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. 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.
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. 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 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-dibromidefrom 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
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 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 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:
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 • 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
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).
Valence shell Changed to three sp2 AO’s with one electron each and one empty p AO Three sp2; Trigonal planar Similar to carbocation Octet rule is satisfied; stable Lewis acid; e- deficient Borane (Lewis acid) + THF ( L. base) complex Borane can be used as a electophile 2. Hydroboration-Oxidation of Alkenes Lets first understand Borane.
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
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).
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.
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, 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