Halogenation, of double bonds

Reactions in monolayers (38b) also provide direct chemical evidence for the dependence of molecular orientation on the surface area. The rates of oxidation and halogenation of double bonds midway along the carbon chain of oleic acid, the rate of lactoniza-... 

Halogenation of double bonds is stereospecific. A reaction is stereospecific when a particular stereoisomeric form of the starting material gives a specific stereoisomeric form of the product. For example, the halogenation of cis- and tra 5-2-butene produces a racemic mixture of 2,3-dibromo-butane and mc5o-2,3-dibromobutane, respectively. 

The introduction of halogen into organic molecules can be carried out by a variety of addition or substitution reactions. The classical methods for the addition of halogen to double bonds or the substitution of halogen for hydroxyl by hydrohalic acids are too well known to bear repetition here. Discussed below, then, are methods that are of interest because of their stereospecific outcome or because they may be used on sensitive substrates. 

One of the most direct routes to vinylsilanes uses vinyl halides as starting materials. Metal-halogen exchange, followed by electrophilic attack by TMSC1, can often provide the vinylsilane quickly and in good yield. As an added bonus, vinyl bromides have been shown (10, II) to proceed through this sequence with retention of double-bond stereochemistry. 

Halogenation of Double and Triple Bonds (Addition of Halogen, Halogen)... 

Retention of the cis configuration at the halogen-bearing double bond confirmed the lack of reactivity at this position. 

In some cases, the functionality may arise due to the presence of double or triple bonds in them. The presence of double and triple bonds in a compound may make it polyfunctional, e.g. the functionality of two in case of ethylene is due to the presence of double bond, ethylene can take up two hydrogen atoms or two halogen atoms as under ... 

Unlike the addition of other halogens to double bonds, where trans addition occurs, Rozen suggested that the syn addition of fluorine procedes by way of a tight ion pair which collapses before any rotation about the C-C bond takes place (Fig. 73) [175]. 

Addition of Fluorine and Halogens to Double Bonds with Potassium... 

For a neutral species, we can calculate the number of double bond equivalents, DBE (Q), by comparing the molecular formula with that of the fully saturated, acyclic parent molecule with the same number of carbons and heteroaloms, C H 2 +2+yN. O ., where H is the total number of hydrogens and halogens. 

The number of double bond equivalents corresponds to the difference between the molecular formula and that for the saturated acyclic parent compound. Each DBE (double bond or ring) results in the subtraction of 2 hydrogens or halogens from the molecular formula of this parent structure. 

A discussion of nuclear electric quadrupole coupling in the vinyl halides has led to the estimate of about 6 percent double-bond character for the C—Cl bond in vinyl chioride and 3 percent for the C—I bond in vinyl iodide.87 Values of electric dipole moments of mono-halogenated benzenes have been interpreted as corresponding to 4 percent of double-bond character for the C—X bonds.68... 

Unlike the addition of halogens across double bonds, addition of acids results in formation of asymmetrical products. Specifically, a different group is added to each side of the double bond. Thus, if this reaction is applied to asymmetrical olefins such as propene, multiple products might be expected to form as illustrated in Scheme 7.8. In fact, while a mixture of products is formed, there is an overwhelming presence of the secondary substituted product compared to that with substitution at the primary position. This preference of reaction products resulting from addition of protic acids across double bonds is governed by Markovnikov s rule. 

---