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Esterification of Vanillin with an Acid [Mar. 25th, 2009|04:17 pm]
Chemistry Help


[mood |confusedconfused]


I am performing a reaction with vanillin, acetic anhydride, and sulfuric acid.  An acylation does NOT occur, because although it can happen with a base, an acid yields a different product. 

Literature value for the melting point is 90 - 91 degrees celsius. 

An IR spectra taken shows peaks at 1609, 1711, 1765, 2850 (moderate peak), 2920, and a 3388 stretch.  1609 indicates an aromatic ring; 1711 is C=O, 1765 indicates a carboxylic ester, right?  I am a bit confused about that.  2850 is probably an sp3 hybridized hydrogen, as is the 2920 stretch; and the 3388 stretch, I believe, is an OH stretch.

The NMR is of the following: there are 5 peaks (5 different kinds of hydrogens).  A peak at 2.2, 2.4, 3.8, 7.2, and 7.6.  The heights (which indicate proportionality) are 8.658, 3, 3, 2.9, and 1, respectively.    2.2 & 2.4 are very close to each other and are indicative of hydrogens on a methyl group attached to a benzene.  The 3.8 peak seems like a hydrogen attached to a methyl group carbon that possesses an OH substituent.  7.2 is the benzene ring, and 7.6 is probably a hydrogen that is part of an OH group that is directly attached to the benzene ring.

NMRs have always troubled me, and the peaks are not as clean as the problems we are assigned...  Along with the proportionalities presented, I'm just confused.

Any help would be GREATLY appreciated.  :)

[User Picture]From: rmpalpha
2009-03-25 10:45 pm (UTC)
I had this exact experiment in my undergrad organic chemistry lab too. It's a good problem. The hydroxyl group in the vanillin molecule is surely acylated (as it is acylated in the base-catalyzed reaction). Notice that there isn't an aldehyde resonance in the NMR data that you've given us - this suggests that something happened to it.

Let's go through the NMR data - I've simplified the integrations a bit...

2.2 ppm (6 H) = ??? (not methyl groups on a benzene ring)
2.4 ppm (3 H) = ??? (not methyl groups on a benzene ring either)
3.8 ppm (3 H) = methoxy group
7.2 ppm (2 H) = benzene ring protons
7.6 ppm (1 H) = benzene ring protons

Hope these hints help ... if you make any more headway and need more help, let me know.
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[User Picture]From: nistha
2009-03-25 11:20 pm (UTC)
on our handout, it said that it could undergo acylation to give the same product as a base (vanillin acetate), but it actually gives a different product.

OH. nevermind... I undergoes acylation, but in a different way...

I'm really curious, though, as to what the 1765 peak is...

I just went back to my apartment and saw in my book that 1735 is indicative of a carboxylic ester...

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[User Picture]From: rmpalpha
2009-03-26 01:08 am (UTC)
Yep, the hydroxyl group on the benzene ring is acylated to produce the acetyl ester, just like the base-catalyzed reaction - vanillin acetate, as you mentioned. The acetyl group there would be the 2.4 ppm peak in the NMR spectrum.

The acid-catalyzed reaction also modifies the aldehyde substituent into something else that integrates to six protons in its NMR spectrum. The only possible source of these additional protons are from the acetic anhydride reagent, and all six protons there are equivalent ...

The structure you propose in your next comment is not correct, however.
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[User Picture]From: nistha
2009-03-26 01:27 am (UTC)
Benzene ring...

the crazy substituent is...

a symmetric carbon, each "side" bearing a carboxylic ester.
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[User Picture]From: rmpalpha
2009-03-26 06:04 pm (UTC)
Okie dokie, from what it sounds like, you seem to have solved the problem :)
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[User Picture]From: nistha
2009-03-26 08:29 pm (UTC)
Thank you so much for your help! :D
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[User Picture]From: nistha
2009-03-26 12:58 am (UTC)
Actually, no. Wait.

I get a benzene ring, with the following sustituents, in ring order:

1. a carbon with a hydrogen, an -OH group, and a ketone as substituents
2. a benzene proton
3. a methoxy group
4. the carboxylic ester
5. a benzene proton
6. a benzene proton

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From: brandilovesfire
2009-03-25 11:28 pm (UTC)
vanillin is the devil! we've had numerous labs with it this semester... and none of them work out very well! sorry I can't be of any help haha
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[User Picture]From: nistha
2009-03-25 11:31 pm (UTC)
that, and I am generally just bad at NMRs. it bothers me, because that's the actual work of organic chem, and I suck at it. X/

thank god I'm a neuroscience major. I will not have to subject the world to my horrific skills as a chemist.
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From: brandilovesfire
2009-03-25 11:33 pm (UTC)
I suck at NMRs as well. I'm a dentistry major so I think I'm safe too! haha
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