Alcohols are often converted into alkyl halides because they are poor leaving groups. Two common reagents used for this transformation are thionyl chloride (SOCl2) and phosphorus tribromide (PBr3), which convert alcohols into alkyl chlorides and alkyl bromides, respectively. This conversion typically proceeds via an SN2 mechanism, characterized by a backside attack, making it suitable only for primary and secondary alcohols. Tertiary alcohols are not compatible with this method due to steric hindrance that prevents effective backside attack.
In an SN2 reaction, the stereochemistry of the product is inverted. This means that if the alcohol has a specific configuration, the resulting alkyl halide will have the opposite configuration. For example, if the alcohol is in a wedge position, the halide will be in a dash position after the reaction.
Examining the mechanism of thionyl chloride, we start with the structure of SOCl2, which consists of a sulfur atom bonded to two chlorine atoms and one oxygen atom. The oxygen acts as a nucleophile due to its electron-rich nature. When it attacks the sulfur atom, which has a partial positive charge, a bond is formed while simultaneously breaking the double bond between sulfur and oxygen. This results in an intermediate where the oxygen carries a negative charge and is bonded to sulfur and a chlorine atom.
Next, the negatively charged oxygen will reform a double bond with sulfur, leading to the expulsion of one chlorine atom as a leaving group. This creates a highly effective leaving group due to the positive charge on the oxygen after the chlorine departs. The remaining chlorine, now a nucleophile, can then perform a backside attack on the carbon atom, resulting in the formation of the alkyl chloride and the inversion of stereochemistry.
Phosphorus tribromide (PBr3) follows a similar mechanism to that of thionyl chloride, making it a straightforward alternative for converting alcohols to alkyl bromides. Understanding these mechanisms is crucial, as they are foundational in organic chemistry and will be encountered frequently in more advanced studies.