Example 3.pdf
Transcript of Example 3.pdf
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1434357584480.055 Example 3
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Spectroscopy Tutorial: Examples
Example 3
C7H16O
MW 116
First calculate the degree of unsaturation: the result is 0. The compound will not have a ring or a doublebond.
IR Spectrum
The compound has an oxygen, but no double bonds, so it must either be an alcohol (OH) or an ether (O). Alcohols are quite distinctive in IR, showing up with a strong, broad band in the region 35003250.Note the broad band at 3391: this indicates that the compounds is indeed an alcohol. You can also lookfor the COH stretch at 12601050.
Think of possible structures
Now we know that the compound has an OH group, but there are lot of ways that this sevencarbonmolecule could be put together. It could be straightchained or it could be branched; the OH group couldbe (theoretically) anywhere in the molecule. Let's look at the NMR to get an idea of how many differenthydrogens are in the molecule so that we can narrow down the number of possible structures.
Proton NMR Spectrum
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The NMR of this compound is below:
There are only four different kinds of protons, and 12 of them (at 0.9 ppm) are next to a carbon that hasonly one hydrogen (because it is a doublet). This indicates that the molecule has two of the substructuresbelow:
These two substructures account for 6 of the 7 carbons in the molecule. Therefore the "something" ineach must be the same carbon. Let's put them endtoend and replace the "something" with a carbon, thensketch in the proper number of bonds:
Now we need to place an OH group in the molecule. It can't be on either of the carbons that areconnected to the two methyl groups, because the position of each hydrogen in blue is dictated by the factthat it splits the 12 hydrogens at 0.9 ppm into a doublet. Therefore, it must be on the middle carbon, likethis:
In the NMR, protons on the same carbon as an OH group the hydrogen in purple above will show upfrom 24 ppm. Sure enough, there is a peak constituting one proton at 3.1 ppm; furthermore, this peak is
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1434357586170.055 Example 3
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a triplet indicating 2 protons (blue) on adjacent carbons.
The peak at 1.71.9 ppm has two protons and is an octet (although it's kind of hard to see all 8 peaks);this peak corresponds to the blue protons, which are split by both the red and purple protons (a total of 7hydrogens on neighboring carbons).
The hydrogen on the oxygen (green) shows up as a singlet at 1.4 ppm. Although hydroxyl protonsusually show up from 24 ppm, the exact position is not always predictable. Hydroxyl protons are usuallynot split.
Summary
Example 3 is 2,4dimethyl3pentanol: