Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: koolguysj on October 21, 2007, 01:16:44 PM

Title: A isolation challenge question
Post by: koolguysj on October 21, 2007, 01:16:44 PM

A mixture contains Benzoic acid, Benzil, and 1,4-dibromobenzene. How do you isolate benzil and 1,4-dibromobenzene by using the following reagents:

1M NaOH
1M HCl
1M NaHCO3
NaCl aq
Diethyl ether
ethanol
methanol
isopropyl alcohol
acetone
hexane
toluene
methylene chloride
granular anhydrous sodium sulfate

Benzoic acid is easily isolated by acid-base reaction. However, the other two are more challenging to seperate. Now I have Benzil and 1,4-dibromobenzene in a test tube. Please help me to isolate them. Thanks

Title: Re: A isolation challenge question
Post by: Donphil07 on October 30, 2007, 10:40:38 PM

What is benzil? it is not exactly bromobenzil or...?please give me full name

Title: Re: A isolation challenge question
Post by: agrobert on October 30, 2007, 10:50:33 PM

benzil <--> 1,2-diphenylethanedione

Title: Re: A isolation challenge question
Post by: macman104 on October 30, 2007, 11:49:58 PM

Is it possible to push the benzil to a hydrate form and increase it's solubility in some more polar solvent?  Not really sure, but it's a thought that came to mind.

You might also try some sort of recrystallization, not sure off the top of my head what solvents to try, but you might consider that method.

Title: Re: A isolation challenge question
Post by: agrobert on October 31, 2007, 12:05:04 AM

Yes maybe dissolve 1,4-dibromobenzene and benzil in methylene chloride and wash with sodium hydroxide.  I believe that benzil exists as benzoin in equilibrium, the enolate formed should be soluble in basic aqueous conditions.

Title: Re: A isolation challenge question
Post by: macman104 on October 31, 2007, 12:08:27 AM

Quote from: agrobert on October 31, 2007, 12:05:04 AM

Yes maybe dissolve 1,4-dibromobenzene and benzil in methylene chloride and wash with sodium hydroxide.  I believe that benzil exists as benzoin in equilibrium, the enolate formed should be soluble in basic aqueous conditions.

Thank you for flushing out my suggestion.  I wasn't exactly sure how to go about forcing the formation of the -OH.  I thought perhaps the 1M HCl would do it.  There is acid-catalyzed enolate formation, right?  or am I mistkaen?

Title: Re: A isolation challenge question
Post by: agrobert on October 31, 2007, 04:12:22 PM

I think it can be acid or base catalyzed.  Haven't tried it.  I have done some NMR with benzil and realized there exists an equilibrium between the keto-enol tautomer.