William H. Brown
Christopher S. Foote
Brent L. Iverson
Eric Anslyn
http://academic.cengage.com/chemistry/brown
Chapter 8
Haloalkanes, Halogenation and Radical
Reactions
William H. Brown • Beloit College
8-1
Structure
‹
‹
‹
Haloalkane (alkyl halide): A compound containing a
halogen atom covalently bonded to an sp3 hybridized
carbon; given the symbol RX.
Haloalkene (vinylic halide): A compound containing a
halogen atom bonded to an sp2 hybridized carbon.
Haloarene (aryl halide): A compound containing a
halogen atom bonded to a benzene ring; given the
symbol ArX. We study these in Chapter 21.
8-2
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.
8-3
Nomenclature
• examples
‹ Common
names: name the alkyl group followed
by the name of the halide.
8-4
Nomenclature
• 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.
8-5
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.
8-6
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
8-7
van der Waals Forces
• The energy minimum is where the attractive and
repulsive forces are balanced.
• 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.
8-8
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.
• 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.
8-9
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.
8-10
Boiling 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.
8-11
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.
8-12
Bond Lengths, Strengths
‹ C-F
bonds are stronger than C-H bonds; C-Cl,
C-Br and C-I bonds are weaker.
8-13
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:
A reaction in which an atom or
group of atoms is replaced by another atom or
group of atoms.
8-14
Halogenation of Alkanes
• If chloromethane is allowed to react with more
chlorine, a mixture of chloromethanes is formed.
8-15
Regioselectivity
‹ Regioselectivity
is high for bromination, but
not as high for chlorination.
8-16
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 the reaction.
8-17
Energetics
‹ Bond
Dissociation Enthalpies (BDE)
8-18
Energetics
‹ Using
BDE, we can calculate the heat of reaction,
ΔH0, for the halogenation of an alkane.
8-19
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.
8-20
Chem Activity 20: Radical Halogenation of Alkanes
‹ Split
quickly in groups of 4
‹ Read Handout Chapter 8 (Chem Activity 20, Part
B)
‹ Work on the critical thinking questions (10-17)
‹ You have 20-25 min
‹ We will discuss the questions in 10 min
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.
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
radical or reactive intermediate and a new
molecule.
‹ 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.
8-24
Chain Propagation Steps
‹ For
any set of chain propagation steps, their
• equations add to the observed stoichiometry.
• enthalpies add to the observed ΔH0.
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 is reached relatively early in the
reaction, and resembles the reactants of that step
more than the products.
• for an endothermic step is reached relatively late in the
reaction and resembles the products of that step more
than the reactants.
‹ This
postulate applies equally well to the
transition state for one-step reactions and to
each transition state in a multi-step reaction.
8-27
Hammond’s Postulate
‹ (a)
A highly exothermic reaction
‹ (b) A highly endothermic reaction
8-28
Hammond’s Postulate
• In the halogenation of an alkane, hydrogen abstraction
(the rate-determining step) is exothermic for
chlorination but endothermic for bromination
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
‹
Transition states and energetics for hydrogen abstraction
in the radical chlorination and bromination of
2-methylpropane (isobutane).
8-32
Stereochemistry
‹ When
radical halogenation produces a chiral
center or takes place at a hydrogen on a chiral
center, the product is a racemic mixture of R and
S enantiomers.
• 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
‹ Radical
bromination of butane.
8-34
Allylic Halogenation
‹ Allylic
carbon: A C adjacent to a C-C double
bond.
‹ Allylic hydrogen: An H on an allylic carbon.
• an allylic C-H bond is weaker than a vinylic C-H bond.
8-35
Allylic Bromination
‹ Allylic
bromination using NBS
8-36
Allylic Bromination
‹A
radical chain mechanism
• Chain initiation
• Chain propagation
8-37
Allylic Bromination
• chain termination
‹ NBS
neutralizes HBr and the protonated amide
then provides Br2 for the chain process.
8-38
The Allyl Radical
‹
A hybrid of two equivalent contributing structures.
8-39
The Allyl Radical
‹
Molecular orbital model of the allyl radical. Combination
of three 2p orbitals gives three π molecular orbitals.
8-40
The Allyl Radical
‹
Unpaired electron spin density map of the allyl radical
• Unpaired electron density (green cones) appears only
on carbons 1 and 3
8-41
Allylic Halogenation
• Problem Account for the fact that allylic bromination of
1-octene by NBS gives these isomeric products
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.
• This radical is stabilized by resonance with both
double bonds.
8-44
Radical Autoxidation
• The doubly allylic radical reacts with O2, 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.
• 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.
8-45
Radical Autoxidation
• vitamin E as an antioxidant
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.
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
8-48
Radical Addition of HBr to Alkenes
‹ Chain
propagation
8-49
Radical Addition of HBr to Alkenes
‹ Chain
‹
termination
This pair of reactions illustrates how the products of a
reaction can be changed by a change in experimental
conditions.
• Polar addition of HBr is regioselective, with
protonation of the alkene preceding the addition of Brto 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