Citral - occurrence - Structural elucidation - Synthesis

1. Citral
Occurence : It occurs in the oils of lemon, oranges and citronella etc.
Structure of citral :
i) The molecular formula of citral given by analytical data and molecular weight
determination is C10
H16
O
ii) It forms tetrahydro and tetrabromo derivatives with hydrogen and bromine respectively.
Therefore it contains two double bonds.
C10
H20
O
2 2H
   C10
H16
O
2 2Br
   C10
H16
OBr4
Tetrahydro citral Citral Citral tetrabromide
iii) It forms an oxime, bisulphite adduct, semicarbazone etc. On oxidation with moist
silver oxide it forms geranic acid containing same number of carbon atom. On reduction it
gives the primary alcohol, geraniol. All these reactions confirm that citral contains an
aldehyde group.
C10
H18
O
( )
/
H
Na Hg
 
C10
H16
O ( )O
Ag O
 
2
C10
H16
O2
Geraniol Citral Geranic acid
iv) On heating with KHSO4
, citral forms p-cymene.
C10
H16
O
Citral
v) Citral on ozonolysis gives acetone, laevulinic aldehyde and glyoxal.

2. vi) When boiled with con. aqueous alkali, citral undergoes dealdolization and gives
acetaldehyde and 6-methyl, hept-5-en-2-one.
vii) Ozonolysis and dealdolization studies suggest the following structure for citral.
viii) Synthesis of citral : Finally the above structure of citral is confirmed by the
following synthesis.

3. Cis-Trans isomerism exhibited by citral
Citral exists in two geometrical isomeric forms as cis-citral (neral; citral-b) and trans-
citral (geranial; citral-a).
Evidence to prove that neral is a cis isomer and geranial is a trans isomer
Usually citral contains a mixture of cis-citral and trans-citral. Therefore this mixture on
reduction gives two isomeric alcohols.
On cyclization both give -terpeniol. But the rate of formation of -terpeniol from
nerol is nine times greater than the rate of its formation from geraniol. This is due to the
proximity of the alcoholic group to the carbon which is involved in cyclization. Thus the
corresponding aldehyde neral is a cis-isomer and geranial is a trans-isomer.