Organic Chemistry Synthesis Question...?

Question

Benzoic acid treated with Et(ethyl)Li and H30+ then treated with PCC...what happens? and please explain what the reagents do in this problem.

Answer 1

Ethyl lithium is a strong base, which means it will deprotonate the carboxylic acid.



PhCO2H + EtLI --> PhCO2Li + Et-H



This lithium salt can be attacked by another equivalent of ethyl lithium, which forms PhC(OLi)2-Et. When you learned about Grignards, you probably learned that Grignards add twice to esters to form tertiary alcohol, and that the second addition is impossible to stop. This reaction is a way of overcoming that. Unlike the PhC(OR)(OMg)Et tetrahedral intermediate, which collapses to eliminate ROMg and produces PhCOEt, the PhC(OLi)2Et intermediate does not collapse to form a ketone because OLi is not a leaving group.



Addition of acid protonates both oxygens. This intermediate - PhC(OH)2Et - is called a hydrate, and under acidic conditions, will form a ketone (in this case phenyl ethyl ketone/1-phenylpropanone/propiophenone).



I'm not sure what the PCC will do at this point. PCC oxidizes alcohols to ketones (and under certain conditions aldehydes to carboxylic acids). In fact I would go so far as to say PCC does nothing here.

Answer 2

The EtLi will attack the carbonyl that is part of the carboxyl group. This will form a tetrahedral intermediate. The acid acts as work-up... so it will look like this:

Ph-C(OH)2Et

The carbon that you see will have an aromatic ring (phenyl), two alcohols (OH), and an ethyl group hanging off of it.



Then, PCC is an oxidizing reagent. So this will make the two alcohols into a single ketone (however, most likely the two alcohols will form into a ketone during work-up). Just a side note, PCC will only oxidize to a ketone or aldehyde. So you will have in the end:

Ph-COEt

This is a carbonyl with an ethyl on one side and a phenyl on the other.



I hope this helped!

Answer 3

You can use an organolithium reagent, followed by acid workup, to convert carboxylic acids to ketones. Instead of memorizing this reaction from scratch, it's helpful to note that it's very similar to a Grignard reaction, which you already know.



PhCOOH + LiCH3 -> PhC(OLi)2CH3

PhC(OLi)2CH3 + H3O+ -> PhC(O)CH3



PCC, an oxidizing reagent, will not react with this ketone. Ketones are already as oxidized as they can get. Your product is therefore acetophenone, PhC(O)CH3.