140 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



If only one of each of the amino or acid groups be present in 

 the molecule the substance is neutral, the basic group neutralising the 

 acid group, but when either two amino groups or two acid groups with 

 only one of the other groups are present, the substance is basic or acidic 

 in character. 



The monoaminomonocarboxylic acids, which have a neutral reac- 

 tion, possess a sweet taste, which is different for the different substances, 

 and serves (with experienced workers) as a useful guide for defining 

 the particular amino acid present in a solution. * 



Except glycine, the amino acids are optically active; some are 

 dextrorotatory, others laevorotatory. An inactive mixture is formed 

 by synthesis. In all cases it has been separated into its stereoisomers. 



Reactions. 



The reactions of glycine are typical of the reactions of all the 

 amino acids, but glycine does not give certain reactions which are 

 characteristic of cystine (p. 143), tyrosine(p. 267), tryptophan (p. 348), 

 the amino acids which contain thio or aromatic groups in their molecules. 



Reactions of Glycine. 



A. As Acid. 



(1) Glycine forms salt with bases : 



CH 2 .NH 2 NH 2 NH 2 



COONa CH 2 . COO Cu OOC . CH 2 



The copper salt is the most characteristic salt of amino acids ; it has 

 a somewhat deep blue colour and generally crystallises well, so that it 

 serves for the isolation of amino acids and for their identification by a 

 determination of the copper content. 



On boiling a solution of glycine with excess of copper hydrate 

 or copper carbonate, filtering off the excess and evaporating the solu- 

 tion until it crystallises and then allowing to cool, the blue copper salt 

 of glycine crystallises out. 



The depth of colour of the copper salt of glycine can be seen by 

 adding a few drops of copper sulphate solution to a solution of glycine. 

 The shade of colour is different to that of copper sulphate and in 

 comparison deeper. 



(2) Glycine forms esters with alcohols : 



CH 2 .NH 2 CH .NH 2 



| +H 2 0. 



COOH + C 2 H 6 OH = COOC 2 H 6 



These esters are prepared by passing dry hydrogen chloride into a 

 suspension of the amino acid in absolute alcohol. The amino acid is 

 converted into its hydrochloride, dissolves and is esterified : - 



