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Chapter 3 Suggested solutions for Chapter 3 Suggested solution two of the Cl atoms are next to each other ('ortho') there are two possible places for the third the 1,2,3- and 1,2,4-isomers. If the first two are 1,3-related ('meta') there are again two structures but we have just used one of them (the 1,2,4-isomer). If the first two are placed 1,4 there is only one place for the third and we have already found that (the 1,2,4-isomer). There are three isomers in all. CI CI CI CI CI CI CI CI CI 1,2,3-trichlorobenzene 1,2,4-trichlorobenzene 1,3,5-trichlorobenzene The ¹³C NMR spectrum will be different for each simply on account of the number of different carbon atoms. The 1,3,5-isomer is most symmetrical, having just two different signals (135.5 and 127.2 p.p.m.). The 1,2,3-isomer has four different signals (134.3, 131.6, 128.7, and 127.5 p.p.m.) but the 1,2,4-isomer has no symmetry and has six signals (133.4, 133.0, 131.1, 131.0, 130.2, and 127.9 p.p.m.). All these signals are in the 100-150 p.p.m. region. Problem 10 How many signals would you expect in the NMR of the following compounds? oH N N N OH A B C D E Purpose of the problem To get you thinking about symmetry. Suggested solution Compound A has tetrahedral symmetry and there are only two types of C atom: all the CHs are the same (a) and all the CH₂s are the same (b). This is the famous compound adamantane - a crystalline solid in spite of its being a hydrocarbon with only ten carbon atoms. If you do not see the symmetry, make a molecular model - it is a beautiful structure. Compound B is symmetrical too: the two C=O groups are the same (a) and so are the other atoms in the ring (b). This compound is called quinone and is an orange solid. Compound C is naphthalene (moth balls) and has high symmetry as shown by the labelling a, b, and Compound D is 'EDTA' (EthyleneDiamineTetracetic Acid - an important metal chelator). This time there are three types of carbon atom - two of type a and four each of b and C. Compound E is 'triethanolamine' used a lot by biochemists. It is also symmetrical with only two types of carbon atom. a H a b b b b OH 0 N b b b a b a b C b a N a b a N a a a C C b b 0 a b a b b b b HO2C OH C A B C D E