fragrances
In the article
below the chemistry of fragrances in household cleaning products is discussed.
This chemistry has to be taken in account because the organoleptic properties
can alter drastically. Reactions which can take place are with oxygen, ammonia
and amines, phophonates, sulphates, sulphonates, light etc.
SATOCONOR.COM 
J.G.
van der Galiën ‘Chemistry With A Smell’ 2.2. (2003)
Full paper
SATOCONOR.COM Journal of Organic Chemistry
Chemistry With A
Smell
The reactions of
fragrances with other ingredients of household cleaning products
By: Johan van der Galiën
For comments: johan.van.der.galien@satoconor.com
The most common
reactions fragrances can undergo due to the chemical environment of household
cleaning products are discussed. To bare in mind that some of them will only be
minor reactions but can determine the shelf live of a cleaning product.
The action
of cleaning products can be based on a number of chemicals. Table 1. gives the most
common ones. Because of the reactivity of these cleaning active chemicals the
stability of other components like fragrance ingredients and dyes must be
guaranteed. To achieve this one must take the chemistry of these components in
to account. In a way that a fragrance ingredient and dye cannot react, or as a
compromise only very slowly (shelf live of many years), with the other
ingredients of the cleaning mixture. Remember that it can be possible that the
ingredients of a fragrance blend react with each other in the chemical
environment of the cleaning products. Also a reaction between a fragrance
ingredient and a dye belongs to the possibilities to be considered. Although
this essay is focused on fragrances the same principles also apply more or less
to dyes, if the dyes contain one or more of the discussed functional groups.
|
Name of cleaning product ingredient |
Chemical formula of active group |
|
Alkyl Benzenesulphonates and Alkyl Sulphonates |
R-SO3- (detergent) |
|
Alkyl Phosphates |
R-PO42- (detergent) |
|
Alkyl Sulphates |
R-SO4- (detergent) |
|
Ammonia |
NH3 and |
|
Ammonium Chlorides |
R-NH3+Cl-, R2NH2+Cl-, R3NH+Cl- and R4N+Cl- (disinfectant) |
|
Calcium Carbonates |
Abrasive no active group |
|
Citric Acid |
H+ (weak acid) |
|
Ethanol |
R-OH (alcohol) |
|
Glycol Esters |
R-CO2-CH2-CH2-OH and R-CO2-CH2-CH2-O2C-R' (emulsifier, thickener and dispenser, ester and alcohol) |
|
Hydrogen Chloride |
H+ (strong acid) |
|
Hydrogen Peroxide |
[O] (bleach by active oxygen) |
|
Isopropanol |
R-OH (alcohol) |
|
ortho-Phenylphenol Sodium Salt and derivatives |
Ar-O-Na+ (aromatic alcohol = phenol) |
|
Silica |
Abrasive no active group |
|
Sodium Bicarbonate |
|
|
Sodium Carbonate |
|
|
Sodium Chloride |
Abrasive no active group |
|
Sodium Hydroxide |
|
|
Sodium Hypochlorite |
[O] (bleach by active chlorine) |
|
Sodium Silicate |
|
Table 1: De most common
active ingredients in household cleaning products1.
But there
are also environmental influences outside the cleaning mixture like reactivity
towards the container material, sunlight and oxygen from the air.
The
discussed reactions can be very slow in the chemical environment of the
cleaning product, also the equilibrium concentrations will strongly depend upon
this environment. All these factors determine the shelf-live (like odor, pH,
colour and cleaning activity changes) of the cleaning product.
Fig. 1: Some
possible structures with epoxides and lactones. All shown lactones can be
considered to have formed from a hydroxycarboxylic acid. The Greek letters
(alpha-delta) comes from the position of the hydroxyl group with regards to the
carboxylic acid group.
Most fragrances contain one or more of the
following functional groups: double bonds (CH2=CH-R, cis and trans
R-CH=CH-R, CR2=CH-R and CR2=CR2), epoxide (see
Fig. 1), hydroxyl (-OH), secondary aromatic amines (Ar-NH-R), (carbonyl
[aldehyde (R-CHO) and ketone (R-CO-R)], carboxylic acid (R-COOH), ester
(R-COOR) and a special case of esters in a ring the lactone-group (see Fig. 1).
These functional groups can under go reactions during storage from chemicals
(mentioned in Table 1) in the cleaning product or due to chemicals in the
container material, heat, light and oxygen from the air. The composition of a
perfumed cleaning product must be such that the fragrances survive the chemical
and environmental influences as long as possible. This is so important because
the intended organoleptic properties can be greatly reduced or even altered
totally!
This essay
will discuss the most fundamental reactions fragrances, with the mentioned
functional groups under the mentioned chemical influences, can undergo.
2. The Reactions With Other Ingredients2,5
2.1. Reactions with
sulphonates, phosphates and sulphates
The
primary alkyl sulphonate-moiety of detergents is a good leaving group when
there is a base and a reactant with active hydrogen present (see Fig. 2). The
reactant with active hydrogen must be Z-CH2-Z', with Z and Z' =
COOR, CHO, COR, COO-, phenyl and many not for fragrances relevant groups.
All these groups are strong electron withdrawing groups. For example fragrances
which fit the criteria of Z and Z' are diphenylmethane, ethyl phenylacetate (1),
methyl phenylacetate, and phenylacetic acid (carboxylic group ionised in basic
media to COO-).
Fig. 2: The SN2-mechanism
of the base induced reaction between sulphonates and a fragrance with active
hydrogens.
1 is a not natural
occurring fragrance, which is synthesised from phenyl acetic acid and ethanol
in an esterification reaction. It has a very intense honey-like odor.3
The methylene group with the active hydrogen can become carbanion 2 when
a base is present. This is energetically favoured because of the strong
electron withdrawing groups (phenyl and carbonyl) at which it is attached.
These groups delocalise and stabilise the negative charge. This carbanion can
attack the carbon atom of for instance octyl sulphonate (3) in an SN2
type reaction. Although the ionised 3 is shown in Fig. 2 it is more
likely that the mechanism goes through the protonated 3. Because two
negative charged particle would repel each other! The leaving group is then HSO3-
of course and not SO32-. The result is a coupling
reaction, with a new carbon-carbon bond, between 1 and the octyl-chain
of 3 resulting in compound 4. Which in principle can undergo the
reaction once more because it still has one active hydrogen. But is now more
sterically hindered so the subsequent reaction will be much slower. The
phosphate (5) and sulphate (6) groups are also good leaving
groups and they can show also an SN2 kind of reaction like in Fig.
2.
The
nucleophile can also be a secondary aromatic amine (Ar-NH-R) like the fragrance
methyl-N-methyl anthranilate. To my knowledge this is the only fragrance
with such functionality. It displays a musty, floral odor recalling orange
blossom and mandarins.3 According to the literature it can react
with sulphonates and sulphates as the leaving groups to tertiary and quaternary
amines (ArRR'N and ArRR'R''N+X- with X = HSO3-
or HSO4-).
The
sulphonate, phosphate and sulphate are even better leaving groups in an acidic
environment. Because the leaving group capacity correlates well with the
acidity (pKa) of the conjugated Brønsted acid. As rule of thumb, for
neutral molecules, a group with a conjugated acid pKa less then 6
can be called a good leaving group.4 This is because the C-X bond
correlates well with the H-X bond. The pKa's for the first acid
dissociation step are respectively H2SO3 = 1.816,
H3PO4 = 2.126 and H2SO4
˜ -102. The second acid dissociation step pKa are HSO3-
= 6.916, H2PO4- = 7.216
and HSO4- = 1.992. Phosphoric acid has a third
acid dissociation step with pKa HPO42- = 12.676.
So under certain acidic conditions all three groups would be fully protonated
and would be good leaving groups! It remains to be seen whether these
conditions would be too harsh for cleaning products. As a matter of fact Ternay5
discusses the phosphate group as a fully protonated good leaving group par
excellence in living systems. The pH conditions in living systems cannot be
too harsh for cleaning products! Because of this fact these groups can induce
another kind of reactions whereby there is solvolysis of the C-X bond leading
to carbocation 7. (See Fig. 3.)
Fig. 3: Alkylation
of a double bond carbon by a carbocation mechanism induced by solvolysis of
sulphonate, phosphate or sulphate C-X bond.
The
carbocation can attack double bonds like the one from ethyl cinnamate (8).
A natural occurring fragrance from styrax and Kaempferia galanga with
sweet, balsamic-fruity, honey like odor.3 The attack can be on the
two places of the double bond but I suppose that it will go as shown in Fig. 3.
Because then the resulting carbocation is delocalised and stabilised by the
phenyl ring. Attack on the other side is unfavourable because of the strong
electron withdrawing carbonyl!
Of course
in aqueous media there is a competing reaction the SN1 with water as
the nucleophile leading to the corresponding alcohol. But make no mistake in
plants the oligomerisations of terpene takes place through a similar mechanism,
which is of course in the aqueous medium of the cytoplasm.5
The result
of this alkyl coupling reaction is two isomers because the carbocation 9
can rearrange the molecule in a trans (10) or cis (11)
position, of the alkyl and phenyl group.
Carbocation
7 may also attack the phenyl ring of
Fig. 4: The
mechanism of electrophylic aromatic substitution with compound 8 and
carbocation 7. The intermediate resonance structures of ortho
attack are given. But both possible products, ortho (16) and para
(17) are shown.
The attack
is most likely on the ortho and para position of the phenyl ring
because the unsaturated ester moiety is a mild activating ortho/para director.
This can be understood from resonance structure 13. Here the double bond
shifts so that the carbocation is adjacent to the strong electron with drawing
carbonyl, which is unfavourable. Because of this field effect the unsaturated
ester moiety is only a mild activating group for electrophylic aromatic
substitution. Would the target molecule been [2-methylpropyl-1-ene]-benzene
than the double bond would haven been a strong activating ortho/para
director. The resulting reaction resonance structure would then involve a
highly stable tertiary carbocation. The ratio of ortho/para product
always depend strongly on the reaction conditions.
2.2.
Reactions with ammonia and ammonium chlorides
The
reactions of ammonia and ammonium chlorides are more or less the same. Of
course the ammonium chlorides as from primary and secondary amines is
concerned, the quaternary ammonium salts from tertiary amines are inert towards
the functional groups of fragrances mentioned in the introduction, give
different substituted reaction products. The crux of the matter is the H-N on
an amine group the react just like the one in ammonia. The reaction is always
through the free amine, which is always present for a certain amount through
the dissociation equilibrium. I will give reaction schemes in the Fig's with
either ammonia or substituted ammonia (amines), but the principles are the
same!
Fig. 5: The
addition of ammonia by the carbonyl of an aromatic aldehyde.
The
addition of ammonia by the carbonyl of aldehydes and ketones usually gives two
main products. Shown in Fig. 5 is this reaction of ethyl vanillin (18),
a synthetic fragrance with an intensely sweet, creamy, vanilla-like odor. The
two products with this compound are a hemiaminal (19) which partly will
undergo an elimination reaction to the imine (20). Non of these two
compounds is stable. 19 can react with carbonyl compounds. And 20
will polymerise because of its hydrogen atom attached to nitrogen, would
ammonia be replaced by a primary amine then the imine would be stable and the
main product. In case of a secondary amine the reaction would give no imines
but more or less stable hemiaminals. Though they can, if there is a-hydrogen in
the molecule, split of water to enamines (R-CH=CH-NR2).
Fig. 6: The
reaction of a lactones with a primary amine. As I said earlier one can
substitute the primary amine with ammonia and one gets a mono substituted amide
moiety. Secondary and tertiary amines do not give this reaction!
In
principle the carboxylic group can react with ammonia and amines but it
requires a pyrolysis step (extreme heating between 100o and 300o
degrees Celsius). This of course beyond the scope of production, storage and
use of household cleaning products. So I will not go into the details! Under
normal conditions though there can be a reaction of carboxylic esters and
lactones between ammonia or amines. In Fig. 6 the reaction of
γ-undecalactone (21) between octyl amine is shown. 21
is also known with the misleading name of peach aldehyde which of course
displays a very intense, fruity, peach like odor. As I said earlier the
lactones are a special case of cyclic carboxylic esters and they also show a
special cyclic reaction product called lactams (23). They net
split of water H+ form the first step and
Fig. 7: The
reaction of ammonia with an epoxide group containing fragrance (andrane 24).
Ammonia can be substituted for amines, as I said earlier, but they will no show
as many reaction steps as ammonia (see text).
Andrane or
8,9-epoxy cedrane or a-cedrane epoxide (24) is one of the few
fragrances with an epoxide group. It has a woody, dry, patchouli and ambergris
odor profile. Its olfactory description is a precious wood odor, reminiscent of
ambergris to sandalwood. Useful in soaps and artificial patchouli.7
It can react with ammonia and ammonium salts containing household cleaning
products like perfumed window cleaners. The products are beta-hydroxyamines like
25, 26 and 27. The main product with 24
is the primary beta-hydroxyamine (25) although also to some less
extent it reacts further to secondary (26) and tertiary
ß-hydroxyamine (27). In case of a primary amine a secondary
beta-hydroxyamine is formed with can react again with the epoxide. In case of a
secondary amine a tertiary beta-hydroxyamine is formed which can not react
further.
The first
reaction of this paragraph is with an aldehyde group containing fragrance like
a -amylcinnamic aldehyde (28) in an ethanol based cleaning
product. 28 is a fragrance with an intense floral, jasmin-like
odor with a warm dry-out.3 Under basic conditions in the first step
of Fig.
Fig. 8: The base
catalysed formation of an acetal from an aldehyde and an alcohol.
The illustration
is only an example, remember this could be any aldehyde group containing
fragrance and any hydroxyl group containing molecule. This reaction can even be
intermolecular. The scope of this reaction is not limited to aldehydes alone,
ketones also show this kind of reaction the products are then called hemiketals
and ketals. Bases (pH > 7) shift the equilibrium to the right and the
presence's of acids (pH < 7) shift it to the left.
Fragrances
like phenylacetic acid (31) can undergo a reaction in a mixture
containing alcohol to yield esters. The alcohol can be the cleaning agent or
even another fragrance like the shown n-undecanol (32). 31
occurs naturally in neroli oil and has a sweet, honey-like odor with a slight
animalic note. 32 is a synthetic fragrance with slightly fatty,
floral-fruity odor.3
Fig. 9: The acid
and base catalysed esterification reaction of phenylacetic acid and n-undecanol.
The
reaction is base and acid catalysed and it depends on the circumstances where
the equilibrium lies. This means that ester groups containing fragrances like
methylsalicylate (from wintergreen and sweet birch oil with a pungent sweet,
rather musty odor with green, medicinal undertones3) can undergo the
reverse reaction if there is enough water present (hydrolysis of esters, when
base catalysed it is called the saponification reaction).
2.4. The
reactions only involving acid or base8
The already
mentioned hydrolysis of esters in paragraph 2.3. is one of them. Also a ring
opening reaction can occur with lactones, which is of course related to the
hydrolysis of esters.
Fig. 10: The base
and acid catalysed ring opening of lactones.
The
smaller the ring the more reactive towards opening. The reaction is base and
acid catalysed. Shown in Fig. 10 is the opening of gamma-nonalactone (34)
to the corresponding gamma-hydroxy carboxylic acid (35) 34
is a synthetic fragrance also known with the misleading name coconut aldehyde.
It has a creamy-sweet, soft coconut odor.3
3. The Reactions Between Fragrances And Fragrances With Dyes.
3.1. The aldol
reaction
A famous
reaction aldehydes and ketones can undergo is the base catalysed aldol
condensation. Here a C-C bond is formed between two molecules with carbonyl
groups where at least one molecule has at least one active hydrogen a to the
carbonyl.
Fig. 11: The aldol
condensation of n-octanal.
This means
that the reaction can be between two different molecules and as a dimerization.
In Fig. 11 is shown the dimerization of n-octanal (36). The
result is alpha,beta-unstaturated aldehyde (37). 36
is a component from orange and rose oils. It has very powerful, wax-like odor
with floral undertones.3
4. The Environmental Reactions
4.1. The reaction
with oxygen8
Oxidation
through oxygen in the air is probably the most common reaction of fragrances.
Oxidation can also happen by exposure to bleaching chemicals, UV-light and
radicals. Here above the oxidation reaction of the aldehyde group containing
citral (38) fragrance is shown. 38 occurs naturally
in oils from lemongrass and Litsea cubeba, it has a powerful lemon odor
with green and bitter notes.3
Fig. 12: The
oxidation of the aldehyde group of citral to a carboxylic acid group.
The
product is a carboxylic acid (39), with can under go follow up
reactions with for instance alcohols, as you have seen earlier in the text. It
is not only the aldehyde group in citral, which can undergo reactions. The
double bonds with its a-hydrogens also possess that capability. They can give
peroxides (R-CH=CH-CHR'-O-O-H) which can undergo a plethora of follow-up
reactions like polymerisation. One can partly undermine these reactions by
means of antioxidants.
4.2. The
reactions induced by (sun)-light8
In
fragrance molecules isomerizations can occur under influence of heat, light, air
and pH. But the light reaction is the most common one. These reactions are
mostly double bond shifts and double bond cis and trans
isomerizations.
Fig. 13: The light
induced isomerization of citral.
Even at
room temperature there is enough heat for these reactions to take place. Take
for instance cis-3-hexenal, it isomerises to trans-3-hexenal
completely in 24 hours. The shown reaction is the double bond shift by
photoisomerization of citral (40). I think that the reaction
product 41 will have the tendency to react further so that the
double bonds will be conjugated C=C-C=C and form one π-system.
-o0o-
1) Anonymous (A+ Cleaning)
'Commercial cleaners' webpage.
2) March J. 'Advanced organic
chemistry: Reactions, mechanisms and structure' 3rd Edition, Wiley-Interscience
(1984)
3) Hall R., Klemme D.,
Nienhaus J. 'The H&R Book: Guide to fragrance ingredients' Johnson
Publications Limited (1985)
4) Berger D. Response in
MadSci Network 'Re: Leaving groups in organic chemistry' (2002) webpage.
5) Ternay A.L. 'Contemporary
organic chemistry' 2nd edition, W.B. Saunders Company (1979)
6) 'Handbook of chemistry and
physics' 57th edition (1976-1977)
7) IFF 'Fragrance
Ingredients: Andrane' webpage
8) Shaikh Y. 'Specialty aroma
chemicals in flavors and fragrances' Allured Publishing Corporation (2002)