HVDROXY,- KETO- AND DIBASIC ACIDS 123 



/?-Hydroxybutyric Acid. 



/3-Hydroxybutyric acid is readily prepared by the reduction of aceto- 

 acetic acid. In this way it probably arises in the animal body. 

 /3-hydroxybutyric acid contains an asymmetric carbon atom and can 

 therefore occur in three forms dextro, laevo and inactive. The 

 laevo form is present in urine in diabetes and under the conditions 

 detailed under aceto-acetic acid. The dextro form is produced by 

 the reduction of aceto-acetic acid by yeast. The inactive form is pro- 

 duced by chemical reducing agents. 



/3-hydroxybutyric acid is usually obtained as a colourless syrup, 

 but has been prepared in colourless crystalline plates by Magnus 

 Levy; these sinter at 47-48-5 and melt at 49-50. It is very hygro- 

 scopic. 



It is easily soluble in water, alcohol, ether, ethyl acetate and 

 acetone but not in benzene and petroleum ether. 



The rotation of /-/5-hydroxy butyric acid is [a]" = - 24-12 at 

 temperatures between 17 and 22 and in concentration less than 

 12 per cent. 



Several salts have been prepared in a crystalline condition, e.g. 

 silver, calcium, zinc. 



Detection. 



/3-hydroxybutyric acid has no reactions like aceto-acetic acid. Its 

 detection is therefore indirect. 



(1) It is converted into <2-crotonic acid by loss of water either 

 by heating alone or more readily by heating with dilute sulphuric 



acid : 



CH 3 . CHOH . CH 2 . COOH = CH 3 . CH= CH . COOH + H 2 O. 



Urine is mixed with an equal volume of concentrated sulphuric acid and 

 distilled ; during the process the volume is kept constant by adding water to 

 the contents of the distilling flask through a tap funnel. Crotonic acid some- 

 times separates out from the first portions of the distillate, but usually the 

 distillate is extracted with ether. The ether is allowed to evaporate spontan- 

 eously ; the crystals of crotonic acid which separate are pressed out on a 

 porous plate and their melting-point determined. Crotonic acid melts 3171-72. 

 The melting-point must be ascertained since benzoic acid also passes into the 

 distillate. They may be purified by solution in ether and precipitation by 

 petrol ether. 



(2) By oxidation with potassium bichromate and sulphuric acid 

 or with hydrogen peroxide and ferrous sulphate it is converted into 

 aceto-acetic acid and then into acetone by decomposition of the aceto- 

 acetic acid (see under estimation). 



The detection of hydroxybutyric acid is carried out most certainly 

 by isolation of the substance by extracting it with ether in the same 

 way as it is estimated (p. 597). 



