How easy is to produce essential oils chemically as compared being extracted from natural?

b.how effective the chemically produced ones for each
of them compared to the ones extracted naturally.Are
there any harmful effects using the chemically
produced ones?
c. in the market ,if one does not know the origin of a
particular oil, how likely are each of them to be
produced chemically as compared it to be the
naturally?
d. apart from laboratory tests, are there any other
way of distinguishing between the two?
e.when the lid is taken off a bottle of the oils , the
oil evaporates to fill the room with its aroma. But
after sometime, no more oil evaporates unless one
stirs it. Why is it so?

a) Most essential oils are mixtures. It is almost impossible to reproduce the bouquet of the natural oil. It is uneconomical to attempt the synthetic route unless the demand is very large.

b) There may be no harmful effects. It is difficult to match the natural oil exactly,

c) Generally, the same manufacturer does not produce both. Buying from a reputed source is best.

d) Your own senses.

e) The oils have low boiling aromatic fractions. These molecules have to diffuse to the surface of the oil before evaporating. Usually, those molecules nearest to the surface evaporate easily. Subsequently, the high viscosity of the rest of the oil reduces the availability of the molecules near the surface and evaporation decreases. By stirring, the volatile molecules are brought to the surface.