Synthetic musk

Musk scented incense. Most modern musk-scented products consist primarily of synthetic musk.

Synthetic musks, known as white musks in the perfume industry, are a class of synthetic aromachemicals to emulate the scent of deer musk or other natural musk. Synthetic musks have a clean, smooth and sweet scent lacking the fecal/"animalic" notes of natural musks and are sometimes attributed as having notes of blackberry, ambrette or ambergris. These compounds are essential in modern perfumery and form the base note foundations of most perfume formulas. Most, if not all musk fragrance used in perfumery today is synthetic.

Synthetic musks can be divided into three major classes — aromatic nitro musks, polycyclic musk compounds, and macrocyclic musk compounds.^[1] The first two groups have broad uses in industry ranging from cosmetics to detergents.

Nitro-musks

Musk ketone, a nitro-musk

An artificial musk was obtained by Albert Baur in 1888 by condensing toluene with isobutyl bromide in the presence of aluminium chloride, and nitrating the product. It was discovered accidentally as a result of Baur's attempts at producing a more effective form of trinitrotoluene (TNT). It appears that the odour depends upon the symmetry of the three nitro group.

• Moskene

Polycyclic musks

Galaxolide, a polycyclic musk

An artificial musk that contains more than one ring in its molecular structure. These musks became popular after World War II and slowly supplanted the nitro-musks in popularity due to the latter's toxicity and molecular instability.

The creation of this class of musks was largely prompted through the need for eliminating the nitro functional group from nitro-musks due to their photochemical reactivity and their instability in alkaline medium. This shown to be possible through the discovery of ambral, a non-nitro aromatic musk, which promoted research in the development of nitro-free musks. This led to the eventual discovery of phantolide, so named due to its commercialization by Givaudan without initial knowledge of it chemical structure (elucidated 4 years later). While poorer in smell strength, the performance and stability of this compound class in harsh detergents led to its common use, which spurred further development of other polycyclic musks including Galaxolide.^[2]

In early 1980s It was discovered in the 1990s that polycyclic musks are also potentially harmful, because the "immediate consequence of inhibition of efflux transporters is that normally excluded xenobiotics will now be able to enter the cell." which could over a long term lead exposure to normally excluded toxicants.^[3] Studies show that exposure to polycyclic musks may break down the body’s defenses against other toxic exposures. Many of these musks were used in large quantities to scent laundry detergents.^{[citation needed]} Levels of these musks in human bodies appear to be associated with the frequency of use of fragranced products, meaning that the more individuals use fragrance, the higher the levels of chemicals like galaxolide and tonalide. Polycyclic musks have been detected in blood, breast milk, and newborns.

Commonly used polycyclic musks include:

• Galaxolide (HHCB)
• Tonalide (Musk Plus, AHTN)
• Phantolide
• Celestolide (Crysolide)
• Traesolide

Macrocyclic musks

Muscone, a macrocyclic musk

A class of artificial musk consisting of a single ring composed of more than 6 carbons (often 10-15). Of all artificial musks, these most resemble the primary odoriferous compound from Tonkin musk in its "large ringed" structure. While the macrocyclic musks extracted from plants consists of large ringed lactones, all animal derived macrocyclic musks are ketones.^[2]

Although muscone, the primary macrocyclic compound of musk was long known, it was only in 1926 that Leopold Ruzicka was able to synthesize this compound in very small quantities. Despite this discovery and the discovery of other pathways for synthesis of macrocyclic musks, compound of this class were not commercially produced and commonly utilized until the late 1990s due to difficulties in their synthesis and consequently higher price.^[4]

About half the human population are anosmic (unable to smell) to macrocyclic musks^{[citation needed]}, possibly due to its high molecular weight. Common macrocyclic musks include:

• Ethylene brassilate
• Globalide (Habanolide)
• Ambrettolide
• Muscone
• Thibetolide (Exaltolide)
• Velvione

Alicyclic musks

Helvetolide, an alicyclic musk

Alicyclic musks, otherwise known as cycloakyl ester or linear musks, are a relatively novel class of musk compounds. The first compound of this class was introduced 1975 with Cyclomusk, though similar structures were noted earlier in citronellyl oxalate and Rosamusk.^[5] Alicyclic musks are dramatically different in structure than previous musks (aromatic, polycyclic, macrocyclic) in that they are modified akyl esters.^[6] Although they were discovered prior to 1980, it was only in 1990 with the discovery and introduction of Helvetolide at Firmenich that a compound of this class was produced at a commercial scale.^[5] Romandolide, a more ambrette and less fruity alicyclic musk compared to Helvetolide, was introduced ten years later.^[6]

Environmental and health issues

The detection of the nitro- and polycyclic chemical groups in human and environmental samples as well as their carcinogenic properties initiated a public debate on the use of these compounds and a ban or reduction of their use in many regions of the world. Research indicates that these musks don’t break down in the environment, can accumulate in human bodies, are potential hormone disruptors and may break down the body’s defenses against other toxic chemical exposures. Macrocyclic musk compounds are expected to replace them since these compounds appear to be safer.^[1]

Synthetic musks (AHTN, HHCB, ATII, ADBI, AHMI, cashmeran (DPMI), musk xylene and musk ketone) enter the aquatic environment through sewer discharges.^[7] They are also present in wastewater treatment plant sludges.^[8]

References