Ochem help

[Tatodotcom]

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[TTUMike]

http://www.cem.msu.edu/~reusch/VirtualText/alcohol2.htm

 

[Tatodotcom]

i found that site but it does not show what happens before that mechanism step, like the activation of the electrophile.....any one else

 

[Venicat]

edit: ok when they say PCC you basically have chromic acid (oxidation state 7 for the Cr)

That's Cr bonded to two OH groups and two double bonded O's. Now what happens is your alcohol does a nucleophilic attack on the Cr to form an ester. Water, or pyrimidine, is going to abstract a hydrogen off the carbon alpha to the O that was part of the original molecule and is now part of the ester bond. As it does that, a double bond is formed between that carbon and the oxygen, and chromate ion is released as the leaving group. It's E2-like. Water might come in and attack the electropositive carbon to form the hydrate, which just rearranges to the aldehyde or ketone.

 

[cassandra123]

http://www.mc.maricopa.edu/~minger/CHM 236/Fall 2008/Homework Set 1 Key.PDF

#9 is i think what you are talking about.

 

[Tatodotcom]

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[Venicat]

i understand everything you guys are saying and i appreciate the help but its still not right...PCC is NOT chromic acid...chromic acid converts primary alcohols to carboxylic acids, not aldehydes.....PCC is pyridinium chlorochromate....thats pyridinium with Cr with two double bonded Oxygens, one single bonded oxygen, and one chlorine.......


anybody else? this is driving me crazy..thanks for the help

Okay it's not the same thing but it's very close and exhibits the same functionality the only difference being that PCC is a bulkier molecule which is why you need a primary or secondary substrate molecule because a tertiary alcohol would be too sterically hindered to undergo the initial esterification step.

With both mechanisms, an aldehyde or ketone is what is formed first (depending on what type of alcohol you started with), it's just under acidic conditions you get additional hydrolysis to form the carboxylic acids. PCC is a relatively mild set of reagents, and the pyrimidine is a good scavenging base because of its bulk-- it will not act as a nucleophile to any appreciable extent.

 

[Short Round]

Is this Ochem "part 2" ? i hope so lol, cuz I'm in part 1 right now and other than the functional groups, I have no idea what you're all talking about. 😱

 

[solzont]

The complete mechanism can be found here: http://www.umich.edu/~chem216/216 W09-Expt 2.pdf