Aromatic Ketones (Carvone, l-Menthone)

2.6.5.4.2 Aromatic Ketones

There are also termed as ‘cyclic terpene ketones’. Generally, the aromatic ketones are classified into two categories, namely:

(i) Monocyclic terpene ketones, and

(ii) Bicyclic terpene ketones.

These two distinct categories shall be discussed separately in the sections that follows:

A. Monocyclic Terpene Ketones A few typical examples of this specific class of ketones are:

l-menthone; carvone.

A.1 l-Menthone

Chemical Structure (2S-trans)-5-Methyl-2-(1-methylethyl) cyclohexanone. As it has two asymmetric carbon atoms (i.e., chiral centres) it can exist in two pairs of enantiomorphs or four optically active isomers (d-; l-; dl-menthone and isomenthone).

Occurrence It is found in a variety of volatile oils, such as: pennyroyal (Mentha pulegium, Family-Lamiaceae); peppermint (Mentha piperita Linn., Family: Labiateae); geranium (Geranium maculatum L., Family: Geraniaceae); and buchu (Barsoma betulina (Berg.) Bartl. and Wendel, Family: Rutaceae).

Isolation l-Menthone usually occurs in association with isomenthone. The former gets solidified at – 6°C whereas the latter at -35°C. In normal practice, the peppermint oil, which contains upto 30% of menthone, is subjected to its oxime or semicarbazone formation and subsequently the l-menthone is regenerated by the aid of dilute sulphuric acid.

Characteristic Features It is a bitter liquid having peppermint-like odour. It is slightly soluble in water, whereas freely soluble in organic solvents. It has the following physical characteristics: bp207°C; mp –6°C; d₄²⁰ 0.895; n²⁰_D 1.4505; [α]²⁰_D -24.8°.

Identification The isomeric forms of menthones may be characterized by the preparation of specific derivatives, for instance: oximes, semicarbazones etc.

Uses

1. It is used extensively in perfume and flavour compositions.

2. It is also employed in the preparation of artificial essential oils.

A.2 Carvone

Chemical Structure 2-Methyl-5-(1-methylethenyl)-2-cyclohexene-1-one.

Occurrence It occurs in the mandarin peel oil (Citrus reticulata Blanco., Family: Rutaceae); spearmint oil upto 70% (Mentha spicata or Mentha cardiaca, Family: Lamiaceae); gingergrass oil (Zingiber officinale Roscoe, Family: Zingiberaceae); oil of caraway upto 50-60% (Carum carvi Linn, Family: Umbelliferae).

Isolation It may be isolated by the following two methods:

Method 1: Formation of Sodium Sulphite Adduct. Carvone may be conveniently isolated from essential oils (e.g., spearmint oil, oil of caraway) by virtue of the fact that it (a lactone) readily forms the water soluble salt of a hydrosulphonic acid (C₁₀H₁₆O₇S₂ Na₂) on treating with a neutral solution of sodium sulphite, whereby the corresponding addition takes place at the both ethylenic linkages.

In order to achieve this, the fraction collected between the boiling range 220-235°C in the case of oil of caraway, is shaken with the requisite quantity of a concentrated aqueous solution of sodium sulphite and the sodium hydroxide thus liberated during the course of reaction is neutralized from time to time with a dilute mineral acid (e.g., HCl) very carefully. As soon as the above process is completed fully, the resulting fractions which have not involved in the above cited reaction may be eliminated by extracting the solution with ether successively (at least three times). At the end, the desired product carvone can be regenerated by the action of sodium hydroxide and finally distilled off with steam.

Method 2: Formation of Hydrogen Sulphide Adduct. Alternatively, carvone may be separated from the volatile oils by the formation of its hydrogen sulphide adduct [(C₁₀H₁₄ O)₂.H₂S]. It is easily accomplished by the passage of a current of hydrogen sulphide (H₂S) gas into an ammoniated alcoholic solution of carvone. Ultimately, the pure ketone i.e., carvone may be regenerated from the corresponding separated adduct by careful digestion with alkali.

Characteristic Feature It is a colourless liquid having a distinct odour typical of caraway seed. The various physical parameters of d-, l- and dl-forms are given below:

It is miscible with ethanol but practically insoluble in water. It congeals at very low temperature.

Identification The different tests for the identification of carvone are stated below, namely;

(a) Bromination of carvone gives rise to a mixture of crystalline derivatives* having distinct melting points:

d- and l-form : mp 120 and 120-122°C

dl-form : mp 112-114°C

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* These are probably the dibromo derivatives because the tetrabromoderivatives are liquids.

(b) Mono-hydrochloride salt is formed when it is treated with HCl in acetic acid.

(c) Hydrobromide salt is obtained by treating d-corvone with HBr (mp 32°C).

(d) Isomerization of Carvone to Carvacrol. It undergoes isomerization to form carvacrol with a number of dehydrating agents, such as: H₂SO₄; H₃PO₄; NaOH; ZnCl₂.

(e) It is also characterized by the preparation of several compounds, for instance: oxime, semicarbazone, H2S-derivative, phenylhydrazone.

Uses

1. It is used as oil of caraway.

2. It is also used for flavouring liqueurs.

3. It is used extensively in perfumery and soaps.

4. It is employed for flavouring many types of food products and beverages.

5. It finds its enormous applications in oral hygiene preparations e.g., toothpastes, gargles, mouthwashes.

6. It is also used in flavouring pharmaceuticals.

B. Bicyclic Terpene Ketones These class of compounds essentially contain two cyclic ring structures fused with each other along with a ketonic function. The two typical examples of pure

chemical entities that belong to this group are camphor and d-fenchone, which have been duly dealt with earlier in this chapter under ‘monoterpenoids’ and ‘bicyclic monoterpenes’ respectively.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar