[DM7000]

So in Organic 2, we recently did a lab in Grignard reactions. My professor being the rogue chemist that he is, decided to say screw it to the reagents that the lab manual said and assign us different ones. I was given Bromobenzene. The reaction would be the Grignard Reagent and Acetone. My predicted product was 2-phenylpropan-2-ol.



 After running the reaction and running H NMR, we saw a very similar spectra to what 2-phenylpropan-2-ol would have except there are two peaks in the NMR at around 5.5ish. These are usually indicative of a double bond so I began to think where a double bond could possibly form...yet any combination I tried didn't seem feasible. I looked up the spectra of the various alternatives and they were all extremely different than what we had. The peak just to the left of 2 also has eluded us.

Can anyone provide some insight? We've been scratching our heads for a while and all we could really come up with was the formation of a double bond, yet that doesn't seem possible. We also toyed with the idea of a water elimination reaction also occuring but the spectra from that wouldn't be anywhere near what we have.

Here are the spectra:
H NMR
IR

[discodermolide]

Looks like you may have a mixture of the olefin and acetone?

[DM7000]

Just quick double check, an olefin is an alkene, right? If so to which are you referring to then? I realize that the acetone is the peak to the left of 2, so the two peaks at 5-6 would be the acetone with the oxygen removed?

[nox]

How did you workup the reaction? Your product looks like it would be quite acid sensitive.

[discodermolide]

Just quick double check, an olefin is an alkene, right? If so to which are you referring to then? I realize that the acetone is the peak to the left of 2, so the two peaks at 5-6 would be the acetone with the oxygen removed?

yes

[DM7000]

How did you workup the reaction? Your product looks like it would be quite acid sensitive.

Most of the reaction was done in an flame-dried flask with Mg, ether, and bromobenze. Acetone was then added after the Grignard reagent was formed. Now my reaction never actually went so I had to rely on my lab partners who actually completed the reaction. After the acetone was added, they allowed the reaction to cool and quenched it by adding it to a flask with ice and aqueous H2SO4 (10%). Do you think it would be possible that it reacted with this?

[discodermolide]

How did you workup the reaction? Your product looks like it would be quite acid sensitive.

Most of the reaction was done in an flame-dried flask with Mg, ether, and bromobenze. Acetone was then added after the Grignard reagent was formed. Now my reaction never actually went so I had to rely on my lab partners who actually completed the reaction. After the acetone was added, they allowed the reaction to cool and quenched it by adding it to a flask with ice and aqueous H2SO4 (10%). Do you think it would be possible that it reacted with this?

Looks like they got the H2O elimination and now have a mixture of olefin and acetone!

[orgopete]

Look up isopropenylbenzene in SDBS.

[DM7000]

Ah, thank you so much guys! I actually had isopropenylbenzene when I originally thought about it but I never accounted for the fact that acetone would still be present!

[orgopete]

What makes you think acetone is present?

[nox]

How did you workup the reaction? Your product looks like it would be quite acid sensitive.

Most of the reaction was done in an flame-dried flask with Mg, ether, and bromobenze. Acetone was then added after the Grignard reagent was formed. Now my reaction never actually went so I had to rely on my lab partners who actually completed the reaction. After the acetone was added, they allowed the reaction to cool and quenched it by adding it to a flask with ice and aqueous H2SO4 (10%). Do you think it would be possible that it reacted with this?

10% sulfuric acid, yep that would definitely give some elimination product.

[discodermolide]

What makes you think acetone is present?

The singlet at 1.85, if this was isopropyl it would be a doublet. The olefin signals are triplets,  the signal at 2.4 integrates for 3H and looks like a triplet with the same coupling constant as the olefinic signals.
The product has eliminated H2O and you have 1-methyl styrene as product.

[discodermolide]

I meant quartets with small J values, not triplets, sorry.

[orgopete]

Re: acetone

By my inspection, it looks like a small amount of TMS is present and the spectrum is shifted by 0.2 ppm. Acetone comes around 2.0 ppm, no peaks there. The largest peak is around 1.6 ppm. Given the reaction and looking at the integral, I would say this is the spectrum of a mixture of the expected product and isopropenylbenzene. The methyl groups of the expected product should be a singlet around where the singlet is present. The integral has been set on the olefinic hydrogens (1 each). From that, you can see the methyl is about 3H but the aromatic is much higher than 5 hydrogens. This leaves plenty of room for the expected Grignard addition product to be present.