**1. Introduction**

Lemon grass belongs to Cymbopogon, a genus of about 55 species of grasses, native to temperate and tropical regions. It is a lofty perennial grass. Common names of Cymbopogon include lemon grass, silky heads, citronella fever grass and barbed wire grass amongst many others. Essential oil called as citral or 3,7-dimethyl-2,6-octadienal is present in leaves and twigs of lemon grass which can be extracted easily by hydrodistillation. The essential oil of lemon grass has many important chemical constituents, which are helpful for many applications. It has cis and trans citral, myrcene, geranial, etc. Citral after distillation can be used for the synthesis of ionones, vitamin A, different types of citral acetals, these acetals has a wide range of applications in perfumery and helpful to reduce antibacterial activities.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1.1. Lemon grass oil**

Lemon grass is a tropical herb of 3–6 feet length, leaves and twigs of this grass have essential oil, which has insect repellent activity. Leaves of this grass dried and stored for making tea, helps to cure many problems of stomach and anesthetic problems. Lemon grass leaves from local garden of PCSIR Laboratories were collected, dried under shade to deactivate starch, and cut into small pieces of 1–2 inches. Essential oil having lemon-like aroma was extracted by steam distillation, which can be used as scent and flavoring agents in medicine. It can help in fever reduction, helpful to improve digestion, reduce diarrhea, and stomachaches. As diluted oil, it is used to ease pain and arthritis, sterile stimulating, antispasmodic, and pain reliever. Lemon grass plant is shown in **Figure 1**.

Odor: lemon like

Density: 0.893 g/cm3 Boiling point: 229°C

soap and cosmetics.

**1.4. Sources of citral**

Refractive index: 1.484–1.490

The Z-isomer is known as neral or citral B.

**1.3. The stereochemistry of geometrical isomers of citral**

There are two major sources of citral, which are explained in below.

A and Z-isomer (neral) is cis and known as citral B.

Lemon grass essential oil (citral, 3,7-dimethyl-2,6-octadienal) or lemonal belongs to monoterpenoids having a formula C10H16O. It is a diastereoisomer of E-isomer (geranial or citral A).

Lemon Grass (*Cymbopogon citratus*) http://dx.doi.org/10.5772/intechopen.69518 123

Commercial citral, is as mixture of two isomers due to cis-trans isomerism at the C=C bond nearly the aldehyde group obtained from oils of plant sources. The geranial has strong lemonlike odor while neral has less. Citral can be used in cosmetic, medicine and food industries. It also has strong antimicrobial activities [1] and pheromonal effects in insects [2]. Citral is a basic intermediate for the synthesis of flavoring and fragrance components such as ionones, methyl ionones, and vitamins A [3] and E [4]. While the essential oil from lemon grass contains 70–75% of citral in addition to myrcene, geraniol, and nerol, is used in a cheap type of

Ciral has two isomers, cis and trans. E-isomer (geranial) is trans and also known as citral

## **1.2. Physical characteristics of citral**

Name: 3,7-dimethyl-2,6-octadienal Molar mass: 152.24 g/mol Appearance: pale yellow liquid

**Figure 1.** Lemon grass plant.

Odor: lemon like Density: 0.893 g/cm3 Boiling point: 229°C Refractive index: 1.484–1.490

**1.1. Lemon grass oil**

122 Grasses - Benefits, Diversities and Functional Roles

**1.2. Physical characteristics of citral**

Name: 3,7-dimethyl-2,6-octadienal

Appearance: pale yellow liquid

Molar mass: 152.24 g/mol

**Figure 1.** Lemon grass plant.

Lemon grass is a tropical herb of 3–6 feet length, leaves and twigs of this grass have essential oil, which has insect repellent activity. Leaves of this grass dried and stored for making tea, helps to cure many problems of stomach and anesthetic problems. Lemon grass leaves from local garden of PCSIR Laboratories were collected, dried under shade to deactivate starch, and cut into small pieces of 1–2 inches. Essential oil having lemon-like aroma was extracted by steam distillation, which can be used as scent and flavoring agents in medicine. It can help in fever reduction, helpful to improve digestion, reduce diarrhea, and stomachaches. As diluted oil, it is used to ease pain and arthritis, sterile stimulating, antispasmodic, and pain reliever. Lemon grass plant is shown in **Figure 1**.

Lemon grass essential oil (citral, 3,7-dimethyl-2,6-octadienal) or lemonal belongs to monoterpenoids having a formula C10H16O. It is a diastereoisomer of E-isomer (geranial or citral A). The Z-isomer is known as neral or citral B.

Commercial citral, is as mixture of two isomers due to cis-trans isomerism at the C=C bond nearly the aldehyde group obtained from oils of plant sources. The geranial has strong lemonlike odor while neral has less. Citral can be used in cosmetic, medicine and food industries. It also has strong antimicrobial activities [1] and pheromonal effects in insects [2]. Citral is a basic intermediate for the synthesis of flavoring and fragrance components such as ionones, methyl ionones, and vitamins A [3] and E [4]. While the essential oil from lemon grass contains 70–75% of citral in addition to myrcene, geraniol, and nerol, is used in a cheap type of soap and cosmetics.

#### **1.3. The stereochemistry of geometrical isomers of citral**

Ciral has two isomers, cis and trans. E-isomer (geranial) is trans and also known as citral A and Z-isomer (neral) is cis and known as citral B.

#### **1.4. Sources of citral**

There are two major sources of citral, which are explained in below.

#### *1.4.1. Essential oils rich in citral*

Percentage of oil (citral) varies according to plant species, about 90–98% oil is present in lemon myrtle, 70–80% in litsea cubeba, 65–85% in lemon grass, 30–35% in lemon verbena, 26% in ironbark lemon, 11% in lemon balm, 6–9% in lime, and about 2–5% in lemon and oranges [5].

#### *1.4.2. Important plant sources of citral*

Lemon myrtle contains chemotypes of two essential oils.

Lemon myrtle oil has typically 90–98% citral and oil yield 1–3% from fresh leaf. It is the highest natural source of citral.

The citronellal chemotype is uncommon and can be used as an insect repellant [6].

#### *1.4.3. Litsea cubeba*

Litsea cubeba fruits yields 3–5% essential oil. The 70–83% oil is obtained by oil's primary isolation. In China, oil production estimated between 500 and 1500 tonnes of oil per annum [7].

#### *1.4.3.1. Lemon grass (Cymbopogon flexuosus)*

Lemon grass (*Cymbopogon flexuosus*) [8], also called Cochin grass or Malabar grass, is native to India, Sri Lanka, Cambodia, Thailand, and Burma. *Cymbopogon citratus*, also known as serai in Malay, is assumed to have origin in Malaysia. While both can be used interchangeably, *C. citratus* is more used in cooking. In India, *C. citratus* is used both as a medical herb and perfumes.

**1.5. Total synthesis of citral**

**Scheme 1.** Synthesis of citral from pinene.

**1.6. Synthetic applications of citral**

*1.6.1. Synthesis of ionones and methyl ionones from citral*

**Scheme 2**.

BASF opened new continuous production of citral from 2-methyl- 4-hydoxy-but-1-ene in 2004 [10]. The annual production of citral is 40,000 tons per annum, which is shown in

Lemon Grass (*Cymbopogon citratus*) http://dx.doi.org/10.5772/intechopen.69518 125

The ionones belong to rose ketones having closely related series of chemical substances including damascones. A variety of essential oils contain these aroma compounds, the ionones, for example, β-ionone has rose aroma to some extent and it is used as a raw material for the production of retinol. Methyl-ionones are not found in the essential oils, but these are

synthesized by aldol condensation of citral with methyl ethyl ketone (MEK).

#### *1.4.4. Chemical synthesis of citral from pinene*

Citral is also commercially produced from pinene [9]. The starting material is β-pinene (1) which is separated from turpentine oil by efficient fractional distillation of turpentine oil under vacuum. The β-pinene obtained in this way is passed through a heated tube with a short contact time at a temperature of about 600°C to furnish myrcene (2). The pyrolysis is thermal reaction and no catalyst is required.

Fortunately, the yield of myrcene under favorable condition can be as high as 90%. Myrcene is converted to the desired terpene mixture of cis and trans-alcohols, that is, geraniol and nerol by hydration of the double bond. This is not as easy as it would appear, to obtain a useful yield it is necessary to proceed via the hydrochloride, resulting a product, containing as much as 80% of linalyl chloride (3). The dry hydrochloric is passed through myrcene below −10°C in the presence of cuprous chloride. Linalyl chloride obtained in this way isomerizes into two isomers: geranyl chloride (4) and neryl chloride (5).

Either of the allylic chlorides can be converted to linayl or geranyl and neryl acetate by reaction with sodium acetate under suitable conditions. Thus if the sodium acetate is reacted in the presence of cuprous copper, the major product is linalyl acetate; whereas in the complete absence of copper, geranyl acetate (5) and neryl acetates (6) are predominate. The mixed terpene acetates undergo saponification to produce geraniol and nerol. These terpene alcohols are selectively oxidized to citral (7). As shown in **Scheme 1**.

**Scheme 1.** Synthesis of citral from pinene.

#### **1.5. Total synthesis of citral**

*1.4.1. Essential oils rich in citral*

124 Grasses - Benefits, Diversities and Functional Roles

*1.4.2. Important plant sources of citral*

*1.4.3.1. Lemon grass (Cymbopogon flexuosus)*

*1.4.4. Chemical synthesis of citral from pinene*

thermal reaction and no catalyst is required.

are selectively oxidized to citral (7). As shown in **Scheme 1**.

est natural source of citral.

*1.4.3. Litsea cubeba*

Lemon myrtle contains chemotypes of two essential oils.

Percentage of oil (citral) varies according to plant species, about 90–98% oil is present in lemon myrtle, 70–80% in litsea cubeba, 65–85% in lemon grass, 30–35% in lemon verbena, 26% in ironbark lemon, 11% in lemon balm, 6–9% in lime, and about 2–5% in lemon and oranges [5].

Lemon myrtle oil has typically 90–98% citral and oil yield 1–3% from fresh leaf. It is the high-

Litsea cubeba fruits yields 3–5% essential oil. The 70–83% oil is obtained by oil's primary isolation. In China, oil production estimated between 500 and 1500 tonnes of oil per annum [7].

Lemon grass (*Cymbopogon flexuosus*) [8], also called Cochin grass or Malabar grass, is native to India, Sri Lanka, Cambodia, Thailand, and Burma. *Cymbopogon citratus*, also known as serai in Malay, is assumed to have origin in Malaysia. While both can be used interchangeably, *C. citratus* is more used in cooking. In India, *C. citratus* is used both as a medical herb and perfumes.

Citral is also commercially produced from pinene [9]. The starting material is β-pinene (1) which is separated from turpentine oil by efficient fractional distillation of turpentine oil under vacuum. The β-pinene obtained in this way is passed through a heated tube with a short contact time at a temperature of about 600°C to furnish myrcene (2). The pyrolysis is

Fortunately, the yield of myrcene under favorable condition can be as high as 90%. Myrcene is converted to the desired terpene mixture of cis and trans-alcohols, that is, geraniol and nerol by hydration of the double bond. This is not as easy as it would appear, to obtain a useful yield it is necessary to proceed via the hydrochloride, resulting a product, containing as much as 80% of linalyl chloride (3). The dry hydrochloric is passed through myrcene below −10°C in the presence of cuprous chloride. Linalyl chloride obtained in

Either of the allylic chlorides can be converted to linayl or geranyl and neryl acetate by reaction with sodium acetate under suitable conditions. Thus if the sodium acetate is reacted in the presence of cuprous copper, the major product is linalyl acetate; whereas in the complete absence of copper, geranyl acetate (5) and neryl acetates (6) are predominate. The mixed terpene acetates undergo saponification to produce geraniol and nerol. These terpene alcohols

this way isomerizes into two isomers: geranyl chloride (4) and neryl chloride (5).

The citronellal chemotype is uncommon and can be used as an insect repellant [6].

BASF opened new continuous production of citral from 2-methyl- 4-hydoxy-but-1-ene in 2004 [10]. The annual production of citral is 40,000 tons per annum, which is shown in **Scheme 2**.

#### **1.6. Synthetic applications of citral**

#### *1.6.1. Synthesis of ionones and methyl ionones from citral*

The ionones belong to rose ketones having closely related series of chemical substances including damascones. A variety of essential oils contain these aroma compounds, the ionones, for example, β-ionone has rose aroma to some extent and it is used as a raw material for the production of retinol. Methyl-ionones are not found in the essential oils, but these are synthesized by aldol condensation of citral with methyl ethyl ketone (MEK).

**Scheme 2.** Total citral synthesis.

#### *1.6.2. Synthesis of ionones through pseudoionones*

Ionone can be synthesized from citral though pseudoionone (PS). Citral and acetone by basic homogeneous or heterogeneous catalysis give PS, and serves as an example of aldol condensation. This is followed by a rearrangement reaction in acidic media to ionones. The nucleophilic addition of the carbanion (3) of acetone (1) to the carbonyl group on citral (4) is a base catalyzed. The aldol condensation product (5) eliminates water through the enolate ion (6) to the pseudoionone (7). The cyclisation reaction of PS to ionone proceeds by acid catalysis where the double bond in (7) opens to form the carbocation (8). A rearrangement reaction of the carbocation follows with ring closure to (10, 11). Finally, a hydrogen atom can be abstracted from (9) to form either (10) (extended conjugated system) or (11) as shown in **Scheme 3**.

Besides the pseudoionones, which are a mixture of two isomers as mentioned above, several side reactions can take place too, especially self-condensation of citral leading to dimerization and polymerization. Secondary reactions are potential problems. This is shown in **Scheme 4**.

*1.6.4. Synthesis of vitamin E (tocopherol)*

**Scheme 3.** Synthesis of ionones pseudoionones from citral.

shown in **Scheme 6**.

Vitamin E can be synthesized from pseudoionones and tri-methyl hydroquinone (TMHQ) as

Lemon Grass (*Cymbopogon citratus*) http://dx.doi.org/10.5772/intechopen.69518 127

Citral has strong lemon-like aroma but it is highly volatile and instable to sunrays and alkalis, thus hardly sustaining its aroma [11]. To resolve this problem, citral dimethyl acetal and citral diethyl acetal have been used these compound have neroli-like citrus green

**1.7. Citral acetals and their importance in cosmetics and toiletries**

#### *1.6.3. Synthesis of vitamin A from ionone*

Ionone is a key intermediate for the production of vitamin A. Ionone (C13 compound) on hydroformation gives C14 compound (2), which is treated with acetylene compound (a C6 compound) to vitamin A having C20, by series of catalytic and isomerization reactions (see **Scheme 5**).

**Scheme 3.** Synthesis of ionones pseudoionones from citral.

#### *1.6.4. Synthesis of vitamin E (tocopherol)*

*1.6.2. Synthesis of ionones through pseudoionones*

**Scheme 2.** Total citral synthesis.

126 Grasses - Benefits, Diversities and Functional Roles

*1.6.3. Synthesis of vitamin A from ionone*

**Scheme 4**.

Ionone can be synthesized from citral though pseudoionone (PS). Citral and acetone by basic homogeneous or heterogeneous catalysis give PS, and serves as an example of aldol condensation. This is followed by a rearrangement reaction in acidic media to ionones. The nucleophilic addition of the carbanion (3) of acetone (1) to the carbonyl group on citral (4) is a base catalyzed. The aldol condensation product (5) eliminates water through the enolate ion (6) to the pseudoionone (7). The cyclisation reaction of PS to ionone proceeds by acid catalysis where the double bond in (7) opens to form the carbocation (8). A rearrangement reaction of the carbocation follows with ring closure to (10, 11). Finally, a hydrogen atom can be abstracted from

Besides the pseudoionones, which are a mixture of two isomers as mentioned above, several side reactions can take place too, especially self-condensation of citral leading to dimerization and polymerization. Secondary reactions are potential problems. This is shown in

Ionone is a key intermediate for the production of vitamin A. Ionone (C13 compound) on hydro-

to vitamin A having C20, by series of catalytic and isomerization reactions (see **Scheme 5**).

compound)

(9) to form either (10) (extended conjugated system) or (11) as shown in **Scheme 3**.

formation gives C14 compound (2), which is treated with acetylene compound (a C6

Vitamin E can be synthesized from pseudoionones and tri-methyl hydroquinone (TMHQ) as shown in **Scheme 6**.

#### **1.7. Citral acetals and their importance in cosmetics and toiletries**

Citral has strong lemon-like aroma but it is highly volatile and instable to sunrays and alkalis, thus hardly sustaining its aroma [11]. To resolve this problem, citral dimethyl acetal and citral diethyl acetal have been used these compound have neroli-like citrus green

**Scheme 4.** Condensation of citral.

aromas. Citral propylene acetals and mono-ether glyceryl acetals are also synthesized and

Lemon Grass (*Cymbopogon citratus*) http://dx.doi.org/10.5772/intechopen.69518 129

These citral acetals are used in cosmetics and toiletries to suppress body smell produced by bacteria instead of antibacterial agents. Citral acetals in cosmetic formulation slowly release

When aldehydes or ketones are treated with alcohols in the presence of acids, it produces an acetal sometimes called ketal [12, 13]. The acetal group has two OR groups attached to the same carbon atom. The formula of acetals is not favored when ketones are treated with simple alcohols and gaseous hydrogen chloride. The formation of cyclic acetals is favored with an excess of 1,2-diol and a trace of acid. The reaction can be reversed by treating the acetal with aqueous acid. Citral ethylene glycol and propylene glycol acetals produced by azeotropic acid

citral, which is antibacterial and also relative save to use.

used in cosmetic.

**Scheme 6.** Synthesis of vitamin E.

**1.8. Synthesis of citral acetals**

catalyzed reaction as shown in **Scheme 7**.

**Scheme 5.** Catalytic isomerization and synthesis of vitamin A.

**Scheme 6.** Synthesis of vitamin E.

aromas. Citral propylene acetals and mono-ether glyceryl acetals are also synthesized and used in cosmetic.

These citral acetals are used in cosmetics and toiletries to suppress body smell produced by bacteria instead of antibacterial agents. Citral acetals in cosmetic formulation slowly release citral, which is antibacterial and also relative save to use.

#### **1.8. Synthesis of citral acetals**

**Scheme 5.** Catalytic isomerization and synthesis of vitamin A.

**Scheme 4.** Condensation of citral.

128 Grasses - Benefits, Diversities and Functional Roles

When aldehydes or ketones are treated with alcohols in the presence of acids, it produces an acetal sometimes called ketal [12, 13]. The acetal group has two OR groups attached to the same carbon atom. The formula of acetals is not favored when ketones are treated with simple alcohols and gaseous hydrogen chloride. The formation of cyclic acetals is favored with an excess of 1,2-diol and a trace of acid. The reaction can be reversed by treating the acetal with aqueous acid. Citral ethylene glycol and propylene glycol acetals produced by azeotropic acid catalyzed reaction as shown in **Scheme 7**.

**2. Material and methods**

a stainless steel tank for distillation.

**2.2. Analytical equipment used**

absorption in the infrared region.

**Figure 2.** Stainless steel equipment for hydrodistillation of lemon grass.

Lemon grass leaves were harvested in the month of July, allowed to dry at room temperature for 2 days and cut into pieces of 2 inches in length then distilled in an appropriate stainless steel equipment having a capacity of 15 L along with a condenser having inlet and outlet for water, a tank is heated with a gas burner to continue and complete distillation. **Figure 2** shows

Lemon Grass (*Cymbopogon citratus*) http://dx.doi.org/10.5772/intechopen.69518 131

The azeotropic mixture of essential oil is collected in a separating funnel and the essential oil of lemon grass (5.7 g) was obtained, dried over anhydrous sodium sulfate, and kept in an air tight stoppered bottle. The percentage yield of oil was determined by infrared spectroscopy (IR) and gas chromatography (GC), and obtained data lemon grass oil were plotted on graphs.

Citral (lemon grass oil) was hydrodistilled by using Dean-Stark apparatus, which consists of a round bottomed flask with the capacity of 2 L. Three-fourth of the flask was filled with 500 g of crushed dried leaves of lemon grass along with water and hydrodistilled. The oil was separated and dried over anhydrous sodium sulfate. It yields 0.38% oil. **Figure 3** shows Dean-Stark apparatus.

**Infrared spectrophotometer**: Model Thermo Nicolet FT-IR 200 (USA) was used for recording

**2.1. Hydrodistillation of essential oil of lemon grass in Dean-Stark apparatus**

**Scheme 7.** Synthesis of citral acetal.
