Download dipole/induced-dipole and dipole/induced

Halogen derivatives of hydrocarbons:
Bonding system and physical properties. Factors affecting the
C-Hlg bond strength of the response relationship skills.
Halides with reduced, normal and increased reactivity. Key
mechanisms and application of nucleophilic substitution (SN1
and SN2). Factors affecting nucleophilic substitution.
Elimination reactions:  - and -elimination, basic mechanism
of -elimination (E1, E2 and E1cB). Substitution and elimination
share of influence. Reaction of halogen compounds with
metals. Preparation of aliphatic and aromatic halogen
compounds.
Classification of C-Hlg compounds
According to
Hlg quality
Numbers of Hlg (di-, tri-, tetra- etc.)
in case of dihalogenids:
n1
Type of carbon chain: • aliphatic (saturated, unsaturated)
• aromatic
The order of C (primary, secondary, or tertiary according to the classification of the
carbon that bears the functional group)
Particular importance of the hybrid form of the  - and  -carbon  reactivity
n2
Nomenclature
• trivial names: eg. chloroform, iodoform, fluothane
• substitution nomenclature (substitutive nomenclature) - halogen name as a prefix only!
• functional group nomenclature (functional class nomenclature): hydrocarbon group +
halide (fluoride, chloride, bromide, iodide) Suffix - assuming that no higher priority
functional group
IUPAC NOMENCLATURE OF ALKYL HALIDES
The IUPAC rules permit alkyl halides to be named in two different ways, called functional
class nomenclature and substitutive nomenclature.
In functional class nomenclature the alkyl group and the halide ( fluoride, chloride, bromide, or iodide)
are designated as separate words. The alkyl group is named on the basis of its longest continuous
chain beginning at the carbon to which the halogen is attached.
Substitutive nomenclature of alkyl halides treats the halogen as a halo- ( fluoro-, chloro-, bromo-, or
iodo-) substituent on an alkane chain. The carbon chain is numbered in the direction that gives the
substituted carbon the lower locant.
When the carbon chain bears both a halogen and an alkyl substituent, the two substituents are considered
of equal rank, and the chain is numbered so as to give the lower number to the substituent nearer the end
of the chain.
Substitutive names are preferred, but functional class names are sometimes more convenient or more
familiar and are frequently encountered in organic chemistry.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 127
Substitutive nomenclature of alkyl halides
John McMurry: Organic Chemistry (7th Edition), ISBN-10: 0840054440 ISBN-13: 9780840054449, Brooks/Cole, 2012, p. 333
Substitutive nomenclature of alkyl halides
John McMurry: Organic Chemistry (7th Edition), ISBN-10: 0840054440 ISBN-13: 9780840054449, Brooks/Cole, 2012, p. 333
Bonding system of C-Hlg compounds
Carbon–halogen bonds are polar covalent bonds, and carbon bears a partial positive charge in
alkyl halides. The presence of this polar bond makes alkyl halides polar molecules.
Typically EN(Hlg) > EN(C)  polarized hetero nuclear
-bond, partial positive charge on carbon atom
(-I effect)
BUT! If the pillar C is a sp2 or sp hybridized 
+M effect also occurs. Result: shorter, stronger bonds
Electronegativity (EN): A measure of the ability of an atom to attract the electrons in a covalent bond
toward itself. Fluorine is the most electronegative element.
Electronic effect: An effect on structure or reactivity that is attributed to the change in electron
distribution that a substituent causes in a molecule.
Inductive effect: An electronic effect transmitted by successive polarization of the bonds within a
molecule or an ion. OR The electron-donating or electron-withdrawing effect of a group that is
transmitted through bonds is called an inductive effect.
Mesomeric effect: The effect is used in a qualitative way and describes the electron withdrawing or
releasing properties of substituents based on relevant resonance structures and is symbolized by the
letter M. The mesomeric effect is a permanent effect and operates in compounds containing at least
one double bond and another double bond or a lone pair separated by a single bond. The
mesomeric effect is negative (–M) when the substituent is an electron-withdrawing group and the
effect is positive (+M) when based on resonance and the substituent is an electron releasing group.
–M effect of a carbonyl group in acrolein
Electronegativity is a chemical property that describes the tendency of an atom to
attract electrons in a covalent bond towards itself.
C-Hlg bond distance
C-Hlg bond distance depends on: the quality of halogen, carbon hybrid status
F
Cl
Br
I
0.139
0.178
0.195
0.214
0.133
0.172
0.188
0.210
Shorter bonds
dC-Hlg (nm)
Longer bonds
Halogens increase in size going down the periodic table, so the lengths of the corresponding carbonhalogen bonds increase accordingly. In addition, C―X bond strengths decrease going down the periodic
table.
C-Hlg bond energy
C-Hlg bond energy depends on: the quality of halogen, carbon hybrid status
Average C-Hlg bond energy (kJ/mol)
C-F
488
C-Cl
326
C-Br 278
C-I
210 (est.)
Reason: weaker overlapping,
smaller charge separation
C-Hlg dissociation energy (kJ/mól)
homolytic
heterolytic
F
Br
F
Br
MeCH2-Hlg
448
282
920
770
PhCH2Hlg
403
229
820
657
CH2=CH-Hlg
497
320
1004
837
Bonding system of C-Hlg compounds 2.
Tendencies:
1. aryl/vinyl halides: stronger bonds (+M effect)
2. allyl/benzyl halides: weaker bonds - greater stability of the formed radical/cation
has a lower energy
3. homolytic bond cleavage requires less energy
BUT!! This is true in gas phase, in solution the solvation energy can overwrite it
C-Hlg dissociation energy (kJ/mól)
homolytic
heterolytic
F
Br
F
Br
alkyl
MeCH2-Hlg
448
282
920
770
benzyl
PhCH2Hlg
403
229
820
657
vinyl
CH2=CH-Hlg
497
320
1004
837
Expected results
• aryl/vinyl halides are less reactive than simple alkyl halides
• allyl/benzyl halides are more reactive than alkyl halides
Dipole moment
Halogens are more electronegative than carbon. The C-X bond is therefore polar, with
the carbon atom bearing a slight positive and the halogen a slight negative charge.
The presence of the polar bonds makes alkyl halides polar molecules.
Due to the ground state polarization: dipole moment appears
Me-F: 1.81 D
Me-Cl: 1.87 D
Distribution of electron density in chloromethane:
Me-Br: 1.80 D
Electrostatic potential maps of
chloromethane.The most positively
charged regions are blue, the most
negatively charged ones red. The
electrostatic potential is most
negative
near
chlorine
in
chloromethane.
The polarization of the bonds to chlorine, as well as its unshared electron pairs, contribute to the
concentration of negative charge on chlorine atoms.
Relatively simple notions of attractive forces between opposite charges are sufficient to account
for many of the properties of chemical substances. You will find it helpful to keep the polarity of
carbon–oxygen and carbon–halogen bonds in mind as we develop the properties of alcohols and
alkyl halides in later sections.
The effective dipole moment is formed by two factors: the value of charge separation (EN
difference) and the bond length.
Bromine is less electronegative than chlorine, yet methyl
bromide and methyl chloride have very similar dipole
moments. Why?
Dipole moment is the product of charge and distance.
Although the electron distribution in the carbon–chlorine
bond is more polarized than that in the carbon–bromine
bond, this effect is counterbalanced by the longer carbon–
bromine bond distance.
BUT! due to the nature of elementary dipole moments the resultant vector may be zero!
=0
 [D]
CH3-Br
CH3CH2-Br
CH3CH2CH2-Br
1.80
1.88
1.92
Cause: Charge separation runs through the chain.
Important lessons for the future: electron-withdrawing groups are not the only  carbon but
also decreasing around the  carbon and reduce the electron density of the  carbon!!!
Don’t forget!
Halogens are more electronegative than carbon. The C-X bond is therefore polar, with the
carbon atom bearing a slight positive and the halogen a slight negative charge.
This polarity results in a substantial dipole moment for all the halomethanes and implies
that the alkyl halide C-X carbon atom should behave as an electrophile in polar reactions.
We’ll see that much of the chemistry of alkyl halides is indeed dominated by their
electrophilic behavior.
Physical properties of C-Hlg compounds
The forces of attraction between neutral molecules are of three types listed here. The first two of these
involve induced dipoles and are often referred to as dispersion forces, or London forces.
1. Induced-dipole/induced-dipole forces
2. Dipole/induced-dipole forces
3. Dipole–dipole forces
Induced-dipole/induced-dipole forces are the only intermolecular attractive forces
available to nonpolar molecules such as alkanes. In addition to these forces, polar molecules
engage in dipole–dipole and dipole/induced-dipole attractions.
The dipole–dipole attractive force is easiest to visualize. Two molecules of a polar substance experience
a mutual attraction between the positively polarized region of one molecule and the negatively
polarized region of the other.
Two molecules of a polar substance are
oriented so that the positively polarized
region of one and the negatively polarized
region of the otherattract each other.
As its name implies, the dipole/induced-dipole force combines features of both the induceddipole/induced dipole and dipole–dipole attractive forces. A polar region of one molecule alters the
electron distribution in a nonpolar region of another in a direction that produces an attractive force
between them.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 130.
Physical properties of C-Hlg compounds 2.
When comparing the boiling points of related compounds as a function of the alkyl group, we
find that the boiling point increases with the number of carbon atoms, as it does with alkanes.
With respect to the halogen in a group of alkyl halides, the boiling point increases as one descends
the periodic table; alkyl fluorides have the lowest boiling points, alkyl iodides the highest. This
trend matches the order of increasing polarizability of the halogens.
Polarizability is the ease with which the electron distribution around an atom is distorted by a
nearby electric field and is a significant factor in determining the strength of induceddipole/induced-dipole and dipole/induced-dipole attractions. Forces that depend on induced
dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the
halogen is a nonpolarizable fluorine.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 131.
Physical properties of C-Hlg compounds 3.
The boiling points of the chlorinated derivatives of methane increase with the number of chlorine
atoms because of an increase in the induced-dipole/induced-dipole attractive forces.
Fluorine is unique among the halogens in that increasing the number of fluorines does not
produce higher and higher boiling points.
The reason for this behavior has to do with the very low polarizability of fluorine and a decrease
in induced-dipole/induced-dipole forces that accompanies the incorporation of fluorine
substituents into a molecule. Their weak intermolecular attractive forces give fluorinated
hydrocarbons (fluorocarbons) certain desirable physical properties such as that found in the “no
stick” Teflon coating of frying pans.
(Teflon is a polymer made up of long chains of -CF2CF2-units.)
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 132.
Physical properties of C-Hlg compounds 4.
Conlusion
Melting point (Mp) and boiling point (bp) – forces: dipole-dipole interaction
Mp and bp greater than in case of the alkanes, alkenes with the same number of C’s.
R-Hlg boiling point (oC)
R-H (oC)
Hlg
F
Cl
Br
Me-Hlg
-78
-24
4
-162
Et-Hlg
-32
12
38
-89
Bu-Hlg
32
79
102
-1
Bp is increasing:
• changes in the quality of halogen (F  I)
• increase in the number of carbon atoms
• increase in the number of Hlg-s (except fluorides)
Physical properties of C-Hlg compounds 5.
Density – increasing F  I, and with the number of Hlg-s, BUT decreasing with the size of
the carbon chain
d(Me-F) = 0.877 g/cm3, d(Pr-F) = 0.779 g/cm3
d(Me-F) = 0.877 g/cm3, d(Me-Br) = 1.732 g/cm3
d(Me-Cl) = 0.991 g/cm3, d(CHCl3) = 1.489 g/cm3 (but! d(CHBr3) = 2.890 g/cm3
Solubility
Properties: low water solubility (all alkyl halides are insoluble in water, H bridges are stronger
than dipole-dipole interactions). Highly dissolves the less polar organic substances, fats.
 good extraction agents
 good cleaners
 modification of biological lipid-lipid systems, high narcotic effect (e.g. fluothane:
CHClBr-CF3)
Because alkyl halides are insoluble in water, a mixture of an alkyl halide and water
separates into two layers. When the alkyl halide is a fluoride or chloride, it is the upper
layer and water is the lower. The situation is reversed when the alkyl halide is a bromide or
an iodide. In these cases the alkyl halide is the lower layer. Polyhalogenation increases the
density. The compounds CH2Cl2, CHCl3, and CCl4, for example, are all more dense than
water.
Preparation of C-Hlg compounds
1. Synthesis of alkyl halides
1.1. Halogenation of alkanes
It involves substitution of a halogen atom for one of the alkane’s hydrogens.
Disadvantages: mixture, no control! (Hlg = Cl, Br)
The alkane is said to undergo fluorination, chlorination, bromination, or iodination according to
whether X2 is F2, Cl2, Br2, or I2, respectively. The general term is halogenation. Chlorination and
bromination are the most widely used.
The reactivity of the halogens decreases in the order F2 > Cl2 > Br2 > I2.
Fluorine is an extremely aggressive oxidizing agent, and its reaction with alkanes is strongly
exothermic and difficult to control. Direct fluorination of alkanes requires special equipment and
techniques, is not a reaction of general applicability.
Chlorination of alkanes is less exothermic than fluorination, and bromination less exothermic than
chlorination.
Iodine is unique among the halogens in that its reaction with alkanes is endothermic and alkyl
iodides are never prepared by iodination of alkanes.
CHLORINATION OF METHANE
The gas-phase chlorination of methane is a reaction of industrial importance and leads to a mixture of
chloromethane (CH3Cl), dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane
(CCl4) by sequential substitution of hydrogens.
The intermediates in the chlorination of methane and other alkanes are quite different; they are
neutral (“nonpolar”) species called free radicals.
Alkyl radicals
Simple alkyl radicals, for example, require special procedures for their isolation and study. We will
encounter them here only as reactive intermediates, formed in one step of a reaction mechanism
and consumed in the next. Alkyl radicals are classified as primary, secondary, or tertiary according to
the number of carbon atoms directly attached to the carbon that bears the unpaired electron.
alkyl substituents stabilize free radicals
Some of the evidence indicating that alkyl substituents stabilize free radicals comes
from bond energies. The strength of a bond is measured by the energy required to break
it. A covalent bond can be broken in two ways.
In a homolytic cleavage a bond between two atoms
is broken so that each of them retains one of the
electrons in the bond.
The more stable the radical, the lower the energy
required to generate it by C-H bond homolysis.
The energy required for homolytic bond cleavage is called
the bond dissociationenergy (BDE).
In contrast, in a heterolytic cleavage one fragment
retains both electrons.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 150.
Alkyl radicals 2.
As the table indicates, C-H bond
dissociation energies in alkanes are
approximately 375 to 435 kJ/mol (90–105
kcal/mol). Homolysis of the H-CH3 bond in
methane gives methyl radical and requires
435 kJ/mol (104 kcal/mol). The
dissociation energy of the H-CH2CH3 bond
in ethane, which gives a primary radical, is
somewhat less (410 kJ/mol, or 98
kcal/mol) and is consistent with the
notion that ethyl radical (primary) is more
stable than methyl.
The dissociation energy of the terminal C-H
bond in propane is exactly the same as that of
ethane. The resulting free radical is primary in
both cases.
Note, however, that Table 4.3 includes two entries for propane.
The second entry corresponds to the cleavage of a bond to one of
the hydrogens of the methylene (CH2) group. It requires slightly
less energy to break a C±H bond in the methylene group than in
the methyl group.
Since the starting material (propane) and one of the products (H) are the same in both processes, the difference in
bond dissociation energies is equal to the energy difference between an n-propyl radical (primary) and an isopropyl
radical (secondary). The secondary radical is 13 kJ/mol (3 kcal/mol) more stable than the primary radical.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 151-152.
MECHANISM OF METHANE CHLORINATION
The reaction itself is strongly exothermic,
but energy must be put into the system
in order to get it going. This energy goes
into breaking the weakest bond in the
system, which we see from the bond
dissociation energy data in Table 4.3, is
the Cl-Cl bond with a bond dissociation
energy of 242 kJ/mol (58 kcal/mol). The
step in which Cl-Cl bond homolysis
occurs is called the initiation step.
Each chlorine atom formed in the
initiation step has seven valence
electrons and is very reactive. Once
formed, a chlorine atom abstracts a
hydrogen atom from methane as shown
in step 2. Hydrogen chloride, one of the
isolated products from the overall
reaction, is formed in this step.
A methyl radical is also formed, which
then attacks a molecule of Cl2 in step 3.
Attack of methyl radical on Cl2 gives
chloromethane, the other product of the
overall reaction, along with a chlorine
atom which then cycles back to step 2,
repeating the process.
Steps 2 and 3 are called the propagation steps of the reaction and, when added together, give the overall equation for
the reaction. Since one initiation step can result in a great many propagation cycles, the overall process is called a
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 153-154.
free-radical chain reaction.
MECHANISM OF METHANE CHLORINATION 2.
The chain sequence is interrupted whenever two odd-electron species combine to give an evenelectron product. Reactions of this type are called chain-terminating steps.
Some commonly observed chain-terminating steps in the chlorination of methane are shown in the
following equations.
Combination of a methyl radical with a chlorine atom:
Combination of two methyl radicals:
Combination of two chlorine atoms:
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 154-155.
1.2. Addition of Hlg2 or HHlg to alkenes, alkynes (radical or electrophile addition mechanism)
Problems with
regioselectivity
Reactivity depends on the quality of the HHlg: HI > HBr > HCl >> HF
1.2.1. ELECTROPHILIC ADDITION OF HYDROGEN HALIDES TO ALKENES
In many addition reactions the attacking reagent is a polar molecule. Hydrogen halides are among the simplest
examples of polar substances that add to alkenes.
The reactivity of the hydrogen halides reflects their ability to donate a proton. Hydrogen iodide is the strongest acid of
the hydrogen halides and reacts with alkenes at the fastest rate.
GENERAL MECHANISM
An alkene can accept a proton from a hydrogen halide to form a carbocation.
Carbocations, when generated in the presence of halide anions, react with them to form alkyl
halides.
This reaction is called electrophilic addition because the reaction is triggered by the attack of an
electrophile
(an acid) on the p electrons of the double bond. Using the two p electrons to form a bond to an
electrophile generates a carbocation as a reactive intermediate; normally this is the ratedetermining step.
REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION: MARKOVNIKOV’S RULE
In principle a hydrogen halide can add to an unsymmetrical alkene (an alkene in which the two carbons of the double
bond are not equivalently substituted) in either of two directions. In practice, addition is so highly regioselective as to
be considered regiospecific.
In 1870, Vladimir Markovnikov, a colleague of Alexander Zaitsev, noticed a pattern in the hydrogen halide addition to
alkenes and assembled his observations into a simple statement. Markovnikov’s rule states that when an
unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the
greater number of hydrogen substituents, and the halogen adds to the carbon having fewer hydrogen substituents.
The preceding general equations illustrate regioselective addition according to Markovnikov’s rule, and the
equations that follow provide some examples.
Markovnikov’s rule
The way we usually phrase it now:
When a hydrogen halide adds to
an alkene, protonation of the
double bond occurs in the
direction that gives the more
stable carbocation.
MECHANISTIC BASIS FOR MARKOVNIKOV’S RULE
Compare the carbocation intermediates for
addition of a hydrogen halide (HX) to an
unsymmetrical alkene!
The activation energy for formation of the more
stable carbocation (secondary) is less than that for
formation of the less stable (primary) one. Both
carbocations are rapidly captured by X- to give an
alkyl halide, with the major product derived from
the carbocation that is formed faster. The energy
difference between a primary carbocation and a
secondary carbocation is so great and their rates of
formation are so different that essentially all the
product is derived from the secondary carbocation.
STRUCTURE OF METHYL CATION
The properties of carbocations are intimately related to their structure, and so let’s think about the
bonding in methyl cation, CH3+ .
The positively charged carbon contributes three valence electrons, and each hydrogen contributes
one for a total of six electrons, which are used to form three C-H bonds. Carbon is sp2-hybridized
when it is bonded to three atoms or groups. Carbon forms bonds to three hydrogens by overlap of
its sp2 orbitals with hydrogen 1s orbitals. The three bonds are coplanar. Remaining on carbon is an
unhybridized 2p orbital that contains no electrons.
The axis of this empty p orbital is perpendicular to the plane defined by the three bonds.
Structure of methyl cation CH3+. Carbon is sp2-hybridized.
Each hydrogen is attached to carbon by a bond formed
byoverlap of a hydrogen 1s orbital with an sp2 hybrid
orbital of carbon. All four atoms lie in the same plane. The
unhybridized 2p orbital of carbon is unoccupied, and its
axis is perpendicular to the plane of the atoms.
STRUCTURE, BONDING, AND STABILITY OF CARBOCATIONS
Carbocations are classified as primary, secondary, or tertiary according to the number of carbons that are directly
attached to the positively charged carbon. They are named by appending “cation” as a separate word after the IUPAC
name of the appropriate alkyl group. The chain is numbered beginning with the positively charged carbon (the
positive charge is always at C-1).
Alkyl groups directly attached to the positively charged carbon stabilize a carbocation. Thus, the observed order of
carbocation stability is
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 140-143.
Because alkyl groups stabilize carbocations, we conclude that they release electrons to the positively charged carbon,
dispersing the positive charge. They do this through a combination of effects.
One involves polarization of the bonds to the positively charged carbon.
Electrons in a C-C  bond are more polarizable than those in a C-H bond, so replacing hydrogens by alkyl groups
reduces the net charge on the sp2-hybridized carbon. The electron-donating or electron-withdrawing effect of a
group that is transmitted Through  bonds is called an inductive effect.
The charge in ethyl cation is stabilized by polarization of
the electron distribution in the bonds to the positively
charged carbon atom. Alkyl groups release electrons better
than hydrogen
A second effect, called hyperconjugation, is also important.
Ethyl cation is stabilized by delocalization of
the electrons in the C-H bonds of the methyl
group into the vacant 2p orbital of the
positively charged carbon.
According to hyperconjugation, electrons in the C-H bond of a +C-C-H unit are more stabilizing than +C-H electrons.
Thus, successive replacement of the hydrogens attached to CH3 by alkyl groups increases the opportunities for
hyperconjugation, which is consistent with the observed order of increasing carbocation stability:
methyl  primary  secondary  tertiary.
Finally, although we have developed this picture for hyperconjugation of a +C-C-H unit, it also applies to +C-C-C as
well as many others.
1.2.2. FREE-RADICAL ADDITION OF HYDROGEN BROMIDE TO ALKENES
When the addition of
hydrogen bromide to
alkenes was performed
in the presence of an
added peroxide, only
1-bromobutane
was
formed.
Peroxides are initiators;
they
are
not
incorporated into the
product but act as a
source
of
radicals
necessary to get the
chain reaction started.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 220-223.
Don’t forget!
Markovnikov addition
ionic addition
The regioselectivity of addition of hydrogen bromide to alkenes under normal (ionic addition)
conditions is controlled by the tendency of a proton to add to the double bond so as to produce the
more stable carbocation.
anti-Markovnikov addition
free-radical addition
Under free-radical conditions the regioselectivity is governed by addition of a bromine atom to give
the more stable alkyl radical.
Free-radical addition of hydrogen bromide to the double bond can also be initiated photochemically,
either with or without added peroxides.
1.2.3. ADDITION OF HALOGENS TO ALKENES
Halogens react with alkenes by electrophilic addition.
The products of these reactions are called vicinal dihalides. Two substituents, in this case the halogens, are vicinal if
they are attached to adjacent carbons. The word is derived from the Latin vicinalis, which means “neighboring.” The
halogen is either chlorine (Cl2) or bromine (Br2), and addition takes place rapidly at room temperature and below in a
variety of solvents, including acetic acid, carbon tetrachloride, chloroform, and dichloromethane.
Mechanism of electrophilic
addition of bromine to
ethylene.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 233-236.
STEREOCHEMISTRY OF HALOGEN ADDITION
The reaction of chlorine and bromine with cycloalkenes illustrates an important stereochemical feature of halogen
addition: Anti addition is observed; the two bromine atoms of Br2 or the two chlorines of Cl2 add to opposite faces of
the double bond.
Anti addition!
1.2.4. ADDITION OF HYDROGEN HALIDES TO ALKYNES
Hydrogen halides, for example, add to alkynes to form alkenyl halides.
The regioselectivity of addition follows Markovnikov’s rule. A proton adds to the carbon that has the greater
number of hydrogens, and halide adds to the carbon with the fewer hydrogens.
In the presence of excess hydrogen halide, geminal dihalides are formed by sequential addition of two molecules
of hydrogen halide to the carbon–carbon triple bond.
The hydrogen halide adds to the initially formed alkenyl halide in accordance with Markovnikov’s rule. Overall,
both protons become bonded to the same carbon and both halogens to the adjacent carbon.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 352-354.
1.2.5. ADDITION OF HALOGENS TO ALKYNES
Alkynes react with chlorine and bromine to yield tetrahaloalkanes. Two molecules of the halogen
add to the triple bond.
A dihaloalkene is an intermediate and is the isolated product when the alkyne and the halogen are
present in equimolar amounts. The stereochemistry of addition is anti.
Francis A. Carey: Organic Chemistry (4th Edition), ISBN 0-07-290501-8; The McGraw-Hill Companies, Inc., 2000, p. 356-357.