Halide, double 151 halogen

A. Werner, in his Neuere Anschauungen auf dem Gebiete der anorganischen Chemie (Braunschweig, 68, 1905),12 assumed that in the double halides, the halogen atoms are united to the central atom so that the group acts as an acid anhydride. The compounds above symbolized then become ... 

Allylic halides and benzylic halides have halogen atoms bonded to sp hybridized carbon atoms and do undergo the reactions described in Chapter 7. Allylic halides have X bonded to the carbon atom adjacent to a carbon-carbon double bond, and benzylic halides have X bonded to the carbon atom adjacent to a benzene ring. The synthesis of allylic and benzylic halides is discussed in Sections 15.10 and 18.13, respectively. 

Simple sulfonyl carbanions which do not contain additional carbanion-stabilising groups, e.g. carbonyl groups or heteroatoms, can be readily alkylated in high yield by modern techniques with the use of alkyllithiums and lithium amide bases. A number of allylic halides have been successfully used. In allylic halides, the halogen directly attached to the double-bonded carbon is relatively inert towards nucleophilic attack (Scheme 41). In this way, sulfones (96) can be transformed via desulfonation into vinyl halides (97) or into ketones (98) by hydrolysis (Scheme 41). In contrast to ordinary alkyl sulfones, triflones (99) can be alkylated under mildly basic conditions (potassium carbonate in boiling acetonitrile) (Scheme 42). The ease of carbanion formation from triflones (99) arises from the additional electron-withdrawing (-1) effect of the trifluoromethyl moiety. 

Addition reactions involve unsaturated compounds containing double or triple bonds, particularly C=C and C=C. Hydrogenation and reactions of hydrogen halides and halogens with alkenes and alkynes are examples of addition reactions. 

Halides are classified as methyl, primary, secondary, tertiary, and vinyl (Fig. 6.2). Chapter 2 (p. 76) introduced this nomenclature. Remember. A primary carbon is attached to only one other carbon, a secondary carbon to two others, and a tertiary carbon to three others. In a primary halide, the halogen atom is attached to a primary carbon, and so on. In vinyl halides, there is always a carbon-carbon double bond to which the halide is directly attached. 

In adding hydrogen halides and halogens to the >C=C< double bond of 1,2-PB, the functionalization degree of the polymer is mostly determined by the reactivity of the electrophilic agent. Relatively low degree of polydiene hydrochlorination (10-15%) at interaction of HCl and syndiotactic 1,2-PB [16, 39, 40] is caused by insufficient reactivity of hydrogen chloride in the electrophilic addition reaction by the double bond (Table 3.2). Due to this, more electron-saturated >C=C< bonds in 1,4 units of butadiene polymerization are subjected to modification. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

A proton and a halogen add to the double bond of an alkene to yield an alkyl halide Addition proceeds in ac cordance with Markovnikov s rule hy drogen adds to the carbon that has the greater number of hydrogens halide to the carbon that has the fewer hydro gens... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Their physieal properties are essentially those of the alkanes. It is the unsaturated linkages that dominate the ehemistry and the main reaetion is one of addition (e.g. hydrogen, halogen, and hydrogen halides) aeross the double bond to produee saturated eompounds. This reaetivity is utilized in the manufaeture of long-ehain polymers, e.g. polyethylene and polypropylene. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

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. 

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