Reaction of Alcohols with Thionyl Chloride

Thionyl chloride (SOCB) is often the best reagent for converting an alcohol to an alkyl chloride. The byproducts (gaseous SO2 and HCI) leave the reaction mixture and ensure there can be no reverse reaction. 

Under the proper conditions, thionyl chloride reacts by the interesting mechanism summarized next In the first step, the nonbonding electrons of the hydroxyl oxygen atom attack the electrophilic sulfur atom of thionyl chloride. A chloride ion is expelled, and a proton is lost to give a chlorosulfite ester. In the next step, the chloro-sulfite ester ionizes (when R = 2° or 3°), and the sulfur atom quickly delivers chloride to the carbocation. When R is primary, chloride probably bonds to carbon at the same time that the C—O bond is breaking. 

This mechanism resembles the S l, except that the nucleophile is delivered to the carbocation by the leaving group, giving retention of configuration as shown in the following example. (Under different conditions, retention of configuration may not be observed.) 

1 Summary of the Best Reagents for Converting Alcohols to Alkyl Halides 1  

Give the structures of the products you would expect when each alcohol reacts with (I) HCI, ZnClj (2) HBr (3) PBrs (4) P/Ij (5) SOCIj- 

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Reaction of alcohols with thionyl chloride (Section... 

Reaction of alcohols with thionyl chloride (Section 4.13) Thionyl chloride is a synthetic reagent used to convert alcohols to alkyl chlorides. 

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). 

Another general method for converting alcohols to halides involves reactions with halides of certain nonmetallic elements. Thionyl chloride, phosphorus trichloride, and phosphorus tribromide are the most common examples of this group of reagents. These reagents are suitable for alcohols that are neither acid sensitive nor prone to structural rearrangement. The reaction of alcohols with thionyl chloride initially results in the formation of a chlorosulfite ester. There are two mechanisms by which the chlorosulfite can be converted to a chloride. In aprotic nucleophilic solvents, such as dioxane, solvent participation can lead to overall retention of configuration.7... 

Alkyl halides are almost always prepared from corresponding alcohols by the use of hydrogen halides (HX) or phosphorus halides (PX3) in ether (see Section 5.5.3). Alkyl chlorides are also obtained by the reaction of alcohols with thionyl chloride (SOCI2) in triethylamine (Et3N) or pyridine (see Section 5.5.3). 

Reactions of Alcohols with Phosphorus Halides 484 Reactions of Alcohols with Thionyl Chloride 485 11-9 Mechanism 11-3 Reaction of Alcohols with PBr3 485 11-10 Dehydration Reactions of Alcohols 487... 

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). 

ROH —> RCl. In some instances zinc chloride has been found to be more effective than pyridine, the usual catalyst (1, 1160-1161), in the reaction of alcohols with thionyl chloride. The reaction proceeds mainly with inversion of configuration. ... 

The mechanism for the reaction of phosphorus tribromide with alcohols is similar to that for the reaction of alcohols with thionyl chloride. The alcohol reacts with PBr3 giving an intermediate of the type ROPBr2 which reacts with the nucleophilic bromide ion. 

The Sni mechanism (substitution-nucleophilic-internal), as originally proposed, postulated a front-side displacement occurring via a four-center transition state. It is now clearly established that such a process has never been observed, and that all the reactions proposed to proceed by SnI mechanisms involve ion pairs. The Sni mechanism was initially suggested to explain the observed stereochemistry of chlorosulfite decomposition. Chlorosulfite esters are formed by reaction of alcohols with thionyl chloride, and are the key reactive intermediates in the conversion of alcohols to chlorides with thionyl chloride. Nucleophilic attack by the chloride of the chlorosulfite ester was considered to be concerted with cleavage of the C-O bond ... 

A few nucleophilic substitution reactions have been observed to proceed with a high degree of retention of configuration. One example is reaction of alcohols with thionyl chloride, which under some conditions gives predominantly product of retained configuration. This reaction is believed to involve formation of a chlorosulfite ester. This can then react with chloride to give inverted product. 

Mechanism of the Reaction of Alcohols with Thionyl Chloride... 

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