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Master Organic Chemistry
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Reagent Friday: Thionyl Chloride (SOCl2)
in ALCOHOLS, ALKYL HALIDES, ORGANIC CHEMISTRY 2,ORGANIC REAGENTS
In a blatant plug for the Reagent Guide and the Reagents App for iPhone, each Friday I profile a different reagent that is commonly encountered in Org 1/ Org 2.
If there’s one thing you learn how to do well in Org 1, it’s make alcohols. Let’s count the ways:hydroboration, acid-catalyzed hydration,oxymercuration for starters, and then substitution of alkyl halides with water or HO(–). If you want to extend it even further, there’s dihydroxylation (to make diols) usingOsO4 or cold KMnO4, and even opening of epoxides under acidic or basic conditions to give alcohols.
There’s just one issue here and it comes up once you try to use alcohols in synthesis. Let’s say you want to use that alcohol in a subsequent substitution step, getting rid of the HO(–) and replacing it with something else. See any problems with that? Remember thatgood leaving groups are weak bases – and the hydroxide ion, being a strong base, tends to be a pretty bad leaving group.
So what can we do?
What you want to do is convert the alcohol into a better leaving group. One way is to convert the alcohol into a sulfonate ester – we talked about that with TsCl and MsCl. Today I’m going to talk about a second approach: converting alcohols into alkyl chlorides with thionyl chloride (SOCl2). This is a useful reaction, because the resulting alkyl halides are versatile compounds that can be converted into many compounds that are not directly accessible from the alcohol itself.