2.6.5.2 Alcohol Volatile Oils
A good number of alcohols occur abundantly in a plethora of volatile oils, which may be judiciously classified into the following heads, namely:
(a) Acyclic (aliphatic) alcohols,
(b) Monocyclic (aromatic) alcohols,
(c) Alicyclic (terpene and sesquiterpene) alcohols.
These three distinct categories of ‘alcohol
volatile oils’ shall be discussed briefly along with certain typical
examples from the plant kingdom.
2.6.5.2.1 Acyclic (Aliphatic) Alcohols In
general, a number of acyclic alcohols, such as: methyl, ethyl, isobutyl,
isoamyl, hexyl and other higher alcohols occur widely in volatile oils, but
being water soluble they are usually eliminated during steam distillation.
They may be further sub divided into two important
categories, namely:
(a) Saturated aliphatic alcohols, and
(b) Unsaturated aliphatic alcohols.
which shall be discussed along with suitable examples.
2.6.5.2.1A Saturated Aliphatic Alcohols Volatile
oils normally contain a few saturated
monohydroxy alcohols belonging to the paraffin series,
most of which are found to be esterified with fatty acids. In the course of
steam distillation these esters undergo hydrolysis to yield the lower members
of saturated aliphatic alcohols together with the lower fatty acids rarely.
A variety of substances are duly formed on account of the
degradation of complex plant
constituents e.g., methanol, ethanol (a by product
of fermentation due to plant starches), furfural and butanedione (diacetyl),
which ultimately are located in the distillation waters of volatile oils.
Isolation of aliphatic alcohols may be accomplished from
the volatile oils by fractional distillation, by forming their respective
derivatives e.g., para-hydroxybenzoates, acid phthalates and
calcium chlorides.
The saturated aliphatic alcohols may be identified by the
preparation of their respective cyrstalline derivatives, such as: para-nitrobenzoates,
3,5 dinitrobenzoates, phenylurethanes, and napthylurethanes.
The presence of ethanol as an ‘adultrant’ in
volatile oil may be carried out by treating it with iodine, potassium iodide,
sodium hydroxide solution (0.5N) and heating the resulting mixture to give rise
to the yellow crystals of iodoform (mp 119oC).
2.6.5.2.1B Unsaturated Aliphatic Alcohols The
unsaturated aliphatic alcohols frequently occurring in volatile oils are
nothing but terpene-derivatives wherein the six membered carbon ring is
found to be broken at one point only. A few important typical members of this
category are as follows:
Examples
1. Geraniol
Chemical Structure 3,7-Dimetyl-2,6-octadien-8-ol;
Lemonol.
Occurrence It is an olefinic terpine alcohol which
constitute the major part of oil of rose, oil of palmarose (95%), oil
of geranium (40-50%), oil of citonella (30-40%) and also in the
essential oil of lemon grass etc.
Isolation
Method 1: Geraniol may be readily isolated
in its pure form from volatile oil fractions by virtue of the fact that it
readily forms a distinct crystalline derivative with anhydrous calcium chloride
[2C10H18O CaCl2]. The
resulting compound is practically insoluble in organic solvents, such as: chloroform,
ether, petroleum ether or benzene and hence can be readily decomposed with pure
distilled water into geraniol and calcium chloride. The separated oil thus
obtained is rapidly washed with luke-warm water and subjected to steam
distillation finally.
Method 2: It may also be isolated and purified, of
course, much less conveniently, by forming its solid acid phthalate (mp 47oC)
which yields a crystalline silver salt.
Characteristic Features It is an oily liquid having
a marked and pronounced agreable rose-like odour. However, the odour of its geometrical
isomeride ‘Nerol’ is definitely found to be more refreshing than
that of geraniol.
Its physical characteristics are as under:
bp757 229-230oC; d204
0.8894; n20D 1.4766; UV max: 190-195 nm (ε 18000).
It is practically insoluble in water, but soluble in
ether, ethanol. The characteristic features of its corresponding acetate,
butyrate and formate analogues are stated below:
1. It is characterized conveniently by preparing its
specific derivatives, for instance: 3-nitrophthalate (mp 109°C) diphenyl
urethane (mp 82.2°C), and α-naphthylurehane (mp 47-48°C).
2. When treated with 5% sulphuric acid geraniol gives
rise to mainly terpin hydrate as given below:
3. The interaction of geraniol with phosphoric acid
and gaseous hydrogen chloride yields diterpene together with other terpenes as
depicted below:
4. In the presence of mineral acids geraniol undergoes
cyclization to give rise to α-terpineol as given under:
Uses
1. It finds its wide application in a plethora of
formulations used as rose scents.
2. It is also employed as insect attractant.
3. It is employed extensively in perfumery e.g.; butyrate
for compounding artificial attar of rose; formate as an important
constituent of artificial neroli oil and of artificial orange blossom
oil.
4. It is used in soap, cosmetic and flavour industries.
2. Linalool
Chemical
Structure 3,7-Dimethyl-1,6-octadien –3-ol; (CH3)2C=CHCH3CH2C-(CH3)(OH)CH=CH2
or
Occurrence It is the major constituent of linaloe
oil. It also occurs in a variety of essential oils, namely: Ceylon Cinnamon
(Cinnamonum verum), Artemisia balchanorum, Acorus calamus, Aloysia triphylla,
Artemisia dracunculus, Camellia sinensis, Cananga odorta, Glechoma hederaceae,
Humulus lupulus, Lantana camara, Laurus nobilis, Lavanchula angustifolia,
Myrica, Myristica fragrans, Narcissus tazetta, O. imum basilicum, Peunus
boldus, Piper nigrum, Prunus armeniaca, Robinia pseudoacacia, Rosmarinus
officinalis, Salvia, Satureja, Syzygium aromaticum, Thymus and Tilla
europaea.
Isolation It is conveniently isolated from the
saponified volatile oil by subjecting it to careful fractional distillation.
Characteristic Features The various typical
examples whereby linalool reacts with organic acids, anhydrides and
inorganic acids are given below:
(a) Organic Acids: It is very sensitive to
organic acids and gets rapidly isomerized to geraniol. Hence, its esters
cannot be obtained in the purest form by ordinary methods.
(b) Inorganic Acids
(i) With Chromic Acid: It undergloes
oxidation to yield Citral.
(ii) With Formic Acid or Conc. Sulphuric Acid: It
undergoes dehydration to yield α-terpinene and dipentene.
(iii) With 5% (w/w) Sulphuric Acid Solution
(iv) With Glacial Acetic Acid and Acetic
Anhydride: Linalool on being heated with glacial acidic acid and acetic
anhydride gives rise to a mixture of esters of geraniol, α-terpineol
and nerol as follows:
It has the following physical characteristic features,
namely:
dl-form: bp720 194-197°C; d15
0.865;
d-form: (Coriandrol): bp760
198-200°C; d204 0.8733; n20D
1.4673; [α]20D + 19.3°;
l-form: (Licareol): colourless
liquid; bp760 198°C; d20 0.8622; n22D 1.4604;
[α]20D -20.1°.
Identification
1. Phenylurethane derivative: mp 65-66°C
2. α-Naphthylurethane derivative: mp 53°C
3. On oxidation with chromic acid mixture it gives rise to
citral that may be further ascertained by forming its semicarbazone
derivative mp 171°C.
Uses
1. It is used extensively in perfumery instead of bergamot
or French lavender oil because it has an odour quite similar to these
essential oils.
2. Its esters, specially the linalyl acetate finds
its abundant usage in perfume, cosmetic, soap and flavour industries.
3. Nerol
Chemical Structure cis-2,6-Dimethyl-2,6-actadien-8-ol; It is the cis-isomer of geraniol
Occurrence Nerol is found in a number of essential oils, specifically oil of Neroli (usually obtained from the fresh and tender flower of orange), oil petit grain (normally prepared from not fully matured fruits of bitter orange) and also in oil of bergamot (conventionally prepared from Citrus auranti var. bergamia).
Isolation Nerol may be isolated from admixture with geraniol in volatile oils by treatment with anhydrous calcium chloride, when the former that does not form any complex with CaCl2 is separated conveniently by either centrifugation or filtration techniques.
Characteristic Features It is an oily liquid having the odour of sweet rose. It is optically inactive. It has the physical parameters as : bp745 224-225°C; d15 0.8813; UV max: 189-194 nm (ε 18000). It is soluble in absolute alcohol.
Identification
1. It is characterized by the preparation of its tetrabromide derivative C10H18Br4O (mp 116-118°C).
2. It also gives rise to the diphenylurethane analogue (mp 52-53°C).
3. It forms needles of allophanate (C12H20N2O3) (mp 84-86°C) from petroleum ether (40-60oC).
Uses It is used extensively as a base for the manufacture of perfumes.
1 Comment:
one of your structures of geraniol (the conversion of linalool) seems to have gained an extra carbon ... the structure is incorrect
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