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Nitrous acid
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<h2 id="structure">Structure</h2> <p>In the gas phase, the planar nitrous acid molecule can adopt both a <i>syn</i> and an <i>anti</i> form. The <i>anti</i> form predominates at room temperature, and <a href="/facts/Infrared_spectroscopy/u1Hzx617">IR measurements</a> indicate it is <a href="/facts/Gibbs_free_energy/bvenDuzh">more stable</a> by around 2.3 kJ/mol.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> </p> <h2 id="preparation-and-decomposition">Preparation and decomposition</h2> <p class="note">See also: <a href="/facts/Dinitrogen_trioxide/3fwlV5UQ">Dinitrogen trioxide</a></p> <p>Free, gaseous nitrous acid is unstable, rapidly <a href="/facts/Disproportionation/67QMkXrV">disproportionating</a> to <a href="/facts/Nitric_oxides/aC4Squ7z">nitric oxides</a>: </p> 2 HNO2 → NO2 + NO + H2O <p>In aqueous solution, the nitrogen dioxide also disproportionates, for a net reaction producing <a href="/facts/Nitric_oxide/UvhdvDzp">nitric oxide</a> and <a href="/facts/Nitric_acid/UDbwGp6s">nitric acid</a>:<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>: 1 <a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> 3 HNO2 → 2 NO + HNO3 + H2O <p>Consequently applications of nitrous acid usually begin with <a href="/facts/Mineral_acid/MaftYXr0">mineral acid</a> acidification of <a href="/facts/Sodium_nitrite/t5p4YpvJ">sodium nitrite</a>. The acidification is usually conducted at ice temperatures, and the HNO2 consumed <i>in situ</i>.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>Nitrous acid equilibrates with <a href="/facts/Dinitrogen_trioxide/3fwlV5UQ">dinitrogen trioxide</a> in water, so that concentrated solutions are visibly blue:<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a>: 2 </p> N2O3 + H2O ⇌ 2 HNO2 <p>Addition of dinitrogen trioxide to water is thus another preparatory technique. </p> <h2 id="chemical-applications">Chemical applications</h2> <p>Nitrous acid is the main chemophore in the <a href="/facts/Liebermann_reagent/E1If8eoF">Liebermann reagent</a>, used to <a href="/facts/Spot_test/3jQIXhak">spot-test</a> for alkaloids. </p><p>At high acidities (<a href="/facts/PH/RnSv8D7y"><i>p</i>H</a> ≪ 2), nitrous acid is protonated to give water and <a href="/facts/Nitrosonium/zmRlEu1k">nitrosonium</a> cations.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a>: 2 </p> <h3>Reduction</h3> <p>With I− and Fe2+ ions, NO is formed:<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> 2 HNO2 + 2 KI + 2 H2SO4 → I2 + 2 NO + 2 H2O + 2 K2SO4 2 HNO2 + 2 FeSO4 + 2 H2SO4 → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4 <p>With Sn2+ ions, N2O is formed: </p> 2 HNO2 + 6 HCl + 2 SnCl2 → 2 SnCl4 + N2O + 3 H2O + 2 KCl <p>With SO2 gas, NH2OH is formed: </p> 2 HNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2 NH2OH <p>With Zn in alkali solution, NH3 is formed: </p> 5 H2O + KNO2 + 3 Zn → NH3 + KOH + 3 Zn(OH)2 <p>With N2H+5, both HN3 and (subsequently) N2 gas are formed: </p> HNO2 + [N2H5]+ → HN3 + H2O + H3O+ HNO2 + HN3 → N2O + N2 + H2O <p>Oxidation by nitrous acid has a <a href="/facts/Kinetic_control/HnnBoou1">kinetic control</a> over <a href="/facts/Thermodynamic_control/HnnBoou1">thermodynamic control</a>, this is best illustrated that dilute nitrous acid is able to oxidize I− to I2, but dilute nitric acid cannot. </p> I2 + 2 e− ⇌ 2 I− <i>E</i>o = +0.54 V NO−3 + 3 H+ + 2 e− ⇌ HNO2 + H2O <i>E</i>o = +0.93 V HNO2 + H+ + e− ⇌ NO + H2O <i>E</i>o = +0.98 V <p>It can be seen that the values of <i>E</i>ocell for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Based on the fact that dilute nitrous acid can oxidize iodide into <a href="/facts/Iodine/JlfMecMf">iodine</a>, it can be deduced that nitrous is a faster, rather than a more powerful, oxidizing agent than dilute nitric acid.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p> <h3>Organic chemistry</h3> <p>Nitrous acid is used to prepare <a href="/facts/Diazonium_salt/5pkN4G5s">diazonium salts</a>: </p> HNO2 + ArNH2 + H+ → ArN+2 + 2 H2O <p>where Ar is an <a href="/facts/Aryl/M3z7QRXd">aryl</a> group. </p><p>Such salts are widely used in <a href="/facts/Organic_synthesis/aprl6Iq7">organic synthesis</a>, e.g., for the <a href="/facts/Sandmeyer_reaction/aJUhDKjY">Sandmeyer reaction</a> and in the preparation <a href="/facts/Azo_dye/5zBT0p8G">azo dyes</a>, brightly colored compounds that are the basis of a qualitative test for <a href="/facts/Aniline/eFXck1Rj">anilines</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Nitrous acid is used to destroy toxic and potentially explosive <a href="/facts/Sodium_azide/4gwc2jzu">sodium azide</a>. For most purposes, nitrous acid is usually formed <i>in situ</i> by the action of mineral acid on <a href="/facts/Sodium_nitrite/t5p4YpvJ">sodium nitrite</a>:<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> It is mainly blue in colour </p> NaNO2 + HCl → HNO2 + NaCl 2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH <p>Reaction with two <a href="/facts/%CE%91-hydrogen/E44SeQ1L">α-hydrogen</a> atoms in <a href="/facts/Ketone/BqQXpLo4">ketones</a> creates <a href="/facts/Oxime/xRlPfB6K">oximes</a>, which may be further oxidized to a carboxylic acid, or reduced to form amines. This process is used in the commercial production of <a href="/facts/Adipic_acid/8l8ahlDC">adipic acid</a>. </p><p>Nitrous acid reacts rapidly with <a href="/facts/Alcohol_(chemistry)/Y0SCTDkh">aliphatic alcohols</a> to produce <a href="/facts/Alkyl_nitrites/A9QmA4ho">alkyl nitrites</a>, which are potent <a href="/facts/Vasodilation/cjzrU8Ro">vasodilators</a>: </p> (CH3)2CHCH2CH2OH + HNO2 → (CH3)2CHCH2CH2ONO + H2O <p>The carcinogens called <a href="/facts/Nitrosamine/3YehbjD6">nitrosamines</a> are produced, usually not intentionally, by the reaction of nitrous acid with <a href="/facts/Secondary_amine/SN7W44fv">secondary amines</a>: </p> HNO2 + R2NH → R2N-NO + H2O <h2 id="atmosphere-of-the-earth">Atmosphere of the Earth</h2> <p>Nitrous acid is involved in the <a href="/facts/Ozone/o0E5gn2p">ozone</a> budget of the lower <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a>, the <a href="/facts/Troposphere/JPDQq8cL">troposphere</a>. The <a href="/facts/Heterogeneous/y84KQJxl">heterogeneous</a> reaction of <a href="/facts/Nitric_oxide/UvhdvDzp">nitric oxide</a> (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric <a href="/facts/Aerosol/X1jXFLp2">aerosols</a>, the product readily <a href="/facts/Photolysis/nzRyzh8I">photolyses</a> to <a href="/facts/Hydroxyl/1Hs2QAEv">hydroxyl</a> <a href="/facts/Radical_(chemistry)/qFymuVoT">radicals</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h2 id="dna-damage-and-mutation">DNA damage and mutation</h2> <p>Treatment of <i><a href="/facts/Escherichia_coli/nR4Gvl56">Escherichia coli</a></i> cells with nitrous acid causes <a href="/facts/DNA_damage_(naturally_occurring)/CjGTLKGl">damage to the cell's DNA</a> including <a href="/facts/Deamination/aBIhMC23">deamination</a> of <a href="/facts/Cytosine/JW20j33u">cytosine</a> to <a href="/facts/Uracil/KUAcYYpQ">uracil</a>, and these damages are subject to repair by specific enzymes.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> Also, nitrous acid causes base substitution <a href="/facts/Mutation/Ee4NnWbY">mutations</a> in organisms with double-stranded DNA.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> </p> <h2 id="see-also">See also</h2> Wikimedia Commons has media related to nitrous acid. <ul><li><a href="/facts/Demjanov_rearrangement/56RhFTpc">Demjanov rearrangement</a></li> <li><a href="/facts/Nitric_acid/UDbwGp6s">Nitric acid</a> (<a href="/facts/Hydrogen/BoYHjl0J">H</a><a href="/facts/Nitrogen/Nf1QpGDz">N</a><a href="/facts/Oxygen/acyn6o8r">O</a>3)</li> <li><a href="/facts/Nitrosyl-O-hydroxide/VK39E22j">Nitrosyl-<i>O</i>-hydroxide</a></li> <li><a href="/facts/Tiffeneau-Demjanov_rearrangement/xDeVxJfV">Tiffeneau-Demjanov rearrangement</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 462. <a href="978-0-08-037941-8" target="_blank">978-0-08-037941-8</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 462. <a href="978-0-08-037941-8" target="_blank">978-0-08-037941-8</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Williams, D. L. H. (1988). Nitrosation. Cambridge, UK: Cambridge University. ISBN 0-521-26796-X. <a href="0-521-26796-X" target="_blank">0-521-26796-X</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Kameoka, Yohji; Pigford, Robert (February 1977). "Absorption of Nitrogen Dioxide into Water, Sulfuric Acid, Sodium Hydroxide, and Alkaline Sodium Sulfite Aqueous". Ind. Eng. Chem. Fundamen. 16 (1): 163–169. doi:10.1021/i160061a031. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Petit, Y.; Larchevêque, M. (1998). "Ethyl Glycidate from (S)-Serine: Ethyl (R)-(+)-2,3-Epoxypropanoate". Org. Synth. 75: 37. doi:10.15227/orgsyn.075.0037. <a href="https://doi.org/10.15227%2Forgsyn.075.0037" target="_blank">https://doi.org/10.15227%2Forgsyn.075.0037</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Smith, Adam P.; Savage, Scott A.; Love, J. Christopher; Fraser, Cassandra L. (2002). "Synthesis of 4-, 5-, and 6-methyl-2,2'-bipyridine by a Negishi Cross-coupling Strategy: 5-methyl-2,2'-bipyridine". Org. Synth. 78: 51. doi:10.15227/orgsyn.078.0051. <a href="https://doi.org/10.15227%2Forgsyn.078.0051" target="_blank">https://doi.org/10.15227%2Forgsyn.078.0051</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Williams, D. L. H. (1988). Nitrosation. Cambridge, UK: Cambridge University. ISBN 0-521-26796-X. <a href="0-521-26796-X" target="_blank">0-521-26796-X</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Williams, D. L. H. (1988). Nitrosation. Cambridge, UK: Cambridge University. ISBN 0-521-26796-X. <a href="0-521-26796-X" target="_blank">0-521-26796-X</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 449. ISBN 978-0-13-175553-6. <a href="978-0-13-175553-6" target="_blank">978-0-13-175553-6</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 449. ISBN 978-0-13-175553-6. <a href="978-0-13-175553-6" target="_blank">978-0-13-175553-6</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Clarke, H. T.; Kirner, W. R. (1922). "Methyl Red". Organic Syntheses. 2: 47. doi:10.15227/orgsyn.002.0047. <a href="http://orgsyn.org/demo.aspx?prep=CV1P0374" target="_blank">http://orgsyn.org/demo.aspx?prep=CV1P0374</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Prudent practices in the laboratory: handling and disposal of chemicals. Washington, D.C.: National Academy Press. 1995. doi:10.17226/4911. ISBN 978-0-309-05229-0. <a href="978-0-309-05229-0" target="_blank">978-0-309-05229-0</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Spataro, F; Ianniello, A (November 2014). "Sources of atmospheric nitrous acid: state of the science, current research needs, and future prospects". Journal of the Air & Waste Management Association. 64 (11): 1232–1250. Bibcode:2014JAWMA..64.1232S. doi:10.1080/10962247.2014.952846. PMID 25509545. <a href="https://doi.org/10.1080%2F10962247.2014.952846" target="_blank">https://doi.org/10.1080%2F10962247.2014.952846</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Anglada, Josef M.; Solé, Albert (November 2017). "The Atmospheric Oxidation of HONO by OH, Cl, and ClO Radicals". The Journal of Physical Chemistry A. 121 (51): 9698–9707. Bibcode:2017JPCA..121.9698A. doi:10.1021/acs.jpca.7b10715. PMID 29182863. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Da Roza, R.; Friedberg, E. C.; Duncan, B. K.; Warner, H. R. (1977-11-01). "Repair of nitrous acid damage to DNA in Escherichia coli". Biochemistry. 16 (22): 4934–4939. doi:10.1021/bi00641a030. ISSN 0006-2960. PMID 334252. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Hartman, Z.; Henrikson, E. N.; Hartman, P. E.; Cebula, T. A. (1994). "Molecular models that may account for nitrous acid mutagenesis in organisms containing double-stranded DNA". Environmental and Molecular Mutagenesis. 24 (3): 168–175. Bibcode:1994EnvMM..24..168H. doi:10.1002/em.2850240305. ISSN 0893-6692. PMID 7957120. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> </ol>