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Chapter 8

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Barb Morra

CHM 247 CH.8 Alkenes: Reactions and Synthesis • Important reactions include the addition of a halogen to give a 1,2-dihalide, addition of a hypohalous acid to give a halohydrin, addition of water to give an alcohol, addition of hydrogen to give a halohydrin, addition of a single oxygen to give a 3-membered cyclic ether called an epoxide, and addition of 2 hydroxyl groups to give a 1,2-diol. 8.1 PreparingAlkenes: APreview of Elimination Reactions • Alkenes are readily available from simple precursors. • Preparation of alkenes dominated by elimination reactions. • Addition tends to break double bonds, where as elimination tends to form them. • Two most common elimination reactions are dehydrohalogenation – the loss of HX from an alkyl halide – and dehydration – the loss of water from an alcohol. • Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. • • Dehydration is often carried out in the laboratory by treatment of an alcohol with a strong acid. • 8.2 Halogenation ofAlkenes: Addition of X 2 • Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called halogenation. + • Possible mechanism below. Electrophilic addition of Br to the alkene, giving a carbocation intermediate that could undergo further reaction with Br to yield the dibromo addition product. • Stereochemistry important with these reactions though. • Anti stereochemistry: The opposite of syn. An anti addition reaction is one in which the 2 ends of the double bond are attacked from different sides. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule. • In CH C2 2 • Stereochemistry observed resulted in an explanation involving a bromonium ion, R Br , 2 + being formed instead of a carbocation. Similarly, a chloronium ion is formed with chlorine. • Bromonium ion: A species with a divalent, positively charged bromine, R Br . 2 + • • The first bromine joins on one side to both carbons. This blocks that side from having the other bromine come in. Therefore, the second bromine has to come in from the top. The result is anti stereochemistry. 8.3 Halohydrins from Alkenes:Addition of HOX • Halohydrin: A 1,2-haloalcohol, such as that obtained on addition of HOBr to an alkene. • Electrophilic addition is the reaction of alkenes with the hypothalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols, or halohydrins. • Addition done indirectly by the reaction of an alkene with either Br or 2l in t2e presence of water. • Br 2nly reacts to form a dibromide, but if the reaction is arried out in the presence of an additional nucleophile, the intermediate bromonium ion can be intercepted by the added nucleophile and diverted to a different product. • In H 2 • The alkene reacts with Br to2form a bromonium ion. The bromonium ion is then attacked by water. Water now has a positive charged on the molecule, and another water molecule comes in to take off an extra hydrogen, making the product plus a hydronium ion. • The reaction is often carried out using NBS as a source of bromine, and water along with DMSO (dimethyl sulfoxide) as a solvent. This reaction is the same as above, just safer. • Aromatic rings are more stable and less reactive causing the double bonds to be broken to be the ones not present in a ring. 8.4 Hydration of Alkenes:Addition of H O by O2ymercuration • Water adds to alkenes to yield alcohol, a process called hydration. Takes place in water in the presence of a strong acid catalyst like H 2O .4 • Requires H S2 or4H PO H3O. 4, 2 • High temperatures also needed. • Oxymercuration: A method for double-bond hydration by reaction of an alkene with aqueous mercuric acetate followed by treatment with NaBH . 4 • • 8.5 Hydration of Alkenes:Addition of H O by2Hydroboration • Hydroboration: Addition of borane (BH ) 3r an alkylborane to an alkene. The resultant trialkylborane products can be oxidized to yield alcohols. • • Hydroboration yields the non-Markovnikov product. • Borane is an electrophile. It will bond to the electrons on the O in THF. When an alkene reacts with BH 3n THF solution, rapid addition to the double bond occurs 3 times and a trialkylborane, R3B, is formed. • • Syn stereochemistry: The opposite
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