Question

A student neglected to add sodium carbonate when extracting the tea leaves with hot water. Yet a very high yield of caffeine was obtained. The crystals melted at 202 - 214 degrees Celsius. Explain.

Answer 1

1)Tea leaves contains tannin , which are acidic , to ensure that this acidic compound remain soluble , and that of caffeine will be present in the free base , sodium carbonate is to be added to the extraction mixture.

2) the high yeild of student is due to the extraction of both caffeine and tannin .

3) The base converts the tannins into their sodium salts - being ionic these salts are not soluble in solvents like methylene chloride so remain in the aqueous layer during extraction. This allows purer caffeine to be extracted.

4) the melting of pure caffine if around 238 ⁰C . As above   the crystals melted at 202 - 214 degrees Celsius also gives an indication that the caffine is not pure and contain impurities ( tannin) .