Reaction with Halogen

FIGURE 17.15 Mechanism of bromination of acetaldehyde under acidic or basic conditions. 

If the carbonyl group in the final product is written in the charge-separated form, then the ring is a two-electron, Hiickel-compliant, aromatic ring (see Section 3.2.1). 

Explain why bromination and chlorination of acetophenone, PhC(=0)CH3, occur at the same rate. Solution 

In general, we would not expect bromine and chlorine to react at the same rate with any substrate. This tells us that the RDS in the process is not the reaction with the halogen, but the formation of the enol, in which the halogen is not involved. Note that we do not now show the first slow step as an equilibrium the forward reaction with the halogen is faster than the reversal to the keto form, so no true equilibrium can be established  

What do these observations tell us about the stability of enols and enolates The fact that the iodoform reaction works and the methyl group is specifically and persistently iodinated tells us that the terminal enolate, 17.23, is more stable than the internal one, 17.24. This impfies that there is significant charge density at carbon in an enolate anion—the stability is following that we would expect for a carbanion. If we consider the enolate as a carbanion, then this also explains why successive iodinations occur at the same site—it is getting successively easier to make the enolate anion, because of the electron-withdrawing effect of the iodine. 

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Reaction with Halogen Nucleophiles. Hydrochloric acid [7647-01-0] hydrobromic acid [10035-10-6] and hydroiodic acid [10034-85-2] react readily with ethyleneimine (3) to give the corresponding P-halogenoethylamines (20,21). 

Reaction with Halogen Electrophiles. The synthesis of l-haloa2iridines, which are prone to explosion, has been carried out using hypohahtes (290,291). l-Chloroa2iridine [10165-13-6] produced in this way reacts with l-Hthiated ethyleneimine to give l,l -dia2iridine [4388-03-8]. Perchloryla2iridine [112405-46-6] has been prepared by reaction of ethyleneimine with dichlorine heptoxide at —20° C (292). 

Alkynes undergo addition reactions with halogens. The reaction has been thoroughly examined from a mechanistic point of view. In the presence of excess halogen, tetrahaloalkanes are formed, but mechanistic studies can be carried out with a limited... 

Scheme 2.39 Ring-opening reactions with halogen nucleophiles.

Peroxides oxidize N,N-DPDD to Wurster s red, a semiquinone diimine derivative [4]. Similarly Wurster s red is also produced from N,N-DPDD by reaction with halogen-containing substances in the presence of sodium ethylate and UV light and by reaction with the chlorinated triazines produced by reaction with chlorine [7]. 

Peroxides oxidize TPDD to Wurster s blue, a product with a semiquinone diimine structure [1]. Similarly Wurster s blue is also produced from TPDD by reaction with halogen-containing substances produced by the reaction of aromatic amines and triazines with chlorine gas. 

Organocobalt(III) complexes are stable towards oxygen unless they undergo spontaneous fission of the Co—C bond to give Co(II), Co(I), or CoH complexes (see Section VI,B,l,a and D). Reactions with halogens are dealt with in Section VI,C,l,c. 

Shokol, V.A. and Kozhushko, B.N., Phosphorohalidites and phospho-ropseudohalidites in the Arbuzov reaction with halogen-containing compounds, Russ. Chem. Rev., 54, 98, 1983. 

The monosubstituted adduct offers the ready synthesis of a whole range of monosubstituted adducts (see Scheme 6) it is often possible to isolate in these reactions intermediates that are not readily obtained by alternative methods. Thus, in the reaction with halogen acids to yield the bridged hydrido complexes HOs3(CO)10X, it is possible to identify the intermediate HOs3(CO)uX complex in which the halogen functions as a one-electron donor bonding to only one metal center (158). 

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