Essentials of Fragrance Chemistry

By Matthew Tittensor, Nicholas Lang, and Sohum Sanghvi

Two more common hygiene products are perfume and cologne.  We know that these sprays smell nice and permeate throughout a room, but what is it that gives them their scent and more importantly why does it disperse?  In today’s blog post we will get into this by discussing the organic structures of esters, specific scents, commercial uses for esters, and the process of diffusion.

           An ester follows the format follows the format of the image to the right, with the R group being any hydrocarbon.  This is written a RCO2R’.  The alcohol component makes up the basis of the alkyl component and R’OH’s root name and is based on the longest chain with an OH attached to it. Meanwhile, RCO2H is the carboxylic acid, from which the –oate in the name is derived from.  The full name for an ester is an alkyl alkanoate. Now that the nomenclature is out of the way, what do esters smell like and would they be used in perfumes?

Esters often have a pleasant fruity aroma as can be seen in the chart to the right.  However, that does not necessarily make them ideal for perfumes.  Most simple esters give off these pleasant smells, but problems arise because they are not prepared to handle the sweat that a human body releases.  This sweat hydrolyzes the simple ester and can replace this seemingly nice smell with a harsh one.  A common example is that butyric acid smells like rancid butter, but ethyl butyrate, an ester it can be derived from, smells like pineapples.  This is one reason that simple esters are not utilized in the perfume industry.  However, perfumeries get around this by often including many esters in their products as well as essential oils to prevent the hydrolysis of the esters.  Esters serve a role in the food and beverage industry as well.

           Would you rather eat a delicious food that smells rancid or a mediocre food that smells delicious, if you did not know how each one tasted?  This is a problem that major manufacturers come to face when they make their products.  These companies utilize a combination of esters and essential oils as well to produce a scent that is please to both smell and taste.  It is not so simple as getting one pleasant odor and taste either, as the human has over 9000 taste receptors on its tongue and smell plays a large role in perception of taste.  To create an ideal, it takes a lot of testing and a wide variety of organic and synthesized compounds to be used.

           Diffusion is the movement of molecules from an area that contains a higher concentration to one with a lower concentration of the molecule.  These molecules are already in constant motion and move in random directions due to the random collisions that they experience with each other and other particles.  The net movement is always towards the lower concentrated expanse as more collisions occur on a more highly concentrated zone, making it more likely for the molecule to be pushed over to the other area.  Dynamic Equilibrium only comes to exist after the concentration gradient, difference in molecule distribution, is removed.  This applies to perfumes and colognes as they emanate from their more highly concentrated location on the wrist or neck to the areas surrounding the wearer.  This creates a nice scent around the user and fulfills the purpose of removing or covering up body odors.

We have taken a look at the concept of esters, specific scents, commercial uses for esters, and basics about the process of diffusion. Using the right ester is vital for obtaining the scent that is wanted, and diffusion is important for making sure the scent remains on the user and covers the body odors. In our next blog post, we will continue our discussion on fragrances and continue to unveil interesting chemistry behind perfumes and colognes.

Fragrance Chemistry: Another Look

In our previous blog post, we introduced concepts from organic chemistry, such as esters, commercial uses of esters, and the process of diffusion. Before we continue in our discussion about esters and fragrances, it is probably important to learn about how smell actually occurs.

How Smell Occurs (the Chem Way!)

Everything that we smell, whether it is food, smoke, wood, soap or shampoo, emits molecules that reach our nose. The molecules that are emitted are all volatile and relatively light. As we have learned, to be a volatile substance, it must be easily to evaporate. Objects that have no smell, such as NaCl, behave this way because they are non-volatile solids.

Behind the inside of one’s nose lie a set of neurons. Unlike many other sets of neurons in our body, they are regularly exposed to oxygen. Also, these neurons have cilia, or tiny hairs, that are special to the nose. the molecule emitted from our volatile substance  attach to the cilia in our nostrils and trigger a sensation in the neurons of our nose. The neurons send a signal to our brain that translates it into a smell.

But, the inquisitive mind may ask, what molecules are emitted from our volatile objects that enter our nostrils? Many natural objects and plants emit molecules that are present when the object undergoes esterification. A banana, for instance, emits the ester isoamyl acetate (CH3COOC5H11.) Oranges produce Octyl Acetate (CH3COOC8H17) when they undergo the same process.

 

How Perfume Works

As you may or may not be aware, perfume is very dilute. Someone may think it is because the producer’s of the perfume are trying to minimize their costs, possibly to the detriment of the product, but this is not the case. In mixtures such as perfume, there are a variety of different alcohols in the same liquid. Alcohols all are very strong-scented; they emit many molecules that are received by the cilia in your nostrils. If it were a very concentrated substance, perfume would give off all the smells of the different alcohols within it simultaneously. Although they might be sweet on their own, the smells together would not be nearly as enjoyable, as we can no longer distinguish one from the other.

Perfumes contain different alcohols mostly due to the way they are supposed to work. There are three stages to perfume smells; top notes, heart notes, and base notes. Top note smells occur within 15 minutes of spraying the perfume on your skin. These chemicals are the first to evaporate, and thus the first molecules to be received by your nostrils. These scents are strange or exotic, and are made that way to interest consumers, yet not stay too long to be sick of them. Heart note smells occur 3-4 hours after application. These are generally the floral smells of a perfume. Base note smells occur 5-8 hours after application, and are usually musky or mossy. The relation to kinetics is the rate it takes for each individual scent to be activated. This can be represented using rate laws and reaction coordinate diagrams, to indicate exactly which scents are being activated at what time. The rate for the top notes will be the fastest, followed by the heart notes and base notes.

One must be careful with perfume, as light has enough energy to speed the decay of top note chemicals. Air also corrodes the fragrance through oxidation, and this occurs quicker after application. Also, top notes will evaporate faster on warmer skin that is less oily than on cold and oily skin. It is because of this that one must store their perfume in a dark, room temperature area to maximize its shelf life.

Classifying Perfumes

In the previous section, we mentioned how the duration of perfumes can help in classifying a perfume. Another way for classifying perfumes is by their smell. As we know, esters give the perfume its fragrance. A consultant in the fragrance industry, Michael Edwards, devised a qualitative way of describing fragrances for perfumes. This classification can be seen in the fragrance wheel, as shown in the picture below.

We know that perfumes can have scents, and that esters have the ability to cover up scents. But, how do we actually create a perfume? That question will be answered in the next section.

 How Perfumes Are Made

First, all ingredients necessary for a perfume must be obtained. A perfume can have over 100 different ingredients, so this step is essential. This process may of extracting oils from plants, extracting scents from fatty substances of animals, or using synthetic fragrances developed by chemists. For example, the citrus tree blossom or the myrrh resins can be extracted and used in perfumes.

 

These ingredients are grouped into 4 categories: primary scents, modifiers, blenders, and fixatives. The primary scents are the most important scents required to give a perfume its scent. The modifiers are often esters, such as having a fruity ester and floral ester to make the scent fruity floral. Modifiers essentially replace one scent with one more geared towards the perfume scent, just like how an ester replaces an odor. The blenders and fixatives add some more scent to the primary scent to make it easier to transition between the three stages (top, heart, and base notes).

After all the scents are gathered, the scents are blended together to create the perfume. After the blending of ingredients, ethyl alcohol and water is added. The amount of alcohol added is based on what strength of the perfume is desired. The concentration of the perfume mixture is often mentioned in units of either volume percent or weight/volume percent. The perfume is then aged in tanks for several weeks and filtered before being put into bottles.

The fragrance industry is a large, growing industry in the world, and new formulas are constantly being developed for new fragrances. It is important to remember that alcohol is added to fragrances to give them scents that come in three notes or phases, the top note, heart note, and base note. Fragrances can be grouped by their qualitative scents by the fragrance wheel as well as by their duration. The process of making fragrances can involve many ingredients of various types, and can be altered to create new fragrances for commercial sale.

We hope you enjoyed reading our series of blog posts! Thank you for your support!

Chemistry of Fragrances: The Intricate Details Within Perfumes

Fragrances are used in countless consumer products, from perfume to soap, and even sunscreen. The chemical details behind these fragrances dictate how the fragrance will smell and how long the scent will last. Luckily, we were able to interview Mr. Herman, a cosmetic chemist, who was able to elaborate on the complexities of fragrance chemistry.

During the interview, we hoped to find out:

What makes up a fragrance?

How does the chemistry influence a scent?

What miniscule chemical interactions are taking place?

BCAH1: What are the main components of a fragrance?

Mr. Herman: In the fragrance world as a whole, the biggest class of fragrances is made up of alcohols and the second biggest class is made up of aldehydes and ketones. RIFM (The Research Institute for Fragrance Materials) conducts a census every two years to gain information on what chemicals fragrance companies buy. A fragrance will most likely be a mix of natural and synthetic products. There are certainly more synthetic products which you can get creative with. The quality control and consistency of synthetics are great. However, natural products can be hard to replicate well. Sometimes it is easier to make a synthetic replicating a natural ingredient like citrus oil.

Although virtually clear to the human eye, perfumes are quite complex. Many components of a perfume are found in nature, like aldehydes and ketones which are oxygen- containing hydrocarbons. These molecules have low molecular weights and have solvent characteristics such as acting as a medium capable of dissolving other substances.

Figure 1: This is a visual representation of aldehyde and ketone. Both are organic compounds. Each structure contains a carbon double bonded to an oxygen (carbonyl group). The carbon has two remaining bonds. If one element bonded to the carbonyl is a hydrogen, the substance is an aldehyde. If not, the substance is a ketone. These functional groups contain polar bonds and can thus dissolve other polar compounds.

Figure 2: This image contains natural aldehydes and ketones. Synthetic products are produced based on the these natural molecules found throughout the world.

Larger aldehydes like cinnamaldehyde tend to have a pleasant smell, whereas the smaller aldehydes have pungent, sharp smells. Aldehydes are derived from alcohols by dehydrogenation. Dehydrogenation is the removal of a hydrogen from a chemical compound.

BCAH1: What effect does light have on fragrance bonds?

Mr. Herman: It has an effect on stability. Alot of things affect fragrance stability. For example, colors affect stability. UV will trigger changes, but beyond that is oxidation. Oxidation is probably the worst thing that can happen to a fragrance. Even when you put a concentrated fragrance into alcohol or base, they can still change. A phenol component is very reactive, it changes color easily because it is easy to take the hydrogen off, leaving a free radical which reacts with all kinds of stuff. This phenol component is fine in the dark, but in the light, the hydrogen gets knocked off and makes the compound very reactive. Chelating agents (EDTA disodium salt), benzophenones (UV absorbers), and antioxidants are added to fragrances to stabilize them. Oxidation is the worst, followed by heat and sunlight because they catalyze reactions. pH is a whole other topic. They relate to esters, which break apart in extreme pH levels. All these factors are important to consider in making a fragrance stable.

Oxidation/Reduction Reactions

Oxidation/Reduction Reactions are chemical reactions where the oxidation state of atoms is changed. It involved the transfer of electrons. Aldehydes are strong reducing agents because of the presence of the hydrogen atom. On the other hand, ketones are resistant to being oxidized. Only very strong oxidizing reagents have the power to oxidize ketones like potassium manganate (VII) solution by destructively breaking the carbon to carbon bond.  Aldehydes (RCHO) are mostly oxidized into carboxylic acids (RCO2H).

The following reactions display the half reaction of oxidation for:

An aldehyde under acidic conditions:

RCHO + H2O → RCOOH + 2 H++ 2 e-

An aldehyde under basic conditions:

RCHO + 3OH- → RCOO- + 2H2O +2 e –

BCAH1:Discuss how size of molecules affects fragrance.

Mr. Herman: Fragrance molecules must be relatively small. The biggest molecule are musks, with the biggest molecular weight of about 350. These are humongous! Most fragrance molecules have a molecular weight of about 120 to 180.  Those molecules have a group coming off of it known as an osmophore. Osmophores are floral fragrance glands which are responsible for making things “smell”.

Aldehydes with larger molecular weights tend to have pleasant, flowery scents.

Different Senses in One Perfume:

There are 3 different notes which unfold after the perfume is sprayed: top notes, heart notes, base note

1. Top Notes

Top notes are smelt immediately after a perfume bottle is sprayed to up to 15 minutes . Since the top notes reach your nose instantly, these molecules must be small and light enough to evaporate fast

2. Heart Notes

Heart notes may appear within 2 minutes to an hour after application. These compounds emerge in the middle of the perfume’s dispersion process

3. Base Notes

Base notes are the most heavy of molecules that tend to “stick” to your skin. These molecules can smelt for hours after first application.

BCAH1: What are the intermolecular forces at play within a fragrance?

Mr. Herman: The simplest way of looking at the polarity of molecules is that they are either water soluble or oil soluble, and that they are polar or nonpolar. Most fragrances can mix together. There are some limitations however, starting with the number of crystals you have. If you have too many crystals, they will fall out of solution. They can make up a maximum of 35% of the fragrance.  Solvents are added to fragrances, and they affect the polarity of the solution. Usually, DPG (dipropylene glycol — colorless odorless solvent) is added. Nonpolar things, such as diethyl phthalate, must also be added to keep everything in solution. Diethyl phthalate, however, has a bad reputation as an endocrine disrupter. If you want to add a fragrance to a shampoo, you need a polar substance, where if you are adding fragrance to a lipstick, you need a nonpolar substance. Perfumes and fragrances can vary from polar and nonpolar. Believe it or not, water soluble fragrances aren’t actually water soluble. You just add surfactants!* A fragrance can either be polar with something like DPG, nonpolar, or water soluble with a surfactant. We usually use nonionic surfactants. A surfactant is a surface-active agent, similar to emulsifiers. Basically you are dealing with surface tension. A drop of water is shaped like a bead. The surface is not inert, the attractive forces in the molecule are big enough to overcome gravity. If you add a detergent to the drop, it spreads out because you effect the surface tension. In a perfume, you want to reduce the surface tension. You add some mechanical energy to the solution, and then you get the emulsion

*Surfactants are responsible for reducing the surface tension of certain components in fragrances. By weakening the strong intermolecular forces that induce surface tension, it is easier to mix different substances.

In conclusion, chemistry is able to explain the function of cosmetics, especially fragrances. Thanks to Mr. Herman, we were able to learn more about the composition of fragrances and perfumes. Fragrances are composed of oils, solvents, and aroma compounds. These compounds, polar and nonpolar, mix together through emulsion, allowing consumers to enjoy many desired properties.