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HaloalkanesHaloalkanes
Chapter 8Chapter 8
8-8-22
8.18.1 Structure Structure
Haloalkane (alkyl halide):Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp3 hybridized carbon; given the symbol RX
Haloalkene (vinylic halide):Haloalkene (vinylic halide): a compound containing a halogen bonded to an sp2 hybridized carbon
Haloarene (aryl halide):Haloarene (aryl halide): a compound containing a halogen bonded to a benzene ring; given the symbol ArX• (vinylic and aryl halides are not studied in this
chapter)
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Halo compoundsHalo compounds
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8.28.2 Nomenclature Nomenclature
1. number the parent chain to give the substituent encountered first the lowest number, whether it is halogen or an alkyl group
2. indicate halogen substituents by the prefixes fluoro-, chloro-, bromo-, and iodo-, and list them in alphabetical order with other substituents
3. locate each halogen on the parent chain by giving it a number preceding the name of the halogen
4. in haloalkenes, number the parent chain to give carbon atoms of the double bond the lower set of numbers
5. In cycloalkenes having halogen substituents begin numbering at the double bond
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NomenclatureNomenclature
• examples
Common names:Common names: name the alkyl group followed by the name of the halide
Br
2-Bromo-4-methyl-pentane
12
34
5
4-Bromo-cyclohexene
1
23
4
56
trans-2-Chloro-cyclohexanol
BrCl
OH
BrCl Cl
2-Bromobutane(sec-Butyl bromide
Chloroethene(Vinyl chloride)
3-Chloropropene(Allyl chloride)
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NomenclatureNomenclature
• several polyhaloalkanes are common solvents and are generally referred to by their common or trivial names
• hydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes
CHCl3CH2 Cl2 CCl2=CHClCH3 CCl3 Dichloromethane(Methylene chloride)
Trichloromethane (Chloroform)
Trichloroethe ne (Trichlor)
1,1,1-Trichloroethane (Methyl chloroform)
PerchloroethylenePerfluoropropanePerchloroethane
C CCl
ClClCl
ClCl
F C C C FF
F
F
F
F
F ClC C
Cl
ClCl
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8.38.3 A.A. Dipole Moments, Table 8-1 Dipole Moments, Table 8-1
Dipole moment of RX depends on:• the sizes of the partial charges
• the distance between them
• the polarizability of the unshared electrons on halogen
CH3Cl
CH3F
CH3Br
CH3I
HalomethaneElectronegativity
of Halogen
Carbon-HalogenBond Length
(pm)
Dipole Moment
(debyes; D)
4.0
3.0
2.8
2.5
139
178
193
214
1.85
1.87
1.81
1.62
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van der Waals Forcesvan der Waals Forces Haloalkanes are associated in the liquid state by
van der Waals forces 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
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van der Waals Forces, Table 8-2van der Waals Forces, Table 8-2
• 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 radiusvan der Waals radius
• nonbonded atoms in a molecule cannot approach each other closer than the sum of their van der Waals radii without nonbonded interaction strain
Increasing van der Waals radius
H F Cl Br CH2 CH3 I
120 135 180 195 200 200 215
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B.B. Boiling Points 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 CH3CH3 CH3Br
bp -89°C bp 4°C
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Boiling PointsBoiling Points
• polarizability: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
• 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
2-Bromo-2-methylbutanebp 72°C
Br
1-Bromobutanebp 100°C
Br
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Boiling PointsBoiling Points
• boiling points of fluoroalkanes are comparable to those of hydrocarbons of similar molecular weight and shape
• 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
Hexane(MW 86.2, bp 69°C)
F
1-Fluoropentane(MW 90.1, bp 63°C)
CH3CHCH3
CH3
CH3CHCH3
F
2-FluoropropaneMW 62.1, bp -11°C
2-MethylpropaneMW 58.1, bp -1°C
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C.C. Density, Table 8-5 Density, Table 8-5 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
CHX3
CH2X2
CX4
Cl Br IX=
1.483
2.497
4.008
Density (g/mL) at 25°C
3.325
2.890
1.594 3.273 4.23
Haloalkane
1.327
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D.D. Bond Lengths, Strengths, Table 8-6 Bond Lengths, Strengths, Table 8-6
C-F bonds are stronger than C-H bonds; C-Cl, C-Br, and C-I bonds are weaker
C-HC-F
C-ClC-BrC-I
Bond
BondLength(pm)
Bond Dissociation Ethalpy
[kJ (kcal)/mol]
109142
178
193214
414 (99)464 (111)
355 (85)309 (78)228 (57)
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8.48.4 Halogenation of Alkanes 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
Substitution:Substitution: a reaction in which an atom or group of atoms is replaced by another atom or group of atoms
CH4 Cl2 CH3Cl HCl+heat
+Methane Chloromethane
(Methyl chloride)
H0
-100 kJ (23.9 kcal)/mol
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Halogenation of AlkanesHalogenation of Alkanes
• if chloromethane is allowed to react with more chlorine, a mixture of chloromethanes is formed
CH3Cl Cl2 CH2Cl2 HCl+heat
+Chloromethane
(Methyl chloride)Dichloromethane
(Methylene chloride)
CH2Cl2Cl2
CHCl3Cl2
CCl4heat heatDichloromethane
(Methylene chloride)Trichloromethane
(Chloroform)Tetrachloromethane
(Carbon tetrachloride)
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A.A. Regioselectivity Regioselectivity
Regioselectivity is high for bromination, but not as high for chlorination
CH3CH2CH3 Br2
Cl2CH3CH2CH3 CH3CHCH3
Cl
CH3CHCH3
Br
CH3CH2CH2Cl
CH3CH2CH2Br HBr
HCl
Propane
+ heator light
+ +
2-Bromopropane(92%)
1-Bromopropane(8%)
Propane
+ heator light
+ +
2-Chloropropane(57%)
1-Chloropropane(43%)
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RegioselectivityRegioselectivity
Regioselectivity is 3° > 2° > 1°• for bromination, approximately 1600:80:1
• for chlorination, approximately 5:4:1
Example:Example: draw all monobromination products and predict the percentage of each for this reaction
+heat
2-Methylpropane
CH3
CH3
CH3CH Br2 C4H9Br + HBr
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B.B. Energetics, Table 8-7 Energetics, Table 8-7
Bond Dissociation Enthalpies (BDE)
CH2=CH-H CH2=CH•
CH3 -H CH3•
CH3 CH2 -H CH3CH2•
(CH3)2CH-H (CH3)2 CH•
(CH3)3C-H (CH3)3 C•
C6H5 CH2 -H C6H5 CH2•
CH2=CHCH2 -H CH2=CHCH2•
Benzyl
Allyl
Vinyl
Methyl
Ethyl
Isopropyl
tert-Butyl
372 (89)376 (90)
405 (97)
414 (99)
422 (101)
439 (105)
464 (111)
Hydrocarbon RadicalName ofRadical
H0
kJ (kcal)/mol
Vinylic
Methyl
Benzylic
Allylic
Type ofRadical
3°
2°
1°
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EnergeticsEnergetics
Using BDE, we can calculate the heat of reaction, H0, for the halogenation of an alkane
CH4 Cl2 CH3Cl HCl
+439(+105)
+ + H0= -96 kJ/mol (-23 kcal/mol)+247
(+59)-351(-84)
-431(-103)
BDE, kJ/mol(kcal/mol)
8-8-2121
8.5 8.5 A.A. Mechanism Mechanism
Radical:Radical: any chemical species that contains one or more unpaired electrons• radicals are formed by homolytic bond cleavagehomolytic bond cleavage
• the order of stability of alkyl radicals is 3° > 2° > 1° > methyl
CH3CH2O OCH2CH3
Cl Cl
CH3 CH3
Cl•
CH3•
CH3CH2O•
•Cl
•CH3
•OCH2CH3 H0 = +150 kJ/mol (+36 kcal/mol)Ethoxy radicalsDiethyl peroxide
80° +
H0 = +377 kJ/mol (+90 kcal/mol)Methyl radicalsEthane
heat +
Chlorine atomsChlorine
light + H0= +150 kJ/mol (+36 kcal/mol)
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B.B. Radical Chain Mechanism Radical Chain Mechanism
Radical Mechanisms have three steps:Radical Mechanisms have three steps:• Chain initiationChain initiation
• Chain PropagationChain Propagation
• Chain TerminationChain Termination
Chain initiation:Chain initiation: a step in a chain reaction characterized by formation of reactive intermediates (radicals, anions, or cations) from nonradical or noncharged molecules
light +Cl Cl Cl• •Clor heat
Step 1:
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Radical Chain MechanismRadical Chain Mechanism
Chain propagation: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
Chain length:Chain length: the number of times the cycle of chain propagation steps repeats in a chain reaction
CH3CH2•
CH3CH2 H CH3CH2•
CH3CH2 Cl
H Cl•Cl
•ClCl Cl
+
+
Step 2:
Step 3:
+
+
8-8-2424
Radical Chain MechanismRadical Chain Mechanism
Chain termination:Chain termination: a step in a chain reaction that involves destruction of reactive intermediates
CH3CH2
CH3CH2
Cl
CH2CH3
Cl Cl
CH3CH2 CH2-CH2
H
Cl Cl
CH3CH2-Cl
CH3CH3
CH3CH2-CH2CH3
CH2=CH2
•
• •
Step 6:
Step 7:
•
••
•
+
Step 4:
Step 5:
+
+
+
+
•
8-8-2525
C.C. Chain Propagation Steps Chain Propagation Steps For any set of chain propagation steps, their• equations add to the observed stoichiometry
• enthalpies add to the observed H0
CH3CH2 -H •Cl
CH3CH2• Cl-Cl
CH3 CH2 -H Cl-Cl
CH3CH2•
CH3CH2 -Cl
CH3 CH2 -Cl
•Cl
H-Cl
H-Cl
+
+ +
+
+422 (+101)
-431(-103)
+247(+59
-355(-85)
-9 (-2)
-108 (-26)
+ + -117 (-28)
H0, kJ/mol(kcal/mol)
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D.D. Regioselectivity? 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)
8-8-2727
Hammond’s PostulateHammond’s Postulate
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
8-8-2828
Hammond’s Hammond’s PostulatePostulate
Fig. 8-1Fig. 8-1
Energy Energy absorbedabsorbed(endothermic)(endothermic)
Energy Energy releasedreleased(exothermic)(exothermic)
8-8-2929
Hammond’s PostulateHammond’s Postulate
• in halogenation of an alkane, hydrogen abstraction (the rate-determining step) is exothermic for chlorination but endothermic for bromination
H
H-Br
H-Br
Cl
Cl
Br
Br
H-Cl
H-Cl
H
-431 (-103)
++ +54 (+13)
-431 (-103)+405 (97)
+ +
+ +
-368 (-88)
-26 (-6)
+37 (+9)
+ +
Reaction step
-9 (-2)
-368 (-88)
+422 (101)
+422 (101)
+405 (97)
17 (4)
17 (4)
H
H
H
[kJ (kcal)/mol]
•
•
•
••
•
•
•
8-8-3030
Hammond’s PostulateHammond’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
8-8-3131
Hammond’s PostulateHammond’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
8-8-3232
Hammond’s Postulate, Fig 8.2Hammond’s Postulate, Fig 8.2
Stabilization of radical character in the transition state results in regioselectivity, Br>ClStabilization of radical character in the transition state results in regioselectivity, Br>Cl
8-8-3333
E.E. Stereochemistry 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
• for simple alkyl radicals, the carbon bearing the radical is sp2 hybridized and the unpaired electron occupies the unhybridized 2p orbital (see next screen)
CH3CH2CH2CH3 Br2 CH3CH2CHCH3
Br
HBr+Butane (R,S)-2-Bromobutane
heator light+
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StereochemistryStereochemistry
8-8-3535
8.6 8.6 Allylic Halogenation Allylic Halogenation
Allylic carbon:Allylic carbon: a C adjacent to a C-C double bond
Allylic hydrogen:Allylic hydrogen: an H on an allylic carbon
• an allylic C-H bond is weaker than a vinylic C-H bond
+ +
Propene 3-Chloropropene (Allyl chloride)
350°CCH2=CHCH3 Cl2 CH2=CHCH2Cl HCl
H
C C
C
H H
HH H
+464 kJ (111 kcal)/mol+372 kJ (89 kcal)/mol
8-8-3636
A.A. Allylic Bromination Allylic Bromination
Allylic bromination using NBS
N Br
O
O
hCH2Cl2
Br
N H
O
OSuccinimide3-Bromo-
cyclohexeneCyclohexene
+
N-Bromo-succinimide
(NBS)
+
8-8-3737
Allylic BrominationAllylic Bromination
A radical chain mechanism• Chain initiation
• Chain propagation
N
O
O
Brh
N•
O
O
•Br+
+ •+•
• ++ •CH2=CHCH2-H Br CH2=CHCH2 H-Br
CH2=CHCH2 Br-Br CH2=CHCH2-Br Br
8-8-3838
Allylic BrominationAllylic Bromination
• chain termination
Br2 is provided by the reaction of NBS with HBr
Br• •Br
CH2=CHCH2•
CH2=CHCH2• •Br
•CH2CH=CH2
Br-Br
CH2=CHCH2-Br
CH2=CHCH2-CH2 CH=CH2
+
+
+
++ NH
O
O
NBr
O
O
HBr Br2
8-8-3939
B.B. The Allyl Radical The Allyl Radical
A hybrid of two equivalent contributing structures
(Equivalent contributing structures)
• •CH2 CH CH2 CH CH2CH2
8-8-4040
The Allyl Radical, Fig 8.3The Allyl Radical, Fig 8.3
Molecular orbital model of the allyl radical
8-8-4141
The Allyl Radical, Fig 8.4The Allyl Radical, Fig 8.4
Unpaired electron spin density map of the allyl radical• the unpaired electron density (green lobes)
appears only on carbons 1 and 3
8-8-4242
Allylic HalogenationAllylic Halogenation
• Example 8.5Example 8.5 Account for the fact that allylic bromination of 1-octene by NBS gives these isomeric products
• Look at the stability of the double bond of the allylic radical
NBS
CH2Cl2 Br
Br12
3 12
3+
1-Octene 3-Bromo-1-octene(racemic, 17%)
1-Bromo-2-octene(83%)
8-8-4343
8.7 8.7 Radical Autoxidation Radical Autoxidation
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 1,4-dienes
• radical abstraction of a doubly allylic hydrogen of a 1,4-diene forms a particularly stable radical
8-8-4444
Radical AutoxidationRadical Autoxidation
• autoxidation begins when a radical initiator, X•, abstracts a doubly allylic hydrogen
• this radical is stabilized by resonance with both double bonds
H H
H H
H H
H
H H
R1 R2
H H
H H
H H
R1 R2
H H
H
X•
HH
R2R1
HH
H 1 21 2
8-8-4545
Radical AutoxidationRadical 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
H H
R1 R2
H H
HO
O
H
R2
H
R1
H H
HO
OH
H H
R1 R2
H H
H OO
H–R
Peroxy radical+ R•
A hydroperoxide
8-8-4646
Radical AutoxidationRadical Autoxidation
Vitamin E as an antioxidant vitamin E, a naturally occurring antioxidant,
reacts preferentially with the initial peroxy radical to give a resonance-stabilized phenoxy radical, which is very unreactive, and scavenges another peroxide radical
8-8-4747
Radical AutoxidationRadical Autoxidation
Vitamin E as an antioxidant
O
H-O
HO
•O
H
ROO•
O
H
OO R
O O
O
H
A phenoxy radical
A peroxide derived from vitamin E
3
a peroxide group
Vitamin E
3
3 3
8-8-4848
8.8 8.8 Radical Addition of HBr to Alkenes 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 when no radicals occur
• non-Markovnikov addition occurs when peroxides or other sources of radicals are present
+ HBr
no peroxides
Br
2-Methyl-propene
2-Bromo-2-methylpropane
Markovnikovaddition
HBrBrperoxides
+
2-Methyl-propene
1-Bromo-2-methylpropane
Non-Markovnikovaddition
8-8-4949
Radical Addition of HBr to AlkenesRadical 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
R O
R-O O-R
H Br
R O RO
HR O Br
Two alkoxy radicalsA dialkylperoxide
Bromineradical
Step 1: +
+Step 2: +
8-8-5050
Radical Addition of HBr to AlkenesRadical Addition of HBr to Alkenes
Chain propagation
Br
BrBr H
Br
BrH Br+
Step 3:
Step 4:
+
+
A 3° radical
1-Bromo-2-methylpropane
8-8-5151
Radical Addition of HBr to AlkenesRadical Addition of HBr to Alkenes Chain termination
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
Br BrBr Br
BrBr Br
BrStep 6:
Step 5: +
+
8-8-5252
HaloalkanesHaloalkanes
End Chapter 8End Chapter 8
