Wednesday, January 14, 2015

Nuclear magnetic resonance spectroscopy (NMR)

Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. When placed in a magnetic field, NMR active nuclei (such as 1H or 13C) absorb electromagnetic radiation at a frequency characteristic of the isotope. The resonant frequency, energy of the absorption, and the intensity of the signal are proportional to the strength of the magnetic field. For example, in a 21 Tesla magnetic field, protons resonate at 900 MHz. It is common to refer to a 21 T magnet as a 900 MHz magnet, although different nuclei resonate at a different frequency at this field strength in proportion to their nuclear magnetic moments.

Solving H-NMR problems

Ok, it’s a puzzle. You have to keep few things in your head and solve the puzzle step by step.

a) How many different sets of signal appear in the spectrum? (This can be determined by the
number of integrated peaks.)

b) What is the relative ratio of all the different hydrogen peaks present? Do they add to the total
number of hydrogen atoms in the molecular formula? 

c) The chemical shift provides information into the chemical environment of the different
hydrogen atoms in the molecule. As a rule of thumb, a chemical shift seen at ≤1.0 ppm and an
integration of 3.0 most possibly indicates an alkyl group whose terminal is a –CH3. An aldehyde
proton will show at ~9.0 ppm and a carboxylic acid ~11.0 ppm. Remember that electronegative
atoms tend to deshield hydrogen atoms moving their chemical shifts downfield by the inductive

d) The splitting patterns are very important because they can help us understand the hydrogen
atoms attached to adjacent carbon atoms.
1. Remember the n+1 rule for the splitting pattern. (n = number of neighboring hydrogen. Neighboring H are located on immediately next carbon atom) 

click on the image to enlarge

2. Remember characteristic splitting patterns. An isolated ethyl group (-CH2CH3) will show a particular pattern. A triplet from the –CH3 (the neighbor is a -CH2-) and a quartet from the –CH2-  An isolated isopropyl group (-CH(CH3)2) will show a doublet from the methyl groups
and a septet from the methyne (-CH-).

click on the image to enlarge

e). After assigning partial structures on the 1H-NMR spectrum try to place them together to form the compound. Next, try to generate a spectrum from the proposed structure and compare it to the spectrum provided.            

Please assign structure for the compounds represented by the following NMR spectra.


Hint: singlet- indicate no neighboring H

click on the image to enlarge


Hint: indicate no neighboring H


Hint: indicate no neighboring H

Hint: triplet and quartet together- indicate ethyl group


Hint: indicate no neighboring H. Look at a chemical shifts table.


Hint: septet together with doublet-indicates presence of isopropyl group


Hint: triplet together with quartet- indicate ethyl group, singlet indicate H without neighboring H. Look at chemical shifts.


Hint: triplet together with quartet- indicate ethyl group, singlet indicate H without neighboring H. Look at chemical shifts.


Tuesday, November 26, 2013

Exploring Conformational Analysis using Molecular Models

1. Draw the molecule methane using the wedge and dash depiction

2.      Draw ethane in wedge and dash depiction, sawhorse depiction and in its Newman projection. Show both eclipsed and anti forms.
Click on the image to enlarge
3.      Draw butane in wedge and dash depiction, sawhorse depiction and in its Newman projection looking down the 2,3 bond. Show eclipse, gauche and anti forms.
Click on the image to enlarge

4.      Draw all Newman projections of 2-methylhexane showing eclipsed, gauche and anti forms. Rotate about the 3,4 carbon-carbon bond and watch what happens to the groups. Draw a potential energy diagram for the rotation about the 3,4 carbon-carbon bond plotting Potential Energy versus Torsion Angle.
Click on image to enlarge
5.      Draw cyclopropane and cyclobutane in three dimensions, be sure to include the hydrogen atoms.
6.       Draw the chair form of cyclohexane.

7.       Draw the following molecules twice, once using the chair conformation AND once using the wedge and dash depiction.

1,2 cis-dimethylcyclohexane

1,2 trans-dibromocyclohexane

1,3 cis-dichlorocyclohexane

1,3 trans-dimethylcyclohexane

1,4 cis-dimethylcyclohexane

1,4 trans-dibromocyclohexane
Click on the image to enlarge


Tuesday, November 13, 2012

How to fill a melting point capillary

First, be sure that you have the right capillary for the machine you are going to use. There are two different mp machines in the lab. One is digital and uses slightly smaller capillaries than the older machines. If you force the old, larger capillaries into the new machine, they will get stuck and will take the new digital mp’s out of service. Please be careful.

Please the opened end of the capillary onto the pile of crystals. The closed end should be pointing up. The crystals will be held in the tube.
fill a melting point capillary

fill a melting point capillary

Turn the tube right side up and tap on the bench top. The crystals will fall to the closed end of the capillary.


Identification and Characterization of an Unknown Solid Using Melting Point and Mixes MP

YOUR SAMPLE FROM THE RECRYSTALLYZATION EXPERIMENT will be analyzed by melting point and mixed melting point. A sample must be dry (free of solvent) before an accurate melting point can be established.

Melting point may be used to determine the purity of a compound based on its melting range or to determine the identity of an unknown given a source of known compounds. An impure compound melts lover than expected and has a larger than two degree range. A higher mp indicates a completely different compound may be present.

The melting point of a solid is really a melting range, the sample should be heated at 1-2 degrees of Celsius  / min. the first drop of liquid is the start if the range and the temperature at which total melting occurs is the upper limit of the range. It is best to take two melting points of an unknown; a quick melting point with heating 5-10 degrees of Celsius / min to get in the area of the real mp. The second, more accurate mp, should be done slowly at 1-2 degrees of Celsius / min per near the mp. Use a new sample each time and be sure the apparatus has cooled to below the mp between runs. The sample cannot be reused because decomposition may have taken place during the first heating. The sample is placed in a capillary tube with one end closed. Fill with about 2-3 mm of crystals. Place the open end of the capillary over the crystals and push onto the crystals. Then turn the capillary over and tap on the lab bench until the crystals fall to the closed end.

When you have an idea of the identity of your compound you should carry out a mixed melting point with the authentic material that is available in the laboratory. To do this, mix a small amount of your unidentified material with the same amount of known material. Determine the mp of the mixture. Of the two compounds are the same, there should be no change in melting point.

However, if the unknown is something else, you will see a depressed mp with a broader range. Why? Try using the wrong compound intentionally. Simply comparing the mp of an unknown with the literature value is incomplete evidence to identify an unknown. Many organic compounds have identical melting points.

You will determine many melting points throughout the semester. Be sure you understand the technique now. Always make sure youy compound is dry (free of water) and free of solvent before putting it in the mp capillary. It will not dry in the capillary and you will get erroneous results.


Thursday, November 1, 2012

Melting Pints of Some Organic Compounds

Compound name                                                                                   MP(in degrees of Celsius)
acetanilide                                                                                                        113-115
p-nitrobenzaldehyde                                                                                         106-108
benzoic acid                                                                                                     122-123
acetylsalicylic acid                                                                                           138-140
4-dimethylaminobenzaldehyde                                                                          73-75
4-hydroxy-3-methoxybenzaldehyde (also known as vanillin)                           81-83
salicylic acid                                                                                                    159-160
p-aminoacetanilide                                                                                          162-163
o-toluic acid                                                                                                     103-105
p-toluic acid                                                                                                     180-182
m-toluic acid                                                                                                    108-110
p-toluic acid                                                                                                     180-182
3-nitroaniline                                                                                                   112-114
o-aminobenzoic acid                                                                                       144-148
m-aminobenzoic acid                                                                                      178-180
t-cinnamic acid                                                                                                134-135
o-nitrobenzoic acid                                                                                          146-148
m-nitrobenzoic acid                                                                                         139-141
p-nitrobenzoic acid                                                                                          237-240


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