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NMR question

Could someone please explain questions 16 and 20 on here please

For 16, I put two singlets as I assumed that two of the methyl groups were symmetrical (the two bold ones) and the third methyl group (italic one) wasn't so it only counted as two peaks but the answer is D, one singlet.

CH3-C(BR)(CH3)CH3


For question 20 I've never heard of this so I assumed they'd all be equal so A but the answer is in fact C, why?

Thank you.
Original post by lovex
Could someone please explain questions 16 and 20 on here please

For 16, I put two singlets as I assumed that two of the methyl groups were symmetrical (the two bold ones) and the third methyl group (italic one) wasn't so it only counted as two peaks but the answer is D, one singlet.

CH3-C(BR)(CH3)CH3


For question 20 I've never heard of this so I assumed they'd all be equal so A but the answer is in fact C, why?

Thank you.


All three methyl groups are attached to the same carbon - they must be equivalent.
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yangg
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Original post by lovex
For question 20 I've never heard of this so I assumed they'd all be equal so A but the answer is in fact C, why?


If it is a triplet there are two H's next to the one you're looking at, as we can tell by the n+1 rule. They can either align or oppose the external magnetic field. If you think of the possible alignments it will make sense.
In case you're still stuck I suggest you read this:
http://www.chemguide.co.uk/analysis/nmr/splitting.html

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