Guaiacol
{{short description|Methoxyphenol compound, constituent of plant lignins and essential oils}}
{{chembox
| Watchedfields = changed
| verifiedrevid = 443849411
| Reference ={{cite book|title=Merck Index|edition=13th|page=4568|title-link=Merck Index}}
| ImageFileL1 =Guaiacol.png
| ImageFileR1 =Guaiacol 3D spacefill.png
| PIN =2-Methoxyphenol
| OtherNames =o-Methoxyphenol
O-Methylcatechol{{cite web|url=http://www.chemindustry.com/chemicals/464173.html|website=Chemindustry|title=List of synonyms for guaiacol|access-date=2009-03-20|archive-date=2016-08-10|archive-url=https://web.archive.org/web/20160810082353/http://www.chemindustry.com/chemicals/464173.html|url-status=dead}}
2-Hydroxyanisole
Pyroguaiac acid
Pyrocatechol monomethyl ether
1-hydroxy-2-methoxybenzene
|Section1={{Chembox Identifiers
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 6JKA7MAH9C
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = D00117
| InChI = 1/C7H8O2/c1-9-7-5-3-2-4-6(7)8/h2-5,8H,1H3
| InChIKey = LHGVFZTZFXWLCP-UHFFFAOYAI
| ChEMBL_Ref = {{ebicite|correct|EBI}}
| ChEMBL = 13766
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C7H8O2/c1-9-7-5-3-2-4-6(7)8/h2-5,8H,1H3
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = LHGVFZTZFXWLCP-UHFFFAOYSA-N
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 90-05-1
| PubChem = 460
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 447
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 28591
| SMILES = COc1ccccc1O
}}
|Section2={{Chembox Properties
| C=7|H=8|O=2
| MolarMass =
| Appearance = colorless oil or crystalline solid
| Density = 1.112 g/cm3, liquid
1.129 g/cm3, crystals
| MeltingPtC = 26-29
| BoilingPtC = 204–206
| BoilingPt_notes =
| Solubility = 23.3 g/L at 25 °C
}}
|Section3={{Chembox Hazards
| MainHazards =
| FlashPt =
| AutoignitionPt =
}}
|Section4={{Chembox Related
| OtherFunction_label = methoxyphenols
| OtherFunction = Mequinol
3-Methoxyphenol
}}
}}
Guaiacol ({{IPAc-en|ˈ|ɡ|w|aɪ|ə|k|ɒ|l}}) is an organic compound with the formula C6H4(OH)(OCH3). It is a phenolic compound containing a methoxy 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 or wood creosote. There also appears to be a petrochemical route to it with great commercial use.{{Cite web|last=Bomgardner|first=Melody M.|date=2016-09-14|title=The Problem with Vanilla|url=https://www.scientificamerican.com/article/the-problem-with-vanilla/|access-date=2020-10-19|website=Scientific American|language=en}}
It is produced by a variety of plants.{{cite journal | doi = 10.1016/0305-0491(77)90029-3 |pmid=830476 |last1=Duffey |first1=S. S. |last2=Aldrich |first2=J. R. |last3=Blum |first3=M. S. | title = Biosynthesis of phenol and guaiacol by the hemipteran Leptoglossus phyllopus | journal = Comparative Biochemistry and Physiology B | year = 1977 | volume = 56 | issue = 2B | pages = 101–102}} It is also found in essential oils from celery seeds, tobacco leaves, orange leaves, and lemon peels.{{cite book |last=Burdock|first=G. A.|date=1995|title=Encyclopedia of Food and Color Additives|location=Boca Raton, FL|publisher=CRC Press|pages=1244–1245|isbn=978-0849394126}} The pure substance is colorless, but samples become yellow upon exposure to air and light. The compound is present in wood smoke, resulting from the pyrolysis of lignin. The compound contributes to the flavor of many substances such as whiskey{{cite news |last=Gallegos |first=Jenna |date=August 17, 2017 |title=The best way to drink whiskey, according to science |newspaper=The Washington Post |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/17/the-best-way-to-drink-whiskey-according-to-science/ |quote=Guaiacol is what gives whiskey that smoky, spicy, peaty flavor.}} and roasted coffee.{{cite journal |last1=Dorfner |first1=R. |last2=Ferge |first2=T. |last3=Kettrup |first3=A. |last4=Zimmermann |first4=R. |last5=Yeretzian |first5=C. |date=Sep 2003 |title=Real-time monitoring of 4-vinylguaiacol, guaiacol, and phenol during coffee roasting by resonant laser ionization time-of-flight mass spectrometry |journal=Journal of Agricultural and Food Chemistry |issn=0021-8561 |doi=10.1021/jf0341767 |pmid=12952431 |volume=51 |issue=19 |pages=5768–5773}}
Preparation
The compound was first isolated by Otto Unverdorben in 1826.{{cite journal|last1=Stevens |first1=M. E. |last2=Ronan |first2=A. K. |last3=Sourkes |first3=T. S. |first4=Boyd |last4=E. M. | title=On the Expectorant Action of Creosote and the Guaiacols | journal=Canadian Medical Association Journal | year= 1943 | volume= 48 | issue= 2 | pages= 124–127 | pmid=20322688 | pmc=1827660}} Guaiacol is produced by methylation of o-catechol, for example using potash and dimethyl sulfate:{{Ullmann|first1=Helmut |last1=Fiege |first2=Heinz-Werner |last2=Voges |first3=Toshikazu |last3=Hamamoto |first4=Sumio |last4=Umemura |first5=Tadao |last5=Iwata |first6=Hisaya |last6=Miki |first7=Yasuhiro |last7=Fujita |first8=Hans-Josef |last8=Buysch |first9=Dorothea |last9=Garbe |first10=Wilfried |last10=Paulus |title=Phenol Derivatives|doi=10.1002/14356007.a19_313}}
: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, can be hydrolyzed via its diazonium derivative. Guaiacol can be synthesized by the dimethylation of catechol followed by selective mono-demethylation.{{OrgSynth | first1= R. N. |last1=Mirrington |first2= G. I. |last2=Feutrill | year = 1988 | title = Orcinol Monomethyl Ether | collvol = 6 | collvolpages = 859 | prep = cv6p0859}}
:C6H4(OCH3)2 + C2H5SNa → C6H4(OCH3)(ONa) + C2H5SCH3
Uses and chemical reactions
=Syringyl/guaiacyl ratio=
Lignin, comprising a major fraction of biomass, is sometimes classified according to the guaiacyl component. Pyrolysis of lignin from gymnosperms gives more guaiacol, resulting from removal of the propenyl group of coniferyl alcohol. These lignins are said to have a high guaiacyl (or G) content. In contrast, lignins derived from sinapyl alcohol affords syringol. A high syringyl (or S) content is indicative of lignin from angiosperms.{{cite journal |doi=10.1105/tpc.010111|title=The Last Step of Syringyl Monolignol Biosynthesis in Angiosperms is Regulated by a Novel Gene Encoding Sinapyl Alcohol Dehydrogenase |year=2001 |last1=Li |first1=Laigeng |last2=Cheng |first2=Xiao Fei |last3=Leshkevich |first3=Jacqueline |last4=Umezawa |first4=Toshiaki |last5=Harding |first5=Scott A. |last6=Chiang |first6=Vincent L. |journal=The Plant Cell |volume=13 |issue=7 |pages=1567–1586 |pmid=11449052 |pmc=139549 }} Sugarcane bagasse is one useful source of guaiacol; pyrolysis of the bagasse lignins yields compounds including guaiacol, 4-methylguaiacol and 4-vinylguaiacol.{{Cite journal |last1=del Río |first1=José C. |last2=Lino |first2=Alessandro G. |last3=Colodette |first3=Jorge L. |last4=Lima |first4=Claudio F. |last5=Gutiérrez |first5=Ana |last6=Martínez |first6=Ángel T. |last7=Lu |first7=Fachuang |last8=Ralph |first8=John |last9=Rencoret |first9=Jorge |date=2015-10-01 |title=Differences in the chemical structure of the lignins from sugarcane bagasse and straw |journal=Biomass and Bioenergy |language=en |volume=81 |pages=322–338 |doi=10.1016/j.biombioe.2015.07.006 |issn=0961-9534|doi-access=free }}
=Chemical intermediate=
Guaiacol is a useful precursor for the synthesis of other compounds.{{cite journal|doi=10.1351/pac200577071221|title=Masked o-benzoquinone strategy in organic synthesis: Short and efficient construction of cis-decalins and linear triquinanes from 2-methoxyphenols|journal=Pure and Applied Chemistry|volume=77|issue=7|pages=1221–1234|year=2005|last1=Liao|first1=Chun-Chen|doi-access=free}} Being derived from biomass, it is a potential component or precursor to "green fuels".{{cite journal|doi=10.1039/C3EE43081B|title=Upgrading of lignin-derived bio-oils by catalytic hydrodeoxygenation|journal=Energy Environ. Sci.|volume=7|pages=103–129|year=2014|last1=Saidi|first1=Majid|last2=Samimi|first2=Fereshteh|last3=Karimipourfard|first3=Dornaz|last4=Nimmanwudipong|first4=Tarit|last5=Gates|first5=Bruce C.|last6=Rahimpour|first6=Mohammad Reza}}
Guaiacol is also a useful reagent for the quantification of peroxidases, as in the presence of hydrogen peroxide these enzymes will catalyse with it the formation of tetraguaiacol,{{cite journal | vauthors = Koduri RS, Tien M | journal = Journal of Biological Chemistry | volume = 270 | issue = 38 | pages = 22254–8 | date = 1995 | pmid = 7673205 | doi = 10.1074/jbc.270.38.22254 | title = Oxidation of Guaiacol by Lignin Peroxidase. Role of veratryl alcohol | doi-access = free }} a coloured compound that can be quantified by its absorbance at 420–470 nm, following the equation:
:4 {{chem|guaiacol (colorless)}} + 2 {{chem|H|2|O|2}} → {{chem|tetraguaiacol (colored)}} + 8 {{chem|H|2|O}}.
=Medicinal and food=
Guaiacol is a precursor to various flavorants, such as eugenol.{{OrgSynth | first1= C. F. H. |last1=Allen |first2=J. W. Jr |last2=Gates | title = o-Eugenol | collvol = 3 | collvolpages = 418 | year = 1955 | prep = cv3P0418}} An estimated 85% of the world's supply of vanillin comes from guaiacol. Because consumers tend to prefer natural vanillin to synthetic vanillin, methods such as microbial fermentation have been adopted. The route entails the condensation reaction of glyoxylic acid with guaiacol to give mandelic acid, which is oxidized to produce phenylglyoxylic acid. This acid undergoes a decarboxylation to afford vanillin.{{cite encyclopedia
| last1 = Esposito
| first1 = Lawrence J.
| first2=K. |last2=Formanek |first3=G. |last3=Kientz |first4=F. |last4=Mauger |first5=V. |last5=Maureaux |first6=G. |last6=Robert |first7=F. |last7=Truchet
| title = Vanillin
| encyclopedia = Kirk–Othmer Encyclopedia of Chemical Technology |edition=4th
| volume = 24
| pages = 812–825
| publisher = John Wiley & Sons
| location = New York, NY
| year = 1997}} The crude vanillin product can then be purified with vacuum distillation and recrystallization.
Guaiacol is also used medicinally as an expectorant, antiseptic, and local anesthetic.{{cite web |url=https://www.drugbank.ca/drugs/DB11359|title=Guaiacol|author= |date=2019-11-02|website=DrugBank|access-date=2019-11-18}}
Guaiacol is produced in the gut of desert locusts, Schistocerca gregaria, by the breakdown of plant material. This process is undertaken by the gut bacterium Pantoea agglomerans (Enterobacter). It is one of the main components of the pheromones that cause locust swarming.{{cite journal|doi=10.1038/35002669|pmid=10706273|title=Pheromones: Exploitation of gut bacteria in the locust|first1=Rod J.|last1=Dillon|first2=Chris T.|last2=Vennard|first3=A. Keith|last3=Charnley|journal=Nature|volume=403|issue=6772|page=851|date=2000-02-24|bibcode=2000Natur.403..851D|s2cid=5207502|doi-access=free}}
Safety
Methoxyphenols 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.{{cite web|last=Smith|first=K. R.|date=2005|url=http://ehs.sph.berkeley.edu/krsmith/publications/2005%20pubs/HC%20woodsmoke%20report%20Mar%2031%2005%20%28rev%29.pdf |title=Critical review of the health effects of woodsmoke|publisher=School of Public Health, University of Berkeley|url-status=dead|archive-url=https://web.archive.org/web/20090710235756/http://ehs.sph.berkeley.edu/krsmith/publications/2005%20pubs/HC%20woodsmoke%20report%20Mar%2031%2005%20%28rev%29.pdf|archive-date=2009-07-10}}