8 julio, 2024

Anisole: what it is, structure, properties, risks and uses

What is anisole?

He anisole, or methoxybenzene, is an organic compound consisting of an aromatic ether whose chemical formula is C6H5OCH3. Its physical state is that of a colorless liquid, which may present yellowish colouring. It is easily recognized by its characteristic smell of anise.

It is a volatile compound with not very high cohesion forces, usual characteristics in light ethers, which are stored in small sealed containers. Specifically, anisole is the simplest of the alkyl aryl ethers, that is, those with an aromatic (Ar) and an alkyl (R) component, Ar-OR.

The group C6H5– comes to denote Ar, and -CH3 to R, thus having C6H5-O-CH3. The aromatic ring, and the presence of -CH3 as a substituent group called methoxy, gives anisole a higher nucleophilicity than benzene and nitrobenzene. Therefore, it serves as an intermediate molecule for the synthesis of compounds with pharmacological activity.

Its characteristic smell of anise has been used to add it to cosmetic and hygiene products that require a pleasant fragrance.

structure of anisole

The upper image shows the molecular structure of anisole using a bar model. The aromatic ring can be seen, whose carbons are sp2 and, therefore, it is flat, like a hexagonal sheet, and linked to it is the methoxy group, whose carbon is sp3, and its hydrogens are above or below the plane of the ring .

The importance of the -OCH3 group in the structure goes beyond that of breaking with the planar geometry of the molecule: it gives it polarity, and consequently, the nonpolar benzene molecule acquires a permanent dipole moment.

dipole moment

This dipole moment is due to the oxygen atom, which attracts the electron densities of both the aromatic ring and the methyl ring. Thanks to this, the anisole molecules can interact through dipole-dipole forces, although it lacks any possibility of forming hydrogen bonds, since it is an ether (ROR does not have H bonded to oxygen).

Its high boiling point (154 °C), experimentally certifies the strong intermolecular interactions that govern its liquid. Likewise, the London dispersion forces, dependent on the molecular mass, and the π-π interactions between the rings themselves are present.

Crystals

The structure of anisole, however, does not allow it to interact strongly enough to become a solid at room temperature (mp = -37 °C). This may also be due to the fact that when the intermolecular distances are reduced, the electrostatic repulsions between the electrons of neighboring aromatic rings begin to gain a lot of force.

Therefore, and according to crystallographic studies, anisole molecules in crystals at a temperature of -173 °C cannot be arranged in such a way that their rings face each other. That is, their aromatic centers do not line up one above the other, but instead an -OCH3 group lies above or below a neighboring ring.

anisole properties

Physical appearance. Colorless liquid, but which may present slight straw-colored tones.
Smell. Lslightly resembling anise seeds.
Flavor. Sweet. However, it is moderately toxic, so this test is dangerous.
Molecular mass. 108.140 g/mol.
Density. 0.995g/mL.
Vapor density. 3.72 (relative to air = 1).
Melting point. -37°C
Boiling point. 154°C.
ignition point. 125°C (open cup).
autoignition temperature. 475°C.
Goo. 0.778 cP at 30°C.
Surface tension. 34.15 dynes/cm at 30°C.
refractive index. 1.5179 at 20°C.
Solubility. Poorly soluble in water (around 1 mg/mL). In other solvents, such as acetone, ethers, and alcohols, it is, however, very soluble.
Nucleophilicity. Anisole’s aromatic ring is rich in electrons. This is due to the fact that oxygen, despite being a very electronegative atom, contributes with the electrons of its π cloud to delocalize them through the ring in numerous resonance structures. Consequently, more electrons travel through the aromatic system and therefore its nucleophilicity increases. Experimentally, the increase in nucleophilicity has been demonstrated by comparing its reactivity, against electrophilic aromatic substitutions, with that of benzene. Thus, the notable effect of the -OCH3 group is evident on the chemical properties of the compound. Likewise, it should be noted that electrophilic substitutions occur in the adjacent (-ortho) and opposite (-para) positions to the methoxy group. That is, it is ortho-para director.
Reactivity. The nucleophilicity of the aromatic ring of anisole already allows us to glimpse its reactivity. Substitutions can occur either on the ring (favored by its nucleophilicity), or on the methoxy group itself; in the latter the O-CH3 bond is broken to replace the -CH3 by another alkyl group: O-alkylation. Therefore, in an alkylation process, anisole can accept an R group (fragment of another molecule) by substituting an H on its ring (C-alkylation), or by substituting CH3 of its methoxy group. The following image illustrates what has just been said:

In the image, the R group is located in the -ortho position, but it can also be in the -para position, opposite to -OCH3. When O-alkylation occurs, a new ether with another -OR group is obtained.

Anisole nomenclature

Name anisole It is the best known and accepted, probably derived from its anise-like odor. However, the name methoxybenzene it is quite specific, since it establishes at once what is the structure and identity of this aromatic ether. This is the name governed by systematic nomenclature.

Another less used name, but equally valid, is phenyl methyl ether, which is governed by the traditional nomenclature. This is perhaps the most specific name of all, as it directly points to the two structural portions of the ether: phenyl-O-methyl, C6H5-O-CH3.

Anisole risks

Medical studies have not yet been able to demonstrate the possible fatal effects of anisole in the body at low doses. However, like almost all chemicals, it is irritating when exposed to the skin, throat, lungs, and eyes in moderate concentrations for long periods of time.

Likewise, due to the nucleophilicity of its ring, a part of it is metabolized and, therefore, it is biodegradable. In fact, due to this property, simulations have shown that it cannot concentrate in aqueous ecosystems, since its organisms first degrade it, and therefore, neither rivers, lakes or seas can accumulate anisole.

In soils, given its volatility, it evaporates quickly and is carried away by air currents. Thus, it does not significantly affect the plant masses or plantations.

On the other hand, atmospherically it reacts with free radicals, and therefore does not represent a risk of contamination for the air we breathe.

Anisole Uses

organic syntheses. Other derivatives can be obtained from anisole by electrophilic aromatic substitution. This makes it possible to use it as an intermediate for the synthesis of drugs, pesticides and solvents, to which it is desired to add its characteristics. The synthetic routes can consist of a C-alkylation or O-alkylation in its majority.
Fragrances. In addition to its use for organic synthesis, it can be used directly as an additive for creams, ointments and perfumes, incorporating anise fragrances to such products.

References

Morrison, RT and Boyd, R, N. Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
Carey, FA Organic Chemistry. (Sixth edition). Mc Graw Hill.
anisole. Retrieved from pubchem.ncbi.nlm.nih.gov.
anisole. Retrieved from en.wikipedia.org.
Methoxybenzene. Recovered from formulacionquimica.com.

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