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What is the most important organic compound found in Fragrance, and is it better, for our health and the environment, to use natural resources to obtain these compounds, as an alternative to synthetic processes used to manufacture these compounds?

Abstract

Perfume is composed of a variety of organic compounds, or are derived from them, most prominently Terpenes, Aldehydes, Benzene, Toluene, Phenol, Naphthalene and Aliphatic materials. Whilst a large proportion of these perfume ingredients can be synthesised in the lab, most of them can also be obtained naturally. Perfumers then have to consider the best way of obtaining the ingredients, to maximise profit, but also the toxicological concerns, to ensure the safe usage by consumers, and to ensure the consumers don't have allergic reactions to the perfume. However essential oils do you have disadvantages. Essential oils can be degraded and can undergo oxidation, and polymerisation, and this can be affected by any of three factors; oxygen availability, light, and temperature (Turek & Stintzing, 2013). Another disadvantage of the extraction of essential oils is that in most cases, many tons of plants must be grown, and harvested in order to extract small amounts of oil, so the perfume becomes more expensive to make and sell. Moreover, some aromas are not present in the natural world and are only available through synthetic routes, such as the long chain aliphatic aldehydes found in Chanel No 5 (Patrick, 2017). This aldehyde, 2-methylundecanal, started the trend of using aldehydes in fragrance and emphasises how important synthetic chemistry can be in Fragrance (Chemistry World, 2012). Terpenes are the most prominently used fragrance ingredients because they are the largest group of odorants that are most commonly found in nature (Sell & Pybus, 1999). It is important to determine the ways in which organic compounds can be manipulated in the best possible way, and therefore the paper will explore how the organic compounds listed are used in fragrance, as well as how safety is monitored, and how essential oils are manipulated, and how stable they really are.

Introduction

This research focuses on the role of Terpenes, Benzene, Toluene, Phenol, Naphthalene, and Aliphatic materials in the fragrances we use every day, and also discusses the advantages of using lab synthesis to obtain these ingredients, and why natural extraction may not be the best route to an ingredient in many situations, as well as looking at safety control and toxicology in fragrance. There are five key terpenes, Linalool, geraniol-nerol, citronellal, citronellol, and citral, and their esters are also of great importance (Sell & Pybus, 1999). Essential oils are crucial in the synthesis of fragrances, whether they are used directly in the perfume itself, or used as inspiration. Chemists use Retrosynthesis, in order to analyse molecules found in nature and reverse engineer these molecules, by "deconstructing" the molecule into available starting materials, to work out how to synthesise these molecules in the lab without the need for oil extraction from plant material.

Synthesis of Terpenes and ingredients derived from it

Terpenes can be identified by the isoprene (2-methylbuta-1,3-diene) rule, which shows terpenes being built up from isoprene units (Figure 1). There are also a variety of subdivisions depending on the number of carbon atoms present, of which monoterpenes (10 Carbons) are the largest group (Taylor, 2002). Turpentine is one of the starting points for the synthesis of major terpenes (synthetic), and it is extracted from softwood that is used in the manufacture of paper, and known as Crude Sulfate Turpentine (CST), after which it is distilled to produce the monoterpene fraction, turpentine  (Sell & Pybus, 1999). Two structural isomers, α- and β-pinene, are extracted from the turpentine by fractional distillation and they occur in a ratio of 7:3 (α:β), so α-pinene is present at about two times as abundant, which means β-pinene is twice as expensive as α-pinene (Sell & Pybus, 1999). This is an example of where cheap natural sources (softwood) are used to obtain a product, which undergoes further syntheses. The two pinene isomers are important in the synthesis of Geraniol, and Linalool.

Figure 1

Synthesis of Linalool

Pinene is converted to pinane by hydrogenation, after which it undergoes pyrolysis. The scheme is shown in figure 2, where pinane is formed, and then oxidised and reduced to pinan-2-ol, which then undergoes pyrolysis, as the alcohol is heated at a high temperature. A retrosynthetic approach to studying Linalool is utilised when looking at lily of the valley, in order to work out different methods of obtaining Linalool for Fragrances.   

Pinan-2-ol

Obtained from (Sell and Pybus, 1999)

Figure 2

Ambergris is one important group of terpenoid compounds, which arise in the sperm whale. This whale produces ambriene, a triterpene, in its intestine, and it is assumed this is a response to some irritation (Sell & Pybus, 1999). However, the ambriene itself undergoes degradation, in a variety of ways, due to sunlight, air, and the salt water in the sea when the whale secretes lumps of ambriene, after which a fossilised resin is then washed up on shore (Sell & Pybus, 1999), so the economic value of ambergris is very high, as attempts to synthesise an ambergris fossil would be quite difficult, so this is an example of where the natural product is valued highly, despite the ability of chemists to synthesise Ambroxan® which is a synthetic compound with an aroma similar to Ambergris.

Cedarwood oil is used in many fragrances and can be classified into two families. The Juniperus species is one of the starting points to the first family of Cedarwood oils. They are derived from sesquiterpenes (15 carbons (Taylor, 2002)), and are named as Chinese, English, or Texan Cedarwoods, whereas on the other hand, the Cedrus species gives rise to Atlas and Himalayan cedarwoods. (Sell & Pybus, 1999). One reason why natural cedarwood oil may be less useful than synthetic cedarwood oil, is because acetylated cedarwood has a much more pungent cedarwood smell, so less would need to be used.

Sandalwood oil is another fragrance valued in the industry and intrinsically linked with terpene chemistry. The oil itself, extracted from a parasitic tree Santalum Album, is reported to be economically unbeaten by synthetically produced oils, yet there are two main families of synthetic sandalwood which are used, nonetheless, campholenic aldehyde derivatives, and terpenophenols (Sell & Pybus, 1999).

Musks have until recently been extracted from animals, such as civet cats and musk deer, and the animalic nature of the smell is attributed to compounds such as 2-methylindole. The fixative ability of these components, mean that they can prevent rapid evaporation of very volatile notes and hold back the evaporation (Sell & Pybus, 1999). However, these natural musks have always been an expensive product, and their structures meant that it was difficult to use in perfume in the late 19th and early 20th century. That was when nitromusks were discovered, when a scientist called Baur was developing explosives, when he noticed a musky odour emanating from the TNT they were working on, and Musk Baur®, began to be used in fragrance (figure 3), yet this became redundant later in the 20th century, when the hazards associated with the preparation outweighed the good, and some nitromusks caused allergic reactions when exposed to the sun (Sell & Pybus, 1999).

Polycyclic musks posed a solution to this problem. It all starts with the production of cumene from propylene and benzene, which accounts for 20% of the demand for benzene in the world (Ceresana, 2018), by a Friedel-Crafts alkylation in the presence of an H3PO4 catalyst. This then undergoes an electrophilic addition

Synthesis of perfume ingredients from Benzene

Benzene is crucial in the synthesis of 2-phenylethanol, which is a very important ingredient in perfumes as it has excellent blending qualities and

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