Thread: NMR and double bonds

when you do an NMR spec of a compund with a double bond.
for example if i cut out a small part. and say the two hydrogen atoms are in different environments in the molecule so it causes two peaks

H(a)----C=C---H(b)


when you do the spectrum, will the hydrogen (a) effect hydrogen (b) and so cause the peak to split. or does it have no effect?

another question.

if 2 hydrogens are on the same carbon in a C=C double bond.
does the fact they cant rotate mean they have different environments therefore causing 2 different peaks

Good question,

firstly, the compound that you mention doesn't exist unless it is some type of radical. Symmetry means that all h-bonds in either an ethylene or acetylene are identical in NMR.

If two H atoms are on the same C atom then they will have different shifts in NMR depending on what is on the other C atom - if one is attached.

When looking at spectroscopy in chemistry, always look at the symmetry, that's what determines everything.

the compound i mentioned doesnt exist because it is a cut out from a compound, i was just comparing two hydrogens A and B.

so say we had CH3CH2CH=CH2

and we take out this part

CH=CH2.

do all 3 hydrogens have different chemical shifts or are there any similair parts?

Originally Posted by organic god

the compound i mentioned doesnt exist because it is a cut out from a compound, i was just comparing two hydrogens A and B.

so say we had CH3CH2CH=CH2

and we take out this part

CH=CH2.

do all 3 hydrogens have different chemical shifts or are there any similair parts?

I believe all 3 hydrogens will have different chemical shifts, but the terminal hydrogens will have similar shifts. However, you can tell them apart because their coupling constants will be quite different.

I think that's right, someone with a better knowledge of organic chemistry can probably confirm/rubbish this...