Hydroxybenzoic acids

Hydroxybenzoic acids
Surname	2-hydroxybenzoic acid	3-hydroxybenzoic acid	4-hydroxybenzoic acid
other names	o -hydroxybenzoic acid salicylic acid	m -hydroxybenzoic acid	p -hydroxybenzoic acid
Structural formula
CAS number	69-72-7	99-06-9	99-96-7
PubChem	338	7420	135
Molecular formula	C 7 H 6 O 3
Molar mass	138.12 g mol −1
Physical state	firmly
Brief description	white crystals		colorless crystals
Melting point	158.3 ° C	200 ° C	215 ° C
boiling point	211 ° C (20  mmHg )	-	-
pK s COOH	2.75	3.90	4.61
pK s OH	12.38	9.78	9.31
Solubility in water	2 g l −1 (20 ° C)		5 g l −1 (20 ° C)
GHS labeling	danger	Caution	danger
H and P phrases	302-315-318-335	302-315-319-335	318-335
	no EUH phrases	no EUH phrases	no EUH phrases
	261-270-280-302 + 352 305 + 351 + 338	261-305 + 351 + 338	260-280-305 + 351 + 338-310

In chemistry , the hydroxybenzoic acids form a group of substances that is derived from both benzoic acid and phenol . The structure consists of a benzene ring with attached carboxy  (–COOH) and hydroxy  (–OH) groups as substituents . Their different arrangement ( ortho , meta or para ) result in three constitutional isomers . They are mainly to be regarded as derivatives of benzoic acid. 2-Hydroxybenzoic acid is known by its common name, salicylic acid . The hydroxybenzoic acids also belong to the group of phenolic acids .

properties

Hydroxybenzoic acids belong to the group of substances:

• 2-hydroxybenzoic acid ( ortho -hydroxybenzoic acid ), also known as salicylic acid
• 3-hydroxybenzoic acid ( meta -hydroxybenzoic acid )
• 4-hydroxybenzoic acid ( para -hydroxybenzoic acid) known as PHB, their esters, the parabens are

They dissolve in diethyl ether and acetone . Similar to many organic acids, they hardly dissolve in water at room temperature, but significantly better in the heat, but well in the alkaline, releasing the proton at the carboxy group and therefore forming anions , and can be precipitated again by adding a stronger acid.

Other representatives

The hydroxybenzoic acids can be found in almost all spices in addition to the hydroxycinnamic acids . Often they are there in the form of their esters or glycosides .

Many derivatives that are multiply substituted with hydroxyl and methoxy groups occur as natural substances :

• Dihydroxybenzoic acids
• Trihydroxybenzoic acids , e.g. B. Gallic acid (3,4,5-trihydroxybenzoic acid)
• Syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid )
• Vanillic acid (4-hydroxy-3-methoxybenzoic acid)

proof

The so-called Vitali reaction is a non-specific proof of hydroxybenzoic acids .

Individual evidence

1. ↑ ^{a b c d e f} CRC Handbook of Tables for Organic Compound Identification , Third Edition, 1984, ISBN 0-8493-0303-6 .
2. ↑ ^{a b} Entry on 2-hydroxybenzoic acid (salicylic acid) in the GESTIS substance database of the IFA , accessed on October 16, 2016(JavaScript required) .
3. ↑ ^{a b} Entry on 4-hydroxybenzoic acid in the GESTIS substance database of the IFA , accessed on October 16, 2016(JavaScript required) .
4. ↑ Data sheet 3-Hydroxybenzoic acid from Sigma-Aldrich , accessed on October 16, 2016 ( PDF ).