HYDROXY-, KETO- AND DIBASIC ACIDS in 



Properties. 



Lactic acid is a syrupy liquid having a sp. gr. of I -248 at 1 5. 

 It is decomposed on distillation at ordinary atmospheric pressure, 

 but at a pressure of -5-1 mm. it distils at about 85 and then sets 

 to a hygroscopic crystalline solid melting at 1 8. It is soluble in 

 water, alcohol or ether, and is only very slightly volatile with steam. 



Lactic acid is the simplest compound which exhibits the pheno- 

 menon of circular polarisation. Most natural compounds exhibit this 

 phenomenon. Circular polarisation is the property of rotating a ray 

 of polarised light to either the right or the left 



According to its source lactic acid may be either dextrorotatory, or 

 laevorotatory, or -inactive. Thus lactic acid from muscle sarcolactic 

 acid is dextrorotatory. Certain bacteria produce laevorotatory 

 lactic acid. Fermentation lactic acid is inactive. 



The examination of natural substances which exhibit circular 

 polarisation has shown that they all contain one or more asymmetric 

 carbon atoms, i.e. carbon atoms combined with four different groups. 

 By representing the carbon atom as a regular tetrahedron and placing 

 the different groups at the four apices, Van't Hoff and Le Bel have 

 given us an explanation of the phenomenon. Adopting any arrange- 

 ment of the groups round the tetrahedron, a reverse arrangement 

 is represented by its image in a mirror. If therefore one variety is 

 represented by the first arrangement, the mirror image of the figure 

 represents the opposite variety. Projected on to a plane surface the 

 following formulae are then obtained : 



CH 3 CH S 



HO.C.H H.C.OH 



COOH COOH. 



One figure will represent laevo or /-lactic acid, the other figure 

 dextro or ^-lactic acid. A mixture of the two in equal quantities re- 

 presents inactive or ^/-lactic acid. This can be proved by the separa- 

 tion of inactive lactic acid into its constituent d- and /-forms by the 

 fractional crystallisation of its strychnine salt 



These optical isomers are known as stereoisomers and the pheno- 

 menon as stereoisomerism. 



Lactic acid has the chemical properties of a secondary alcohol and 

 of an acid. The most characteristic salt of lactic acid is the zinc salt. 

 This is prepared by boiling a solution of lactic acid for some time with 

 excess of zinc carbonate and filtering whilst hot. On cooling, zinc 

 lactate crystallises out, if the solution be sufficiently concentrated. 



