In early 1828, the Swedish chemist J. Y. Berzelius established that amber consists of a volatile aromatic oil, two soluble fractions of a resin, succinic acid, and 90% of the insoluble residue. At that time, amber was already used in medicine, zoologists widely used its preservative properties, and botanists, by the nature of the insoluble residue, tried to determine from which species of pine it originated.
Sometime later, in the Baltic amber – succinite (from the Latin name for pine, which grew in the distant past on the territory of the modern Baltic) discovered the bi-cyclic alcohol borneol and succinic acid. Various amounts of succinic acid were contained not only in unchanged amber, but also in products made of them, which have lain in the ground for more than one millennium. Attempts to ascertain the geographical affiliation of such amber served as an important factor for its further study in the last century. It turned out that succinic acid is a characteristic feature of succinite. This feature has become widely used in the determination of amber from archaeological tombs. According to the German scientist O.Helma, the content of succinic acid in succinite ranges from 3 to 8%: the smallest is in transparent succinite (3.2-4.5%), the highest is in the weathered surface crust (8.2%). This allowed O. Helm to determine the birthplace of amber, which was discovered during archaeological excavations in Italy.
Later instrumental methods of analysis were involved in the study of amber. It turned out that amber of various ages (and from different fields) is quite well diagnosed by IR absorption spectra. In some, acidic functions predominate, in others, essential functions. For example, the infrared spectra of amber from the cretaceous sediments of Spain contain absorption bands of free acid groups, and the infrared spectra of oligocene amber have no such bands, instead they have clearly shown strong absorptions corresponding to ether groups. Absolutely no acid functions was in Baltic amber.
The method of IR-spectrometry allows you to trace all the changes that occur with amber in the process of weathering. It turns out that this decreases the number of C = 0 bonds. Such spectra, in particular, are characterized by amber, extracted from the burial grounds of thousands of years old.
When comparing the IR spectra of amber with the spectra of the resins of modern conifers, was determined the botanical identity of some amber. Amber from the lower cretaceous sediments of Lebanon formed from Araucanian resin. In the Cretaceous, these trees occupied extensive spaces in the southern hemisphere. They still give a significant amount of copal resin.
Currently, the composition and structure of amber is being studied. It’s volatile part (about 10% of its weight) has been known for a long time. These are aromatic compounds – terpenes with 10 carbon atoms and sesquiterpenes with 15 carbon atoms in a molecule. These compounds are in significant quantities in turpentine.
In early 1930s, succinicobietinol acid (C25H40O4) with two OH groups (one carboxyl) was recovered from the nonvolatile residue of amber. As shown by later mass spectrometry studies, amber contains more than 40 compounds. Many of them are still unknown.
Only abietic acid and its isomers (levopimaric, palustrin, neoabietic, dextropimaric, and isodextropimaric acids) are exctracted from amber In pure form. The first four acids form a chemically related group; they differ only in the position of the double bonds, which are easily displaced inside both rings. Abietic acid is the most stable. From it ancient fossil resins usually consist.
New data on the structure of amber were obtained using gas and thin layer chromatography. The amber contains acids: dehydroabietic, isodextropimaric, dehydroisopimaric, sandaracopimaric, diagenic and abietic. They make up a part (20-25%) of Baltic amber that soluble in organic solvents.
The rest of amber, not soluble in any of the known solvents, the German scientist I.Joon in early 1816 called succininin. Data from IR spectrometry showed that succinin contains lactone (ester) groups, i.e. it is an ester.
So, how did the conversion of abietic acid to ester happened and how did the alcohol function of amber molecule emerge? A significant role in this process was done by the influence of sunlight. If a solution of abietic acid is subjected to ultraviolet irradiation for a long time, it forms formic acid and dehydroabiethen – a compound with an extracyclic double bond. We cant also say that the absorption bands characteristic of an extra – cyclic double bond are found in the spectra of many amber samples. Under the influence of water, the double bond gives an alcohol group, which further participates in the formation of ether.
One thing should be noted, that using a mass spectrometer, the molecular weight of compounds isolated from the insoluble amber residue was set to 604. This is twice the mass of abietic acid. It means that it is not abietic acid that is involved in the formation of esters, but its dimer. This assumption is confirmed experimentally: multi – day ultraviolet irradiation of amber contributes to the formation of a compound completely insoluble in organic solvents, with a molecular weight of 604. The melting point of the crude and purified diabietic acid obtained in this way is equal to 365 and 390°C, it is coincides well with the point melting the insoluble residue of amber – succinin (365°C).
According to the above, amber consists of three groups of compounds:
- volatile terpenes and sesquiterpenes
- soluble organic acids
- insoluble polyesters of these acids with alcohols formed from the same acids
The first two groups of compounds were contained in resin (sap), which flowed from pines in the long past epochs. The third group is a product of various transformations of the original resin.
The disclosure of the secret of amber allows us to hope that in the near future it will be possible to obtain synthetic amber in much the same way as in the technique of obtaining polymers. At present, synthetic products with the structure of abietic acid are already patented (but not yet produced).