![]() This fact is demonstrated by the following equations. In this case, two inversions lead to retention.Īnother characteristic of thionyl chloride reactions is their tendency to yield allylic rearrangement products with allylic alcohols. An alternative explanation for the retention of configuration, involving an initial solvent molecule displacement of the chlorosulfite group (as SO 2 and chloride anion), followed by chloride ion displacement of the solvent moiety, has been suggested. These reactions are illustrated by the following equations. The carbocation partner in the ion pair may also rearrange. This is classified as an S Ni reaction (nucleophilic substitution internal). In ether and similar solvents, the chlorosulfite reacts with retention of configuration, presumably by way of a tight or intimate ion pair. In the presence of a base such as pyridine, the intermediate chlorosulfite ester reacts to form a "pyridinium" salt, which undergoes a relatively clean S N2 reaction to produce the inverted chloride. The reaction of thionyl chloride with chiral 2º-alcohols has been observed to proceed with either inversion or retention. Rearrangement (pink arrows) of the carbocations leads to isomeric products. ![]() In the presence of HBr, a series of acid-base and S N2 reactions occur, along with the transient formation of carbocation intermediates. The initially formed trialkylphosphite ester may be isolated if the HBr byproduct is scavenged by a base. Hindered 1º- and 2º-alcohols react sluggishly with the former and may form rearrangement products, as noted in the following equation.īelow, an abbreviated mechanism for the reaction is displayed. It should be noted that 3º-alcohols are not good substrates for the new procedures.ĭisadvantages to using \(PBr_3\) and \(SOCl_2\)ĭespite their general usefulness, phosphorous tribromide and thionyl chloride have shortcomings. For these and other reasons, alternative mild and selective methods for transforming such alcohols by nucleophilic substitution of the hydroxyl group have been devised. ![]() In this two-step approach, a clean configurational inversion occurs in the first S N2 reaction however, the resulting alkyl halide may then undergo repeated S N2 halogen exchange reactions, thus destroying any stereoisomeric identity held by the initial carbinol carbon. Of course, it is possible to avoid such problems by first preparing a mesylate or tosylate derivative, followed by nucleophilic substitution of the sulfonate ester by the appropriate halide anion. These reagents are generally preferred over the use of concentrated HX due to the harsh acidity of these hydrohalic acids and the carbocation rearrange ments associated with their use. replacement of the hydroxyl group) are treatments with thionyl chloride and phosphorus tribromide, respectively. The most common methods for converting 1º- and 2º-alcohols to the corresponding chloro and bromo alkanes ( i.e. ![]()
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