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Characterization Techniques CHEM 241-08B, IMG_2893 - Coggle Diagram
Characterization Techniques CHEM 241-08B
Melting Point Analysis
Overview:
The melting point of a substance is a range from when the substance starts to melt and when the substance completely becomes a liquid.
Mixture Melting Point Method:
Step 1: Using a MEL-TEMP apparatus, take a quick/rapid melting point reading of your unknown compound to obtain an approximate melting point range
Step 2: Obtain the actual melting point of your unknown by slowing the rate of heating as the temperature of the apparatus reaches within 10 degrees of the approximate melting point range
Step 3: Make 3 mixtures: your unknown + unknown A, your unknown + unknown B, and your unknown + unknown C. Obtain melting points for all 3 mixtures
Step 4: Since your unknown is one of the three compounds A, B, or C, one of the melting points will have a melting pattern similar to that of your starting unknown sample. This allows for the identification of your unknown.
Melting Point Analysis Video
Pros:
Helps with determining the purity of an organic compound
Helps with identifying or verifying the identity of an organic compound
Only requires a very small amount of sample
Cons:
Can be difficult to determine the exact temperature at which a substance melts
If unknowns A, B, or C are not known you would only be able to identify your unknown as A, B, or C. You wouldn't have enough information to identify your unknown by name
When Appropriate:
When purity of a compound is unknown
When identity of compound is unknown
Thin-layer Chromatography
Pros:
Very fast process and carries out in less than 5 minutes per plate.
Very cheap process, and one of the most analytical processes for an organic chemistry lab.
Can reliably monitor the process of a reaction, and at the end compare Rf values.
When Appropriate:
When wanting to find active ingredients in a mixture by comparing Rf values.
Finding Rf values which then identifies the similarities between compounds.
Cons:
Only viable use of identification an unknown if you have something to compare it to.
Many Rf values are the same for molecules so it is not useful to rely on this only.
Not often used to determine the identity of an unknown.
Overview:
Thin-layer chromatography is a chromatography technique used to separate non-volatile mixtures. Thin-layer chromatography is performed on a sheet of glass, plastic, or aluminium foil, which is coated with a thin layer of adsorbent material, usually silica gel, or cellulose.
Step 1: Obtain a TLC Chamber with a lid and place a sheet of filter paper inside it to keep paper saturated. Then placing a portion of the prepared solvent for chromatography.
Step 2: On the TLC plate make two lines lightly in pencil on it, with one being being 1 cm from the bottom, and then making the lanes each at least 6mm apart.
Step 3: Spot the plates with samples 2mm in a diameter on these lanes.
Step 4: Place the plate in the Chamber remembering to place the lid back on top of it. From there let it run until the solvent reaches .5 cm from the other pencilled line.
Step 5: Remove the TLC Plate for the chamber and place a line for how far the solvent traveled. Then record the Rf values for each of the separated analytes, if the analyte does not show up use a UV light and circle where the spots ended up.
Thin Layer Chromatography Video
Gas Chromatography
Cons
Can only be used for volatile substances
Sample can't be something that reacts with the column
Sample must be able to remain constant at varying temperatures
Pros
Allows for the separation of materials in a timely manner
Accurate measures of the qualities of the compounds
Can be used in tandem with mass spectroscopy
When Appropriate
Can be used for any volatile substance
Should be used when there is a substance with multiple compounds, as it is a technique which separates and characterizes individual compounds
Can be used to inform the purity of the compound
Overview
Gas Chromatography is a characterization technique that is used to separate a mixture of compounds based on polarities and affinity for a mobile or stationary phase.
Step 1: Stationary phase is put into a thin column.
Step 2: Compounds are absorbed into resin to varying degrees, depending on their polarity.
Step 3: The compounds travel through the column at different speeds, allowing them to be separated.
Step 4: As the gas exits the column, the particles are detected by the GC device and it creates a GC printout, from which the different molecules present can be determined. The molecules with the higher boiling point will have a longer retention time; the molecules with the lower boiling point will have a shorter retention time
GC Instructional Video
NMR Spectroscopy
Cons
Big and expensive
machine
.
The neighboring protons must be equivalent to one another and not equivalent to the proton of interest
Need to isolate the product that you want to look at.
Make sure you're examining the correct molecule
Big and expensive machines.
Pros
Nondestructive: This means that you can still use the sample after it has gone through the spectroscopy.
Xavier has one! So it can be done relatively quickly.
Shows the relative concentrations to compare between each other.
Overview:
NMR is a technique which analyzes the various protons in a molecule. A molecule is placed inside a strong magnet and irradiated with low-energy, radio-wave photons. Hydrogen nuclei absorb photons of certain energy based on the amount of electron density around the nucleus. The energy of the radiowave photons is characteristic for different kinds of hydrogen nuclei, and the results provide information about the structure of the molecule.
How to use:
A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of a powerful magnet, and is spun to average any magnetic field variations, as well as tube imperfections. Radio frequency radiation of appropriate energy is broadcast into the sample from an antenna coil. A receiver coil surrounds the sample tube, and emission of absorbed rf energy is monitored by dedicated electronic devices and a computer. An nmr spectrum is acquired by varying or sweeping the magnetic field over a small range while observing the rf signal from the sample. An equally effective technique is to vary the frequency of the rf radiation while holding the external field constant.
What can we learn:
Chemical shifts/changes in concentrations
Atomic ratios within a product/molecule
Makeup of different molecules.
Presence of functional groups/atoms in molecules.
LAB Procedure
Pick a hydrogen atom (on an sp3
-hybridized carbon atom) to analyze and find the base chemical
shift value. Note if it is a CH3 group, a CH2 group, or a CH group, find that column on the table,
and start with the base chemical shift value.
b. Look at the α-carbon and note the substituents attached to the α-carbon (not including the αcarbon). Find that substituent in the substituent column, follow across to the α-carbon column
corresponding to the hydrogen atom, and add that value to the base value.
c. Look at the β-carbon and note the substituents attached to the β-carbon (not including the βcarbon). Find that substituent in the substituent column, follow across to the β-carbon column
corresponding to the hydrogen atom, and add that value to the base value.
d. Add the base value and all the various substituent values to get the predicted chemical shift
value for that hydrogen.
