Abstract:
In research into the structure of complex molecules of animal or vegetable origin, such as hormones, steroids and vitamins, it has been noticed that very often from the carbon atom next to a point of unsaturation in the molecule, there extends a side chain. The chemist interested in the laboratory synthesis of such compounds quite frequently has been able to build up the greater part of the molecule and is then faced with the problem of how to build on such a side chain. Sometimes this can be effected by such reactions as Grignard and Reformatsky if on the carbon atom in question there is a halogen atom; so that the real problem is how to replace by halogen a hydrogen atom attached to the carbon vicinal to the double bond.
With regard to the chemical reactivity of halogenated derivatives of unsaturated compounds, the position of the halogen with respect to the double bond is critical. For example, in the series CH2 = CH (CH2)X C1 there is the very reactive vinyl chloride (x=0), the extremely labile allyl compound (X=1) and then the higher members (X ≥ 2) which has reactivity comparable with that of the halo paraffins. Compounds with "allyl" halogen, therefore, provide the chemist with excellent starting material for synthetic work. It is fortunate that, in general, atoms attached to the carbon in the "allyl" position have a reactivity which is quite pronounced e.g. in the oxidation of pinene A. St Pfau and P. Plattner Halv. Chem Acta 19, 858 (1936) the allyl hydrogen is oxidised preferentially. Hydrogen atoms in such positions should be capable of replacement by halogen with relative ease. But when the direct halogenation of a simple unsaturated compound is attempted, it is found that the additive power of the double bond so greatly exceeds the replacing ability of the halogen that only the addition product obtains. However, under exceptional conditions it is possible to effect substitution in preference to addition as exemplified in the side-arm halogenation of toluene where the "double bond" is contained in the ring as an aromatic link.
