Aqueous or alcoholic solutions of ketones or aldehydes can be titrated by solutions of halogens or i — Organic Chemistry Chemistry Question
Keto-enol tautomerism
Aqueous or alcoholic solutions of ketones or aldehydes can be titrated by solutions of halogens or interhalides. In order to obtain reproducible results, the titration should be performed fast in the presence of buffer salts, such as NaHCO3.
Thus, to 10 g of cyclohexanone in aqueous methanol were added 2.00 mmol NaHCO3, and 1.00 cm3 2.00 N methanolic solution of ICl. After thorough mixing an excess of aqueous NaI solution was added, followed by titration by 1.594 cm3 of 1.00 N Na2S2O3 using starch as indicator.
A colorless substance A with the empirical formula C2H2O shows in 13C NMR only two signals at 94 and 159 ppm. The reactions of A with halogens or interhalides are instantaneous, but titration, as described above, is not useful as more than one mole halogen per mole A is consumed to give off heavy precipitates.
A readily reacts with aldehydes in the presence of either acidic or basic catalysts, to form products of 1:1, 1:2 or 1:3 stoichiometry (depending on reagent ratio). Such products are often colored, which is used in many well-known qualitative reactions for aldehyde-containing materials. For example, carbohydrates give red coloration when treated by A and a drop of HCl.
Under alkaline conditions A reacts with methyl iodide to give a mixture of products. With a large excess of MeI a single compound B is produced. B turned out to be identical to a known trimer of dimethylketene formed under the conditions of basic catalysis. On the other hand, if the reaction of A with excess MeI is performed in the presence of NaHCO3 a different compound C is formed. This compound possesses a fine odor and has been identified as one of important constituents of rose flavor. In 1H NMR compound B shows a single resonance, while C shows two sharp singlets with integral intensities ratio of 1:3.
The reaction of A with NaHSO3 on heating gives colorless water-soluble material (brutto-formula C6H5NaO5S) showing a purple coloration with FeCl3 solution. The 13C NMR spectrum in D2O shows 4 signals at 157, 144, 106, 105 ppm.
The reaction of A with hydroxylamine gives a compound D (brutto-formula C2H3NO), which is cleanly reduced by H2 over Raney-Ni catalyst to give a compound E (brutto-formula C2H3N) rapidly darkening in the air. The compound is poorly soluble in water, but readily dissolves in dilute HCl. Boiling of this solution gives back A.
Write the reactions involved in the analysis.
Model Answer
The reactions involved in the analysis are:
1. Keto-enol tautomerization (catalyzed by acids/bases):
Cyclohexanone ⇌ Cyclohex-1-en-1-ol
2. Addition of iodine chloride to the double bond of the enol:
Cyclohex-1-en-1-ol + ICl → 2-iodocyclohexanone + HCl
3. Conversion of excess ICl to iodine by adding NaI:
ICl + I^- → I2 + Cl^-
4. Titration of the liberated iodine with sodium thiosulfate:
2 S2O3^2- + I2 → S4O6^2- + 2 I^-
What compound reacts with ICl? Estimate the content of this compound in cyclohexanone.
Model Answer
The compound that reacts directly with ICl is cyclohex-1-en-1-ol (the enol tautomer of cyclohexanone).
Estimation of enol content:
- Initial ICl added: 1.00 cm^3 × 2.00 N = 2.00 meq.
- Excess ICl (measured via S2O3^2- titration): 1.594 cm^3 × 1.00 N = 1.594 meq.
- ICl consumed by enol: 2.00 - 1.594 = 0.406 meq.
- Cyclohexanone quantity: 10 g / 98.15 g/mol = 0.1019 mol = 101.9 mmol.
According to the original source text, the calculated enol content value is estimated to be 1.18% (though this is kinetically overestimated, and the real pK is 5-6).
What is the role of buffer salt? What can happen if Na2CO3 is taken in place of NaHCO3?
Model Answer
The role of the buffer salt (NaHCO3) is to neutralize the hydrochloric acid liberated during the electrophilic addition of ICl to the enol. This prevents the acid-catalyzed enolization of cyclohexanone during the titration, ensuring that only the enol initially present is consumed and analyzed.
If Na2CO3 is used instead of NaHCO3, the solution becomes too alkaline. This alkaline environment triggers base-catalyzed enolization of cyclohexanone during the titration, causing continuous reaction and an artificially high and incorrect enol content estimation.
Determine the structures of A, B, C, D, E.
Model Answer
The structures of the compounds are as follows:
- A: Phloroglucinol (1,3,5-trihydroxybenzene, which is the fully enolized tautomer of cyclohexane-1,3,5-trione), C6H6O3.
- B: 2,2,4,4,6,6-hexamethylcyclohexane-1,3,5-trione (a C-alkylated trimer of dimethylketene formed under basic catalysis), C12H18O3.
- C: 1,3,5-trimethoxybenzene (an O-alkylated derivative and an important constituent of rose flavor), C9H12O3.
- D: Cyclohexane-1,3,5-trione trioxime (phloroglucinol trioxime), C6H9N3O3.
- E: 1,3,5-triaminobenzene, C6H9N3.
Write the reactions mentioned in the text
Model Answer
The reactions mentioned in the text are:
1. Acid-catalyzed condensation of phloroglucinol (A) with aldehydes (RCHO) to yield colored quinoid compounds (stoichiometry 1:1, 1:2, or 1:3).
2. Alkylation of A with excess MeI under basic conditions (NaOH) to yield the C-methylated compound B.
3. Alkylation of A with excess MeI in the presence of NaHCO3 to yield the O-methylated compound C.
4. Nucleophilic addition/tautomerization of A with NaHSO3 upon heating to form 3,5-dihydroxybenzenesulfonic acid (C6H5NaO5S).
5. Reaction of A with hydroxylamine to yield phloroglucinol trioxime (D).
6. Reduction of D over Raney-Ni catalyst to yield 1,3,5-triaminobenzene (E).
7. Acid-catalyzed hydrolysis (boiling E in dilute HCl) to regenerate phloroglucinol (A).