Orientation electrophilic addition

Markovnikov s rule (Section 6.8) A guide for determining the regiochemistry (orientation) of electrophilic addition reactions. In the addition of HX to an alkene, the hydrogen atom bonds to the alkene carbon thal has fewer alkyl substituents. 

A. Synthetic Methods.—Electrophilic addition of P compounds to olefinic compounds is a well-established route to phosphonic acids, although yields are often disappointing. With phosphorus pentachloride it has been found that yields are greatly improved when phosphorus trichloride is added to the reaction mixture. Since the orientation of the addition implies that electrophilic addition to phosphorus rather than chlorine is the initial step, it seems likely that the trihalide participates by decreasing the free concentration of chlorine rather than by a more active role. This... 

A very similar situation is encountered in the electrophilic addition of unsymmetrical adducts (e.g. HBr) to vinyl halides (e.g. CH2=CHBr), where the inductive effect of halogen controls the rate, but relative mesomeric stabilisation of the carbocationic intermediate controls the orientation, of addition (p. 185). 

Electrophilic addition to 1-haloalkenes (e.g. 27), presents a number of parallels to the electrophilic substitution of halobenzenes (p. 155). Thus it is the involvement of the electron pairs on Br that controls the orientation of addition (cf. o-/p-direction in C6H5Br) ... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

Fields et alf and Schmidt made closely parallel observations concerning polar cycloaddition of ethylenes substituted at the a-position by an electron-withdrawing group and having no substituent at the jS-position. In both cases the product observed was that to be expected if the electrophile had added to the j3-carbon atom. Since it is clear that the normal ground-state polarization of acrylonitrile (127) and methyl methacrylate (128) should tend to destabilize the cation produced by j8-addition, it was concluded that the orientation of polar cycloadditions could not be predicted by the rules of electrophilic addition and that this apparent anomaly pointed toward a more concerted type of cycloaddition reaction. 

Electrophilic addition of HBr to propene gives predominantly the so-called Markovnikov orientation Markovnikov s rule states that addition of HX across a carbon-carbon multiple bond proceeds in such a way that the proton adds to the less-substituted carbon atom, i.e. that already bearing the greater number of hydrogen atoms (see Section 8.1.1). We rationalized this in terms of formation of the more favourable carbocation, which in the case of propene is the secondary carbocation rather than the alternative primary carbocation. 

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

Exercise 10-13 Explain how Markownikoff s rule for orientation in electrophilic additions can be accounted for in terms of the modern view of how these reactions occur, using the reaction of HCI with 1-methyicyclohexene as an example. 

Electrophilic addition of hydrogen halides to a,/f-unsaturated aldehydes and ketones places the halogen on the /3 carbon. This orientation is opposite to that observed for related additions to conjugated dienes ... 

Despite the fact that this reaction proceeds at much higher temperature and results in the formation of a mixture of products, all of them are derived from the attack of sulfur intermediately and exclusively on the terminal carbon of the olefin, which is not consistent with the orientation of electrophilic addition to polyfluropropenes where an electrophile attacks the central carbon of the double bond exclusively (see Eqs. 29,37,38). 

Thermal addition of SXC12 to fluoroolefins was often referred to as electrophilic addition [136, 137], although observed orientation of addition actually resembles that in radical reactions [139], Regardless of the mechanism, this reaction is highly regioselective and undoubtedly has synthetic value as a high yield route to polyfluoroalkyldisulfides. 

Strategy All of these reactions are electrophilic additions of HX to an alkene. Use Markovnikov s rule to predict orientation. 

Regio- and Stereoselectivity of the Addition Reactions Like proton-induced HAT additions [66-68], additions of carbocations to alkenes proceed with strict regioselectivity, the orientation being determined by the stabilities of the intermediate carbocations (Markovnikov rule). In this respect, carbocation additions differ from other electrophilic additions, as sulfenylations or selenylations, where the orientation is controlled by the nucleophilic attack at the bridged cationic intermediate (Scheme 13) [67, p. 860]. 

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