Converting Alcohols to Alkyl Halides

[90210]

I'll explain what I understand first. If anyone could provide some input or correct me, that would great! 🙂

The -OH of an Alcohol is a very poor leaving group (very unstable). Therefore, it must be made into a good leaving group. This could be done in a variety of ways:

For a tertiary or secondary alcohol: Reacting the alcohol with a Hydrogen Halide (ie. HBr, HCl, or HI) first protonates the Alcohol into a (H2O+) group. The H2O+ falls off resulting in a tertiary or secondary carbocation. Afterwards, the conjugate base (good nucleophile) attacks the carbocation forming an Alkyl Halide -- an SN1 mechanism.

I'm a little unsure how this works for Primary Alcohols though. For a primary alcohol, there wouldn't be a carbocation intermediate (ie. SN1 wouldn't happen). Instead the reaction would need to proceed through SN2. The problem with this though is that if you used HBr for instance, after the -OH group gets protonated -- the nucleophile (Br-) in this case isn't strong enough to displace it. It's a good nucleophile but not a great one. To make matters worse, the solution is protic making the reaction even more unfavorable.

Therefore, is it true to say the way to convert a primary alcohol into an alkyl halide is via one of these two methods:

- PBr3, PCl3, etc. (with pyrimidine solvent)
- SOCl2 (with pyrimidine solvent)

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[90210]

Bumping this thread 😀

 

[movax]

Bumping this thread 😀

It's only been like...4 hours.

Anyways, yes you are correct. You can generate an alkyl halide via phosphorous halids (PBr3, PCl3, etc) and SOCL2, starting with a primary or secondary alcohol (via SN2). Tertiary of course will only undergo SN1.

(looked it up in my EK Orgo Manual)

 

[90210]

It's only been like...4 hours.

Anyways, yes you are correct. You can generate an alkyl halide via phosphorous halids (PBr3, PCl3, etc) and SOCL2, starting with a primary or secondary alcohol (via SN2). Tertiary of course will only undergo SN1.

(looked it up in my EK Orgo Manual)

So even if the primary alcohol was placed in an acidic solution (ie. HBr), SN2 wouldn't happen? That was mainly my question. I was trying to figure out if the nucleophile (Br-) was strong enough to displace the H2O+ under protic conditions.

 

[bharat008]

So even if the primary alcohol was placed in an acidic solution (ie. HBr), SN2 wouldn't happen? That was mainly my question. I was trying to figure out if the nucleophile (Br-) was strong enough to displace the H2O+ under protic conditions.

If you have protic solvent then small nucleophiles such as C, N, O would be unable to react. But larger nucleophiles could be solvated by the protic solvent but not as much as the smaller nucleophiles. So the reaction could occur.

In Aprotic Solvent, nucleophilicity increases as you go bottom to top in periodic table.
In protic solvent, nucleophilicity increases as you go from top to bottom. Reason being, F- is a smaller nucleophile compared to I-; so F- would be solvated by the protic solvent and become really stable; decreasing its reactivity. While I- is a large nucleophile so protic solvent won't be able to solvate it as much as the smaller nucleophile..

In conclusion, Br- is a large and strong nucleophile so the protic solvent won't affect it as much; ;; REACTION OCCURS.......

HOpe this clears up the concept for you...

 

[orgohacks]

So even if the primary alcohol was placed in an acidic solution (ie. HBr), SN2 wouldn't happen? That was mainly my question. I was trying to figure out if the nucleophile (Br-) was strong enough to displace the H2O+ under protic conditions.

It goes through SN2. Under protic conditions, the OH2 is a great leaving group and Br- is a competent nucleophile.

SN1 would not go here. You definitely don't want a primary carbocation.

Hope this helps - James

 

[plzNOCarribbean]

So even if the primary alcohol was placed in an acidic solution (ie. HBr), SN2 wouldn't happen? That was mainly my question. I was trying to figure out if the nucleophile (Br-) was strong enough to displace the H2O+ under protic conditions.

No, it definitely can go SN2. Br is not hindered, and it is charged, and although it is a weak base, it can result in SN2 occuring here. I tutored Ochem for a year and my professor always said that with alcohols reacting with halogen acids, the reaction always can occur, but the Reaction type depends on whether your dealing with methly, primary, secondary, or tertiary halides.

For secondary and tertiary halides, assume that reaction with HBr or HCl occurs via an Sn1 scheme and for methyl and primary, and SN2 scheme. However, for synthetic purposes:

it would be better to use PBr3 to convert methyl, 1, and 2 alcohols in to alkyl bromides
and SOCl2 to convert methyl, 1, and 2 alcohols in alkyl chlorides

^these reagents are good because no re-arrangements occur when dealing with 2 alcohols. These reagents do not work for 3 alcohols. Lastly, someone mentioned POCl3, and I just want to remind you that this reagent doesn't do substitution. It does elimination. so it would not work for converting alcohols to alkyl halides.

 

[movax]

Lastly, someone mentioned POCl3, and I just want to remind you that this reagent doesn't do substitution. It does elimination. so it would not work for converting alcohols to alkyl halides.

I said PCl3, not POCl3.