[Lhg07]

Hi, I really need some help figuring out this molecular structure for the problem in the picture below.

I've tried so many isomers of C₆H₁₂ and I just can't get it. It's been hours...  I really need some guidance and a really dumbed-down explanation!

Thank you so much

[Dan]

Have you tried thinking about the questions and clues given to you already (red text in picture)?

The 1:6 septet and doublet fragment is very characteristic - what do you think this is?

It might help us to help you if you can explain the logic behind your proposed structures and share your ideas so we can guide you.

[Lhg07]

Yes, I've tried making sense of the clues, but I feel it's just evident that I'm really confused about this.

The coupling constant doesn't make too much sense to me. This is the first problem I've gotten where it didn't say (3H) or (1H) as the integration and I can't piece together how the coupling works with just J values. It says the septet and doublet are coupled, but I don't know what that means.

I'm sure I'm going about this really embarrassingly wrong, but this is my thought process. (Don't judge...it's a six week orgo 2 course and it's all happening so fast).

IHD of 1 and IR values indicate a double bond.

The most downfield doublet should be nearest to the double bond so I was figuring that it would be attached to a CH since a doublet indicates only 1 neighbor:

CH2=CH

I know the septet needed 6 neighbors so I imagined that it would be something like a CH with two methyl groups attached.

But when I try to figure out what's in between, I can't make it all work. I know I'm missing something about the coupling but I just can't figure it out.

[Borek]

This is the first problem I've gotten where it didn't say (3H) or (1H) as the integration

Assume the smallest of the numbers given means 1H, try to calculate what others mean (numbers given are integration values, just in terms of some internal machine units).

[Lhg07]

Borek, do you mean of the pink numbers? How does that relate to the J value of 6.9 Hz?

[Lhg07]

Borek, thank you for that. I was able to figure out that the integrations were (2H, d), (1H, sept), (3H, s) and (6H, d).

That gave me:

2,3-dimethyl-1-butene.

My question though is which protons correspond with the doublet at about 4.6 ppm? I have all the others accounted for and I'm left with the protons on the methylene, but they have no neighbors within 3 sigma bonds. How is it a doublet?

[Borek]

I agree



looks plausible. Why a doublet? The only thing I can think of is that it is actually not a doublet, but two singlets. Double bond is rigid (no rotation), so protons at the "free" end of double bond can be not equivalent (one is close to methyl, the other is close to isopropyl). But I can be awfully wrong, we will have to wait for someone better trained in NMR.

If this is the correct answer, the comment about lack of terminal vinyl is misleading.

[discodermolide]

That structure looks ok. A terminal methylene is a CH₂=C(R₂)

[Lhg07]

Borek that was the right answer! I was so thrown off by the vinyl comment and that doublet that I kept straying far from what the answer was. I'm still not sure why that would be a doublet, but thank you for you helping. It lead me to the right answer 

[Borek]

That structure looks ok. A terminal methylene is a CH₂=C(R₂)

Ah, OK. So the terminal vinyl is -CH=CH₂.

[Lhg07]

So how do you differentiate between a terminal methylene and a vinyl? They seem the same.

[Borek]

Not exactly. Terminal vinyl has a hydrogen as one of the R groups (which is enough to make its IR spectra different from a terminal methylene).

[Lhg07]

Ohhh. Thanks for all your help Borek