Carbon-heteroatom bonds brominations

Only the C-Br bond is polarised to any significant extent, and it is polarised towards the bromine atom. Hence, it is likely that this is the bond that will be broken most readily. Write down the first step of the SN1 reaction in which the carbon/heteroatom bond is broken, using curly arrows to indicate the movement of electrons. Identify the products. 

Reactions involving the formation of carbon heteroatom bonds include the industrially best known photochemical reactions. In fact, chlorination, bromination and sulfochlorination are major processes in industrial chemistry, and oxygenation has likewise an important role. Due to the focus on fine chemistry of this chapter, the discussion below is limited to laboratory-scale preparations and in particular to some bromination and oxygenation reactions illustrating the advantage of the photochemical approach, as well as to some alkoxylation, hydroxylation and amination reactions. 

Due the nature of the substituents, all the stable singlet carbenes exihibit some carbon-heteroatom multiple-bond character and for some time their carbene nature has been a subject of controversy. One has to keep in mind that apart from dialkyl-carbenes, all the transient singlet carbenes present similar electronic interactions. As early as 1956, Skell and Garner drew the transient dibromocarbene in its ylide form based on the overlap of the vacant p-orbital of carbon with the filled p orbitals of the bromine atoms (Scheme 8.31). 

Carbon compounds may also incorporate atoms of other elements, called heteroatoms, such as nitrogen, sulfur, chlorine, bromine, or others. But whatever the identity of the bonded species, carbons will always sport four bonds, be they single, double, or triple bonds. When a carbon has four single bonds, an interesting property can be created handedness. 

With regard to optical absorptions, influence of substituents and molecular structure all observed trends follow the same rules as outlined for the (S S)+ radical cations. The most stable (S . X)+ radical cations are those intramolecular species which attain five-membered ring structures, i.e., those with, for example, three linking carbons between the two heteroatoms. Particularly interesting are the S Br bonded systems since they represent a rare example of bromine-based cationic species in organic chemistry. 

It is known that 4 reacts faster than 2 to form dibromoacetone, and this is usually associated with a higher percentage of enol for 4 when compared to 2. In principle, the 0-H bond of the enol is polarized such that H is and will be attracted to the 6- bromine atom by intramolecular hydrogen bonding. The net result is stabilization of the enol form. The presence of heteroatoms at the a-carbon should stabilize the enol form, and this is generally correct. Titration of 2,4-pentanedione (7 known as acetylacetone) with bromine, as with the reaction of 2, gives 3-bromo-2,4-pentanedione. This experiment shows that there is 79.7% of the enol, 8, which is consistent with the concept of enol stabilization by an adjacent heteroatom substituent. 

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