
(CH 3 ) 2 CuLi + CH 3 (CH 2 ) 8 CH2 I
Lithium
1-Iododecane
dimethylcopper
CH 3 (CH 2 ) 6
H
trans-1-Iodo-1nonene
3 Cu + LiI
7-45
with a vinylic halide
H
+ (CH 3 CH 2 CH 2 CH 2 ) 2 Cu Li
C C
diethyl ether
or THF
CH3 (CH 2 ) 8 CH2 CH3 + CH
Undecane
u Cross-coupling
Et 2 O
or THF
I
CH 3 (CH 2 ) 6
H
C C
H
CH 2 CH 2 CH 2 CH 3
trans-5-Tridecene
7-46