[ponyoponyo]

I need help answering this question. I know the answer is 1=H   2=NH2   3=H   4=OH,
but I can't get the right answer.


I have a modeling kit. I started off by making a Newman projection with on carbon-1 the NH2 group facing away from me and the H towards me. On carbon-2 I have the OH group towards me and the H group away from me. Then I positioned the model vertically with those two groups on each carbon coming out towards me and the Ph and CH3 groups facing away from me. I get 1=NH2   2=H   3=H   4=OH which is only partially correct. Please tell me what I am doing wrong.

[Dan]

Either your model is incorrect, or the configuration you've put on the paper is not the same as the model you're looking at. Can you post a photo of the model?

I started off by making a Newman projection with on carbon-1 the NH2 group facing away from me and the H towards me. On carbon-2 I have the OH group towards me and the H group away from me.

This is OK if the methyl and phenyl groups are configured anti - this may be a mistake in the model (can't tell without a picture though).

[ponyoponyo]

This is OK if the methyl and phenyl groups are configured anti - this may be a mistake in the model (can't tell without a picture though).

Oh, if I have the Ph and CH3 groups antiperiplanar then I get the right answer. So should the end backbone groups always be antiperiplanar when going from Newman to Fischer?

[Dan]

You should build your model as it is drawn in the first diagram (with Me and Ph anti). If they are syn in diagram, do the same in the model.

[ponyoponyo]

So if the methyl group was pointing down in the diagram on the left and the NH2 was still dashed but positioned up, then 1=NH2   2=H   3=H   4=OH would be correct?

[Dan]

So if the methyl group was pointing down in the diagram on the left and the NH2 was still dashed but positioned up, then 1=NH2   2=H   3=H   4=OH would be correct?

Yes.

[ponyoponyo]

Thanks I understand a lot better now!

[ponyoponyo]

I have another question. I was given this Newman projection and have to make a Fischer projection

I made a model that looks exactly like the Newman projection with the two largest groups oriented antiperiplanar like in the diagram. The I looked at the model from top and got this Fischer projection:


Why is this Fischer projection incorrect?

[sjb]

Remember that horizontal and vertical lines in Fischer projections have implicit stereochemical information