PBr3 and SOCl2

PBr₃ and SOCl₂: Reagents For Converting Alcohols To Good Leaving Groups

• Alcohols can be converted into alkyl halides with phosphorus tribromide (PBr₃) or thionyl chloride (SOCl₂).
• The reaction with PBr₃ occurs with inversion of configuration at carbon.
• The reaction with SOCl₂ also occurs with inversion of configuration [but check with your instructor to see if they cover the S_Ni mechanism]
• Using PBr₃ and SOCl₂ is much more mild and predictable than using HBr or HCl to convert alcohols to alkyl halides since it avoids the possibility of carbocation rearrangements.

Table of Contents

1. Making Alcohols Into Good Leaving Groups, Part 3
2. Why Do We Need Yet Another Method? (Hint: Grignard Formation)
3. Phosphorus Tribromide (PBr₃) and Thionyl Chloride (SOCl₂)
4. PBr₃ For Converting Alcohols To Alkyl Bromides: The Mechanism
5. SOCl₂ For Converting Alcohols To Alkyl Chlorides: The Mechanism
6. Summary: PBr₃ and SOCl2
7. Notes
8. (Advanced) References and Further Reading

1. Making Alcohols Into Good Leaving Groups, Part Three.

[Before we get too far into this, let me say that there’s some differences as to how the mechanism of the reaction of SOCl₂ with alcohols is taught. Most schools teach inversion, but it is also (rarely) taught as retention via a different mechanism. For the whole discussion, see this article: SOCl₂ and the S_Ni mechanism

So far we’ve covered two different ways of making alcohols into good leaving groups.

– Conversion of alcohols to alkyl halides with strong acid. This works well for tertiary alcohols when nothing “bad” can happen (i.e. no side reactions). However, when certain secondary alcohols are used, rearrangements can occur.

– Conversion of alcohols into tosylates or mesylates – here, we break O-H and “cap” the oxygen with a “sulfonyl” group (“tosyl” and “mesyl” are popular choices). Very simple. No rearrangements.   This does not affect the stereochemistry.

2. So why might we need more than these two ways to make alcohols to good leaving groups? Isn’t two methods enough? 

Fair question!

We mentioned that strong acid (HCl, HBr, HI) can lead to rearrangements with certain secondary alcohols. So an alternative that doesn’t lead to rearrangements would be useful from that perspective. Secondly, strong acid is a pretty blunt instrument, like a sledgehammer. It gets the job done, but can lead to some collateral damage if you have a molecule containing functional groups with various levels of acid sensitivity (esters, alkenes, alkynes). Using a milder, more targeted reagent would help us avoid undesired side reactions in more complex situations.

A harder point to address is this: why not just, for example, always make alcohols into mesylates or tosylates if we want to make them good leaving groups? This is actually a great idea most of the time!  As for exceptions, I can think of at least one situation where when you would need to make a halide. For example, if you haven’t already, you will learn about Grignard reagents at some point. These can be made from alkyl halides but not from mesylates or tosylates, so an alternative to what we’ve already learned is good to know.

OK. Let’s dig in.

3. Phosphorus Tribromide (PBr₃) and Thionyl Chloride (SOCl₂)

The reagents we’ll talk about today are thionyl chloride (SOCl₂) and phosphorus tribromide (PBr₃). These are two representatives of a family [Note 1] of reagents that can convert alcohols to alkyl halides (Later on, when you learn about carboxylic acids, you’ll see that these can also be used to convert carboxylic acids to acyl halides).

Here’s examples of each of these reagents in action.

What do you notice?

• First of all, check out the bonds formed and bonds broken: break C-OH, form C-Br or C-Cl
• Note the change in stereochemistry. Both occur with inversion.
• Note the lack of rearrangement. Had we used HCl or HBr, it would have led to a ring expansion.

Nice and clean way to convert alcohols to alkyl halides.

4. PBr₃ For Converting Alcohols To Alkyl Halides: Mechanism

So how do they work? Let’s look at PBr₃.

This reaction proceeds in two steps that you can think of as “activation” and “substitution”. In the “activation” step,  the alcohol is converted into a good leaving group by forming a bond to P (O-P bonds are very strong) and displacing Br from P [note that this is essentially nucleophilic substitution at phosphorus].

Now that the oxygen has been “activated” (i.e. converted to a good leaving group) a substitution reaction at carbon can occur.

The bromide ion that was displaced from phosphorus attacks carbon via backside attack (S_N2), forming C-Br and breaking C-O and we are left with a new alkyl bromide (with inversion of configuration) and the Br₂P-OH leaving group.

5. SOCl₂ For Converting Alcohols To Alkyl Chlorides: Mechanism

The reaction of thionyl chloride with alcohols similarly goes through an “activation” step and a “substitution” step. In the first step, oxygen attacks sulfur, displacing chloride ion. In the second step the chloride ion attacks carbon in an SN2 reaction, leading to inversion of configuration. [Note 2]

For our purposes, the mechanism ends here, but it’s worth noting that the sulfur byproduct (HO-S(O)-Cl) can further break down to SO₂ gas and HCl through the mechanism shown [similar to the breakdown of carbonic acid to CO₂ and water]. Removal of SO₂ from the reaction vessel renders this reaction irreversible and helps drive the reaction to completion.

[I recall TA’ing a lab where a student dropped a round bottom flask with 5 mL of SOCl₂ into a rotovap bath – there was immediate bubbling and the stench of SO₂ made us have to evacuate the entire lab of about 120 people outside for fresh air. We were lucky it was a pleasant day and not in the depths of Montreal’s epic winters]

The process shown works well for primary and secondary alcohols. A process that goes through an S_N2 mechanism shouldn’t work so well for tertiary alcohols.  I find textbooks extremely vague as to how they cover the use of these reagents with tertiary alcohols, so I’m not going to go into more detail on this point. [Note 3]. Ask your instructor.

6. Summary: PBr₃ and SOCl₂

The bottom line for today is to learn about these two methods for converting alcohols into alkyl halides, and pay particular attention to their stereochemistry. Extremely testable! 

I think that’s about all we have to say about converting alcohols to good leaving groups!

There’s just one more thing here. We’ve finished covering substitution reactions of alcohols. But what about elimination reactions of alcohols? How would we go about making alkenes? (aka “dehydration”). Many of the steps will look familiar – but there will be new wrinkles too.

Next Post – Elimination Reactions Of Alcohols

Notes

Note 1. By family of reagents, I mean that there are related reagents that go through the same mechanisms , that we won’t talk about today (PCl₃, SOBr₂, PCl₅, PBr₅)

Note 2. Again, things in “real life” are a bit more complicated. You might want to double check that your instructor follows this mechanism. If not, check out this post on the SNi mechanism.

Note 3.  March mentions that SOCl₂ can be used to convert tertiary alcohols to tertiary alkyl chlorides. So “in the lab”, things are a bit more complex than alluded to here.

(Advanced) References and Further Reading

The conversion of alcohols into alkyl bromides with PBr₃ is quite general. The reaction conditions for this are varied, and all 3 bromine atoms in PBr₃ are available for reaction.

1. Convenient synthesis of labile optically active secondary alkyl bromides from chiral alcohols
   Robert O. Hutchins, Divakar. Masilamani, and Cynthia A. Maryanoff
   The Journal of Organic Chemistry 1976, 41 (6), 1071-1073
   DOI: 10.1021/jo00868a034
2. Synthesis of Optically Active Alkyl Halides
   Harry R. Hudson
   Synthesis 1969, 112-119
   DOI: 10.1055/s-1969-34195
   The main utility of PBr₃ is that it allows the conversion of chiral alcohols to bromides with retention of configuration, as the above two papers demonstrate. They also illustrate the mechanism of the reaction, going through the intermediate alkyl phosphites.
3. TETRAHYDROFURFURYL BROMIDE
   H. Smith
   Org. Synth. 1943, 23, 88
   DOI: 10.15227/orgsyn.023.0088
   This procedure from Organic Synthesis, a source of reliable and independently tested experimental organic chemistry procedures, shows how PBr₃ is compatible with ethers.