Furan

The name "furan" comes from the Latin furfur, which means bran{{cite book | first = Alexander | last = Senning | title = Elsevier's Dictionary of Chemoetymology | publisher = Elsevier | date = 2006 | isbn = 0-444-52239-5}} (furfural is produced from bran). The first furan derivative to be described was 2-furoic acid, by Carl Wilhelm Scheele in 1780. Another important derivative, furfural, was reported by Johann Wolfgang Döbereiner in 1831 and characterised nine years later by John Stenhouse. Furan itself was first prepared by Heinrich Limpricht in 1870, although he called it "tetraphenol" (as if it were a four-carbon analog to phenol, C₆H₅OH).{{cite journal | title = Ueber das Tetraphenol C₄H₄O | pages = 90–91 | first = H. | last = Limpricht | author-link = Heinrich Limpricht | volume = 3 | issue = 1 | year = 1870 | journal = Berichte der Deutschen Chemischen Gesellschaft | doi =10.1002/cber.18700030129| url = https://zenodo.org/record/1424994 }}{{cite book | first = Ernest Harry | last = Rodd | title = Chemistry of Carbon Compounds: A Modern Comprehensive Treatise | publisher = Elsevier | year = 1971}}

Industrially, furan is manufactured by the palladium-catalyzed decarbonylation of furfural, or by the copper-catalyzed oxidation of 1,3-butadiene:{{Ullmann | doi = 10.1002/14356007.a12_119.pub2 | title = Furfural and Derivatives |date = 15 April 2007| first1 = H. E. | last1 = Hoydonckx | first2 = W. M. | last2 = Van Rhijn | first3 = W. | last3 = Van Rhijn | first4 = D. E. | last4 = De Vos | first5 = P. A. | last5 = Jacobs}}

:File:Manufacture of furan.png

In the laboratory, furan can be obtained from furfural by oxidation to 2-furoic acid, followed by decarboxylation.{{OrgSynth | last = Wilson | first = W. C. | title = Furan | collvol = 1 | collvolpages = 274 | year = 1941 | prep = cv1p0274}} It can also be prepared directly by thermal decomposition of pentose-containing materials, and cellulosic solids, especially pine wood.

The Feist–Benary synthesis is a classic way to synthesize furans. The reaction involves alkylation of 1,3-diketones with α-bromoketones followed by dehydration of an intermediate hydroxydihydrofuran.{{cite journal|last1=Hou |first1=X. L. |last2=Cheung |first2=H. Y. |last3=Hon |first3=T. Y. |last4=Kwan |first4=P. L. |last5=Lo |first5=T. H. |last6=Tong |first6=S. Y. |last7=Wong |first7=H. N. | year = 1998 | volume = 54 | pages = 1955–2020 | title = Regioselective syntheses of substituted furans | doi = 10.1016/S0040-4020(97)10303-9 | journal = Tetrahedron| issue = 10}} The other traditional route involve the reaction of 1,4-diketones with phosphorus pentoxide (P₂O₅) in the Paal–Knorr synthesis.{{cite book |last1=Gilchrist |first1=Thomas L. |title=Heterocyclic Chemistry |date=1997 |publisher=Longman |location=Liverpool |page=209-212 |edition=3rd}}

Many routes exist for the synthesis of substituted furans.{{cite journal | doi = 10.15227/orgsyn.084.0199 | title = Electrophilic Cyclization with N-Iodosuccinimide: Preparation of 5-(4-Bromophenyl)-3-Iodo-2-(4-Methyl-Phenyl)Furan | journal = Organic Syntheses | date = 2007 | volume = 84 | page = 199|first1=Adam|last1=Sniady|first2=Marco S.|last2=Morreale|first3=Roman|last3=Dembinski}}{{cite journal |doi=10.15227/orgsyn.076.0263 |title=Isomerization of b-Alkynyl Allylic Alcohols to Furans Catalyzed by Silver Nitrate on Silica Gel: 2-Pentyl-3-Methyl-5-Heptylfuran |journal=Organic Syntheses |date=1999 |volume=76 |page=263|author=James A. Marshall, Clark A. Sehon}}

File:Ranitidine Structural Formulae.png|The drug Zantac, also known as ranitidine.

File:Rosefuran-2D-skeletal.png|Rosefuran, an aroma compound found in rose oil.

File:Furfural.svg|Furfural, derived from sugars, is the major source of furans

File:MethanofuranFeb2011.png|methanofuran is a cofactor in methanogenesis.

Furan has aromatic character because one of the lone pairs of electrons on the oxygen atom is delocalized into the ring, creating a {{math|4n + 2}} aromatic system (see Hückel's rule). The aromaticity is modest relative to that for benzene and related heterocycles thiophene and pyrrole. The resonance energies of benzene, pyrrole, thiophene, and furan are, respectively, {{cvt|152|,|88|,|121|, and|67|kJ/mol}}. Thus, these heterocycles, especially furan, are far less aromatic than benzene, as is manifested in the lability of these rings.{{March6th|page=62}} The molecule is flat but the C=C groups attached to oxygen retain significant double bond character. The other lone pair of electrons of the oxygen atom extends in the plane of the flat ring system.

Examination of the resonance contributors shows the increased electron density of the ring relative to benzene, leading to increased rates of electrophilic substitution.{{cite book |title=Organic Chemistry |edition=5th |last=Bruice |first=Paula Y. |year=2007 |publisher=Pearson Prentice Hall |location=Upper Saddle River, New Jersey |isbn=978-0-13-196316-0|pp=955-958,969}}

::File:Furan resonance with arrows.svg

Because of its partial aromatic character, furan's behavior is intermediate between that of an enol ether and an aromatic ring. It is dissimilar vs ethers such as tetrahydrofuran.

Like enol ethers, 2,5-disubstituted furans are susceptible to hydrolysis to reversibly give 1,4-diketones.

Furan serves as a diene in Diels–Alder reactions with electron-deficient dienophiles such as ethyl (E)-3-nitroacrylate.{{cite journal | title = The oxanorbornene approach to 3-hydroxy, 3,4-dihydroxy and 3,4,5-trihydroxy derivatives of 2-aminocyclohexanecarboxylic acid |last1=Masesane |first1=I. |last2=Batsanov |first2=A. |last3=Howard |first3=J. |last4=Modal |first4=R. |last5=Steel |first5=P. | journal = Beilstein Journal of Organic Chemistry | year = 2006 | volume = 2 | issue = 9 | doi = 10.1186/1860-5397-2-9 | pages = 9 | pmid = 16674802 | pmc = 1524792 |doi-access=free }} The reaction product is a mixture of isomers with preference for the endo isomer:

::File:Furan cycloaddition.svg

Diels-Alder reaction of furan with arynes provides corresponding derivatives of dihydronaphthalenes, which are useful intermediates in synthesis of other polycyclic aromatic compounds.{{cite journal|last1=Filatov|first1=M. A.|last2=Baluschev|first2=S.|last3=Ilieva|first3=I. Z.|last4=Enkelmann|first4=V.|last5=Miteva|first5=T.|last6=Landfester|first6=K.|author-link6=Katharina Landfester|last7=Aleshchenkov|first7=S. E.|last8=Cheprakov|first8=A. V.|year=2012|title=Tetraaryltetraanthra[2,3]porphyrins: Synthesis, Structure, and Optical Properties|url=https://s3.amazonaws.com/academia.edu.documents/51537332/Tetraaryltetraanthra23porphyrins_synthes20170128-30836-19uulyz.pdf?response-content-disposition=inline%3B+filename%3DTetraaryltetraanthra_2_3_porphyrins_Synt.pdf&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWOWYYGZ2Y53UL3A%2F20200219%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20200219T215659Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=c00193208ea0765b1027679f11da29ad6222abb3f1a4ef8bcdc900375ea22418|url-status=dead|journal=J. Org. Chem.|volume=77|issue=24|pages=11119–11131|doi=10.1021/jo302135q|pmid=23205621|archive-url=https://web.archive.org/web/20200219215759/https://s3.amazonaws.com/academia.edu.documents/51537332/Tetraaryltetraanthra23porphyrins_synthes20170128-30836-19uulyz.pdf?response-content-disposition=inline%3B+filename%3DTetraaryltetraanthra_2_3_porphyrins_Synt.pdf&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWOWYYGZ2Y53UL3A%2F20200219%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20200219T215659Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=c00193208ea0765b1027679f11da29ad6222abb3f1a4ef8bcdc900375ea22418|archive-date=2020-02-19}}

::File:Reaction_of_furan_with_benzyne.svg

• It is considerably more reactive than benzene in electrophilic substitution reactions, due to the electron-donating effects of the oxygen heteroatom. It reacts with bromine at 0 °C to give 2-bromofuran.

• Hydrogenation of furans sequentially affords dihydrofurans and tetrahydrofurans.{{cn|date=September 2023}}
• In the Achmatowicz reaction, furans are converted to dihydropyran compounds.
• Pyrrole can be prepared industrially by treating furan with ammonia in the presence of solid acid catalysts, such as SiO₂ and Al₂O₃.{{Ullmann|first=Albrecht Ludwig |last=Harreus |title=Pyrrole |doi=10.1002/14356007.a22_453}}

Furan is found in heat-treated commercial foods and is produced through thermal degradation of natural food constituents.{{Cite journal | pmid = 23627283 | year = 2013 | last1 = Anese | first1 = M. | last2 = Manzocco | first2 = L. | last3 = Calligaris | first3 = S. | last4 = Nicoli | first4 = M. C. | url = http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%207_Derivaty%20karboxylovych%20kyselin_Vlastnosti,%20priprava%20a%20reakcie/Doplnkove%20studijne%20materialy/Acid%20halides/Industrially%20Applicable%20Strategies%20for%20Mitigating%20Acrylamide.pdf | archive-url = https://web.archive.org/web/20170808072346/http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%207_Derivaty%20karboxylovych%20kyselin_Vlastnosti,%20priprava%20a%20reakcie/Doplnkove%20studijne%20materialy/Acid%20halides/Industrially%20Applicable%20Strategies%20for%20Mitigating%20Acrylamide.pdf | url-status = dead | archive-date = 2017-08-08 | title = Industrially Applicable Strategies for Mitigating Acrylamide, Furan and 5-Hydroxymethylfurfural in Food | doi = 10.1021/jf305085r | journal = Journal of Agricultural and Food Chemistry | pages = 10209–14 | volume=61 | issue=43}}{{Cite journal | pmid = 22641279 | year = 2012 | last1 = Moro | first1 = S. | last2 = Chipman | first2 = J. K. | last3 = Wegener | first3 = J. W. | last4 = Hamberger | first4 = C. | last5 = Dekant | first5 = W. | last6 = Mally | first6 = A. | title = Furan in heat-treated foods: Formation, exposure, toxicity, and aspects of risk assessment | volume = 56 | issue = 8 | pages = 1197–1211 | doi = 10.1002/mnfr.201200093 | journal = Molecular Nutrition & Food Research| hdl = 1871/41889 | s2cid = 12446132 | url = https://research.vu.nl/ws/files/674101/296515.pdf }} It can be found in roasted coffee, instant coffee, and processed baby foods.{{cite journal | title = Update on furan levels in food from monitoring years 2004–2010 and exposure assessment | journal = EFSA Journal | year = 2011 | volume = 9 | issue = 9 | pages = 2347 | author = European Food Safety Authority | doi = 10.2903/j.efsa.2011.2347| author-link = European Food Safety Authority | doi-access = free }} {{open access}} {{Cite journal | pmid = 22035212 | year = 2012 | last1 = Waizenegger | first1 = J. | last2 = Winkler | first2 = G. | last3 = Kuballa | first3 = T. | last4 = Ruge | first4 = W. | last5 = Kersting | first5 = M. | last6 = Alexy | first6 = U. | last7 = Lachenmeier | first7 = D. W. | title = Analysis and risk assessment of furan in coffee products targeted to adolescents | volume = 29 | issue = 1 | pages = 19–28 | doi = 10.1080/19440049.2011.617012 | journal = Food Additives & Contaminants: Part A| s2cid = 29027966 }} Research has indicated that coffee made in espresso makers and coffee made from capsules contain more furan than that made in traditional drip coffee makers,{{cite journal|first1=M. S.|last1=Altaki|first2=F. J.|last2=Santos|first3=M. T.|last3=Galceran|title=Occurrence of furan in coffee from Spanish market: contribution of brewing and roasting|journal=Food Chemistry|year=2011|volume=126|issue=4|pages=1527-1532|doi=10.1016/j.foodchem.2010.11.134}} although the levels are still within safe health limits.{{cite news|url=https://www.sciencedaily.com/releases/2011/04/110413090012.htm |title=Espresso makers: Coffee in capsules contains more furan than the rest |website=Science Daily |date=April 14, 2011}}

Exposure to furan at doses about 2,000 times the projected level of human exposure from foods increases the risk of hepatocellular tumors in rats and mice and bile duct tumors in rats.{{Cite journal | pmid = 20237914 | year = 2010 | last1 = Bakhiya | first1 = N. | last2 = Appel | first2 = K. E. |url= https://media.ellinikahoaxes.gr/uploads/2016/12/10.1007@s00204-010-0531-y.pdf | title = Toxicity and carcinogenicity of furan in human diet | volume = 84 | issue = 7 | pages = 563–578 | doi = 10.1007/s00204-010-0531-y | journal = Archives of Toxicology| s2cid = 19389984 }} Furan is therefore listed as a possible human carcinogen.

• BS 4994 – Furan resin as thermoset FRP for chemical process plant equipments
• Furanocoumarin
• Furanoflavonoid
• Furanose
• Furantetracarboxylic acid
• Simple aromatic rings
• Furan fatty acids
• Tetrahydrofuran

{{Reflist|30em}}

{{commons category|Furan}}

• [https://www.organic-chemistry.org/synthesis/heterocycles/furans/furans.shtm Recent synthetic methods]

{{Simple aromatic rings}}

{{Authority control}}

Category:IARC Group 2B carcinogens

Category:Simple aromatic rings

Category:Substances discovered in the 19th century