Given five samples of a known ketone derivative, the purpose of this experiment is to identify which unknown ketone derivative corresponds to the five known samples. In other words, using specific methods of compound detection, it is possible to match an unknown compound with a known compound because similar compounds will display similar characteristics. In this experiment, identification of the unknown ketone is accomplished through thin layer chromatography, melting point, and 1H-NMR spectroscopy. The unknown ketone is from a homologous series of methyl ketones. CH3CO (CH2) nCH3
The first step in the lab is the preparation of the solvent used in the developing chamber for thin layer chromatography. The solvent used is a 3:1 mixture of toluene and petroleum. After the developing chamber is prepared, it is essential to begin preparation of the unknown DNPH derivative. The preparation of the 1,2 DNPH derivative of a ketone is in fact a small organic synthesis which produces a fraction of a gram of product.
The second part of the lab makes use of NMR Spectrometry. NMR takes advantage of the magnetic properties of the 1H & 13C nuclei. We are only concerned with 13C because 12C does not have a magnetic spin and will go undetected in the NMR spectrum. Atoms with spin act like bar magnets and when placed in a large magnetic field the atoms tend to align with the field. There are two fundamental ways of obtaining an NMR spectrum; continuous-wave (CW) where a sample (unknown) is constantly irradiated with RF waves while the magnetic or RF frequency is varied, this induces a change in nuclei spin and these changes are measured and converted into peaks on a chart, Fourier-transform NMR (FT-NMR) spectrometer, is where the sample is irradiated with a short intense pulses of full-spectrum RF radiation; this action displaces the nuclei from its equilibrium division. This displacement response is recorded; data is converted by a processor which transforms it into an NMR spectrum. The experimental techniques used are weighing, measuring volume, vacuum filtration, drying, recrystalization, and melting point measurement. A further exploration of the aforementioned techniques will be explained in the discussion. Results:
Table #1: Physical Properties of Various Homologous Methyl Ketones
Table #2: Physical Properties for Unknown Ketone
Assume 2 – Octanone is unknown # 3
% error = ((63 – 58)/58)* 100% = 8.6%
Sample calculations of Rf(Unknown # 3)
Rf = Δ spot / Δ Solvent front = 4 .8 cm/ 5.4 cm = 0.89
Assume 2 – Octanone is unknown # 3
% error = (.89 – .83/ .83)*100 % = 7.2%
Table #3: Rf Values Obtained from Experiment
Table #4:1H-NMR Spectra Data of Unknown
The first step in the lab is the preparation of the solvent used in the developing chamber for thin layer chromatography. The solvent used is a 3:1 mixture of toluene and petroleum, the chamber itself is a glass jar for which the solvent and TLC plate will be placed and capped off with foil. The TLC plates are polar absorbent and the unknown as well as the 5 knowns are eventually spotted to this plate. It is through this phase where it is first possible to detect and match the unknown with a known sample.
After the developing chamber is prepared, it is essential to begin preparation of the unknown DNPH derivative. This is an important step because 2,4-Dinitrophenylhydrazone can be detected on the TLC plate. The DNPH gives color to the spots on the plate making it easier to determine the appropriate Rf values. This is first accomplished by mixing the ketone with the DNPH reagent which contains 1,2 DNPH with sulphuric acid and aqueous ethanol. After mixing in 3 ml of ethanol, the solution remains colorless. Only when 7 ml of DNPH is mixed in does the solution turn a milky orange color. The preparation of the 1, 2 DNPH...