Proton NMR Flashcards
what does it mean if the relative areas under 2 peaks are the same
E.g. if there were two peaks in a 3:3 or 2:2 ratio
the number of hydrogen atoms in each of those environments is the same
What do you do if you don’t know if something is an aldehyde or a ketone, how do you differentiate which one it is?
Look for peaks of the same ratio (ie. 3:3) which could represent x2 CH3 groups. This would mean it is a ketone because ketones have two CH3 groups on either side of the C=O.
Steps to approach proton NMR
1) look at the molecular formula
2) determine the variety of types of compound it could be
3) Look at the chemical shifts to work out which functional groups it has in it and determine the exact type of compound
4) Look at the ratios underneath the peaks to work out the number of hydrogens in each environment and to see if there are any of the exact same atomic groups but in different locations (eg. CH3 same atomic group but at different ends of a molecule so different hydrogen environments) - if same number in the ratio (eg. 2:2:2)
5) Work out splitting patterns (using the number of hydrogens on the neighboring carbon) and check the graph to reassure you of your structural formula - hopefully their will be the correct number of splits according to your displayed formula
What to do if it doesn’t give you a molecular formula
you can only really determine the functional group it belongs to
What does the ratio of areas under the peak tell you?
number of hydrogen atoms in that particular environment
What does the splitting pattern tell you?
the number of hydrogens bonded to the carbon neighboring the group we are looking at
relationship of adjacent hydrogens and splits
number of adjacent hydrogens + 1 = the number of splits
eg. 3 adjacent hydrogens means 4 splits so a QUARTET
If they give C4H8O2 what is it usually…
an ester
General formula for PURE aldehydes and ketones (when there is no other oxygen apart from in the C=O bond)
CnH2nO
general formula for a carboxylic acid
CnH2n+1COOH
ester general formula
RCOOR’
what does a relative area under the peak ratio of 2:2:4 mean?
that there are three groups in the whole molecule which are involved with hydrogens but two of these are identical groups of atoms (but in different places in the molecule because if they were the same place then there would only be one peak because it would be exactly the same environment)
what does a relative area under the peak ratio of 3:3:3 mean?
there are three groups within the molecule involved with hydrogens and all three of these have the same number of hydrogen atoms in their environment (but they are in different areas of the molecule)
so all three groups have 3 hydrogen atoms in their environment but each group is in a different place in the molecule. (eg. it could be three CH3 groups)
What are the solvents for proton NMR and why?
CCl4 (carbon tetrachloride) and CDCL3 (deuterated chloroform)
- don’t contain hydrogen so don’t produce peaks
When might you have a large peak ratio number? (eg. 6)
If there are lots of CH2 groups in a lengthy alkane
The more asymmetrical a molecule is …
the more different hydrogen environments it will have and so the more peaks it will produce.
The more symmetrical a molecules is…
it will have fewer different hydrogen environments and so less peaks
What does this mean - R3CH>R2CH2>
RCH3
that R3CH has the bigger shift and its peak appears closer to 2.0 rather than 1.0
What to remember about counting hydrogens to work out splitting patterns?
don’t count the hydrogen’s bonded to the carbon bonded to the hydrogen you’re interested in
If it gives you a displayed formula molecule, NMR spectrum and peak ratios
and asks you to label the peaks
what do you do?
1) label the different hydrogen environments (then check these against peak ratios - eg. if the ratio is 2:2:4 then check there are 3 different H environments in total but two of those both involve 2 hydrogen’s whilst one involves 4)
2) because they give you the shifts you use the shifts to correlate that to the data sheet and work out which groups (ie. CH3 of CH2OH etc) relate to which peak
3) identify the splitting pattern produced by each hydrogen environment and then look at the correlating peaks to check their are the same number of splits
