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  1. #1

    Default Maturing reactions

    Hello ..

    I hope everyone is doing well.

    I am sorry if you have already discussed this issue.

    If anyone please could explain the reactions which take places between the molecules or the functional groups during the maturity period.

    Some notes are totally faded or disappeared and some become clearer.

    What is the mechanism of this ?

    Thank you

  2. #2

    Default Re: Maturing reactions

    Hello Mind,

    I am really glad you asked this question because I am very interested in this topic as well. You did not specify which material you were thinking of so I will lay down a foundation of common chemical reactions. This is my first post so forgive it's bad form.

    Simple Oxidation

    -Alcohol dehydrogenation into ketones (Examples: borneol -> camphor / R-C-O-H -> R-C=O). This reaction can be catalyzed by trace metals (Ni,Fe,Cu etc) and only occurs on secondary alcohols (Isoproanol -> Acetone) because carbon can only make a maximum of 4 bonds.

    - Alcohol dehydrogenation into aldehydes (Examples: Nerol -> Neroli aldehyde / R-C-O-H -> R-C=O). This reaction occurs when primary alcohols react with oxidizing chemicals/enzymes. Aldehydes are a source of allergenic/toxic potential of fragrances because aldehydes can react with nucleophiles such as RNH2 and ROH groups found in important biological structures (DNA/Proteins). One example of an aldehyde reaction is the schiff base reaction used to produce the fragrance aurantinol (follow link below). A Schiff base can hydrolyze into a carboxylic acid.

    Aldehydes can be further oxidized into carboxylic acids (RC=O -> R-COOH). For example, acetaldehyde can react to form acetic acid (Vinegar).

    Complicated Oxidations

    Organic molecules with double bonds (called alkenes C=C) absorb light. This energy can go into exciting the electrons into an "anti bonding" state (C*-*C). This is a highly reactive state and can undergo many reactions. For example, dioxygen (O2) can react with double bonds by forming an adduct in a reaction called a Diels Alder reaction. This can lead to hydro-peroxides (ROOH), and epoxides (R=R -> ROR).

    Sunscreen ingredients are often added to fragrances to remove the photo-excited energy before it can lead to double bond breaking/photo-oxidation reactions. The choice of ingredient is important for the specific composition and use of the fragrance since properties depend on composition of fragrance base. Toxicity of sunscreen is also a major concern.

    Hydroperoxides are generally removed by adding sacrificial antioxidants (Vitamin E) or stabilizers (BHT), which can destroy hydroperoxides (ROOH -> ROH + H2O). This leads to the irreversible loss of the antioxidant (IE vitamin E). A pathway in the formation of hydroperoxides is catalyzed by metal ions (Fe2+, etc). This is why "chelating" molecules which sequester metal ions (Fe2+) are added to food/fragrances.

    I will list other reactions below:

    Acid Catalyzed Reactions:
    - Acid catalyzed ester breaking (Borneol acetate -> borneol + acetic acid)
    - Water elimination (dehydration) / ROH -> R=R
    -Acid catalyzed rearrangement (Hard to describe but occurs for strained carbon rings such as cyclobutane, cyclopropane, etc)
    - Epoxide ring opening (Nucleophilic addition)
    - Acid catalyzed hydration (May explain accumulation of patchoulol alcohol in aged patchouli, see link below (Fig 7C))
    - Transesterification in which a ester bond is transferred between molecules.

    Base Catalyzed Reactions:
    - Epoxide ring opening (Nucleophilic addition)

    Light Catalyzed Reactions
    Photoisomerization (IE: Shifts between Alpha, beta, gamma Eudesmol)
    Photochemical rearrangments (IE: Santonin synthesis)
    Photo dimerization (IE: Limonene dimers found in aged frankincense)

    More important details

    - Epoxides can be extraordinarily allergenic (IE Linalool epoxide), but are sometimes benign (IE Eucalyptol).
    - Peroxides do not always form on alkenes, and can form in compounds with electrophilic behavior (Diethyl ether). Low water environments allow them to accumulate until they explode or are decomposed.
    - When a ketone is next to a double bond it can form a reactive "Michael acceptor". These are generally reactive and toxic/carcinogenic for the same reasons as aldehydes. Examples include Beta Damascone, Furan, Coumarin.


    All the above reactions occur and lead to the formation of compounds which may or may not posses odor. As more reactive compounds decompose, their notes may decrease while others increase. I have started doing some experiments with aging and found that amyris can develop a strong smoky aroma, but this may be an illusion. My reason for doing this was to isolate the gamma eudesmol isomer from the alpha and beta, since it is more stable than the others. I also recently obtained some beta damascone and methyl anthranilate from perfumers apprentice, and when mixed, I believe it formed a cheesy odor.

  3. #3

    Default Re: Maturing reactions

    Quote Originally Posted by Mind View Post
    If anyone please could explain the reactions which take places between the molecules or the functional groups during the maturity period.

    Some notes are totally faded or disappeared and some become clearer.

    What is the mechanism of this ?
    This is a really good question. To some extent it really depends on the exact fragrance chemicals, and I feel that certain fragrance chemicals in particular may have more potential likelihood for compatibility problems.

    Chemistry (all the more so organic chemistry) is a pretty complicated field so it would be hard for most perfumers to predict this.

    One problem, amines tend to react with aldehydes. You get a condensation product.
    Indole, commonly used in trace amounts in perfumery, is one example of an amine. Methyl anthranilate is another.

    This is something I wish more professional perfumers would be on the lookout for.

    The reaction tends not to be immediate, since the aldehydes are tied up in the form of hemiacetals.

    (There's no problem here with ketones though, it's only aldehydes)

    There are several non-obvious aldehydes like vanillin and citral.
    (they are aldehydes chemically but are normally not considered "aldehydes" within the context of perfumery)
    Citral is an important fragrance component in many natural essential oils like lemon, elemi, lemongrass, and lemon myrtle.
    Vanillin and benzaldehyde in trace quantities are probably what gives benzoin resin most of its distinctive smell.

    Fortunately I think amines in general are rather rare in perfumery.
    But there is also dimethyl anthranilate used in honeysuckle accords.

  4. #4

    Default Re: Maturing reactions

    Yes, in general, it's best to keep perfume fragrances from exposure to too much air, if they are going to be stored for long periods of time.
    And important to keep the bottle out of exposure to direct sunlight (this also includes bright sources of artificial lighting), especially if the bottle is not darkened.
    That might mean not the most optimal thing to have a bottle that's two-thirds empty if you're going to be storing it for many years.
    The oxidation reaction takes place in the presence of both air and light at the same time.

    Not all fragrance chemicals are so easily vulnerable to oxidation. Typically it's the ones with carbon-carbon double bonds, which includes terpenes and citrus scents.
    (If the chemical name as an "ene" in the name, typically it has a carbon-carbon double bond)

    Often times it doesn't matter if there's some oxidation, but in some cases it really does, when the oxidation products are sensitizers.
    This is the case for limonene (tangy orange smell) found in citrus oils (mandarin/orange), for example.
    If there was any limonene, you'd really want to avoid oxidation, and might want to throw a little antioxidant in the formula as a precaution.
    (Antioxidants will not prevent oxidation, but they will help)

    Typically, natural essential oils will be more vulnerable to oxidation than most synthetic fragrance chemicals. (Just a general trend, there are plenty of exceptions)
    Ocimene and myrcene are terpenes, so are vulnerable to oxidation.

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