Editing Aspirin (section)

==Physical properties==
Aspirin, an [[acetyl]] derivative of salicylic acid, is a white, crystalline, weakly acidic substance that melts at {{convert|136|C|F}},<ref name=b92>{{RubberBible92nd|page=3.8}}</ref> and decomposes around {{convert|140|C|F}}.<ref name="Myers2007">{{cite book |vauthors=Myers RL |title=The 100 most important chemical compounds: a reference guide |url=https://books.google.com/books?id=MwpQWcIKMzAC&pg=PA10 |year=2007 |publisher=ABC-CLIO |isbn=978-0-313-33758-1 |page=10 |url-status=live |archive-url=https://web.archive.org/web/20130610170345/http://books.google.com/books?id=MwpQWcIKMzAC&pg=PA10 |archive-date=10 June 2013}}</ref> Its [[acid dissociation constant]] (p''K''<sub>a</sub>) is 3.5 at {{convert|25|C|F}}.<ref name="asaaciddissconst">{{cite web |title=Acetylsalicylic acid |publisher=Jinno Laboratory, School of Materials Science, Toyohashi University of Technology |date=4 March 1996 |url=http://chrom.tutms.tut.ac.jp/JINNO/DRUGDATA/07acetylsalicylic_acid.html |access-date=12 April 2014 |url-status=dead |archive-url=https://web.archive.org/web/20120120224558/http://chrom.tutms.tut.ac.jp/JINNO/DRUGDATA/07acetylsalicylic_acid.html |archive-date=20 January 2012}}</ref>

===Polymorphism===
[[Polymorphism (materials science)|Polymorphism]] is the ability of a substance to form more than one [[crystal structure]]. Until 2005, there was only one proven polymorph of aspirin (form I), though the existence of another polymorph was debated since the 1960s, and one report from 1981 reported that when crystallized in the presence of aspirin anhydride, the [[diffractogram]] of aspirin has weak additional peaks. Though at the time it was dismissed as mere impurity, it was, in retrospect, form II aspirin.<ref name="Bučar-2015">{{cite journal | vauthors = Bučar DK, Lancaster RW, Bernstein J | title = Disappearing polymorphs revisited | journal = Angewandte Chemie | volume = 54 | issue = 24 | pages = 6972–6993 | date = June 2015 | pmid = 26031248 | pmc = 4479028 | doi = 10.1002/anie.201410356 }}</ref>

Form II was reported in 2005,<ref>{{cite journal | vauthors = Vishweshwar P, McMahon JA, Oliveira M, Peterson ML, Zaworotko MJ | title = The predictably elusive form II of aspirin | journal = Journal of the American Chemical Society | volume = 127 | issue = 48 | pages = 16802–16803 | date = December 2005 | pmid = 16316223 | doi = 10.1021/ja056455b | bibcode = 2005JAChS.12716802V }}</ref><ref>{{cite journal | vauthors = Bond AD, Boese R, Desiraju GR | title = On the polymorphism of aspirin: crystalline aspirin as intergrowths of two "polymorphic" domains | journal = Angewandte Chemie | volume = 46 | issue = 4 | pages = 618–622 | year = 2007 | pmid = 17139692 | doi = 10.1002/anie.200603373 }}</ref> found after attempted co-crystallization of aspirin and [[levetiracetam]] from hot [[acetonitrile]]. Pure form II aspirin can be prepared by seeding the batch with aspirin anhydrate in 15% weight.<ref name="Bučar-2015" /> 

In form I, pairs of aspirin molecules form centrosymmetric [[Dimer (chemistry)|dimers]] through the [[acetyl]] groups with the (acidic) [[methyl]] proton to [[carbonyl]] [[hydrogen bond]]s. In form II, each aspirin molecule forms the same hydrogen bonds, but with two neighbouring molecules instead of one. With respect to the hydrogen bonds formed by the [[carboxylic acid]] groups, both polymorphs form identical dimer structures. The aspirin polymorphs contain identical 2-dimensional sections and are therefore more precisely described as polytypes.<ref>{{cite web |title=Polytypism - Online Dictionary of Crystallography |url=http://reference.iucr.org/dictionary/Polytypism |website=reference.iucr.org}}</ref>

Form III was reported in 2015 by compressing Form I above 2 GPa, but it reverts to form I when pressure is removed.<ref>{{Cite journal | vauthors = Crowell EL, Dreger ZA, Gupta YM |date=2015-02-15 |title=High-pressure polymorphism of acetylsalicylic acid (aspirin): Raman spectroscopy |url=https://www.sciencedirect.com/science/article/abs/pii/S0022286014011065 |journal=Journal of Molecular Structure |volume=1082 |pages=29–37 |doi=10.1016/j.molstruc.2014.10.079 |bibcode=2015JMoSt1082...29C |issn=0022-2860}}</ref> Form IV was reported in 2017, which is stable at ambient conditions.<ref>{{Cite journal | vauthors = Shtukenberg AG, Hu CT, Zhu Q, Schmidt MU, Xu W, Tan M, Kahr B |date=2017-06-07 |title=The Third Ambient Aspirin Polymorph |url=https://pubs.acs.org/doi/10.1021/acs.cgd.7b00673 |journal=Crystal Growth & Design |volume=17 |issue=6 |pages=3562–3566 |doi=10.1021/acs.cgd.7b00673 |bibcode=2017CrGrD..17.3562S |osti=1373897 |issn=1528-7483}}</ref>