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The Chemistry of Essential Oils
Essential oils, which are widely used for aromatherapy, are a complex mixture of many
different chemical components. Spearmint oil is extracted by steam distillation of the
flower tops. It contains both limonene and carvone.
Spearmint
Limonene
Carvone
Identify the functional group present in carvone and describe a chemical test which could
be used to differentiate between the two components of spearmint oil.
(3)
Carvone contains a chiral carbon atom. Indicate this (*) on the compound, and then, with
reference to this – or another named compound explain what a chiral compound is.
(3)
(+)carvone, which is isolated from caraway seed oil and (–)carvone, the component found
in spearmint oil, are optical isomers. If synthesized in the laboratory, the carvone
produced would be a racemic mixture. Explain what this is and why it happens. (3)
Carvone can be reduced. Describe how, showing the mechanism for the reaction. (6)
Answers
Carvone is a ketone. (1)
Test with 2,4,DNPH (Brady’s reagent) (1) – a yellow/orange precipitate indicates the
presence of the carbonyl group found only in Carvone.(1)
Identifies the chiral carbon as carbon atom number 4 (1), if counting carbon
atom number 1 as that attached to the methyl, C2 as the carbonyl carbon and so on. This
is because you trace the ring back, in either direction from C4 and there are different
groups each way. C4 is also attached to C attached to C=C and a methyl group, and to a
hydrogen atom – hence satisfying the requirement for being attached to four different
groups.
Accept any other suitable alternative.
A chiral compound is one in which a carbon atom is attached to four different groups (1)
– each of which should be evident from the diagram or in the explanation (1).
Laboratory synthesized carvone would be a racemic mixture because there would be
equal quantities of the (+) and (– )isomers or enantiomers (1). However, when an
optically active compound is synthesised naturally – for example in a plant such as
spearmint – only one of the enantiomers is formed. (1) This is because biological
systems involve enzymes which are stereospecific (1).
Carvone is a ketone which can be reduced by a reducing agent such as Sodium
tetrahydridoborate (NaBH4) (1)
Mechanism should show:
delta + carbonyl carbon and delta - =O (1)
Hydride ion showing lone pair of electrons, with negative charge – curly arrow from lone
pair to delta + carbon (1)
Curly arrow from centre of =O bond to O (1)
Intermediate showing lone pair on O (attached to C – previously the carbonyl carbon) and
negative charge on O, with H attached to C – previously the carbonyl carbon. (1)
Final compound – where carbonyl group of carvone is replaced by OH group. (1)