Exp.11) Identification of unknown ketones.
Introduction:
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[6]. 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[4]. 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:

Reaction Mechanism:

Table #1: Physical Properties of Various Homologous Methyl Ketones

Table #2: Physical Properties for Unknown Ketone

%error (m.p.)
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
%error (Rf):
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

Discussion:
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.[6]

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.[6] 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 derivative of a ketone is in fact a small organic synthesis which produces a fraction of a gram of product. This is an addition elimination reaction (Condensation reaction).

From the above reaction mechanism, the ketone reacts with 2,4 Dinitrophenylhydrazine, this is called condensation; there’s the addition where 2,4 Dinitrophenylhydrazine adds across the carbon-oxygen double bond to give an intermediate wich then goes through the elimination stage by losing or giving off a water molecule.

However the preparation is not complete. With the presence of sulfuric acid, the melting point of the derivative will be lower than theoretically expected due to catalyzed isomerization reactions, the acid must be removed for this reason. The acid is removed using sodium bicarbonate after vacuum filtration. The derivative is then purified through recrystallization. Mixed solvent recrystallization is done in this experiment due to 2,4-DNPH dissolving too readily in ethanol and too sparingly in water which would not make the derivative  a good recrystallization solvent itself. Therefore both water and ethanol is mixed into the derivative and vacuum filtered. After the unknown is prepared, the 5 knowns will be prepared in much the same way as the unknowns. Once everything is ready, 0.01 g of the unknowns and knowns are dissolved in 0.5ml of ethyl acetate. The TLC plates are spotted with the 2 unknowns and the 5 knowns placed in the developing chamber which contains the solvent, and at this point it is possible to see the spots rise due to the color from the DNPH. Not only do the spots rise but the solvent front as well, after the process stops it is easy to match the unknown with a known by calculating the Rf values of all spots and matching melting points with the knowns.[6] The final step for this experiment is to deduce the structure of the ketone through H NMR spectrum. As mentioned in the introduction, a sample of the unknown is placed in the spectrometer, more specifically, in the midst of a magnetic field.[1] To conclude, it is evident from the Rf values obtained that unknown # 3 Rf is closest to that of 2 – Octanone. With a 7.2 % error, this can be due to the reagent being diluted with too much ethanol or still containing traces of the acid, however if traces of the acid are present then the melting point[3] should be significantly lower. From the melting points we can determine that unknown # 3’s melting point is closest to that of 2 – Octanone with an 8.6% error. This can be attributed by either the unknown being diluted with too much ethanol and water or the mixture containing traces of acid. However, as aforementioned, traces of acid would only lower the melting point not raise it, therefore it is unlikely that traces of acid can be attributed to the error.[3] From the H NMR spectrum, integration suggests that at 1.9 and 2.25 there is some indication of possible ketone fragments. 1H-NMR Spectra for 2-octanone:

[5]

2-Octanone’s structure contains a hydrocarbon chain therefore the spectrum has indicated the presence of methyl groups giving further evidence that the unknown is in fact 2-Octanone.

To conclude, TLC, melting point measurement, and H NMR spectrometry are powerful tools relevant to deduction and detection of unknown compounds. Together, the three have pointed out in different ways that the unknown compound is in fact 2-Octanone. References:

[1]“1H NMR – Intro.” Wake Forest University. Web. 21 Aug. 2011. http://www.wfu.edu/~ylwong/chem/nmr/h1/ [2] Brown, William Henry. “13.11 13C-NMR.” Organic Chemistry. Belmont, CA: Brooks/Cole, Cengage Learning, 2012. 521-25. Print. [3] Chemglue4u. “Melting Points.” | Chemglue4u.com – The Lab Report Guide. Web. 21 Aug. 2011. http://chemglue4u....

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