You may find spectroscopy to be one of the most challenging parts of the course, however with a little patience and practice the subject should begin to become easier. In fact, once students have mastered the subject, some say that spectroscopy is their favorite part of organic chemistry. So let's begin.
The typical spectral problem usually includes three parts. You will be provided with the molecular formula of a compound and both the Infrared Spectra and the proton (1H) NMR of the molecule. It is up to you to put these three pieces together to determine the structure of the molecule.
* Please note that you may be provided with other information regarding the molecule, however, at this level you should be able to determine the structure of the molecule using only these three parts.
Please follow the steps in the order given!
Step 1: Calculate the Degree of Unsaturation
Most students underestimate the value of performing this simple calculation, however it will give you a lot of valuable information regarding the molecule. To teach you how to perform this calculation I have provided the following hypothetical molecular formula to use:
A) Count the carbons and plug that number into the fomula 2n + 2 where n = the number of carbons in the molecular formula.
SInce there are two carbons in the molecular formula given, we plug this number into the formula as follows:
2 (2) + 2 =
6
B) Count the number of hydrogens in the molecular formula.
1.Halogens (F, Br, Cl, I) count as hydrogens.
2. Nitrogens count as one less hydrogen.
3. Ignore Oxygen
There are 4 hydrogens in the molecule + 1 halogen (Br) - 1 nitrogen =
4
C) Take the value from part A that is located in the aqua colored circle minus the value from part B and divide this anwer by two.
Value from A - Value from B
2
= the degree
of unsaturation
6 -4
2
=
1
This is the degree of unsaturation for this molecular formula.
So what does degree of unsaturation mean?
Each degree of unsaturation tells you that you have either a double bond or a ring. Therefore, since the degree of unsaturation is one for this molecular formula, this tells us that the molecule has either one ring or one double bond.
*For arguments sake, if you calculate a value of two for the degree of unsaturation for a problem this can mean the following: the molecule has 2 rings or 2 double bonds or 1 ring and 1 double bond or 1 triple bond (a triple bond is worth two degrees of unsaturation).
*Perhaps the most valuable part of the degree of unsaturation is this:
If you calculate the degree of unsaturation for a molecule and you get a value of 4 or more degrees of unsaturation the molecule has a benzene ring in it. Remember, a benzene ring looks like this:
Notice how the benzene ring has three double bonds plus one ring for a total value of four degrees of unsaturation.
Valuable
Information
This Way!
Step 2: Interpret The Infrared Spectra (I.R.)
Most teachers will give you a huge list of the necessary I.R. peaks that you will need in order to answer the questions on an exam. Unfortunately, there is limited time during an exam to keep flipping back and forth to this spectral date sheet. Therefore, I highly recommend that you memorize the following ranges before the exam. There really aren't that many peaks (only eight), and it will make exam day a less stressful experience. An Infrared Spectrum looks similar to the one below. Almost like someone scribbled onto the page. However, with a little practice it will begin to look less like scribble and more like data that you can interpret.Here are the peak ranges that you should memorize:
WAVENUMBER (cm-1)
3500-3300
3000
2200
1700
1600
T
R
A
N
S
M
I
T
A
N
C
E
(%)
sp3 H's
sp2 H's
NH
NH2
Here is a brief explanation of what you see above:
1. If you see a peak at approximately 1600 (cm -1) this usually tells you that you have a benzene ring in the molecule.
2. If you see a peak at approximately 1700 (cm -1) this usually means that you have a carbonyl (C=O) present in the molecule.
* This peak is often very strong and often looks like it is almost touching the bottom of the page.
3. If you see a peak at approximately 2200 (cm-1) it can mean that you either have a nitrile (C N) or a triple bond (C C).
4. Slightly below 3000 (cm -1) (to the right of the dotted line) you will usually see something that looks like someone scribbled on the page. This tells you that there are sp3 hydrogens in the molecule. Sp3 hydrogens are located on carbons with four single bonds and at least one hydrogen directly attached to it.
*Keep in mind that this will show up in most molecules so it really won't help you to interpret the structure of the molecule. However you should be aware that it exists.
5. Slightly above 3000 (cm-1) (to the left of the dotted line) you can see something that also looks like somebody scribbled on the page. This tells you that there are sp2 hydrogens present in the molecule. Sp2 hydrogens are carbons with one double bond and two single bonds with at least one hydrogen directly attached to the carbon.
*Again, this really won't help too much in interpreting the structure of the molecule, however you should be aware that it exists.
6. There are three different peaks that show up in the 3300 cm-1
range:
A) A more rounded peaks usually means that there is an
alcohol (OH) present. *see the black line
B) A more pointy peak usually means that there is a NH
group present. *see the green line
C) A peak that usually looks like a "W" means that there is
a NH2 group. *see the blue line
*For the 3300 cm-1 range it is sometimes difficult to tell the difference between an OH or NH. In this case be sure to look at the molecular formula to see if there is an oxygen or a nitrogen in the molecule. This should help clear things up.
=
_
=
_
MEMORIZE ALL I.R. PEAKS!
Step 3: Interpret The Proton NMR (1H NMR)
Just like in I.R., 1H NMR has certain peaks that you must memorize.
Here are the ranges that you should memorize:
12
11
10
9
8
7
6
5
4.1
0
CHEMICAL SHIFT
Downfield
Upfield
__
__
__
__
Here is a brief explanation of what you see above:
(*Keep in mind that 1H NMR shows hydrogen's in a molecule)
1. If you see lines in the region between 12-11, this usually tells you that you have a hydrogen that is part of a carboxylic acid.
(see the yellow underlined hydrogen)
2. If you see lines in the region between 9-10, this tells you there is a hydrogen that is part of an aldehyde present in the molecule.
3. If you see lines in the region between 6-8, it usually means that you have a benzene ring with at least one hydrogen directly attached to it.
4. If you see lines in the region between 5-6, this tells you that you have a double bond with at least one hydrogen directly attached to it.
5. If you see lines at exactly 4.1 this tells you that you have an ester with at least one hydrogen on the carbon that is next to the oxygen (the carbon with the yellow star on it).
6.*In 1H NMR usually only hydrogen's that are directly attached to carbons show up. These hydrogen's will show up in the specta as straight lines. However, there is an exception to the rule:
A) If there is a hydrogen attached to an oxygen (an OH)
it will show up as something that is called a broad
singlet with the number one on the top. You will
see it above in the region between 0-4.1(an aqua
blinking bump) however, it can almost show up in
any region.(a bump with a #1 on top)
B) If there is a H attached to a nitrogen (NH) it will also
look like the same thing (a bump with a #1 on top)
C) A bump with a #2 on top means that you have an
(NH2).
*For a bump with the #1 on top it is sometimes difficult to tell if it is an OH or NH. In this case be sure to look at the molecular formula to see if there is an oxygen or a nitrogen in the molecule. This should help clear things up.
7. Additionally, there are two regions in the NMR: downfield and upfield. Downfield is the region closer to the higher numbers (#12) (to the left) and upfield is the region closer to the lower numbers (#1) (to the right).
1
2
Finally, you need to be able to interpret 1H NMR data. Remember, the way that chemists get a 1H NMR spectrum is by taking a molecule and placing it in a NMR machine. The computer then prints out a sheet with data on it (called a specta) and it is up to us to interpret the data.
*This is the most tricky part of this section so pay close attention:
Very Tricky Material Ahead!
How To Draw A 1H NMR Spectrum
There are two basic types of molecules that we will go over.
1. The Most Basic Molecule:
Carbon A: The information regarding carbon A is enclosed by a lavender square. You should notice that there is a blue number 3 in the top region of this square. This number shows that there are 3 hydrogens directly attached to carbon A. You should also notice that there are three green lines under the blue number 3. When three lines are drawn it is called a triplet. These lines tell you how many hydrogens are located on the carbon next to carbon A. Aways subtract the number 1 from this number and this will tell you how many hydrogens are on the carbon next to A. Therefore, three lines(a triplet) minus one = 2. This means that there are two hydrogens on the carbon next to carbon A.
A
B
3
2
12
O
8
6
Carbon A
Carbon B
Carbon B: The information regarding carbon B is enclosed by a dark purple square. You should notice that there is a blue number 2 in the top region of this square. This number shows that there are 2 hydrogens directly attached to carbon B. You should also notice that there are four green lines under the blue number 2. When three lines are drawn it is called a quartet. These lines tell you how many hydrogens are located on the carbon next to carbon A. Aways subtract the number 1 from this number and this will tell you how many hydrogens are on the carbon next to B. Therefore, four lines(a quartet) minus one = 3. This means that there are three hydrogens on the carbon next to carbon A.
* Finally, the last thing that you should notice is how far downfield carbon B is in respect to carbon A. Remember, that downfield is the region closer to the number 12 (to the left) on the spectrum. The rule is, is that if a carbon is directly attached to an electronegative element (Br, Cl, I, F, O, N) it brings the carbon farther downfield. Therefore, it makes sense that since carbon B is directly attached to a bromine that it is located further downfield than carbon A. At this point do not worry about how far downfield a carbon is. You should just be able to tell which carbon should be located furthest downfield in a molecule.
2. A More Complicated Molecule:
A
B
C
6
1
12
0
6
Carbon's A and C
Carbon B
Carbon B: Let's work on carbon B first because it is the least complicated of the three carbons.The information regarding carbon B is enclosed by a dark purple square. You should notice that there is a blue number 1 in the top region of this square. This number shows that there is 1 hydrogen directly attached to carbon B. You should also notice that there are seven green lines under the blue number 2. When seven lines are drawn it is called a septet. These lines tell you how many hydrogens are located on the carbon next to carbon B. Aways subtract the number 1 from this number and this will tell you how many hydrogens are on the carbon next to B. Therefore, seven lines(a septet) minus one = 6. This means that there are six hydrogens on the carbon next to carbon B. If you look at the molecule you will see that this is the case. There are 3 hydrogens on one side of carbon B and three hydrogens on the other side; for a total of six hydrogens. Also notice that carbon B is the farthest downfield because it is the carbon with the electronegative element directly attached to it.
Downfield
Upfield
Carbons A and C: The information regarding carbons A and C are enclosed by a lavender square. You should notice that there is a blue number 6 in the top region of this square. This number shows that there are 6 hydrogens directly attached to carbon A and C. This is the most confusing part of the molecule because you know that there can never be one carbon with six hydrogens attched to it (remember a carbon can only have four bonds). Therefore, what the computer is really telling you is that the nmr should really look like the close up picture below:
Finally, you should also notice that there are two green lines under the blue number 6. When three lines are drawn it is called a doublet. These lines tell you how many hydrogens are located on the carbon next to carbons A and C. Aways subtract the number 1 from this number and this will tell you how many hydrogens are on the carbon next to A. Therefore, two lines(a doublet) minus one = 1. This means that there is one hydrogen on the carbon in between carbons A and C.
0
3
3
3
Carbon C
Carbon A
Notice that since carbon A and carbon C are identical (they both have three H's attached to them and are both directly attached to a carbon with only 1 H and 1 Br located on it) the computer prints them out directly on top of one another. The end result is that you only see one set of green lines, but instead of seeing two blue 3's on top, the computer adds them together to give you a total value of 6.
A Closeup View
Okay, so now you have enough information to attempt some spectral problems of your own. You may want to print out the spectroscopy review sheet before you begin. You will find both links at the top of the page.Keep in mind that it is normal to have difficulty with these problems at first. But I think you will find that with a little practice you will become very good at these problems. Good Luck!
A CLOSE UP VIEW
Step 4: Draw the structure of the molecule
Remember, that when determining the structure of the molecule that:
