Chapter 5

Addition Reaction of Alkenes

Addition of Halogens to AlkenesAddition of Cl2 and Br2 to give vicinal dihalides. The other halogens are notcommonly used because F2 is too reactive and I2 is not reactive enoughInert solvents, such as CCl4, CHCl3, or CH2Cl2 are used.

H3C CH

CH CH3

2-butene+ Br2

CH2Cl2

(solvent)

H3CHC

HC CH3

Br Br

2,3-dibromobutane

Test of Unsaturation: The addition of bromine to most alkenes are so fastthat when bromine is added dropwise to a solution of alkene, the red brominecolor disappear almost immediately. This test is used to qualitatively identifyalkenes.

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 mighthappen? Note that the species onthe right hand side of the resonancestructure will be more stable, sinceeach atom has an octet and there is an extra bond.

Step 2.In symmetric bromonium ions, attack from the other sideof the ring is equally probable at either carbon leading toracemic or meso products.

• 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.

Bromonium Ions DO Exist

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-1,2-dibromocyclohexane is formed.

Only anti-addition is observed (So only trans product). The product is racemicsince the initial attack of bromine can occur with equal probability at eitherface 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 thereis a competing nucleophile (here water) that attacks the bromonium ion to give the product. A proton is released in the process.

• 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 of Formation of a Chlorohydrin

Mechanism:

Homework: Draw resonance structures of the chloronium ion intermediate andreason why water, the nucleophile, attacks the carbon that is highly substituted.

Br2

(H2O)

Predict the product

CH2Br

OH

Mechanism:

Some Reagents of the type A-B, in which A acts as the electrophile, A+, and B the nucleophile, B-, can undergo stereo- and regiospecific addition reactions to alkenes:

• 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)

Reduction of Alkenes: Hydrogenation

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

• Heterogeneous – reaction between phases• Addition of H-H is syn

Mechanism of Catalytic Hydrogenation

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 greatsolvent 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 waterin the above reaction)

Mechanism of Oxymercuration – Reduction Reaction

1. Markovnikov’s rule2. No rearrangement

More examples of Oxymercuration – Reduction Reaction

Notice the High yields of reaction. Markovnikov orientationis followed and no rearrangement observed (Since no

Carbocations involved).

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)

Borane can be used as a electophile

complex

2. Hydroboration-Oxidation of Alkenes

Lets first understand Borane.

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.

So Hydroboration-Oxidation Reaction…

Note the anti-Markovnikovproduct

An example of a concerted reaction

Mechanism: Regioselective (Syn addition) and Anti-Markovnikov addition.

Why Syn Addition & an Anti-Markovnikov 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).

• 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

In Summary, Overall, Hydroboration-Oxidation Forms an Alcohol from an 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 ProductIn 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 mechanismleads 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, whichis much faster than the ionic mechanism operating in the absence of peroxides.

Mechanism: Note below a different kind of arrow, a Fishhook-like, to depict a radical reaction.

- 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