Review Before Radicals Were Free – the Radical Particulier of de Morveau Edwin C. Constable * and Catherine E. Housecroft Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, CH-4058 Basel, Switzerland; [email protected] * Correspondence: [email protected]; Tel.: +41-61-207-1001 Received: 31 March 2020; Accepted: 17 April 2020; Published: 20 April 2020 Abstract: Today, we universally understand radicals to be chemical species with an unpaired electron. It was not always so, and this article traces the evolution of the term radical and in this journey, monitors the development of some of the great theories of organic chemistry. Keywords: radicals; history of chemistry; theory of types; valence; free radicals 1. Introduction The understanding of chemistry is characterized by a precision in language such that a single word or phrase can evoke an entire back-story of understanding and comprehension. When we use the term “transition element”, the listener is drawn into an entire world of memes [1] ranging from the periodic table, colour, synthesis, spectroscopy and magnetism to theory and computational chemistry. Key to this subliminal linking of the word or phrase to the broader context is a defined precision of terminology and a commonality of meaning. This is particularly important in science and chemistry, where the precision of meaning is usually prescribed (or, maybe, proscribed) by international bodies such as the International Union of Pure and Applied Chemistry [2]. Nevertheless, words and concepts can change with time and to understand the language of our discipline is to learn more about the discipline itself. The etymology of chemistry is a complex and rewarding subject which is discussed eloquently and in detail elsewhere [3–5]. One word which has had its meaning refined and modified to an extent that its original intent has been almost lost is radical, the topic of this special issue. This article has two origins: firstly and most importantly, on the occasion of his 80th birthday, it is an opportunity to express our gratitude and thanks for the friendship and assistance of Bernd Giese in our years together in Basel, and secondly to acknowledge a shared interest with Bernd in the history of our chosen discipline. 2. Modern Understanding It seems relevant to present the IUPAC definition of a radical in full at this point in the text as it both provides a precision for modern usage and also contains hints of the historical meaning: “A molecular entity such as CH , SnH , Cl possessing an unpaired electron. (In these formulae · 3 · 3 · the dot, symbolizing the unpaired electron, should be placed so as to indicate the atom of highest spin density, if this is possible.) Paramagnetic metal ions are not normally regarded as radicals. However, in the ‘isolobal analogy’, the similarity between certain paramagnetic metal ions and radicals becomes apparent. At least in the context of physical organic chemistry, it seems desirable to cease using the adjective ‘free’ in the general name of this type of chemical species and molecular entity, so that the term ‘free radical’ may in future be restricted to those radicals which do not form parts of radical pairs. Depending upon the core atom that possesses the unpaired electron, the radicals can be described as carbon-, oxygen-, nitrogen-, metal-centered radicals. If the unpaired electron occupies an orbital having Chemistry 2020, 2, 19; doi:10.3390/chemistry2020019 www.mdpi.com/journal/chemistry Chemistry 2020, 2 2 Chemistryradicals 2020can, 2be, 19 described as carbon-, oxygen-, nitrogen-, metal-centered radicals. If the unpaired1 of 11 electron occupies an orbital having considerable s or more or less pure p character, the respective considerableradicals are termed s or more σ- or or π-radicals. less pure In p character,the past, the the term respective ‘radical’ radicals was used are to termed designateσ- ora substituentπ-radicals. Ingroup the past,bound the to term a molecular ‘radical’ wasentity, used as toopposed designate to ‘free a substituent radical’, groupwhich boundnowadays to a molecularis simply entity,called asradical. opposed The to bound ‘free radical’, entities which may nowadaysbe called groups is simply or called substituents, radical. Thebut boundshould entities no longer may be be called groupsradicals” or [6]. substituents, but should no longer be called radicals” [6]. To summarize,summarize, inin acceptedaccepted modernmodern usage,usage, aa radicalradical possessespossesses anan unpairedunpaired electron.electron. 3. A Radical Birth 3.1. de Morveau’s Introduction The word radical was introduced by the French politician and chemist, Louis-Bernard Guyton, Baron dede Morveau Morveau (1737–1816, (1737–1816, prudently prudently identified identifi aftered theafter French the revolutionFrench revolution without the without aristocratic the rankaristocratic as Louis-Bernard rank as Louis-Bernard Guyton-Morveau, Guyton-Morveau, Figure1)[ Figure7]. In 1) 1782, [7]. In de 1782, Morveau de Morveau published published an article an entitledarticle entitledSur les Sur Dé nominationsles Dénominations Chymiques, Chymiques, La né Lacessit nécessitéé d’en d’ perfectioneren perfectioner le syst le systême,ême, et leset les règles règles pour pour y parveniry parvenirin in which which he he identified identified the the need need for for a a new new systematic systematic nomenclature nomenclature in in chemistrychemistry [[8].8]. InIn thisthis paper, hehe notnot only only formulated formulated his his five five principles principles of of nomenclature nomenclature which which later later became became embodied embodied in the in Mtheéthode Méthode de Nomenclature de Nomenclature Chimique Chimique[9,10], but[9,10], also but introduced also introduced the word radicalthe wordto describe radical ato multiatomic describe a entity;multiatomic in his entity; own words in his own “Having words found “Having the adjectives found the arsenicaladjectives and arsenical acetic and consecrated acetic consecrated by usage, itby was usage, necessary it was tonecessary preserve to them preserve and formthem onlyand suchform closeonly such nouns close to the nouns radicals to the of radicals these terms of these that theyterms could that they be understood could be understood without explanation. without explan Arseniatesation. Arseniates and acetates and seemed acetates to seemed me to fulfil to me this to condition.”fulfil this condition.” He makes He no furthermakes no comment further on comment the term on in the this term paper, in whichthis paper, also includeswhich also a table includes which a liststable acids, which the lists generic acids, names the generic of salts names derived of salts from de theserived acids, from bases these or acids, substances bases or that substances bind to acids. that Thisbind tableto acids. also This confirms table thatalso heconfirms was still that a phlogistonist he was still a [ 11phlogistonist,12] in 1782, [11,12] as phlogiston in 1782, is as listed phlogiston amongst is thelisted bases amongst or substances the bases that or substances bind to acids. that The bind word to acids.radical Theitself word seems radical to derive itself seems from theto derive Latin wordfrom radixthe Latin (root). word radix (root). Figure 1. Louis-BernardLouis-Bernard Guyton, Baron Baron de de Morveau Morveau ( (1737–1816,1737–1816, subsequently subsequently Louis-Bernard Louis-Bernard Guyton- Guyton- Morveau) was a FrenchFrench chemistchemist andand politicianpolitician whowho introducedintroduced thethe wordword radicalradical inin 1782.1782. (Public(Public domain image. Source image.https:// en.wikipedia.orgSource/wiki /Louis-Bernard_Guyton_de_Morveau#https://en.wikipedia.org/wiki/Louis-/media/File: Louis-Bernard_Guyton_de_Morveau.jpgBernard_Guyton_de_Morveau#/media/File:L). ouis-Bernard_Guyton_de_Morveau.jpg). By the timetime ofof thethe publicationpublication of of the theM Méthodeéthode, the, the concept concept of of radicals radicals was was embedded embedded in in the the core core of theof the model model in in five five classes classes of of substances substances which which had had not not been been decomposed decomposed into into simplersimpler materialsmaterials (the second class includes all the acidifiableacidifiable bases or radicalradical principles of the acids) [[9,10].9,10]. In this work, the “radical“radical of of the the acid” acid” was preciselywas precisely defined defined as “the expressionas “the expression of acidifiable of base”.acidifiable The explanationsbase”. The givenexplanations in the textgiven are in di thefficult text for are the difficult modern for reader the modern to follow reader as the to conversionfollow as the of conversion the radical of (such the as nitrate or acetate) to the parent acid did not involve the addition of protons but rather oxygen. Chemistry 2020, 2, 19 2 of 11 Although the credit for the discovery of oxygen should be shared between William Scheele, Joseph Priestley and Antoine Laurent de Lavoisier [13,14], Lavoisier’s contribution included the name oxygène, from the Greek ὀξύ& (acid, sharp) and -γενή& (producer, begetter), on the basis of his belief that oxygen was a constituent of all acids. On this basis, the Méthode continues to clarify the nomenclature of radicals defining known acids as arising from the addition of oxygen to “pure charcoal, carbon or carbonic radical ::: Sulphur or sulphuric ::: radical and phosphorus or phosphoric radical”. The identification of oxygen as