Chlorination reaction with alkenes

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

Benzonitrile oxide (C in Figure 15.44) is an isolable 1,3-dipole. It can be generated from benzaldoxime and anNaOH/Cl2 solution. Under these reaction conditions the oxime/nitroso anion (A B) is initially formed and chlorine disproportionates into Cl—O and chloride. An SN reaction of the negatively charged C atom of the anion A B at the Cl atom of Cl— O or of Cl—O—H affords the oc-chlorinated nitroso compound E, which tautomerizes to the hydroxamic acid chloride D. From that species, the nitrile oxide C is generated via a base mediated 1,3-elimination. Isoxazoles are formed in the reactions of C with alkynes (Figure 15.44), while isoxazolines would be formed in its reactions with alkenes. 

The reactions of the non-5-chlorinated 1,3,2,4,6-dithiatriazines are, in fact, reactions of the dimers. These undergo addition reactions with alkenes to give adducts (85CC929,85MCLC447,861C47, 86IC2119,91CB39>. 

The cycloaddition of an arylnitrile oxide to methyl ethynylmethylphosphinate in an inert solvent at 5 °C produces a (5) -phosphinoylated-isoxazole 513 in high yield " and phosphorylated 1,2,4-oxadiazoles 514 are obtainable in a similar addition to (R0)2P(Z)CN (Z = O or Phosphorylated nitrile oxides, e.g. 515, are generally prepared by the Et3N-dehydrobromination of the product from the bromination of the oxime of a j5-(dialkoxyphosphinoyl)acetaldehyde, and then used in situ in reactions with alkenes to give isoxazolines and with alkynes to give isoxazoles . The chlorination and subsequent dehydrochlorination of diethyl (nitromethyl)phosphonate yield the nitrile oxides 516 (R = EtO Pr O or morpholinyf and these, with unsaturated centres, yield isomerically phosphorylated isoxazolines and isoxazoles. 

The addition of halogens is also used to quantitatively determine the degree of unsaturation in vegetable oils, margarines, and shortenings (Section 8.4). Chlorine reacts with alkenes to give dichloro products in an addition reaction similar to that of bromine. However, it is not used as a test for unsaturation because it is difficult to see the pale green color of chlorine in solution. 

What about diatomic fluorine, F-F For many years, elemental fluorine was thought to be too reactive and too dangerous for reaction with alkenes. To void such problems, fluorine is typically mixed with an inert gas such as nitrogen or argon. Diluted in this manner, fluorine does react with alkenes, but the yields are often poor and, in some cases, solvents for the alkene, such as methanol, participate in the reaction. 1-Phenylpropene (PhCH=CH2), for example, reacted with fluorine in methanol to give 51% of the corresponding difluoride, along with 49% of 2-fluoro-l-methoxy-l-phenylpropane. The problems associated with fluorine lead to a simpliflcation. In this chapter, alkene reactions are reported only for chlorine, bromine, or iodine but not fluorine. 

An aqueous solution of bromine or chlorine reacts with alkenes to form addition products called halohydrins. These compounds have a halogen and a hydroxyl group on adjacent carbon atoms. The reaction of aqueous chlorine with propene is shown below. 

Reaction of HOCl, formed from calcium hypochlorite and CO2, with highly substituted alkenes in CH2CI2 is a convenient route to aHyUc chlorides (111). Ketones are chlorinated to a-chloroketones by reaction with HOCl Acetone initially gives CH2COCH2CI (112). Methyl ethyl ketone gives 78% CH3CHCICOCH3, 15% CH3CH2COCH2CI, and 7% dichlorides (113). 

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