Radical theory is an obsolete scientific theory in

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

describing the structure of

organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...

. The theory was pioneered by

Justus von Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at th ...

Friedrich Wöhler Friedrich Wöhler () FRS(For) Hon FRSE (31 July 180023 September 1882) was a German chemist known for his work in inorganic chemistry, being the first to isolate the chemical elements beryllium and yttrium in pure metallic form. He was the fi ...

and

Auguste Laurent Auguste Laurent (14 November 1807 – 15 April 1853) was a French chemist who helped in the founding of organic chemistry with his discoveries of anthracene, phthalic acid, and carbolic acid. He devised a systematic nomenclature for organic che ...

around 1830 and is not related to the modern understanding of

free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spon ...

. In this theory, organic compounds were thought to exist as combinations of radicals that could be exchanged in chemical reactions just as chemical elements could be interchanged in inorganic compounds.

Preamble

The term radical was already in use when radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase "radical" in 1785 and the phrase was employed by

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (

Traité Élémentaire de Chimie ''Traité élémentaire de chimie'' (''Elementary Treatise on Chemistry'') is a textbook written by Antoine Lavoisier published in 1789 and translated into English by Robert Kerr in 1790 under the title ''Elements of Chemistry in a New Systemati ...

. A radical was identified as the root base of certain acids (The Latin word "radix" meaning "root"). The combination of a radical with oxygen would result in an acid. For example the radical of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

was called "acetic" and that of muriatic acid (

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...

) was called "muriatic".

Joseph Louis Gay-Lussac Joseph Louis Gay-Lussac (, , ; 6 December 1778 – 9 May 1850) was a French chemist and physicist. He is known mostly for his discovery that water is made of two parts hydrogen and one part oxygen (with Alexander von Humboldt), for two laws ...

found evidence for the cyanide radical in 1815 in his work on

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on a ...

and a number of cyanide salts he discovered. He also isolated

cyanogen Cyanogen is the chemical compound with the formula ( C N)2. It is a colorless and highly toxic gas with a pungent odor. The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups – analogous to diatomic halogen molec ...

((CN)₂) not realizing that cyanogen is the cyanide dimer NC-CN.

Jean-Baptiste Dumas Jean Baptiste André Dumas (14 July 180010 April 1884) was a French chemist, best known for his works on organic analysis and synthesis, as well as the determination of atomic weights (relative atomic masses) and molecular weights by measuring ...

proposed the ethylene radical from investigations into

diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...

and

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

. In his Etherin theory CHANDLER LECTURE M. Gomberg Industrial & Engineering Chemistry 1928 20 (2), 159-159 he observed that ether consisted of two equivalents of ethylene and one equivalent of water and that ethylene and ethanol could interconvert in chemical reactions. Ethylene was also the base fragment for a number of other compounds such as

ethyl acetate Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...

. This Etherin theory was eventually abandoned by Dumas in favor of radical theory. As a radical it should react with an oxide to form the hydrate but it was found that ethylene is resistant to an oxide like

calcium oxide Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "'' lime''" connotes calcium-containing inorganic ...

Henri Victor Regnault Henri Victor Regnault (21 July 1810 – 19 January 1878) was a French chemist and physicist best known for his careful measurements of the thermal properties of gases. He was an early thermodynamicist and was mentor to William Thomson in ...

in 1834 reacted ethylene dichloride (CH₂CH₂.Cl₂) with KOH forming

vinyl chloride Vinyl chloride is an organochloride with the formula H2C=CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC ...

, water, and KCl. In etherin theory it should not be possible to break up the ethylene fragment in this way. Radical theory replaced electrochemical dualism which stated that all molecules were to be considered as salts composed of basic and acidic oxides.

Theory

Liebig and Wöhler observed in 1832 in an investigation of

benzoin resin Benzoin or benjamin (corrupted pronunciation) is a balsamic resin obtained from the bark of several species of trees in the genus '' Styrax''. It is used in perfumes and some kinds of incense and as a flavoring and medicine (see tincture of b ...

(

benzoic acid Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin ...

) that the compounds

almond oil The almond (''Prunus amygdalus'', syn. ''Prunus dulcis'') is a species of tree native to Iran and surrounding countries, including the Levant. The almond is also the name of the edible and widely cultivated seed of this tree. Within the genus ' ...

(

benzaldehyde Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. It is a colorless liquid with a characteristic almond-like odor. ...

), "''Benzoestoff''" (

benzyl alcohol Benzyl alcohol is an aromatic alcohol with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn" (not to be confused with "Bz" which is used for benzoyl), thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is a colorless liquid w ...

), benzoyl chloride and

benzamide Benzamide is a organic compound with the chemical formula of C6H5C(O)NH2. It is the simplest amide derivative of benzoic acid. In powdered form, it appears as a white solid, while in crystalline form, it appears as colourless crystals. It is slig ...

all share a common C₇H₅O fragment and that these compounds could all be synthesized from almond oil by simple substitutions. The C₇H₅O fragment was considered a "radical of benzoic acid" and called benzoyl. Organic radicals were thus placed on the same level as the inorganic elements. Just like the inorganic elements (''simple radicals'') the organic radicals (''compound radicals'') were indivisible. The theory was developed thanks to improvements in

elemental analysis Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualit ...

by von Liebig. Laurent contributed to the theory by reporting the isolation of benzoyl itself in 1835, however the isolated chemical is today recognised at its dimer dibenzoyl.

Raffaele Piria Raffaele Piria ( Scilla 20 August 1814 – Turin 18 July 1865) was an Italian chemist from Scilla, who lived in Palmi. He converted the substance Salicin into a sugar and a second component, which on oxidation becomes salicylic acid, a major c ...

reported the salicyl radical as the base for

salicylic acid Salicylic acid is an organic compound with the formula HOC6H4CO2H. A colorless, bitter-tasting solid, it is a precursor to and a metabolite of aspirin (acetylsalicylic acid). It is a plant hormone, and has been listed by the EPA Toxic Substa ...

. Liebig published a definition of a radical in 1838 Berzelius and

Robert Bunsen Robert Wilhelm Eberhard Bunsen (; 30 March 1811 – 16 August 1899) was a German chemist. He investigated emission spectra of heated elements, and discovered caesium (in 1860) and rubidium (in 1861) with the physicist Gustav Kirchhoff. The Bu ...

investigated the radical

cacodyl Cacodyl, also known as dicacodyl or tetramethyldiarsine, (CH3)2As–As(CH3)2, is an organoarsenic compound that constitutes a major part of " Cadet's fuming liquid" (named after the French chemist Louis Claude Cadet de Gassicourt). It is a poisono ...

(reaction of cacodyl chloride with zinc) around 1841, now also known as a dimer species (CH₃)₂As—As(CH₃)₂.

Edward Frankland Sir Edward Frankland, (18 January 18259 August 1899) was an English chemist. He was one of the originators of organometallic chemistry and introduced the concept of combining power or valence. An expert in water quality and analysis, he was ...

and Hermann Kolbe contributed to the radical theory by investigating the ethyl and the methyl radicals. Frankland first reported

diethylzinc Diethylzinc (C2H5)2Zn, or DEZ, is a highly pyrophoric and reactive organozinc compound consisting of a zinc center bound to two ethyl groups. This colourless liquid is an important reagent in organic chemistry. It is available commercially as a ...

in 1848. Frankland and Kolbe together investigated the reaction of ethyl cyanide and zinc in 1849 reporting the isolation of not the ethyl radical but the methyl radical (CH₃) which in fact was ethane. Kolbe also investigated the

electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...

of potassium salts of some fatty acids.

Acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

was regarded as the combination of the methyl radical and

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early invest ...

and electrolysis of the salt yielded as gas again ethane misidentified as the liberated methyl radical. In 1850 Frankland investigated ethyl radicals. In the course of this work butane formed by reaction of ethyl iodide and zinc was mistakenly identified as the ethyl radical.

Demise

August Wilhelm von Hofmann August Wilhelm von Hofmann (8 April 18185 May 1892) was a German chemist who made considerable contributions to organic chemistry. His research on aniline helped lay the basis of the aniline-dye industry, and his research on coal tar laid the g ...

and

Charles Frédéric Gerhardt Charles Frédéric Gerhardt (21 August 1816 – 19 August 1856) was a French chemist, born in Alsace and active in Paris, Montpellier, and his native Strasbourg. Biography He was born in Strasbourg, which is where he attended the gymnasium (an a ...

challenged Frankland and Kolbe by suggesting that the ethyl radical was in fact a dimer called dimethyl. Frankland and Kolbe countered that ethyl hydride was also a possibility and in 1864

Carl Schorlemmer Carl Schorlemmer FRS (30 September 1834 – 27 June 1892) was a German chemist who did research on hydrocarbons and contributed to the study of the history of chemistry. Early life and education Schorlemmer was born in 1834, the son of a joi ...

proved that dimethyl and ethyl hydride were in fact one and the same compound. Radical theory was eventually replaced by a number of theories each advocating specific entities. One adaptation of radical theory was called theory of types (theory of residues), advocated by

Charles-Adolphe Wurtz Charles Adolphe Wurtz (; 26 November 181710 May 1884) was an Alsatian French chemist. He is best remembered for his decades-long advocacy for the atomic theory and for ideas about the structures of chemical compounds, against the skeptical opinio ...

and

. Another was water type as promoted by

Alexander William Williamson Prof Alexander William Williamson FRS FRSE PCS MRIA (1 May 18246 May 1904) was an English chemist. He is best known today for the Williamson ether synthesis. Life Williamson was born in 1824 in Wandsworth, London, the second of three childr ...

and

(an early supporter of radical theory) challenged radical theory in 1840 with a Law of Substitution (or Theory of Substitution). This law acknowledged that any hydrogen atom even as part of a radical could be substituted by a

halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this grou ...

. Eventually Frankland in 1852 and

August Kekulé Friedrich August Kekulé, later Friedrich August Kekule von Stradonitz ( , ; 7 September 1829 – 13 July 1896), was a German organic chemist. From the 1850s until his death, Kekulé was one of the most prominent chemists in Europe, especially ...

in 1857 introduced valence theory with as central theme the tetravalency of carbon. making trivalent carbon obsolete for the time being.

Legacy

In 1900

Moses Gomberg Moses Gomberg (February 8, 1866 – February 12, 1947) was a chemistry professor at the University of Michigan. He was elected to the National Academy of Sciences and served as president of the American Chemical Society. Early life and education ...

unexpectedly discovered true trivalent carbon and the first

radical Radical may refer to: Politics and ideology Politics * Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe an ...

in the modern sense of the word in his (unsuccessful) attempt to make

hexaphenylethane Hexaphenylethane is a hypothetical organic compound consisting of an ethane core with six phenyl substituents. All attempts at its synthesis have been unsuccessful. The trityl free radical, Ph3C, was originally thought to dimerize to form hexap ...

. In current organic chemistry concepts such as

benzoyl In organic chemistry, benzoyl (, ) is the functional group with the formula C6H5CO-. It can be viewed as benzaldehyde missing one hydrogen. The term "benzoyl" should not be confused with benzyl, which has the formula C6H5CH2. The benzoyl group ...

and

acetyl In organic chemistry, acetyl is a functional group with the chemical formula and the structure . It is sometimes represented by the symbol Ac (not to be confused with the element actinium). In IUPAC nomenclature, acetyl is called ethanoyl, ...

persist in chemical nomenclature but only to identify a

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...

having the same fragment.

References

{{Reflist Obsolete scientific theories