Reaction with alkyl chlorosulfites

The reactions of thionyl chloride with organic compounds having hydroxyl groups are important. Alkyl chlorides, alkyl sulfites, or alkyl chlorosulfites form from its reaction with aUphatic alcohols, depending on reaction conditions, stoichiometry, and the alcohol stmcture ... 

The example shown is the most important case of this mechanism yet discovered, since the reaction of alcohols with thionyl chloride to give alkyl halides usually proceeds in this way, with the first step in this case being ROH -I- SOCI2 ROSOCl (these alkyl chlorosulfites can be isolated). 

The thionyl chloride reaction apparently can proceed from the alkyl chlorosulfite stage by more than one mechanism an ionic SN2 chain reaction with chloride ion,... 

The reaction with thionyl chloride affords a chlorosulfite, the decomposition of which may generate an alkyl chloride by the S i (substitution, nucleophilic, internal) mechanism (Scheme 2.16). This reaction, w hich may proceed by an ion pair, can lead to the retention of configuration of an asymmetric secondary alcohol in the conversion to the alkyl chloride. This is in contrast to the inversion of configuration found with the reaction with phosphorus pentachloride and with the nucleophilic displacement of a leaving group. 

As models for the investigation of the S i and S i mechanisms the thermal decompositions of alkyl chlorosulfites and allyl chlorosulfites have received considerably more attention than that of other desulfonylation reactions. The usual method of converting alcohols into alkyl chlorides is by treatment with thionyl chloride. This reaction has been shown to proceed through an alkyl chlorosulfite which can often be isolated and shown to decompose by a homogeneous first-order process yielding alkyl chloride and 802 " , although frequently substantial amounts of alkene are formed in the reaction , viz. 

Alkyl chlorides can be prepared by the reaction of alcohols with thionyl chloride (SOCl2) in the presence of a nitrogen base (e.g. triethylamine or pyridine). An intermediate alkyl chlorosulfite (ROSOC1) is formed by nucleophilic attack of ROH on the sulfur atom of thionyl chloride. The OH group is converted to an OSOCl-leaving group, which is displaced on reaction with the chloride anion (e.g. in an SN2 mechanism when R is a primary alkyl group). 

Reaction with SOCij and SOBtj (Section 10.5C) Alcohols react with SOClj to initially give an alkyl chlorosulfite intermediate, which is displaced from the backside by Cl to give the chloroalkane. This is often the method of choice for converting a primary or secondary alcohol to an alkyl chloride or alkyl bromide. 

It is tempting to employ the same steps for this problem as we used in the conversion of an alcohol into the corresponding alkyl chlorosulfite, ROSOCl, upon reaction with SOCI2 (Section 9-4), namely, displacement of chloride from sulfur by the hydroxy group of the alcohol ... 

If a base such as pyridine is present to neutralize the HCl generated in the steps leading to the alkyl chlorosulfite, a substitution reaction occurs with inversion of configuration. 

In dioxane as solvent, the substitution reaction occurs with retention of configuration. The alkyl chlorosulfite undergoes heterolytic cleavage of its C—O bond to give a pair of oppositely charged ions. These ions, a carbocation and chlorosulfite, remain together as an ion pair. Dioxane solvates the carbocation on the side opposite the chlorosulfite. 

When thionyl chloride is in excess, or when alkyl chlorosulfites are treated with thionyl chloride, alkyl chlorides are produced and the byproducts are HCl and SOi- The mechanism of this reaction may be Sn 1, Sn2, SNi, or Sn2 (with allylic or propargylic alcohols). In the absence of base the SnI mechanism operates retention of configuration is observed in the alkyl chloride (eq 3). Considerable ionic character may be involved in this process and small amounts of elimination or rearrangement are common as side reactions. ... 

The reaction occurs by a nucleophilic acyl substitution pathway in which the carboxylic acid is first converted into a chlorosulfite intermediate, thereby replacing the -OH of the acid with a much better leaving group. The chloro-sulfitc then reacts with a nucleophilic chloride ion. You might recall from Section 17.6 Hint an analogous chlorosulfite is involved in reaction of an alcohol with SOCb to yield an alkyl chloride. 

Because tertiary alcohols are so readily converted to chlorides with hydrogen chloride, thionyl chloride is used mainly to prepare primary and secondary alkyl chlorides. An early step in the mechanism of this reaction is the conversion of the alcohol to a chlorosulfite, which then reacts with chloride ion to yield the alkyl chloride. 

We ve already seen one general reaction of alcohols—their conversion to alkyl halides (Section 12.3). Tertiary alcohols react with HGl and HBr by an SnI mechanism through a carhocation intermediate. Primary and secondary alcohols react with SOCI2 and PBra by an Sn2 mechanism through backside attack on a chlorosulfite or dibromophosphite intermediate. 

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