Chen-Kao reaction

Chelate complex of the Chen-Kao reaction starting from (1S,2S)-pseudoephedrine

The Chen-Kao reaction (named after Ko Kuei Chen and Chung-Hsi Kao, Madison, Wisconsin 1926)[1][2] is a chemical method for determining the presence of pseudoephedrine, ephedrine, and similar phenylalkylamines.[3] The reaction is used in spot tests and is also known as Chen-Kao test (or simply as Chen, Test T in UN Precursor Test Kit). The test is often used to distinguish ephedrine, pseudoephedrine, norephedrine, cathinone and methcathinone from amphetamine and methamphetamine, which do not react with Chen’s test reagent.[4]

Testing Method

The Chen-Kao test is performed by creating an acidic solution of the compound to be tested and adding dilute Copper (II) Sulfate and Sodium hydroxide solutions.
The procedure is as follows:

  1. Place small amount of material to be tested on a spot plate.
  2. Add 2 drops of reagent A
  3. Add 2 drops of reagent B
  4. Add 2 drops of reagent C
  5. A violet color indicates presence of ephedrine or pseudoephedrine

Further, the coordination complex might be extracted with organic solvent like diethyl Ether or n-butanol (see Table II), as proposed in the literature,[8][9] which provides additional confirmation of the original results, but no further differentiation. This modification is not included in the UN Precursor Test Kit and other spot tests alike. In order to further differentiate between ephedrine enantiomers - ephedrine, pseudoephedrine and other ephedrine-related compounds, Simon’s test (with acetaldehyde) and Simon’s test (with acetone) are used. (see Table III)[4]

Reaction mechanism

When acidic solution of ephedrine or pseudoephedrine is mixed with copper sulfate and sodium hydroxide solutions, a violet color is formed. Upon shaking with ether, the organic phase turns red/purple and the aqueous phase turns blue. The staining is based on the formation of a coordination complex of two ephedrine molecules and a copper ion Cu2+[4][10]

This works, because, in an alkaline solution, the hydroxyl and amine groups are de-protonated, leaving a negative charge on the central atoms. They can then form a coordination complex with the positively-charged cupric ions from the copper sulfate. Phenethylamines (and their derivatives) that have a β-ketone group will also react, but less predictably and dramatically (the carbonyl oxygen is less nucleophilic than the deprotonated hydroxyl group). They usually form a deeper blue color, and often a grey-ish precipitate. This includes the cathinones and presumably βK-2C-x compounds.[citation needed]

Chen-Kao reaction of 2-amino-1-phenylpropan-1-ol (Cathine) without consideration of the stereochemistry.

Description

The execution of the Chen-Kao reaction is simple, needs little practice and limited skills. Also, the violet color obtained in the reaction is easy to define. For a correct execution, it is important to note that the typical colors develop relatively slowly, and that a good color intensity requires a sample of a few milligrams of the substances tested (i.e., more than what would typically be required for most other tests included in the UN test kits).

Of all ephedrine-related compounds, only ephedrine and pseudoephedrine produce the typical, stable violet color. Other ephedrine-related compounds produce a blue to greenish-blue precipitate. This precipitate could be seen as characteristic for the members of the ephedrine group other than pseudoephedrine and ephedrine itself, hence, the Chen-Kao test appears to show a significant specificity within the ephedrine group. However, it is known from previously published cross-testing work that various pharmaceuticals not related to the ephedrine group may produce similar blue copper complexes.

The two keto-amines, cathinone and methcathinone, initially also produce blue-colored complexes with the Chen-Kao reagent. However, a slow transition of the initial color into yellow, followed by an orange-brown color can be observed with both compounds, thus indicating the instability of the complexes initially formed, and an obvious decomposition of the two compounds under the alkaline conditions of this color reaction.

The results of the solvent extractions, summarized in Table II appear to add little novelty to the results of the original Chen-Kao reactions. However, in cases of doubts, they may serve as confirmatory steps.[4]

Results

Table I - Results of the Chen-Kao test[4]
Compound tested Color
Ephedrine Violet
Pseudoephedrine Violet
Norephedrine Bright blue precipitate
Norpseudoephedrine Blue precipitate
Chloropseudoephedrine Bright blue (light greenish) precipitate
N-Methylephedrine Pale blue precipitate (the crystals in the precipitate are violet)
Cathinone Pale blue precipitate, turns to violet, through grey (greenish) in 2–3 minutes it turns to orange(brownish)
Methcathinone Bright blue precipitate, through green it turns to brown, after 10 minutes the precipitate and the solution are orange
Table II - Results of solvent extractions of the complex formed in the Chen-Kao reaction[4]
Compound tested Chen-Kao D-Ether D-Ether - Water layer N-Butanol N-Butanol - Water layer
Ephedrine Violet Purple Blue Purple Transparent
Pseudoephedrine Violet Purple Blue Purple Blue
Norephedrine Blue Transparent Pale blue, precipitate Blue-Violet Opaque
Norpseudoephedrine Blue Precipitate forming a ring between layers Blue Blue-Violet Transparent
Table III - Results of the Simon and Chen-Kao reactions used for the distinction of ephedrine-related compounds[4]
Compound tested Chen-Kao Simon test I(with acetaldehyde) Simon test II(with acetone)
Ephedrine Violet Light blue (in 5 min. deeper) No color
Pseudoephedrine Violet No color No color
Norephedrine Bright blue precipitate Olive green(brownish) Light Pink
Norpseudoephedrine Blue precipitate No color Light Pink
Chloropseudoephedrine Bright blue (light greenish) precipitate Bright blue, after 5 min it turns to grey (brownish sediment) No color
N-Methylephedrine Pale blue precipitate Light Pink Light Orange

See also

References

  1. ^ K. K. Chen and C. H. Kao (1926). "Ephedrine and Pseudoephedrine, their Isolation, Constitution, Isomerism, Properties, Derivatives and Synthesis". Journal of the American Pharmaceutical Association. 15 (8): 625–639. doi:10.1002/jps.3080150804.
  2. ^ James Reardon-Anderson (1991). The Study of Change: Chemistry in China, 1840–1949 (1. ed.). Cambridge/New York: Cambridge University Press. pp. 224, 235. ISBN 978-0-521-53325-6. {{cite book}}: |journal= ignored (help)
  3. ^ Gábor Nagy, István Szöllősi, Kálmán Szendrei (December 2005). "Color Tests for Precursor Chemicals of Amphetamine-Type Substances" (PDF). United Nations International Drug Control Programme. Szeged University, Hungary. Retrieved 2018-04-05.{{cite web}}: CS1 maint: date and year (link) CS1 maint: multiple names: authors list (link)
  4. ^ a b c d e f g Gábor Nagy, István Szöllősi, Kálmán Szendrei (December 2005). "Color Tests for Precursor Chemicals of Amphetamine-Type Substances" (PDF). United Nations International Drug Control Programme. Szeged University, Hungary. Retrieved 2020-06-24.{{cite web}}: CS1 maint: date and year (link) CS1 maint: multiple names: authors list (link)
  5. ^ United Nations (1994). Rapid Testing Methods of Drugs of Abuse (Manual for Use by National Law Enforcement and Narcotic Laboratory Personnel). New York. pp. 71, 105. ISBN 978-92-1-148230-0.{{cite book}}: CS1 maint: location missing publisher (link)
  6. ^ Kaitlyn E Toole, Shanlin Fu, Ronald G Shimmon, Nadine Kraymen - University of Technology Sydney (2007). "Color Tests for the Preliminary Identification of Methcathinone and Analogues of Methcathinone". Microgram Journal. 9 (1). Sydney, New South Wales, Australia.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Kelly M. Elkins (October 18, 2018). Introduction to Forensic Chemistry. CRC Press. ISBN 9781498763103.
  8. ^ Wiegrebe, W. und Vilbig (1981). "36b". Zeitschrift für Naturforschung: 1297. doi:10.1515/znb-1981-1020. S2CID 36268717.
  9. ^ S.L. Ali (1986). "Ephedrine hydrochloride". K. Florey (Ed.): Analytical Profiles of Drug Substances. Analytical Profiles of Drug Substances. 15. New York: Academic Press: 233–281. doi:10.1016/S0099-5428(08)60415-3. ISBN 9780122608155.
  10. ^ Siegfried Ebel and Hermann J. Roth, ed. (1987). Lexikon der Pharmazie (1 ed.). Stuttgart: Georg Thieme Verlag. p. 134. ISBN 3-13-672201-9.