5-Amino-o-cresol
[CAS# 2835-95-2]

Identification

• Classification: Organic raw materials >> Hydrocarbon compounds and their derivatives >> Aromatic hydrocarbon
• Name: 5-Amino-o-cresol
• Synonyms: 5-Amino-2-methylphenol; 3-Hydroxy-4-methylaniline; 4-Amino-2-hydroxytoluene
• Molecular Formula: C₇H₉NO
• Molecular Weight: 123.15
• CAS Registry Number: 2835-95-2
• EC Number: 220-618-6
• SMILES: CC1=C(C=C(C=C1)N)O

Properties

• Density: 1.2±0.1 g/cm³ Calc.^*
• Melting point: 160 - 162 °C (Expl.)
• Boiling point: 268.6±20.0 °C 760 mmHg (Calc.)^*
• Flash point: 116.3±21.8 °C (Calc.)^*
• Index of refraction: 1.616 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07;GHS09 Warning
• Risk Statements: H315-H317-H319-H335-H400-H410-H411
• Safety Statements: P261-P264-P264+P265-P271-P272-P273-P280-P302+P352-P304+P340-P305+P351+P338-P319-P321-P332+P317-P333+P317-P337+P317-P362+P364-P391-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Specific target organ toxicity - single exposure	STOT SE	3	H335
Eye irritation	Eye Irrit.	2	H319
Skin irritation	Skin Irrit.	2	H315
Skin sensitization	Skin Sens.	1	H317
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Skin sensitization	Skin Sens.	1A	H317
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411
Acute toxicity	Acute Tox.	4	H302
Eye irritation	Eye Irrit.	2A	H319
Acute toxicity	Acute Tox.	4	H332
Acute toxicity	Acute Tox.	4	H312

Discovery and Applications

5-Amino-o-cresol, also known as 5-amino-2-methylphenol, is an aromatic compound characterized by a benzene ring substituted with an amino group (–NH₂) at the 5-position and a hydroxyl group (–OH) at the 2-position, along with a methyl group (–CH₃) ortho to the hydroxyl group. Its molecular formula is C₇H₉NO. This compound appears as a pale solid and is moderately soluble in water and polar organic solvents.

The compound is a derivative of o-cresol (2-methylphenol), modified by the introduction of an amino group at the 5-position. It can be synthesized through various methods, including nitration of o-cresol followed by reduction of the nitro group, or by direct amination reactions on suitably substituted phenolic precursors. These synthetic pathways require careful control of reaction conditions to achieve regioselective substitution and to avoid overreaction or polymerization.

5-Amino-o-cresol has important applications primarily in the cosmetic and pharmaceutical industries. It is widely used as a key intermediate in the formulation of hair dye products. The presence of both amino and hydroxyl groups on the aromatic ring allows it to participate effectively in oxidative coupling reactions with other dye precursors under alkaline conditions, leading to the formation of colored compounds within the hair shaft. Its properties contribute to hair coloring by producing a range of shades from browns to blacks when combined with oxidizing agents such as hydrogen peroxide.

In addition to its role in hair dye chemistry, 5-amino-o-cresol is also employed in the synthesis of various heterocyclic compounds and pharmaceuticals. The reactive amino and hydroxyl groups enable it to serve as a versatile building block for the construction of molecules with potential biological activity. For example, derivatives of 5-amino-o-cresol have been explored for antioxidant, antimicrobial, and enzyme inhibitory properties, although its primary commercial importance remains within the field of cosmetic chemistry.

The compound's chemical reactivity allows it to undergo electrophilic substitution, condensation, and coupling reactions. The amino group, being nucleophilic, can be diazotized to form diazonium salts that serve as intermediates in azo dye synthesis. Meanwhile, the hydroxyl group provides hydrogen bonding capability and affects the compound’s solubility and reactivity. The methyl group contributes to the hydrophobic character and influences the electron density of the aromatic system, affecting substitution patterns.

From an analytical perspective, 5-amino-o-cresol can be characterized by nuclear magnetic resonance (NMR) spectroscopy, which provides detailed information on the position of substituents and the electronic environment of the protons and carbons in the aromatic ring. Infrared (IR) spectroscopy confirms the presence of amino and hydroxyl functional groups through characteristic absorption bands. High-performance liquid chromatography (HPLC) is commonly used to assess purity and monitor its concentration in formulations.

In terms of safety, 5-amino-o-cresol should be handled with appropriate precautions. As with many aromatic amines and phenols, it can cause skin irritation or allergic reactions in sensitive individuals. Proper personal protective equipment, including gloves and eye protection, should be used when handling the compound. Its use in cosmetic products is regulated, and formulations must comply with safety standards set by regulatory authorities to prevent adverse effects.

In conclusion, 5-amino-o-cresol is a functionally versatile aromatic compound primarily valued for its role in hair dye formulations. Its combination of amino, hydroxyl, and methyl substituents provides unique chemical properties that enable its participation in oxidative dye coupling reactions and its use as an intermediate in organic synthesis. Its industrial relevance is supported by well-established synthetic routes and characterization methods, ensuring its continued application in cosmetic and chemical manufacturing.

References

2009. Skin metabolism of aminophenols: Human keratinocytes as a suitable in vitro model to qualitatively predict the dermal transformation of 4-amino-2-hydroxytoluene in vivo. Toxicology and Applied Pharmacology, 235(1).
DOI: 10.1016/j.taap.2008.11.014

2009. Metabolite screening of aromatic amine hair dyes usingin vitrohepatic models. Xenobiotica; the fate of foreign compounds in biological systems, 39(10).
DOI: 10.1080/00498250903134443

2009. Contact dermatitis to 4-amino-2-hydroxytoluene in hair dye. Contact Dermatitis, 60(2).
DOI: 10.1111/j.1600-0536.2008.01491.x