Edexcel Jun 2014 (R) Paper 5 Q23

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 23 Anaesthetics Substances have been used to relieve or eliminate pain during medical procedures for well over two thousand years. However, uncertainty about the nature and purity of the plant extracts being used and in standardising their application prompted Fallopius, a leading sixteenth century physician, to complain When soporifics are weak, they are useless, and when strong, they kill. Modern analytical techniques have all but eliminated these problems. Anaesthetics are divided into two broad categories: general anaesthetics, which cause a reversible loss of consciousness, and local anaesthetics, which result in the absence of pain sensation in the area where they are applied. General anaesthetics may be delivered by inhalation or injection, while local anaesthetics may be injected or applied directly to the surface. The structures of some anaesthetics are shown below. Ethoxyethane (Tb = 34.5(cid:113)C) OH Desflurane (Tb = 23.5(cid:113)C) Procaine Propofol H2N H2N Benzocaine Ethoxyethane was first synthesized in 1540 by distilling a mixture of ethanol and concentrated sulfuric acid. Although its anaesthetic properties were established at this time, it was not until the nineteenth century that it was used as an inhalational general anaesthetic in medical procedures. However, ethoxyethane is highly flammable and has unpleasant side effects, so it has been entirely replaced by compounds like desflurane in modern medical practice. Propofol is a general anaesthetic which is administered by injection. Procaine and benzocaine are local anaesthetics and are examples of aminoester anaesthetics. 22 *P42989rA02228*<br /> (a) (i) Explain why ethoxyethane has a lower boiling temperature than ethanol (Tb = 78.5(cid:113)C) even though an ethanol molecule has fewer electrons. A detailed explanation of the forces involved is not required. (3)*(ii) Desflurane is approximately 3700 times more potent as a greenhouse gas than carbon dioxide and it has been estimated that the worldwide use of inhalational anaesthetics contributes the equivalent of 1 million cars to global warming. Suggest three factors that might be considered before deciding whether this type of general anaesthetic should be banned. (3)*P42989rA02328* 23 Turn over<br /> (iii) Suggest why desflurane is much more stable than ethoxyethane and why the chemical stability of desflurane increases its potential to cause damage to the environment. (2)*(b) Procaine is often used by dentists and administered by injection. Due to the presence of the amine groups, procaine is a basic compound. Explain why the presence of an amine group makes a compound basic and hence compare the effect of each amine group on the basicity of procaine. (3)24 *P42989rA02428*<br /> (c) The aminoester, benzocaine, is a local anaesthetic used in creams and cough drops. It can be made from 4-aminobenzoic acid, the structure of which is shown below. OH H2N 4-aminobenzoic acid (i) One possible preparation of benzocaine would be from 4-aminobenzoic acid using ethanol and a sulfuric acid catalyst. Suggest a disadvantage of this method. (1)(ii) Suggest an alternative two-stage synthesis of benzocaine from 4-aminobenzoic acid. The reagents used must be identified and the structure of the intermediate compound given. (3) *P42989rA02528* 25 Turn over<br /> (d) The structure of an anaesthetic may be determined by proton nmr spectroscopy, while its purity may be established by high performance liquid chromatography. (i) Low resolution nmr spectra for phenol and for propofol are shown below. In these simplified spectra, the signal strength is proportional to the number of protons of each type, but only within each spectrum. Signal strength phenol 10 7 3 Chemical shift/ ppm propofol Signal strength 10 7 3 Chemical shift/ ppm Use the phenol spectrum and your Data Booklet to identify the protons of propofol responsible for each of the peaks (A to E) in its nmr spectrum. Label all of the protons in the diagram below. (3) H3C HC H3C CH3 CH CH3 OH 26 *P42989rA02628*<br /> (ii) Explain why high performance liquid chromatography, rather than nmr, is used to show that an anaesthetic contains no trace impurities. (2)(Total for Question 23 = 20 marks) TOTAL FOR SECTION C = 20 MARKS TOTAL FOR PAPER = 90 MARKS *P42989rA02728* 27<br />

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