Substitution or Type Theory
From WikidChem
[edit]Slide 5
In 1853, water (H20), hydrogen (H2), hydrochloric acid (HCl), and ammonia (NH3) were recognized as four types of molecules. In this notation, the atom to the left of the bar, if there is one, is the central or core atom of the molecule (though, surprisingly to us, without geometric implications). In the case of water, both of the hydrogen atoms are bonded to associated with (bonds came later - JMM) the oxygen atom because it is the central atom. The theory shown here is a combination of radical and type theories. It does not involve the idea of dualism, which says that all things are associated with positive or negative electrical properties. In the examples shown here, the four basic types of molecules undergo reactions in which an atom is replaced (substitution or type theory) by an atom or a radical (radical theory). For instance, an ethyl radical replaces a hydrogen atom in the ammonia to form ethyl amine. This can take place several times, forming diethyl amine or triethyl amine. This slide also shows an ethyl group replacing a hydrogen from water, then a potassium atom replacing the other hydrogen. In the slide the structure of ethyl iodide (C2H5I - I don't really know the correct nomenclature - iodoethane in type nomenclature - JMM) is equated to a molecule of diatomic hydrogen. Then a substitution reaction is shown between the modified water molecule and the ethyl iodide, producing diethyl ether and potassium iodine. This reaction is an example of Williamson ether synthesis, first performed in 1850.
[edit]Slide 6
This slide contains quotes by Dumas and Williamson on Unitary theory, rather than dualistic theory. Dumas (1840)
(Molecules are like) planetary systems held together by a force resembling gravitation, but acting in accord with much more complicated laws.
- W. Williamson (1851, at age 27)
Formulae may be used as an actual image of what we rationally suppose to be the arrangement of constituent atoms in a compound, as an orrery is an image of what we conclude to be the arrangement of our planetary system.
This slide also includes a picture of an orrery. It is a device that serves as a model of our solar system, complete with moving paths for the planets.
Dumas is saying that, although molecules behave according to very complicated physical laws, they have structures that we can know and understand. Williamson's quote says that formulas should suggest the arrangement of atoms in a molecule. This idea can be seen in modern nomenclature. Just like the radical theory shows up when C4H9Br is called butyl bromide (the two names and the space showing that the compoun forms from two distinct radicals), the Unitary theory shows up when this same compound is called bromobutane (the name suggests that the molecule is like a molecule of butane with a bromine atom).
[edit]Slide 7
Slide 7 shows a quote by Berzelius in 1837 in which he tries to disprove Unitary theory: "By reacting chlorine with ordinary ether (Dumas) pro-duced a very interesting compound which he reckoned, according to the theory of substitutions, to be an ether in which 4 atoms of chlorine replace 4 atoms of hydrogen. An element as eminently electronegative as chlorine would never be able to enter into an organic radical: this idea is contrary to the first principles of chemistry…"
He is saying that chlorine is too electronegative to substitute for a hydrogen atom in an organic compound, which Dumas claimed it had done in this reaction.
This is not the most important part of the slide, however. The key part of slide 7 is that Berzelius used "R" to denote a radical for the first time ever. R has now become a very important part of notation for organic chemists.
