Conversions of Alcohols into Alkyl Halides SN1 and SN2 Style

[@VictortheOrganicChemistryTutor]

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[@MasterD1213]

For the bonus question: can we use 1. PBr3/py then 2. HBr?

[@VictortheOrganicChemistryTutor]

Nope, that won't work.

[@memphisvelasco1258]

for the bonus questions is it 1. TsCl and 2. Br ?

[@VictortheOrganicChemistryTutor]

You're on the right track thinking that you need a few steps there but think carefully about the stereochemistry and how many times you'd need to invert it to have the same orientation of the groups in the final product.

[@BeatrizValdevieso]

Is the answer, Sn2 mechanism with PBr3 and Pyridine as the solvent?

[@VictortheOrganicChemistryTutor]

If you do it as a simple SN2, you'll have an inversion of stereochemistry. Also, what would be a purpose for pyridine?

[@marounxd3871]

with hydrogen halides will the alcohol mainly do SN1 or SN2 ?

[@VictortheOrganicChemistryTutor]

Eh… those are literally the only two options for a simple substitution reaction. What else can they do?

[@marounxd3871]

I meant to ask about how could I tell if HBr for example will undergo SN1 with an alcohol instead of SN2@@VictortheOrganicChemistryTutor

[@VictortheOrganicChemistryTutor]

@@marounxd3871 Oh, I see. That depends on the alcohol. 1° alcohols can't make carbocations unless they are stabilized by resonance (like, for instance, benzylic or allylic alcohols), so they will do SN2 reaction. 2° and 3° alcohols can form carbocations (especially easily in the case of the 3° ones), so they will typically do SN1. 2° alcohols are the tricky ones as they can do either but they still tend to do SN1 in these conditions.

[@marounxd3871]

and if the 1° alcohol was next to a secondary carbon as in cyclopentanemethanol, is the H able to kick out the alcohol and do a carbocation rearragement ?@@VictortheOrganicChemistryTutor

[@efekeskin655]

1. PCl5 and 2. PBr3 ?

[@VictortheOrganicChemistryTutor]

Assuming you're talking about the last q, no, it won't work but you have the right idea 😉 Once you've replaced your -OH with Cl, how would you replace it with Br now? PBr3 is a reagent for alcohols, not alkyl halides.

[@efekeskin655]

@@VictortheOrganicChemistryTutor ahahaha i didnt know that it was for alcohols, i would use PBr5 then, since Br is a better nucleophile than Cl it can replace it.

[@VictortheOrganicChemistryTutor]

@@efekeskin655 Yep, that would be a good way to approach it. Also, when it comes to reactions, keep in mind that reagents and starting materials do matter. Just because something like PBr3 replaces -OH in an alcohol, doesn't mean that it would do the same thing for other compounds. So, always pay attention to the mechanisms and what those reagents actually do to avoid mistakes like this in the future.

[@steveng724]

Step 1: Use PBr3 Step 2: NaBr

[@VictortheOrganicChemistryTutor]

Yeah, you could do something like that. But not exactly that. Edit: more details in the comment below.

[@coltrane1331]

​@@VictortheOrganicChemistryTutor how would you avoid reverse reactions? In Step 2 ... attacking the now S configuration chiral center with Bromine attached to force the Bromine to leave and become a Bromide ion... with a Bromide ion from a dissolved salt like NaBr. Wouldn't this be reversible and continue to swap configuration?

[@VictortheOrganicChemistryTutor]

@@coltrane1331 True, if you're not careful with your experimental conditions, you'll end up flip-flopping the stereo there and make a racemic mix instead. The trick is in the experimental procedure and how you do it. Notice that I said you could do "something like that" but not "that's right" 😉 A better way here would be to use PI3 or a sequence of 1. TsCl, 2. KCl and then substitute it with Br using KBr. Also, I need to stop replying from my phone, so I can make more meaningful comments 🤣

[@adityabasu2001]

One doubt: which condition makes Br- substitute I-? Afaik finkelstein protocol does the reverse

[@adityabasu2001]

I think a plausible approach is via the Mitsunobu: 1) PPh3, DEAD; 2) Aq. NaOH; 3) PBr3, pyridine

[@Qjel-o7q]

Hbr only