Electrophilic addition reaction regiospecificity

Addition of HBr to a double bond with an ether (-OR) substituent occurs regiospecifically to give a product in which the -Br and -OR are bonded to the same carbon. Draw the two possible carbocation intermediates in this electrophilic addition reaction, and explain using resonance why the observed product is formed. 

Reactions in which one product predominates are termed regioselective and those in which one is formed exclusively are regiospecific. The electrophilic addition reactions in this chapter are... 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

Electrophilic Addition.— The olefinic bond in an ergost-22-ene undergoes stereospecific and regiospecific addition reactions, controlled by its dissymmetric environment. The preferred conformation, revealed by A-ray analysis, is represented by (77). Frontal hindrance by the steroid nucleus, and the 24-methyl... 

Polyacetylene reacts with chlorine rapidly to give a white polymer that is equivalent to poly(l,2-dichloroethylene) and no stereospecificity has been reported on the chlorinated polymer. The addition of halogens is an important general reaction of carbon-carbon double bonds in a stereospecific and regiospecific sense. For instance, the electrophilic additions of chlorine to ethylene, buta-1,3-diene and hexa-l,3,5-triene have been shown to proceed by 1,2-trans (anti), 1,4-cis (syn) and 1,6-trans (anti) attack, respectively. This stereospecificity has been rationalized with a mixing rule of a-n orbital interaction. The reaction of a long conjugated polyene like polyacetylene with chlorine may produce an atactic chlorinated polyene, because random l,2n additions occur to result in a random addition product. ... 

Addition of psendohalogens, such as BrNj and INg, to unsaturated compounds has been shown to occur either by a free radical via Nj) or by an electrophilic via stepwise mechanism, depending on the reaction conditions (Hassner and Boerwinkle, 1969). Electrophilic addition of BrNg to 1-phenylpropyne is non-regiospecific (Hassner, 1969) and both products 50 and 51 are formed. The addition of INg is instead... 

In general, alkenes can undergo stereo- and regiospecific addition reactions with reagents of the type A-B, in which the A-B bond is polarized such that A acts as the electrophile A and B as the nucleophile B . Table 12-2 shows how such reagents add to 2-methylpropene. 

As conjugated systems with alternating 7T-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized 7T-bond. If the nucleophilic and electrophilic reactions are charge-controlled, reactants can attack regiospecifically. 

Despite the fact that orientation of addition across the double bond in polyfluoroolefins is usually unambiguous and in most cases regiospecific, it is not always straightforward several exceptions have been reported, mostly in the reactions involving neutral electrophiles. For instance, formation of a significant amount of regioisomer 17b was observed in reaction of CFH=CF2 with I0S02F ... 

In contrast to most reported electrophilic reactions of CF2=CFCF3, addition of CF3OCl to this olefin is not regiospecific, and it results in formation of two isomers [9], which maybe an indication that a competetive radical pathway also operates in this reaction ... 

C-NMR spectra show that the negative charge in the pentadienylmetal derivatives of groups I, II, and III resides preferentially at the terminal and central positions (21). Electrophiles can therefore attack either at the terminal or at the central position Z and E isomers can result from terminal addition. For the reactions to be useful synthetically, high regiospecificity at one or other site is desirable. 

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