protection of alcohols

[thebillsfan]

what are the relevant mcat ways to protect alcohols. i know attacking it with a diol to form a acetal or ketal would work. but i get mixed up with SOCl2. is SOCL2 used from protection or to transfer a Cl group? and if SOCl2 is used to transfer chlorine, then what sulfur compound is used as a protecting group for alcohols? I know there's one...can't rmr what it is. tosylates, perhaps?

---

Members don't see this ad.

 

[RogueUnicorn]

i believe the point of acetals is to protect the carbonyl, not the alcohol

 

[wanderer]

what are the relevant mcat ways to protect alcohols. i know attacking it with a diol to form a acetal or ketal would work. but i get mixed up with SOCl2. is SOCL2 used from protection or to transfer a Cl group? and if SOCl2 is used to transfer chlorine, then what sulfur compound is used as a protecting group for alcohols? I know there's one...can't rmr what it is. tosylates, perhaps?

I believe you're thinking of SiCl(R)3, which forms silyl ether when reacted with an alcohol.

R-OH + SiCl(CH3)3 ---> RO-Si(CH)3 + HCl

RO-Si(CH)3 + F- ----> RO- + SiF(CH3)3

F- is often in the form of a flouride salt such as TBAF

http://en.wikipedia.org/wiki/Silyl_ether
http://en.wikipedia.org/wiki/Tetra-n-butylammonium_fluoride

 

[sleepy425]

yeah, as bleargh said, using ethylene glycol to make an acetal/ketal is common, but is used for protecting aldehydes/ketones, not alcohols.

For alcohols, as wanderer said, you can use silyl ethers. Another very common alcohol protecting group is ethers. So the most common one is benzyl ether (you use benzyl bromide or benzyl chloride to do an Sn2 displacement to make a benzyl ether). Other ethers are used as well.

 

[thebillsfan]

yeah, my bad i meant carbonyl. now, in all situations you can remove the group w/ acid, correct? and "benzyl ethers"--isn't that closely related, if not actually the same thing as, tosylates? would more electron withdrawing groups on the tosylate be better or worse for protection (i know thats a random ? but i'm trying to think outside the box)

 

[sleepy425]

yeah, my bad i meant carbonyl. now, in all situations you can remove the group w/ acid, correct? and "benzyl ethers"--isn't that closely related, if not actually the same thing as, tosylates? would more electron withdrawing groups on the tosylate be better or worse for protection (i know thats a random ? but i'm trying to think outside the box)

First of all, no, tosylates are a very bad choice for a protecting group. Tosylates are very good leaving groups, so unless you want to get rid of the alcohol, you shouldn't use a tosylate. Tosylates are not very similar to benzyl ethers at all. Tosylates are not ethers, they are sulfonate esters, and have very different reactivity.

As wanderer mentioned, silyl ethers are most commonly removed by fluoride deprotection, but they are also usually mildly acid labile (depending on which silyl ether you use).

Benzyl ethers are almost always removed by hydrogenolysis. Hydrogenolysis involves using H2 and Pd/C (same as hydrogenation conditions) to deprotect. Benzyl ethers can also be removed by HCl, but this is less common. Other ether protecting groups are usually removed by HCl.

 

[loveoforganic]

And I doubt you'll need to know hardly any of that for the MCAT Might be good background though.