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Is there any series of chemical reactions that will help in distinguishing propanone and butanone? A simpler series of reactions or tests would be optimal. I know it's easy to distinguish and possibly separate by physical properties like boiling point but I need a chemical test or reaction.

So far I have thought up using NBS (N-Bromosuccinimide) to brominate both ketones. And possibly converting products into alcohols and then using Lucas Reagent to distinguish between the different alcohols. In the case of propanone the $\ce{Br}$ and hence the $\ce{OH}$ group is attached to the methyl group whereas in butanone it is attached to the ethyl group. I am not sure if this will work.

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    $\begingroup$ What about physical properties such as boiling points? $\endgroup$ – Mathew Mahindaratne Nov 7 '19 at 16:44
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    $\begingroup$ You may also need to edit your posy to show your work to find the difference. Otherwise, this may get on hold tag as homework question. $\endgroup$ – Mathew Mahindaratne Nov 7 '19 at 16:46
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    $\begingroup$ The boiling points are $\pu{25°C}$ apart, it should be easy. Failing that, NMR or refractive index. $\endgroup$ – Jan Nov 7 '19 at 16:56
  • $\begingroup$ I appreciate the answers and I know we can practically use physical properties but this was asked in a test and I'm not sure my answer is the simplest. $\endgroup$ – Tarun Nov 7 '19 at 18:00
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    $\begingroup$ Do analytic methods count? E.g burning and determining the elemental composition from the CO2 / H2O content would be a chemical solution to the problem. $\endgroup$ – Greg Nov 7 '19 at 19:59
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Different carbonyl compounds were identified in the past by making stable crystalline imines, which have sharp melting points and these melting points differ greatly from compound to compound.
This was accomplished by making imines out of semicarbazide or 2,4-dinitrophenylhydrazine.

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  • $\begingroup$ They're hydrazones or semicarbazones, but your approach is correct $\endgroup$ – Waylander Nov 7 '19 at 18:02
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Carbonyl compounds such as aldehydes and ketones reacts readily with derivatives of ammonia containing a hetero atom attached to the ammonia nitrogen to give stable solid derivatives with sharp melting points. A well known examples of those derivatives of ammonia containing a hetero atom attached to the ammonia are hodroxyl amine ($\ce{H2N-OH}$), semicarbazide ($\ce{H2N-NH-C(=O)-NH2}$), and phenylhydrazine ($\ce{Ph-NH-NH2}$). The most common phenylhydrazine used is 2,4-dinitrophenylhydrazine (2,4-DNP), because it gives hydrazone derivative, which can be used as qualitative test for these carbonyl compounds.

Traditionally, carbonyl compounds can be identified using sharp melting points of two of their nitrogen derivatives, e.g., corresponding semicarbozones and 2,4-dinitrophenylhydrazones. For example, one of the laboratory experiment for sophomore undergraduates taking Organic Chemistry II course at Texas A&M University is Reactions of Aldehydes and Ketones. During this lab experiments, students needs to make two derivatives (semicarbozones and 2,4-dinitrophenylhydrazones) of their unknown compound, and identify them by their melting points and other classification tests (to judge the unknown is aldehyde or ketone, and is the type of $\ce{CH3C(=O)-R}$). I listed the given values of several melting point values in the corresponding Laboratory Manual:

$$ \begin{array}{ccc} \\\hline \text{Name of Unknown} & \text{Boiling point} & \text{M.p. of semicarbazone} & \text{M.p. of 2,4-DNP}\\\hline \text{2-Propanone} & \pu{56 ^\circ C} & \pu{190 ^\circ C} & \pu{126 ^\circ C} \\ \text{2-Butanone} & \pu{80 ^\circ C} & \pu{145 ^\circ C} & \pu{118 ^\circ C} \\ \text{3-Methyl-2-butanone} & \pu{94 ^\circ C} & \pu{113 ^\circ C} & \pu{124 ^\circ C} \\ \text{2-Pentanone} & \pu{102 ^\circ C} & \pu{112 ^\circ C} & \pu{144 ^\circ C} \\ \text{3-Pentanone} & \pu{102 ^\circ C} & \pu{139 ^\circ C} & \pu{156 ^\circ C} \\ \text{2,4-Dimethyl-3-pentanone} & \pu{124 ^\circ C} & \pu{160 ^\circ C} & \pu{88 ^\circ C} \\ \text{2-Hexanone} & \pu{128 ^\circ C} & \pu{125 ^\circ C} & \pu{110 ^\circ C} \\ \text{4-Methyl-3-penten-2-one} & \pu{130 ^\circ C} & \pu{164 ^\circ C} & \pu{205 ^\circ C} \\\hline \end{array} $$

The first two is two compounds you want to identify: 2-propanone and 2-butanone. By the values given,, you'd be able to identify them exclusively by chemical and physical means. Needless to say that preparation of corresponding semicarbozones and 2,4-dinitrophenylhydrazones of given carbonyl compounds are so simple (simplest possible) that even undergraduate students with no prior experience in organic chemistry can also do it.

Then again, we have now introduced simple analytical method to identify unknown carbonyl compounds: $\ce{^1H}$-$\mathrm{NMR}$, aiming students to have well understand of spectroscopy. Following is two $\ce{^1H}$-$\mathrm{NMR}$ spectra of propanone (acetone) and butanone (ethylmethylketone). As you see, you can easily distinguish the two compounds in minutes: Propanone gives only one resonance for the methyl group, a singlet. Butanone, on the other hand, gives three signals - a quartet ($\ce{-CH2 -}$), a singlet ($\ce{-CH3}$), and a triplet ($\ce{-CH3}$).

Propanone Butanone

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