12188 14 Pentane-2,4-dione (acetylacetone) is an unusual compound. It exists as a mixture of isomers. O C C H CH3 C O H3C keto form                            enol form O C C O H3C CH2 CH3 H (a) The nmr spectrum of pentane-2,4-dione at room temperature shows that it is a mixture of isomers. (i) On the following nmr axes draw the expected nmr spectrum, including an integration curve, of pentane-2,4-dione as the keto form. Use the data leaflet for your answer. 5 4 3 2 1 0 chemical shift / ppm [2] 12188 [Turn over (ii) The following is the nmr spectrum of pentane-2,4-dione as an enol at room temperature. Using the data in the data leaflet, label the peaks in the spectrum with the appropriate hydrogen atoms from the enol form. 6 5 4 3 2 1 0 chemical shift / ppm [2] (b) The infrared spectrum of pentane-2,4-dione is shown below. Explain how it suggests that pentane-2,4-dione is a mixture of isomers. 3000 2000 1000 wavenumber / cm−1 1 0.8 0.6 0.4 0.2 [2] 12188 (c) It is possible to titrate the enol isomer of pentane-2,4-dione using bromine because the C C bond reacts faster in an addition reaction than in any possible substitution reaction with bromine. (i) How would you slow down the substitution reaction before the titration was carried out? [1] (ii) How would you know that the end point had been reached? [1] (iii) In a titration, 25.0 cm3 of a 0.008 M solution of pentane-2,4-dione was reacted with a 0.002 M solution of bromine. The titration value was 4.6 cm3 of bromine solution. Use these values to calculate the percentage of pentane-2,4-dione which exists in the enol form. [4] 12188 [Turn over (d) Pentane-2,4-dione is a ligand represented by the symbol acac. It combines with many transition metal ions to form complexes. (i) How many coordinate bonds can pentane-2,4-dione form? [1] (ii) It reacts with copper(II) ions to form [Cu(acac)3]2+ which has an octahedral structure. Draw a 3D structure of this octahedral complex using dotted lines and wedges. [2] (iii) Explain why [Cu(acac)3]2+ reacts with edta. [2] (e) Pentane-2,4-dione does not form a hydrazone with phenylhydrazine. Instead, it forms a substituted pyrazole which has a melting point of 107–108 °C. Explain how this reaction can be used to identify pentane-2,4-dione. [1]