Transcript Organic Chemistry - Home | Rutgers

Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
8-1
Haloalkanes
Chapter 8
8-2
Structure
 Haloalkane
(alkyl halide): a compound containing
a halogen covalently bonded to an sp3 hybridized
carbon; given the symbol RX
 Haloalkene (vinylic halide): a compound
containing a halogen bonded to an sp2
hybridized carbon
 Haloarene (aryl halide): a compound containing a
halogen bonded to a benzene ring; given the
symbol ArX
• (we do not study vinylic or aryl halides in this chapter)
8-3
Nomenclature
• number the parent chain to give the substituent
encountered first the lowest number, whether it is
halogen or an alkyl group
• indicate 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 by giving it a
number preceding the name of the halogen
• in haloalkenes, number the parent chain to give carbon
atoms of the double bond the lower set of numbers
8-4
Nomenclature
• examples
Cl
Br
5
4
3
2
OH
1
2-Bromo-4-methylpentane
trans-2-Chlorocyclohexanol
3
2
4
1
5
Br
6
4-Bromocyclohexene
 Common
names: name the alkyl group followed
by the name of the halide
Br
2-Bromobutane
(sec-Butyl bromide
Cl
Cnhloroethene
(Vinyl chloride)
Cl
3-Chloropropene
(Allyl chloride)
8-5
Nomenclature
• several polyhaloalkanes are common solvents and are
generally referred to by their common or trivial names
CH2 Cl2
CHCl3
Dichloromethane
Trichloromethane
(Methylene chloride)
(Chloroform)
CH3 CCl3
1,1,1-Trichloroethane
(Methyl chloroform)
CCl2 =CHCl
Trichloroethyne
(Trichlor)
• hydrocarbons in which all hydrogens are replaced by
halogens are commonly named as perhaloalkanes or
perhaloalkenes
Cl Cl
Cl C C Cl
Cl Cl
Perchloroethane
F F F
F C C C F
F F F
Perfluoropropane
Cl
Cl
C C
Cl
Cl
Perchloroethylene
8-6
Dipole Moments
 Dipole
moment of RX depends on:
• the sizes of the partial charges
• the distance between them
• the polarizability of the unshared electrons on halogen
Carbon-Halogen
Dipole
Electronegativity Bond Length
Moment
Halomethane
of Halogen
(pm)
(debyes; D)
139
CH3 F
1.85
4.0
CH3 Cl
3.0
178
1.87
CH3 Br
2.8
193
1.81
CH3 I
2.5
214
1.62
8-7
van der Waals Forces
 Haloalkanes
are associated in the liquid state by
van der Waals forces
 van der Waals forces: a group intermolecular
attractive forces including
• dipole-dipole forces
• dipole-induced dipole forces
• induced dipole-induced dipole (dispersion) forces
 van
der Waals forces pull molecules together
• as molecules are brought closer and closer, van der
Waals attractive forces are overcome by repulsive
forces between electron clouds of adjacent atoms or
molecules
8-8
van der Waals Forces
• the energy minimum is where the attractive forces are
the strongest
• nonbonded interatomic and intermolecular distances
at these minima can be measured by x-ray
crystallography and each atom and group of atoms
can be assigned a van der Waals radius
• nonbonded atoms in a molecule cannot approach each
other closer than the sum of their van der Waals radii
without causing nonbonded interaction strain
H
F
Cl
Br
CH2
CH3
I
120
135
180
195
200
200
215
Increasing van der Waals radius
8-9
Boiling Points
 For
an alkane and a haloalkane of comparable
size and shape, the haloalkane has the higher
boiling point
• the difference is due almost entirely to the greater
polarizability of the three unshared pairs of electrons
on halogen compared with the low polarizability of
shared electron pairs of covalent bonds
CH3 CH3
bp -89°C
CH3 Br
bp 4°C
• polarizability: a measure of the ease of distortion of
the distribution of electron density about an atom in
response to interaction with other molecules and ions;
fluorine has a very low polarizability, iodine has a very
high polarizability
8-10
Boiling Points
• among constitutional isomers, branched isomers have
a more compact shape, decreased area of contact,
decreased van der Waals attractive forces between
neighbors, and lower boiling points
Br
1-Bromobutane
bp 100°C
Br
2-Bromo-2-methylbutane
bp 72°C
8-11
Boiling Points
• boiling points of fluoroalkanes are comparable to
those of hydrocarbons of similar molecular weight and
CH3
F
shape
CH3 CHCH3
2-Methylpropane
MW 58.1, bp -1°C
CH3 CHCH3
2-Fluoropropane
MW 62.1, bp -11°C
F
Hexane
(MW 86.2, bp 69°C)
1-Fluoropentane
(MW 90.1, bp 63°C)
• the low boiling points of fluoroalkanes are the result of
the small size of fluorine, the tightness with which its
electrons are held, and their particularly low
polarizability
8-12
Density
 The
densities of liquid haloalkanes are greater
than those of hydrocarbons of comparable
molecular weight
• a halogen has a greater mass per volume than a
methyl or methylene group
 All
liquid bromoalkanes and iodoalkanes are
more dense than water
 Di- and polyhalogenated alkanes are more dense
than water
Density (g/mL) at 25°C
Haloalkane X=
CH2 X2
CHX3
CX4
Cl
Br
I
1.327
2.497
2.890
3.325
4.008
3.273
4.23
1.483
1.594
8-13
Bond Lengths, Strengths
 C-F
bonds are stronger than C-H bonds; C-Cl,
C-Br, and C-I bonds are weaker
Bond Bond Dissociation
Length
Ethalpy
Bond (pm)
[kJ (kcal)/mol]
C-F
C-H
C-Cl
C-Br
C-I
142
109
178
193
214
464 (111)
414 (99)
355 (85)
309 (78)
228 (57)
8-14
Halogenation of Alkanes
 If
a mixture of methane and chlorine is kept in the
dark at room temperature, no change occurs
 If the mixture is heated or exposed to visible or
ultraviolet light, reaction begins at once with the
evolution of heat
CH4 + Cl2
Methane
heat
CH3 Cl + HCl
Chloromethane
(Methyl chloride)
H0
-100 kJ (23.9 kcal)/mol
 Substitution:
a reaction in which an atom or
group of atoms is replaced by another atom or
group of atoms
8-15
Halogenation of Alkanes
• if chloromethane is allowed to react with more
chlorine, a mixture of chloromethanes is formed
CH3 Cl + Cl2
Chloromethane
(Methyl chloride)
heat
CH2 Cl2 + HCl
Dichloromethane
(Methylene chloride)
Cl2
Cl2
CH2 Cl2
CHCl3
CCl4
heat
heat
Dichloromethane
Trichloromethane
Tetrachloromethane
(Methylene chloride)
(Chloroform)
(Carbon tetrachloride)
8-16
Regioselectivity
 Regioselectivity
is high for bromination, but
not as high for chlorination
CH3 CH2 CH3 + Br2
Propane
CH3 CH2 CH3 + Cl2
Propane
heat
or light
heat
or light
Br
CH3 CHCH3
+ CH 3 CH2 CH2 Br
2-Bromopropane
(92%)
Cl
CH3 CHCH 3
+ HBr
1-Bromopropane
(8%)
+ CH3 CH2 CH2 Cl
+ HCl
2-Chloropropane 1-Chloropropane
(57%)
(43%)
8-17
Regioselectivity
 Regioselectivity
is 3° > 2° > 1°
• for bromination, approximately 1600:80:1
• for chlorination, approximately 5:4:1
Example: draw all monobromination products and predict
the percentage of each for this reaction
CH 3
CH3 CH
+ Br2
heat
C4 H 9 Br + HBr
CH 3
2-Methylpr opane
8-18
Energetics
 Bond
Dissociation Enthalpies (BDE)
Hydrocarbon
Radical
CH2 =CHCH2 -H CH2 =CHCH2 •
0
Name of
Radical
H
Type of
Radical kJ (kcal)/mol
Allyl
Allylic
C6 H5 CH2 -H
C6 H5 CH2 •
Benzyl
Benzylic
372 (89)
376 (90)
(CH3 ) 3 C-H
(CH3 ) 3 C•
tert-Butyl
3°
405 (97)
(CH3 ) 2 CH-H
(CH3 ) 2 CH•
Isopropyl
2°
414 (99)
CH3 CH2 -H
CH3 CH2 •
Ethyl
1°
421 (101)
CH3 -H
CH3 •
CH2 =CH-H
CH2 =CH•
Methyl
Vinyl
Methyl
Vinylic
439 (105)
464 (111)
8-19
Energetics
 Using
BDE, we can calculate the heat of reaction,
H0, for the halogenation of an alkane
CH4 + Cl2
BDE, kJ/mol +439
(kcal/mol)
(+105)
+247
(+59)
CH3 Cl + HCl
-351
(-84)
-431
(-103)
0
H = -96 kJ/mol
(-23 kcal/mol)
8-20
Mechanism
 Radical:
any chemical species that contains one
or more unpaired electrons
• radicals are formed by homolytic bond cleavage
• the order of stability of alkyl radicals is 3° > 2° > 1° >
methyl
Cl
Cl
light
Chlorine
CH3 CH2 O
OCH2 CH3
+
•
Cl
Chlorine atoms
80°
CH3 CH2 O• + • OCH2 CH3
Ethoxy radicals
Diethyl peroxide
CH3 CH3
Ethane
Cl•
heat
CH3 • + • CH3
Methyl radicals
H0= +150 kJ/mol
(+36 kcal/mol)
H0 = +150 kJ/mol
(+36 kcal/mol)
H0 = +377 kJ/mol
(+90 kcal/mol)
8-21
Radical Chain Mechanism
 Chain
initiation: a step in a chain reaction
characterized by formation of reactive
intermediates (radicals, anions, or cations) from
nonradical or noncharged molecules
Step 1:
Cl
Cl
light
or heat
Cl•
+
•
Cl
8-22
Radical Chain Mechanism
 Chain
propagation: a step in a chain reaction
characterized by the reaction of a reactive
intermediate and a molecule to form a new
reactive intermediate and a new molecule
Step 2: CH3 CH2
Step 3: CH3 CH2 •
CH3 CH2 •
H + • Cl
+
Cl
Cl
+ H Cl
CH3 CH2 Cl + • Cl
 Chain
length: the number of times the cycle of
chain propagation steps repeats in a chain
reaction
8-23
Radical Chain Mechanism
 Chain
termination: a step in a chain reaction that
involves destruction of reactive intermediates
Step 4:
CH3 CH2 • +
•
CH2 CH3
Step 5:
CH3 CH2 • +
•
Cl
Step 6:
Cl • +
•
CH3 CH2 - CH2 CH3
CH3 CH2 -Cl
Cl
Cl
Cl
H
Step 7:
CH3 CH2 • +
CH2 -CH2 •
CH3 CH3 + CH2 =CH2
8-24
Chain Propagation Steps
 For
any set of chain propagation steps, their
• equations add to the observed stoichiometry
• enthalpies add to the observed H0
0
H , kJ/mol
(kcal/mol)
CH3 CH2 -H + • Cl
+422
(+101)
CH3 CH2 • + H-Cl
-431
(-103)
-9 (-2)
CH3 CH2 • + Cl-Cl
+247
(+59
CH3 CH2 -Cl + • Cl
-355
(-80)
-108 (-26)
CH3 CH2 -H + Cl-Cl
CH3 CH2 -Cl + H-Cl
-117 (-28)
8-25
Regioselectivity?
 The
regioselectivity of chlorination and
bromination can be accounted for in terms of the
relative stabilities of alkyl radicals (3° > 2° > 1° >
methyl)
 But how do we account for the greater
regioselectivity of bromination (1600:80:1)
compared with chlorination (5:4:1)
8-26
Hammond’s Postulate
 Hammond’s
Postulate: the structure of the
transition state
• for an exothermic step looks more like the reactants of
that step than the products
• for an endothermic step looks more like the products
of that step than the reactants
 This
postulate applies equally well to the
transition state for a one-step reaction and to
each transition state in a multi-step reaction
8-27
Hammond’s
Postulate
8-28
Hammond’s Postulate
• in halogenation of an alkane, hydrogen abstraction
(the rate-determining step) is exothermic for
chlorination but endothermic for bromination
H
[kJ (kcal)/mol]
Reaction step
H
•
+ •Cl
+
+422 (101)
H
+
•Cl
•
+
+405 (97)
-9 (-2)
H-Cl
-26 (-6)
17 (4)
-431 (-103)
H
+
H-Cl
-431 (-103)
•
• Br
+
+422 (101)
H + •Br
+405 (97)
H-Br
+54 (+13)
-368 (-88)
•
+
H-Br
-368 (-88)
17 (4)
+37 (+9)
8-29
Hammond’s Postulate
 Because
hydrogen abstraction for chlorination is
exothermic:
• the transition state resembles the alkane and a
chlorine atom
• there is little radical character on carbon in the
transition state
• regioselectivity is only slightly influenced by radical
stability
8-30
Hammond’s Postulate
 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
• regioselectivity is greatly influenced by radical stability
• radical stability is 3° > 2° > 1° > methyl, and
regioselectivity is in the same order
8-31
Hammond’s Postulate
8-32
Stereochemistry
 When
radical halogenation produces a chiral
center or takes place at a hydrogen on a chiral
center, the product is a mixture of R and S
enantiomers as a racemic mixture
CH3 CH2 CH2 CH3 + Br2
Butane
heat
or light
Br
CH3 CH2 CHCH3 + HBr
(R,S)-2-Bromobutane
• for simple alkyl radicals, the carbon bearing the radical
is sp2 hybridized and the unpaired electron occupies
the unhybridized 2p orbital (see next screen)
8-33
Stereochemistry
8-34
Allylic Halogenation
 Allylic
carbon: a C adjacent to a C-C double bond
 Allylic hydrogen: an H on an allylic carbon
350°C
CH2 = CHCH 3 + Cl 2
Pr opene
CH2 = CHCH 2 Cl + HCl
3-Chloropr opene
(Allyl chloride)
• an allylic C-H bond is weaker than a vinylic C-H bond
H
H
+464 kJ (111 kcal)/mol
C
C
H
C
H
H
H
+372 kJ (89 kcal)/mol
8-35
Allylic Bromination
 Allylic
bromination using NBS
Br
O
+
Cyclohexene
N Br
O
N-Bromosuccinimide
(NBS)
h
CH2 Cl2
O
+
3-Bromocyclohexene
N H
O
Succinimide
8-36
Allylic Bromination
A
radical chain mechanism
• Chain initiation
O
N Br
O
h
N• + •Br
O
O
• Chain propagation
CH2 = CHCH 2 - H +
• Br
CH2 = CHCH 2 •
Br- Br
+
CH2 = CHCH 2 • + H- Br
CH2 = CHCH 2 - Br + • Br
8-37
Allylic Bromination
• chain termination
Br•
+ • Br
Br- Br
CH2 = CHCH 2 • + • Br
CH2 = CHCH 2 • +
 Br2
•
CH2 = CHCH 2 - Br
CH2 CH= CH 2
CH2 = CHCH 2 -CH2 CH= CH2
is provided by the reaction of NBS with HBr
O
N Br + HBr
O
O
N H + Br2
O
8-38
The Allyl Radical
A
hybrid of two equivalent contributing
structures
CH 2
CH
•
CH2
•
CH2
CH
CH2
(Equivalent contributing structures)
8-39
The Allyl Radical
 Molecular
orbital model of the allyl radical
8-40
The Allyl Radical
 Unpaired
electron spin density map of the allyl
radical
• the unpaired electron density (green lobes) appears
only on carbons 1 and 3
8-41
Allylic Halogenation
• Example 8.5 Account for the fact that allylic
bromination of 1-octene by NBS gives these isomeric
products
NBS
CH2 Cl2
1-Octene
3
2
Br
3-Bromo-1-octene
(racemic, 17%)
1
+
3
2
1
Br
1-Bromo-2-octene
(83%)
8-42
Radical Autoxidation
 Autoxidation:
oxidation requiring oxygen, O2,
and no other oxidizing agent
• occurs by a radical chain mechanism similar to that for
allylic halogenation
• in this section, we concentrate on autoxidation of the
hydrocarbon chains of polyunsaturated triglycerides
• the characteristic feature of the fatty acid chains in
polyunsaturated triglycerides is the presence of 1,4dienes
• radical abstraction of a doubly allylic hydrogen of a
1,4-diene forms a particularly stable radical
8-43
Radical Autoxidation
• autoxidation begins when a radical initiator, X•,
abstracts a doubly allylic hydrogen
X•
R1 H H R2
H
H
H
R1
H
R1 1 H 2 R2
R2
H
1
H
H
H
H
H
H
H
H
H
2
H
H
H
H
H
H
• this radical is stabilized by resonance with both double
bonds
8-44
Radical Autoxidation
• the doubly allylic radical reacts with oxygen, itself a
diradical, to form a peroxy radical
• the peroxy radical then reacts with another 1,4-diene to
give a new radical, R•, and a hydroperoxide
R1
H
R2
H
O
H
H
H
R1
O
H
H
H–R
R2
O
O
H
H
H
Peroxy radical
R1
H
H
R2
O
H
O
+ R•
H
H
H
A hydroperoxide
• vitamin A, a naturally occurring antioxidant, reacts
preferentially with the initial peroxy radical to give a
resonance-stabilized phenoxy radical, which is very
8-45
unreactive, and scavenges another peroxide radical
Radical Autoxidation
• vitamin E as an antioxidant
O
3
O
H
3
H
•O
H-O
A phenoxy radical
Vitamin E
O R
O
a peroxide
group
ROO•
O
O
3
H
3
H
O
O
A peroxide derived from vitamin E
8-46
Radical Addition of HBr to Alkenes
 Addition
of HBr to alkenes gives either
Markovnikov addition or non-Markovnikov
addition depending on reaction conditions
• Markovnikov addition occurs when radicals are absent
• non-Markovnikov addition occurs when peroxides or
other sources of radicals are present
Markovnikov
addition
Non-Markovnikov
addition
+ HBr
no
peroxides
2-Methylpropene
2-Bromo-2methylpropane
+
2-Methylpropene
Br
HBr
peroxides
Br
1-Bromo-2methylpropane
8-47
Radical Addition of HBr to Alkenes
• addition of HCl and HI gives only Markovnikov
products
• to account for the the non-Markovnikov addition of
HBr in the presence of peroxides, chemists proposed a
radical chain mechanism
 Chain
initiation
Step 1: R-O O-R
A dialkyl
peroxide
Step 2: R O
+ H Br
R O
+
O R
Two alkoxy radicals
R O H + Br
Bromine
radical
8-48
Radical Addition of HBr to Alkenes
 Chain
propagation
Step 3:
+
Br
Br
A 3° radical
Step 4:
Br
H +
Br
Br
H
+ Br
1-Bromo-2methylpropane
8-49
Radical Addition of HBr to Alkenes
 Chain
termination
Step 5:
Step 6:
Br +
Br
Br Br
Br
Br +
Br
Br
 This
pair of addition reactions illustrates how the
products of a reaction can often be changed by a
change in experimental conditions
• polar addition of HBr is regioselective, with Br adding
to the more substituted carbon
• radical addition of HBr is also regioselective, with Br
adding to the less substituted carbon
8-50
Haloalkanes
End Chapter 8
8-51