6.3.2 Spectroscopy

Question 1

what are some shortcuts to figure out if carbons are in the same environment

Answer 1

If we can reflect a molecule so that it looks exactly the same as it did before we reflected it, any two carbon atoms which swap places must be in the same environment.

2. If we can rotate a molecule so that it looks exactly the same as it did before we reflected it, any two carbons which swap places must be in the same environment.
3. If we can rotate a bond such that molecule so that it looks exactly the same as it did before we rotated it, any two carbons which swap places must be in the same environment.

Question 2

what happens if there is a unique atom

Answer 2

Whenever there is a unique atom in a molecule, any two carbon atoms that are a different number of bonds away from that unique atom must be in different environments.

Question 3

what does the number of peaks in an NMR spectrum tell us

Answer 3

the number of carbon environments in our sample

Question 4

what does the position of the NMR spectrum tell us

Answer 4

The positions (AKA “shifts”) of the peaks in an NMR spectrum tells us about the atoms bonded to the carbon atoms in each environment.

Question 5

what is the standard chemists add to there sample before running NMR

Answer 5

tetramethylsilane (TMS)

Question 6

why do chemists add TMS

Answer 6

It only produces one peak
The peak it produces doesn’t interfere with other peaks
It is easy to remove from a sample due to its low boiling point

Question 7

what does each peak in a carbon NMR correspond to

Answer 7

Eachpeakin ac-13spectrum corresponds to acarbonenvironment in the molecule which produces it.

Question 8

how do we know a molecule is more symmetrical

Answer 8

The larger the difference between:the number of carbons in a molecule’s molecular formula the number of peaks in itsCarbon-13 NMRspectrum

Question 9

How can we justify if a molecule matches the carbon-13 NMR spectrum

Answer 9

table which contains
shift then type of carbon then assignment

Question 10

how can we identify molecules using carbon NMR

Answer 10

Analyse the number of peaks to find the number of environments
Analyse the position of the peaks to find any information about what each carbon is bonded to
Use this information, alongside the molecular formula, to suggest a structure for the unknown molecule