u6 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Properties. 



Tartaric acid crystallises in large hemihedral monoclinic prisms 

 which are colourless and transparent. It melts at 167-170 and is easily 

 soluble in water and alcohol, but insoluble in ether. 



Tartaric acid is optically active ; the ordinary tartaric acid is dextro- 

 rotatory having [a] D = 13-1 for a 15 per cent, solution and = 147 

 for a 2 per cent, solution. A laevo-tartaric acid and two inactive forms 

 of tartaric acid also exist. Tartaric acid contains two asymmetric 

 carbon atoms in its molecule and to these carbon atoms the same groups 

 are attached. If the arrangement of these groups round each carbon 

 atom is the same, an optically active form will result ; but if different an 

 inactive form will result ; in the former case both asymmetric carbon 

 atoms are rotating to the right or to the left ; in the latter case the 

 one carbon atom rotates in one direction as much as the other carbon 

 atom rotates in the other direction and they neutralise one another. 

 Internal compensation occurs. 



This inactive acid is known as meso-tartaric acid and is produced 

 by prolonged heating of dextrorotatory tartaric acid to 165 with a 

 small quantity of water. 



The other inactive form is a mixture of the dextro and laevo forms 

 n equal proportions. As a mixture it can be separated into its two 

 constituents. It occurs with ^/-tartaric in crude tartars. 



Reactions. 



(1) On heating, tartaric acid melts and chars giving off an odour 

 resembling that of burnt sugar. 



(2) Tartaric acid chars almost immediately when it is heated with 

 concentrated sulphuric acid. 



(3) A white precipitate of silver tartrate is formed on adding 

 silver nitrate to a neutral solution of a tartrate. The precipitate 

 dissolves in ammonia and when this solution is slowly warmed a 

 silver mirror is formed on the sides of the vessel. 



(4) On adding calcium chloride to a cold solution of a neutral tartrate 

 (sodium potassium tartrate) a white precipitate of calcium tartrate is 

 formed. This precipitate, after filtering and washing, is soluble in 

 acetic acid and caustic soda (free from carbonate) ; on boiling the 

 solution in the latter, it is reprecipitated (distinction from calcium 

 oxalate). 



(5) A precipitate of potassium hydrogen tartrate is formed on 

 adding potassium chloride and acetic acid to a not too dilute 

 solution of a tartrate. 



This reaction is used in estimating tartaric acid, 



