Editing Benzophenone (section)

===Radical anion<span class="anchor" id="Benzophenone radical anion"></span>===
{{See also|Air-free technique}}
[[File:Making benzophenone radical anion.webm|left|thumb|Addition of a solution of benzophenone in [[Tetrahydrofuran|THF]] to a vial containing THF, sodium metal, and a stir bar, yielding the deep blue benzophenone anion radical. Playback speed 4x original recording. Notice that the stirbar is not Teflon-coated, which would be attacked by the ketyl.]]
[[File:Solvent pot.JPG|thumb|left|upright|A solvent pot containing [[dibutyl ether]] solution of sodium benzophenone ketyl, which gives it its purple color.]]

Alkali metals reduce benzophenone to the deeply blue colored [[Radical ion|radical anion]], diphenylketyl:<ref name=":0">{{Cite journal|title = Chemical Redox Agents for Organometallic Chemistry|last1 = Connelly|first1 = Neil|date = March 28, 1996|journal = Chemical Reviews|doi = 10.1021/cr940053x|last2 = Geiger|first2 = William|pmid=11848774|volume=96|issue=2|pages=877–910}}</ref>
:M + Ph<sub>2</sub>CO → M<sup>+</sup>Ph<sub>2</sub>CO<sup>•−</sup>

Generally sodium is used as the alkali metal. Sodium-benzophenone ketyl is used in the purification of organic solvents, particularly ethers, because it reacts with water and oxygen to give non-volatile products.<ref>{{cite book |last1=Armarego |first1=W. L. F. |title=Purification of laboratory chemicals |last2=Chai |first2=C. |publisher=Butterworth-Heinemann |year=2003 |isbn=978-0-7506-7571-0 |location=Oxford}}</ref><ref>{{cite book |last1=Harwood |first1=L. M. |url=https://archive.org/details/experimentalorga0002harw |title=Experimental Organic Chemistry: Standard and Microscale |last2=Moody |first2=C. J. |last3=Percy |first3=J. M. |publisher=Blackwell Science |year=1999 |isbn=978-0-632-04819-9 |location=Oxford |url-access=registration}}</ref> Adsorbents such as alumina, silica gel, and especially [[molecular sieves]] are superior and far safer.<ref>{{cite journal|last1=Williams |first1=D. B. G. |last2=Lawton |first2=M. |title=Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants |journal=The Journal of Organic Chemistry |date=2010 |volume=75 |issue=24 |pages=8351–4 |doi= 10.1021/jo101589h |pmid=20945830|s2cid=17801540 }}</ref> The sodium-benzophenone method is common since it gives a visual indication that water, oxygen, and peroxides are absent from the solvent. Large scale purification may be more economical using devices which utilize adsorbents such as the aforementioned alumina or molecular sieves.<ref>{{Cite journal |last1=Simas |first1=Alessandro B. C. |last2=Pereira |first2=Vera L. P. |last3=Barreto Jr. |first3=Cleber B. |last4=Sales |first4=Daniel L. de |last5=Carvalho |first5=Leandro L. de |date=2009 |title=An expeditious and consistent procedure for tetrahydrofuran (THF) drying and deoxygenation by the still apparatus |journal=Química Nova |language=en |volume=32 |issue=9 |pages=2473–2475 |doi=10.1590/S0100-40422009000900042 |issn=0100-4042|doi-access=free }}</ref> The ketyl is soluble in the organic solvent being dried, which leads to faster purification. In comparison, sodium is insoluble, and its heterogeneous reaction is much slower. When excess alkali metal is present a second reduction may occur, resulting in a color transformation from deep blue to purple:<ref name=":0" />
:M  +  M<sup>+</sup>Ph<sub>2</sub>CO<sup>•−</sup>   →   (M<sup>+</sup>)<sub>2</sub>(Ph<sub>2</sub>CO)<sup>2−</sup>