Chapter 5
Addition Reaction of Alkenes
Addition of Halogens to Alkenes
Addition of Cl2 and Br2 to give vicinal dihalides. The other halogens are not
commonly used because F2 is too reactive and I2 is not reactive enough
Inert solvents, such as CCl4, CHCl3, or CH2Cl2 are used.
H3C
C
H
CH
2-butene
CH3
+ Br2
CH2Cl2
H3C
H
C
H
C
Br
Br
CH3
(solvent)
2,3-dibromobutane
Test of Unsaturation: The addition of bromine to most alkenes are so fast
that when bromine is added dropwise to a solution of alkene, the red bromine
color disappear almost immediately. This test is used to qualitatively identify
alkenes.
Mechanism: Experimental evidence suggests that the
mechanism of bromination proceeds via bromonium ion.
The polarizability of the Br-Br bond allows heterolytic
cleavage when attacked by a nucleophilic π bond, forming
a cyclic bromonium ion.
Step 1.
How the first step of Step 1 might
happen? Note that the species on
the right hand side of the resonance
structure will be more stable, since
each atom has an octet and there is
an extra bond.
Step 2.
In symmetric bromonium ions, attack from the other side
of the ring is equally probable at either carbon leading to
racemic or meso products.
Bromonium Ions DO Exist
• Bromonium were postulated more than 60 years ago
to expain the stereochemical course of the addition (to
give the trans-dibromide from a cyclic alkene
• Olah showed that bromonium ions are stable in liquid
SO2 with SbF5 and can be studied directly.
By the way, you can have cis/trans isomers in
cyloalkanes even though no double bonds
• This is possible because of the greater
rigidity of cycloalkanes than regular
alkanes
X
X
X
X
cis
trans
Stereochemistry: syn or anti addition
Why Only Trans-Product in
Bromination of Cyclic Alkenes?
Consider the bromination of cyclohexene. No cis-1,2dibromocyclohexane is formed.
Only anti-addition is observed (So only trans product). The product is racemic
since the initial attack of bromine can occur with equal probability at either
face of the cyclohexene.
Homework: Draw a mechanism of this reaction.
Addition of Br2 to Cyclopentene
•
Addition is again exclusively trans
+
Halohydrin Formation from Alkenes
(A vicinal (i.e. adjacent) halohydrin)
Reaction does proceed through the same bromonium ion intermediate, but there
is a competing nucleophile (here water) that attacks the bromonium ion to give
the product. A proton is released in the process.
Mechanism of Formation of a Chlorohydrin
• Cl2 forms chloronium ion, then water attacks.
– Orientation toward stable C+ species, so regioselectivity observed.
[The most highly substituted carbon has the most positive charge, so
nucleophile attacks there]
Mechanism:
Homework: Draw resonance structures of the chloronium ion intermediate and
reason why water, the nucleophile, attacks the carbon that is highly substituted.
Predict the product
CH2Br
OH
Br2
(H2O)
Mechanism:
Some Reagents of the type A-B, in which A acts as the
electrophile, A+, and B the nucleophile, B-, can undergo stereoand regiospecific addition reactions to alkenes:
Reduction of Alkenes: Hydrogenation
•
•
•
•
•
Addition of H-H across C=C – A Syn Addition
Reduction in general is addition of H2 or its equivalent
Requires Pt or Pd as powders on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous (process is not in solution)
Hydrogen Addition- Selectivity
• Selective for C=C. No reaction with C=O,
C=N
• Polyunsaturated liquid oils become solids
• If one side is blocked, hydrogen adds to
other
Mechanism of Catalytic Hydrogenation
• Heterogeneous – reaction between phases
• Addition of H-H is syn
Conversion of Alkenes to Alcohols:
Two Other Methods Used Widely in labs
1.Oxymercuration-Reduction of Alkenes
- Highly Regioselective – Markovnikov product formed
(Hydroxyl group is added to the more branched carbon of
the double bond. Reaction proceeds through mercurinium ion
- No rearrangements
2. Hydroboration-Oxidation of Alkenes
- Regioselective – Anti-Markovnikov Product formed.
- A Syn Addition of H and OH (means they add to same face
of the double bond)
- No Rearrangements
1. Oxymercuration-Reduction of Alkenes:
Reaction of alkene with mercuric (II) acetate in THF/water,
followed by reaction with sodium borohydride. THF is a great
solvent because it dissolves both water and many other organics.
The reagent mercuric (II) acetate dissociates slightly to form
+Hg(OAc) which acts as a electrophile that is attacked by the
pi bond. (Home work: Try to write this reaction)
(Think what kind of product would you get if you use alcohol (ROH) instead of water
in the above reaction)
Mechanism of Oxymercuration – Reduction Reaction
1. Markovnikov’s rule
2. No rearrangement
More examples of Oxymercuration – Reduction Reaction
Notice the High yields of reaction. Markovnikov orientation
is followed and no rearrangement observed (Since no
Carbocations involved).
2. Hydroboration-Oxidation of Alkenes
Lets first understand Borane.
Valence shell
Changed to three sp2 AO’s
with one electron each
and one empty p AO
Three sp2; Trigonal planar
Similar to carbocation
Lewis acid;
e-
deficient
Octet rule is satisfied; stable
Borane (Lewis acid) + THF ( L. base)
complex
Borane can be used as a electophile
So Hydroboration-Oxidation Reaction…
Addition of H and OH (elements of H2O) to alkenes.
Two step reaction:
1. The alkene is reacted with a complex of BH3 and THF
2. Treatment with hydrogen peroxide in basic solution.
Note the anti-Markovnikov
product
Mechanism: Regioselective (Syn addition) and Anti-Markovnikov
addition.
An example of a concerted reaction
Why Syn Addition & an AntiMarkovnikov Additon?
• The electron-deficient borane adds to the least substituted carbon (i.e.
Sterically less crowded one)
• The other carbon acquires a partial positive charge.
• H adds to adjacent C on same side (Syn). The hydroxy group replaces
the boron with complete retention of configuration (Syn).
In Summary, Overall, Hydroboration-Oxidation
Forms an Alcohol from an Alkene
• Addition of H-BH2 (from BH3-THF complex) to three
alkenes gives a trialkylborane
• Oxidation with alkaline hydrogen peroxide in water
produces the alcohol derived from the alkene
Think: How can you use this reaction to generate an ether, R-O-R (e.g. CH3-O-CH3)
Ozonolysis of Alkenes
- Reaction of ozone(O3) with alkenes produces ozonide
which then is reduced to give aldehydes or ketones.
Ozone is the mildest reagent capable of breaking both
the  and  bonds in a double bond.
- O3 acts both like a nucleophile and an electrophile
- This reaction is used for structure determination of alkenes.
e.g.
Structure Elucidation With Ozone
•
Cleavage products reveal an alkene’s structure
Free Radical Addition to Alkenes:
Peroxide Effect – Anti-Markovnikov Product
In the presence of peroxides, HBr adds to an alkene to form
the “anti-Markovnikov” product.
Only HBr has the right bond energy. HCl bond is too strong.
HI bond tends to break heterolytically to form ions.
In the presence of oxygen, a radical chain sequence mechanism
leads to the anti-Markovnikov product. Small amounts of peroxides (RO-OR)
are formed in alkene samples stored in the presence of air (O2).
The peroxides initiate the radical chain sequence mechanism, which
is much faster than the ionic mechanism operating in the absence of peroxides.
Mechanism: Note below a different kind of arrow,
to depict a radical reaction.
a Fishhook-like,
- Note how the propagation step is exothermic in the reaction
of alkenes with HBr.
- The halogen’s attack is regioselective, generating the more
stable secondary radical rather than the primary one. The
radical stability follows a similar pattern as observed with
carbocations (remember this).
- In the final step, the alkyl radical subsequently abstracts a
hydrogen from HBr which regenerates the chain-carrying
bromine atom.
- Termination is by radical recombination or by some other
removal of the chain carriers.
- Some commonly used peroxides for
initiating radical additions
Are radical additions general?
HCl and HI do not give anit-Markovnikov addition products with
alkenes. The chain propagation steps involving these hydrogen
halides are endothermic which leads to very slow reactions and
chain termination.
HCl and HI give Markovnikov products by ionic mechanisms
irregardless of the presence of radicals.
Other reagents, such as thiols, do however undergo successful
radical additions to alkenes:
e.g.
Suggested problems (Chapter 5)
5.1, 5.3, 5.5, 5.8, 5.10, 5.12, 5.17, 5.23, 5.24, 5.28, 5.31