Determination of drugs based on their functional groups Dr. Mohammad A. Khanfar

Determination of drugs based on their functional groups A functional group is an atom or group of atoms that exhibit the same chemical behavior no matter what the rest of the molecule consists of. Example: R-OH, R-SH, R-NH2

Determination of aldehyde and ketons The determination of aldehydes depends upon the reaction with hydroxylamine hydrochloride: The liberated hydrochloric acid can be titrated with standard alkali.

Determination of aldehyde and ketons Example. Determination of the %w/w of cinnamic aldehyde in cinnamon oil. 100 g of cinnamon oil was reacted with adequate excess of hydroxylamine hydrochoride (15 ml). The reaction mixture was titrated with 160ml of 0.5M KOH until the red color of methyl orange changes to full yellow. Calculate the %w/w of cinnamic aldehyde (132.16 g/mol).

Determination of alcohol and phenol 1) Acylation: Their analysis based on their ability to form esters. Two problems: Too slow reactions, equilibrium is unfavored (low % yield). These problems are solved by: 1) Catalysis. 2) Temperature. 3) Anhydrides or acyl chloride are used instead of carboxylic acid because they have better leaving group. Thiols are similar to alcohol regarding their acylation:

Determination of alcohol and phenol Titrimetric acylation method ROOH and R-SH can be analyzed by acylation. The total acid (R2COOH) is titrated by standard base solution (back or direct titration). A blank is carried out, it should consume more base upon titration. The difference between the number of equivalents of base consumed by the blank and by the sample is equal to the number of equivalents of hydroxy group in the sample.

Determination of alcohol and phenol When the number of OH groups per molecule is known, the purity of a compound can be calculated in the usual way. Samples analyzed by this method should be pure and free from any external OH-containing compound (H2O for example). Acetic anhydride is the usual acylating agent; good reactivity, and small enough to react with hindered OH groups. However, it has disadvantage by reacting with aldehyde in non-stoichiometric manner. So alcohol (or thiol) compounds must be free from aldehyde groups. Acetyl chloride is used for hindered hydroxy groups unable to react with acetic anhydride, because it’s more reactive than acetic anhydride.

Determination of alcohol and phenol Phthalic anhydride and pyromellitic anhydride are used to determine alcohol in the presence of phenols.

Determination of alcohol and phenol 2) Titration of acidic OH groups. Titration of acidic OH groups. Phenols are stronger acids than alcohol due to resonance stabilization of the conjugate base anion. Generally, it’s possible to determine the concentration of phenols by non-aqueous acid-base titration. Non-aqueous acid-base titration is good to determine phenols in the presence of other acylable groups such as alcohol or amines.

Determination of alcohol Titration of system. Such compounds exist in two tautomaric forms. Similar titration to phenols, by non-aqueous acid-base titration of the enol form.

Determination of alcohol and phenol 3) Bromination of phenols 4) Oxidation of alcohol. Both techniques are described in redox titration. 5) Spectrophotometric analysis of phenols. Phenols can be directly determined by U.V. absorbance. Or, determined by colorimetry. a) Phenols with Fe3+ give colors that can be used to determine the concentration of either Fe3+ or phenols. b) Another colorimetric approach is by coupling to diazonium salts to yield colored dyes. Coupling takes place at the para position, and if it is substituted, it goes to the ortho position.

Determination of phenol Colorful which is measured by colorimetry

Determination of Thiols 1) Thiols are much stronger acids than alcohol, as the conjugate’s anion charge is distributed over large sulfur atom. Thiols are acidic enough to be titrated by aqueous titration. Non-aqueous titration is generally more accepted for thiols. 2) AgNO3 reacts with thiols to ppt silver-thiolates: The silver method can be conducted by adding excess AgNO3 and back-titrate the excess using SCN- (thiocynate) with Fe3+ as indicator. Alternatively, HNO3 produced from the silver reaction can be determined by direct acid/base titration.

Determination of Thiols 3) The third method for analyzing thiols depend on its oxidation to disulfides. In this method the unreacted I2 is back titrated with thiosulfate using starch as indicator.

Determination of Carboxylic acids 1) Carboxylic acids are weak acids with an average pka value of 4. Therefore, it can be determined by direct (or back) titration with standard alkali titrant. 2) Organic acids including formic, malonic, benzoic and salicylic acids are all oxidized to CO2 and H2O by known excess of Ce4+ in strongly acidic medium, heating is essential and remaining unreacted Ce4+ is back-titrated with Fe2+.

Determination of esters. The determination of ester is performed by hydrolysing the substance to an alcohol and an acid using excess of standard ethanolic KOH solution, and then back-titrating the excess alkali using strong acid (HCl). A blank determination is performed.

Determination of esters. Saponification value as an example of back-titration with blank determination. The saponification value for a fixed oil (vegetable oil) is the number of mg of KOH equivalent to 1 g of oil. A high value means rancidity, low value means possible a adulteration with mineral oil. Almost all edible oils have a saponification value between 188 and 196. Saponification value = (b – a) x molarity x 28.05/weight of sample (g) b = burette reading for blank a = burette reading for sample

Determination of Nitrogen 1) Elemental analysis of nitrogen (Kjeldahl determination) The most common method for determining organic nitrogen (protein, synthetic drugs, and protein content in blood product), which is based on a neutralization titration. In the Kjeldahl method, the sample is decomposed in hot concentrated sulfuric acid to convert the bound nitrogen to ammonium ion. The resulting solution is then cooled, diluted, and made basic. The liberated ammonia is distilled, collected in an acidic solution, and determined by a neutralization titration.

Determination of Nitrogen Kjeldahl method

Determination of Nitrogen Kjeldahl method

Determination of Nitrogen Kjeldahl method

Determination of Nitrogen Modifications were introduced to: 1) Reduce the digestion time. 2) To decompose resistant compounds. 3) To simplify ammonia analysis. K2SO4 (Potassium sulfate) is added to increase the boiling point of H2SO4 and then reduce the digestion time. Some compounds are resistant to decomposition, and these include: Azo compounds, N-containing heterocycle, Nitroso and nitro compounds. Heavy metals are added to obtain quantitative decpmosition of organic compounds, e.g. Mercury (oxide or Sulfate), Cupper Special treatment ease the decomposition of these resistant compounds. e.g. : preliminary reduction of nitro group.

Determination of Nitrogen One of the following way is applied to quantify NH3 concentration: A) By adding the distillate to a known excess of HCl or H2SO4 then determine unreacted HCl (or H2SO4) using back titration with NaOH. B) Directly receiving NH3 vapor on boric acid (H3BO3) to form ammonium borate ((NH4)3BO3), then determine NH3 by direct titration with HCl

Determination of Nitrogen C) Calorimetry: Used for very low concentrations of NH3 The most important caloremtric determination of NH3 is by Nessler’s reagent. Which is alkaline potassium mercuric iodide that gives brownish color with NH3, with an absorbance at 410nm (λmax).

Determination of Nitrogen 2) Aliphatic amines (1o, 2o, and 3o amines) are weak acids with an average pKa value of 9. Can be titrated (direct or back) with standard acid titrant. 3) Diazotization of aromatic primary amine. The end-point in the sodium nitrite titration is determined by the liberation of iodine from iodide. The small excess of HNO2 present at the end-point can be detected visually by employing starch-iodide 4) Bromination reaction. Titrant