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=Classifying d-limonene, beta-ionone, and vanillin, as top, mid, or base notes and the vapor pressure effects in fragrances when notes are combined=

Jessica Sisco
(Final Paper: 1500 word minimum)

Abstract
Fragrance oils are composed of numerous natural or synthetic chemicals, known as raw materials. Raw materials can be liquids or solids and are classified by structure into three main categories known as aliphatics, terpenes, and benzenoids. Most importantly, however, fragrances are constructed in an artistic manner, in which the Perfumer, knowing how each raw material smells and reacts in solvents, loosely based on what is known as the fragrance pyramid. The fragrance pyramid structurally defines the portions of raw materials based on vapor pressures/volatility.The suggested proportions for top notes are 15–25%, where middle notes range from 30 – 40%, base notes comprise of about 45–55%. Top, middle, and base notes are terms used for raw materials based on their vapor pressures; top notes are the most volatile are the first scents detected in fragrances, followed by middle notes, and lastly base notes which are the lasting odors of fragranced products. Since vapor pressures are dependent on chemical structure, d-limonene, a terpene, beta-ionone, an aliphatic, and vanillin, a benzenoid, were researched based on vapor pressures to show the correlation between structural fragrance chemical categories and the fragrance pyramid. Ultimately proving that d-limonene, with a vapor pressure of 213 Pa, is a top note, beta-ionone, 7.2 Pa, is a mid note, and vanillin, 0.23 Pa, is a base note, due to numerical orders of vapor pressures. Most importantly, when notes are combined into fragrances, the overall chemical properties alter based on specific components used, which can higher or lower the overall vapor pressure.

Introduction
All marketed cosmetic and personal care products are created for specific consumer needs; almost all of these products contain ingredients that perform specific physical functions, such as cleansing or moisturizing, while others play more commercial roles in helping the product gain consumer appeal. Fragrance components are among these commercial ingredients. Fragrance components, volatile aroma compounds, are used in marketed products for either odor masking or aesthetic purposes. Many scented products are used on a daily basis, making exposure to fragrances unavoidable. Even certain unscented products can contain some fragrance chemicals, depending on regulations. [1, 2]

Each fragrance is a complex mixture of chemicals, known as raw materials, which can be synthetic or natural. These raw materials are carefully blended to produce specific scents. Some fine fragrances may contain as many as 6,000 individual raw materials ranging from 0.001 to 100 parts per kilo, for example. Based on their molecular structures, fragrance raw materials fall into three broad categories: aliphatics, benzenoids and terpenes. Aliphatic compounds are straight-chain organic chemicals. They can be subclassified into C6-C12 alcohols, C6-C12 aldehydes, esters, ketones and lactones. Benzenoids are compounds whose benzene rings are substituted with different functional groups. The terpene group comprises of terpenoids that are compounds that share a common 5-carbon unit, known as isoprene. Different numbers of the isoprene unit account for different scents of terpenoids. [1-6]

In the perfumery industry special attention is given to the performance of these materials in the composition of perfumes, as well as individually. These raw materials are also classified into fragrance notes of three types: top, middle, and base notes. Top notes are the most volatile and their odor usually lasts for less than a few minutes, whether tested individually or analyzed in a fragrance. Examples of top notes are citrus oils, such as lemon, lime, or orange, in which the citrus oils are extracted naturally. Middle notes, also called heart notes, represent the body of the perfume, which are noticed minutes to hours after application, after top notes have disappeared. Examples of mid notes are floral notes, such as extracted rose or jasmine oils. Base notes have the lowest volatility and are evident hours after application and can last from several hours to days. Base notes are used as fixatives of the whole perfume and lower the volatility of top and middle notes when combined. Examples are vanillin and musk oils. The structure of a perfume has been created as a triangle that is divided horizontally in three parts; each one representing top, middle, and base notes and their proportions to each other. The suggested proportions for top notes are 15–25%, where middle notes range from 30 – 40%, and lastly base notes comprise of about 45–55% of the overall fragrance. The different characteristics of these raw materials are based on the properties of their main constituents, such as volatility, odor threshold, polarity, and affinity to the liquid media and odor value. [4, 5, 7, 8]

Volatility is the tendency of a substance to vaporize and essentially one of the most important aspects in the fragrance industry.The odorant molecule is solubilized by a transport protein in the nose and brought to the membrane of the receptor cells. The vapor pressures of combined fragrances result in the odor depth consumers smell thus many scientists have studied and tested how quickly raw materials vaporize by testing the strength of their odors, tested on paper blotters, over time. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure. [9, 10]

D-Limonene ((+)-(R)-limonene), shown in Figure 1, known as Compound (1), for example, is the main constituent of orange oil and occurs in lemon, mandarin, lime, grapefruit, and bergamot oils, as well as a very large number of other essential oils; thus, it can be extracted as a raw material and used in perfumery. Limonene is a cyclic terpene and can thus be classified in the terpene/terpenoid category. [8, 11, 12]
 * D-Limonene**

Figure 1:

To determine whether or not Compound (1) can be categorized as a top, mid, or base note, the Vapor Pressure, P, of Compound (1) must be known. The experimental vapor pressure of Compound (1) is 213 Pascalls according to [11]. Since a substance with a high vapor pressure at normal temperatures is referred to as volatile, it would appear as though this is a volatile chemical. Yet, in order to classify Compound (1), it must be compared to other fragrance raw materials, to determine whether it is a top, mid or base note. The chart from source [11], shows vapor pressures for other chemicals that fall in other groups besides the terpene/terpenoid group.

For example, beta-ionone, as seen in Figure 2, known as Compound (2), has a vapor pressure of 7.2 Pascalls [11]. This pressure is less than the terpene, Compound (1).
 * Beta-Ionone**

Figure 2:

Thus Compound (2) is less volatile; considering the fact that Compound (2) is a ketone, it is classified as an aliphatic. The ionones, alpha, beta, and gamma, are a series of closely related chemicals that are part of a group of compounds known as rose ketones. Considering floral notes are classified as mid notes, it might be possible to assume that Compound (2) and possibly all aliphatics are also classified as mid notes. However, the vapor pressure of many other aliphatics must be established before that assumption is validated. Most importantly, rose essential oils are composed of many aliphatics and terpenes. Although terpenes are present in rose oils, the combination of different chemical constituents change the over all vapor pressure for the extracted oil, thus making most floral extracts mid notes in perfumery, despite the fact that the chemical makeup is composed of top and mid notes. [13, 14]

An example of a base note is vanillin, pictured in Figure 3, known as Compound (3). Compound (3) is a benzenoid and also widely used in the perfume industry.
 * Vanillin**

Figure 3:

The vapor pressure of Compound (3) is 0.0017mmHg at room temperature, which is equivalent to 0.23 Pa. Therefore, the vapor pressure of Compound (3) is the lowest when compared to Compound (1) and (2). [15,16]

This confirms that Compound (1) is a top note, Compound (2) is a middle note, and Compound (3) is a base note in comparison with each other. Yet, does this prove that all terpenes, like Compound (1) are top notes; or that all aliphatics are mid notes and all benzenoids are base notes? Each classification of chemicals has specific structures, which directly relate to vapor pressure. Analytical fragrance chemists use GC-MS for completed fragrance compositions and are able to analyze the structures in the mixture as well as identify volatile compounds; this allows them to relate the fragrance makeup back to the fragrance pyramid and see what needs to be added or removed to make the fragrance more suitable for marketing. [5, 8, 17]
 * Conclusion / Effect on Overall Fragrance Vapor Pressure**

Ultimately each classification of fragrance raw materials is divided by structure and consequently volatility. The terpene raw materials have the highest vapor pressures and although they are top notes, many terpenes are present in extracted floral oils. Just like every natural raw material, the oils extracted oils are comprised of different chemical compositions. The vapor pressure of a mixture is equal to the vapor pressure of the pure component, at that temperature, multiplied by their specific mole fractions in the mixture. Thus although certain floral oils comprise of Compound (1), they could react as middle notes in fragrances based on their chemical compositions. Depending on properties like volatility of the raw material molecules or the molecular interactions existing in the mixtures, all fragrant species start to evaporate at a faster or slower rate with the application of the perfume or scented product. So, despite the fact that each individual chemical can be interpreted as top, mid, or base notes, the interactions when combined with other raw materials and solvents could result in reactions and unpredicted results than previously considered. As the evaporation and diffusion processes take place when the perfume or scented product is applied or used, a blending of vapor fragrances is evolving in the air above the liquid and the human nose simultaneously perceives those single odorants in different intensities. [18]

In binary fragrance mixtures, AB, consisting of two fragrant chemical compounds with different volatilities, each one belonging to one type of fragrant notes; for example, A is a top note and B, a middle note. If component A is Compound (1) and component B is Compound (2), for the mixture, there exists nA moles of component A and nB moles of component B. The molar fraction of A is represented by xA=nA(nA+nB) and the molar fraction of B is xB=nB(nA+nB). If there are only these two components, xA+xB=1. For each mixture (xA, xB) the odor values of each component can be calculated using the equation in Figure 4. [18,19]

Figure 4 [4]:

The odor values can change using different solvents or different quantities of each fragrant chemical for a specific multicomponent mixture. This is important because initial chemicals have different properties before they are combined into fragrances. Depending on solvents used in fragrance, their corresponding vapor pressures initially have an evaporation path; when combined, the odor value changes and thus also changes the path of evaporation for the individual component. Chemical reactions occur either initially or with time once a fragrance is created and combined. Ultimately the different chemical structures and their overall volatility effect the fragrant composition. Thus, the Perfumer needs to know how to combine selected materials based on their chemical properties and know how to compose a fragrance with an end result and reaction in mind, opposed to thinking as a fragrance as individual components. Ultimately, each raw material used in fragrance chemistry is classified by structure as aliphatic, terpene, or benzenoid; these structure have different vapor pressures, depending on specific chemical and substituents that can thus be classified as top, middle, or base notes. The importance of vapor pressures, relative to perfumery, is due to the fact that when combining fragrance components, the overall properties will change in mixtures, thus when composing a fragrance, while the initial properties are important, they will eventually change and create something new. [4, 18-20]

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