Guaiacol () is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
C
6H
4(OH)(OCH
3). It is a phenolic compound containing an
methoxy
In organic chemistry, a methoxy group is the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula .
On a benzene ring, the Hammett equation classifies a methoxy substituent at the ''para'' position as ...
functional group. Guaiacol appears as a viscous colorless oil, although aged or impure samples are often yellowish. It occurs widely in nature and is a common product of the pyrolysis of wood.
[
]
Occurrence
Guaiacol is usually derived from guaiacum
''Guaiacum'' (''OED'' 2nd edition, 1989.[Entry "guaiacum"](_blank)
in
creosote
Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel. They are typically used as preservatives or antiseptics.
Some creosote types were ...
.
It is produced by a variety of plants. It is also found in essential oils from celery
Celery (''Apium graveolens'') is a marshland plant in the family Apiaceae that has been cultivated as a vegetable since antiquity. Celery has a long fibrous stalk tapering into leaves. Depending on location and cultivar, either its stalks, lea ...
seeds, tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
leaves, orange
Orange most often refers to:
*Orange (fruit), the fruit of the tree species '' Citrus'' × ''sinensis''
** Orange blossom, its fragrant flower
*Orange (colour), from the color of an orange, occurs between red and yellow in the visible spectrum
* ...
leaves, and lemon
The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China.
The tree's ellipsoidal yellow fruit is used for culin ...
peels. The pure substance is colorless, but samples become yellow upon exposure to air and light. The compound is present in wood smoke
Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product ...
, resulting from the pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
of lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
. The compound contributes to the flavor of many substances such as whiskey
Whisky or whiskey is a type of distilled alcoholic beverage made from fermented grain mash. Various grains (which may be malted) are used for different varieties, including barley, corn, rye, and wheat. Whisky is typically aged in wooden cask ...
and roasted coffee
Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world.
Seeds of th ...
.
Preparation
The compound was first isolated by Otto Unverdorben
Otto Unverdorben (13 October 1806 – 28 November 1873) was a German chemist and merchant who was born in Dahme/Marke. After completing his schooling in Dresden, he studied chemistry at Halle, Leipzig and Berlin.
In 1826 at the age of 20, Unver ...
in 1826. Guaiacol is produced by methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
of ''o''-catechol
Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amoun ...
, for example using potash
Potash () includes various mined and manufactured salts that contain potassium in water-soluble form. and dimethyl sulfate
Dimethyl sulfate (DMS) is a chemical compound with formula (CH3O)2SO2. As the diester of methanol and sulfuric acid, its formula is often written as ( CH3)2 SO4 or Me2SO4, where CH3 or Me is methyl. Me2SO4 is mainly used as a methylating agent ...
:
:C6H4(OH)2 + (CH3O)2SO2 → C6H4(OH)(OCH3) + HO(CH3O)SO2
Laboratory methods
Guaiacol can be prepared by diverse routes in the laboratory. ''o''-Anisidine, derived in two steps from anisole
Anisole, or methoxybenzene, is an organic compound with the formula CH3OC6H5. It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances. The compound is ...
, can be hydrolyzed via its diazonium
Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide.
General properti ...
derivative. Guaiacol can be synthesized by the dimethylation of catechol
Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amoun ...
followed by selective mono-demethylation Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen ato ...
.
:C6H4(OCH3)2 + C2H5SNa → C6H4(OCH3)(ONa) + C2H5SCH3
Uses and chemical reactions
Syringyl/guaiacyl ratio
Lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
, comprising a major fraction of biomass, is sometimes classified according to the guaiacyl component. Pyrolysis of lignin from gymnosperm
The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, ''Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμνό ...
s gives more guaiacol, resulting from removal of the propenyl In organic chemistry, 1-propenyl (or simply propenyl) has the formula CH=CHCH3 and 2-propenyl (isopropenyl) has the formula CH2=C-CH3. These groups are found in many compounds. Propenyl compounds are isomeric with allyl compounds, which have the f ...
group of coniferyl alcohol
Coniferyl alcohol is an organic compound with the formula HO(CH3O)C6H3CH=CHCH2OH. A colourless or white solid, it is one of the monolignols, produced via the phenylpropanoid biochemical pathway. When copolymerized with related aromatic compounds, ...
. These lignins are said to have a high guaiacyl (or G) content. In contrast, lignins derived from sinapyl alcohol
Sinapyl alcohol is an organic compound structurally related to cinnamic acid. It is biosynthetized via the phenylpropanoid biochemical pathway, its immediate precursor being sinapaldehyde. This phytochemical is one of the monolignols, which are ...
affords syringol
Syringol is the organic compound with the formula HO(CH3O)2C6H3. The molecule is a phenol, with methoxy groups in the flanking (2 and 6) positions. It is the symmetrically dimethylated derivative of pyrogallol. It is a colorless solid, althoug ...
. A high syringyl (or S) content is indicative of lignin from angiosperm
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s. Sugarcane bagasse is one useful source of guaiacol; pyrolysis of the bagasse lignins yields compounds including guaiacol, 4-methylguaiacol and 4-vinylguaiacol.
Chemical intermediate
Guaiacol is a useful precursor for the synthesis of other compounds. Being derived from biomass, it is a potential component or precursor to "green fuels".
Guaiacol is also a useful reagent for the quantification of peroxidase
Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides.
Functionality
Peroxidases typically ca ...
s, as in the presence of hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
these enzymes will catalyse with it the formation of tetraguaiacol, a coloured compound that can be quantified by its absorbance at 420–470 nm, following the equation:
4 + 2 → + 8 .
Medicinal and food
Guaiacol is a precursor to various flavorant
A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive used to improve the taste or smell of food. It changes the perceptual impression of food as determined primarily by the chemoreceptors of the gust ...
s, such as eugenol
Eugenol is an allyl chain-substituted guaiacol, a member of the allylbenzene class of chemical compounds. It is a colorless to pale yellow, aromatic oily liquid extracted from certain essential oils especially from clove, nutmeg, cinnamon, basil ...
. An estimated 85% of the world's supply of vanillin
Vanillin is an organic compound with the molecular formula . It is a phenolic aldehyde. Its functional groups include aldehyde, hydroxyl, and ether. It is the primary component of the extract of the vanilla bean. Synthetic vanillin is now used ...
comes from guaiacol. Because consumers tend to prefer the taste of natural vanillin to synthetic vanillin, methods such as microbial fermentation have been adopted to chemically simulate the taste of natural vanillin. The route entails the condensation reaction
In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...
of glyoxylic acid
Glyoxylic acid or oxoacetic acid is an organic compound. Together with acetic acid, glycolic acid, and oxalic acid, glyoxylic acid is one of the C2 carboxylic acids. It is a colourless solid that occurs naturally and is useful industrially.
Str ...
with guaiacol to give mandelic acid, which is oxidized to produce phenylglyoxylic acid
Phenylglyoxylic acid is the organic compound with the formula C6H5C(O)CO2H. The conjugate base, known as benzoylformate is the substrate of benzoylformate decarboxylase, a thiamine diphosphate-dependent enzyme:
:benzoylformate + H+ benzaldehyde ...
. This acid undergoes a decarboxylation to afford vanillin. The crude vanillin product can then be purified with vacuum distillation and recrystallization.
Guaiacol is also used medicinally as an expectorant
Mucoactive agents are a class of chemical agents that aid in the clearance of mucus or sputum from the upper and lower airways, including the lungs, bronchi, and trachea. Mucoactive drugs include expectorants, mucolytics, mucoregulators, and muco ...
, antiseptic
An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...
, and local anesthetic
An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia — in other words, to result in a temporary loss of sensation or awareness. They may be divided into two ...
.
Guaiacol is produced in the gut of desert locust
The desert locust (''Schistocerca gregaria'') is a species of locust, a periodically swarming, short-horned grasshopper in the family Acrididae. They are found primarily in the deserts and dry areas of northern and eastern Africa, Arabia, and sou ...
s, ''Schistocerca gregaria'', by the breakdown of plant material. This process is undertaken by the gut bacterium ''Pantoea agglomerans
''Pantoea agglomerans'' is a Gram-negative bacterium that belongs to the family Erwiniaceae.
It was formerly called ''Enterobacter agglomerans'', or ''Erwinia herbicola'' and is a ubiquitous bacterium commonly isolated from plant surfaces, seed ...
'' (''Enterobacter''). It is one of the main components of the pheromones that cause locust
Locusts (derived from the Vulgar Latin ''locusta'', meaning grasshopper) are various species of short-horned grasshoppers in the family Acrididae that have a swarming phase. These insects are usually solitary, but under certain circumstanc ...
swarming
Swarm behaviour, or swarming, is a collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving ''en masse'' or migrating in some direction. ...
.
Safety
Methoxyphenol Methoxyphenol or hydroxyanisole may refer to:
* 2-Methoxyphenol (guaiacol, ''o''-methoxyphenol, methylcatechol, 2-hydroxyanisole)
* 3-Methoxyphenol (''m''-methoxyphenol, ''m''-guaiacol, resorcinol monomethyl ether, 3-hydroxyanisole, ''m''-hydroxyan ...
s are potential biomarkers of biomass smoke exposure, such as from inhalation of woodsmoke. Dietary sources of methoxyphenols overwhelm the contribution from inhalational exposures to woodsmoke.
See also
* Creosote
Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel. They are typically used as preservatives or antiseptics.
Some creosote types were ...
* Guaifenesin
Guaifenesin, also known as glyceryl guaiacolate, is an expectorant medication that aids in the elimination of sputum from the respiratory tract. Chemically it is an ether of guaiacol and glycerine. It is often used in combination with other me ...
References
{{Authority control
Flavors
Insect pheromones
O-methylated natural phenols