Leaf alcohol: Properties, Production process and Uses

Leaf alcohol, also known as (Z)-hex-3-en-1-ol, cis-hex-3-en-1-ol, cis-3-hexen-1-ol, cis- 3-hexene-1-ol, cis-3-hexenol, 3-(Z)-hexenol; 3-(Z)-hexen-1-ol, (Z)-3-hexen-1-ol, is a derivative of acetylene and ethylene oxide. CAS Registry Number: 928-96-1. Molecular formula: C6H12O, molecular weight: 100.1589. The chemical structure is:

Properties of leaf alcohol

Leaf alcohol is colorless oily liquid with boiling point of 156–157°C, density of 0.8460 g.cm^–3 and refractive index of 1.4395 at 20°C. It is miscible with most oils and soluble in alcohol and most organic solvents, but only slightly soluble in water. Leaf alcohol has a pleasant and aroma of fresh green grass. It is a representative fragrance with green aroma. Natural leaf alcohol exists in almost all green plants, mainly in tea, mint, jasmine, Osmanthus, Acacia, mulberry, grapefruit, and so on. The isomer of leaf alcohol is trans-hex-2-en-1-ol. The specific fragrant properties of the two isomers are caused by the presence of a double bond and geometric structures on their molecules wherein cis-hex-3-en-1-ol possesses a very pleasant fragrance while trans-hex-2-en-1-ol lacks typical green odor but has fruity odor.

Process for manufacture of leaf alcohol

Leaf alcohol can be isolated from the essential oils extracted from various plants, but only in limited quantities. Commercial leaf alcohol is mainly obtained by chemical synthesis. According to the starting material, the main methods for synthesis of leaf alcohol include acetylene route, vinylacetylene route, butyne route and propargyl alcohol route. The butyne route follows exactly the same procedure as the acetylene route after 1-butyne has been prepared.

Acetylene route

In liquid ammonia, acetylene reacts with metal sodium to form sodium acetylide, and then sodium acetylide reacts with bromoethane to form 1-butyne. Further reaction of 1-butyne with sodium in liquid ammonia gives butynyl sodium, which is then reacted with ethylene oxide and hydrolyzed to yield 3-hexyne-1-ol. Partial hydrogenation of 3-hexyne-1-ol produces leaf alcohol.

The reaction of triethylaluminum with acetylene can also give the cis-configuration of the double bond, which can also be used for the synthesis of leaf alcohol

Vinylacetylene route  

In liquid ammonia, vinylacetylene reacts with metal sodium to form sodium vinylacetylene, and then sodium vinylacetylene reacts with ethylene oxide to give 3-hexyne- 5-en-1-ol, and partial hydrogenation of 3-hexyne-5-en-1-ol produces leaf alcohol.

Propargyl alcohol route

Propargyl alcohol was converted to propyne chloride. The latter reacted with methyl Grignard reagent to obtain butynyl magnesium chloride, which was then reacted with ethylene oxide and hydrolyzed to give 3-hexyne-1-ol. Partial hydrogenation of 3-hexyne-1-ol in the presence of palladium catalyst yields leaf alcohol. The purity of leaf alcohol was up to 99% and the overall yield was 37.8%.

The propyne chloride may be replaced with p-toluenesulfonyl propynyl ester under substantially the same reaction conditions. The yield of leaf alcohol is slightly improved and the purity is almost the same.

Uses of leaf alcohol

At present, hundreds of tons of leaf alcohol are consumed in the world, of which 35% is directly used for preparation of essence and 65% is used for production of esters, mainly formate, acetate, benzoate and salicylate of leaf alcohol.

1. Application of leaf alcohol in food

In Japan, leaf alcohol is widely used in the preparation of natural fresh flavor essences such as banana, strawberry, citrus, rose and apple. It is also used together with ester flavors such as acetate, valerate and lactate to improve the taste of food, mainly for inhibiting the sweet aftertaste of cool drinks and fruit juices. In the national standard of food additive of China (GB2760-1996), leaf alcohol is specified to be used in food flavor according to the production needs.

2. Application of leaf alcohol in cosmetics industry

Leaf alcohol possess an intensely green grassy odor, which is a popular type of delicate fragrance. Essences prepared with leaf alcohol are also used in cosmetics in large quantities. More than 40 famous essence formulations in the world contain leaf alcohol. Usually, addition of only 0.5% or less of leaf alcohol can obtain obvious aroma of fresh green leaves. It is used as component of various artificial essential oils similar to natural fragrances such as Convallaria, clove, mint and lavender essentialoils. It is also used for preparation of various flower essences, giving the artificial essential oils and essences pleasant odors of green leaves.

3. Application of leaf alcohol in biological control

Leaf alcohol is also an indispensable physiological active substance in both plants and insects. Insects use leaf alcohols as pheromones or sex hormones for alarm and assembly. Therefore, leaf alcohol can be used in biological control. For example, the mixture of leaf alcohol and benzoquinone in a certain proportion can induce the aggregation of male scarab and beetle, so as to kill such forest pests in a large area.  

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