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What is Cresol?

Cresols are defined as organic compounds which are methyl phenol. They are a widely occurring manufactured and natural group of aromatic organic compounds, which can be categorized as phenols (also referred to as phenolics, sometimes). Cresols are categorised as solid or liquid based on their melting point, which is similar to room temperature.

About Cresols

These, like other forms of phenols, oxidise slowly when exposed to oxygen, and impurities frequently give cresol samples a yellowish to brownish-red tint. Also, cresols hold an odour characteristic to that of other simple phenols, reminiscent of a somewhat "coal tar" smell. The term cresol reflects their structure, being phenols, and the traditional source, creosote.

Cresols also have other names such as hydroxytoluene.

Structure and Production of Cresols

Cresols, in its chemical structure, a molecule has a methyl group substituted onto the phenol ring. There exist three forms (otherwise, isomers) of cresol which are given as meta-cresol (m-cresol), para-cresol (p-cresol), and ortho-cresol (o-cresol). These forms take place either separately or as a mixture, which can also be known as cresol or, more specifically as tricresol.

About half of the supply of cresols of the world is extracted from coal tar. The rest can be produced by the hydrolysis of chlorotoluenes or by the related sulfonates. The other method entails methylation of phenol with methanol over a solid acid catalyst, often comprising alumina or magnesium oxide. Temperatures ranging above 300°C are given as typical. Anisole converts to cresols as per the given conditions.

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Health Effects

When the cresol compounds are ingested, inhaled, or applied to the skin, they might become very harmful. The effects that are observed in people are burning of skin, mouth, throat, and eyes, irritation; vomiting and abdominal pain; anaemia; heart damage; kidney and liver damage; coma; facial paralysis; and death.

Breathing this compound at high levels for a short time will result in irritation of the throat and nose. Besides these effects, very little is known about the breathing cresol effects, for example, at the lower levels, over longer times.

Ingesting this compound at high levels results in mouth and throat burns, kidney problems, vomiting, abdominal pain, and also effects on the nervous system and blood.

Skin contact with high levels of cresol compounds can damage the kidneys, blood, liver, brain, and lungs and burns the skin.

Cresol use has been found to have the same effects in both short- and long-term animal experiments. No animal or human studies have shown the harmful effects of cresols on re-production.

Also, it is not known what the effects are from skin contact or the long-term ingestion with low levels of cresols.

Why is m-Cresol More Acidic Compared to p-Cresol?

m-cresol is more acidic compared to p-cresol. This is completely true in water (the p-cresol is 10.3 and pKa of m-cresol is 10.1), and it is also completely true in the gas-phase with the difference in the acidity of ca 0.7 kcal/mol.

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The classic explanation of the cresol is that a methyl should provide a positive inductive effect (which is +I) to the ring. They increase the negative charge on phenolate oxygen, consequently making all the cresols less acidic compared to phenol.

Since the inductive effect is the field-effect, it should provide a larger effect for the lower effect and ortho-derivative for the para. Therefore, the order should be as:

p-cresol << m-cresol << o-cresol << phenol.

As we can see from the above-given Pka data, the meta-compound cannot follow the prediction.

Let us look at the resonance of phenolate ions: the para and ortho-positions are partially negative.

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Hyperconjugation can be given as somewhat the obscure effect and involves the sigma bond interaction (in the case of the C-H bonds) and a non-binding pi or sigma bond.

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Hyperconjugation is an example that is used to explain the stability of a tertiary carbocation over the primary carbocation.

If an extra methyl stabilizes a positive charge, it completely makes sense that a negative charge can be destabilized by methyl.

The resonance structures also clearly describe that in both para and ortho positions, there exists a partially negative charge (a lone pair of electrons). Hence, an extra methyl in the given position should contain a destabilizing (i.e., negative) effect on the acidity in para and ortho, but not in the meta.

Thus, meta is indeed more acidic compared to para.

It means, if we combine both these effects, the para and ortho derivative should be the least acidic because of the resonance or hyperconjugation, and the meta should still be less acidic in comparison with the phenol because of the positive inductive effect. And due to similar reasons, the ortho should contain the largest +I effect. Thus, it should be the least acidic of all the isomers.

FAQs (Frequently Asked Questions)

1. Give the Applications of Cresols?

Answer: Let us look at the use of cresols or the applications of cresols. Cresols are the precursors or synthetic intermediates to the other materials and compounds, including pesticides, plastics, dyes, and pharmaceuticals.

For disinfectants or cresol bactericides, the mechanism of action is because of the destruction of bacterial cell membranes.

Very recently, cresols have been used in creating a breakthrough in the manufacture of carbon nanotubes at a scale that is separated but not twisted, without any additional chemicals that change the nanotube's surface properties.

2. Explain Why o-Fluorophenol is Considered to be More Acidic than p-Fluorophenol?

Answer: o-fluorophenol is considered to be more acidic compared to p-fluorophenol. This is due to the reason that fluorine is more electronegative compared to oxygen and when both conjugate bases are produced, the negative charge present on oxygen is more stabilized by the ortho fluorine than para fluorine because of the lesser distance and thus greater inductive effect. Hence, a more stable conjugate base forms more acidic acid.

3. Explain if Picric Acid is More Acidic than Phenol?

Answer: Yes, picric acid is more acidic compared to phenol due to the presence of three nitro groups (which is the deactivating group) at o-positions and p- positions, which makes it more acidic compared to phenol (the carbolic acid) which contain only one hydroxyl group.

4. Why is Phenol Acidic in Nature?

Answer: The proton present on the OH group of a phenolic compound is replaced by a metallic cation easily using the alkali, such as in the reaction of alkali and mineral acid. The reason for this is the negative charge, which is left on the oxygen of phenoxide ion after the proton removal is stabilized because of the delocalization onto the benzene ring.