Unsaturated Compounds

UNSATURATED COMPOUNDS. A tetravalent carbon with hydrogen gives a saturated compound, methane, CH₄; monovalent methyl radicals, CH₃, joining in pairs, also form a saturated hydrocarbon, ethane, C₂H₆. But bivalent radicals CH₂, joining in pairs, give an unsaturated (unlimited) hydrocarbon—ethene, C₂H₄, in which two atoms of hydrogen are lacking for complete saturation. Consequently, in ethene there must exist two free valences, CH₂-CH₂, but it is assumed that these free valences mutually saturate each other, forming the so-called double bond, which is denoted by two dashes (CH₂=CH₂) or two dots (CH₂:CH₂). Hydrocarbons with a double bond are called ethene (by the first representative, ethene) or olefins. Olefins and compounds with a double bond in general are characterized by the high reactivity of the carbon atoms connected by a double bond. The double bond is less strong than a single one; it easily breaks, and at the place of breakage addition reactions are possible, which characterize compounds with a double bond as unsaturated. For example, ethene adds hydrogen, forming a saturated hydrocarbon, ethane: CH₂:CH₂+H₂=CH₃.CH₃; it adds halogen, hydrogen halide, forming halogen derivatives of saturated hydrocarbons: CH₂:CH₂+Cl₂=CH₂Cl.CH₂Cl; CH₂:CH₂+HCl=CH₃.CH₂Cl; it adds sulfuric acid, forming ethylsulfuric acid: CH₂:CH₂+HOSO₂.OH=CH₃.CH₂.OSO₂.OH. Especially often, to prove the unsaturated character of a compound, two reactions are used: 1) the decolorization of bromine at low temperature, for example CH₂:CH₂+Br₂=CH₂Br.CH₂Br, and 2) the reduction of an alkaline solution of potassium permanganate, also at low temperature, with the release of manganese peroxide and the formation of glycol: CH₂:CH₂+O+H₂O=CH(OH).CH(OH). At the place of the double bond, ozone can further add, forming an ozonide, for example CRR':CR''R''+O₃=CRR'CR''R'' and the O-O-O group as well. Trivalent radicals, CH₂, joining in pairs, give an unsaturated hydrocarbon, acetylene, C₂H₂, in which the two free valences of each carbon mutually saturate each other, forming a triple bond: CH≡CH or CH:CH. The triple bond is even weaker than the double one; when it breaks, it first passes into a double one, and this further into a single one: CH:CH+H₂=CH₂:CH₂; CH₂:CH₂+H₂=CH₃.CH₃. Because of this, hydrocarbons with a triple bond, called acetylene, are very prone to addition reactions, just like ethene, and are also unsaturated compounds. The assumption of double and triple bonds in the classical theory of unsaturated compounds, however, does not fully correspond to facts. Double and triple bonds are not equivalent to 2 or 3 single bonds; they are weaker than single bonds; consequently, in the formation of a multiple bond, some reserve of chemical affinity must remain, the so-called residual affinity. The greater reserve of energy in compounds with a double bond finds its physical expression in an increase in molecular refraction and in higher values for heats of combustion. According to Baeyer, assuming that the units of affinity of a carbon atom are directed to the corners of a tetrahedron, the angle between two valences will be 109° 28'; in a single bond, the forces of valence are directed along the line connecting the centers of the atoms; in a double bond, the direction of the valences deviates from their direction in a single bond, causing tension proportional to the angle of deviation; this angle for a double bond is 54° 44', for a triple bond is 70° 32'. According to Thiele, the forces of valence decompose into two components: along the line connecting the centers of the atoms and perpendicular to it. A multiple bond is formed only at the expense of the first component, the second one does not participate in it; it is this that explains the residual affinity, residual or partial valences, denoted by Thiele with a dash: CH₂=CH₂. If a compound has two double bonds between two neighboring pairs of atoms, then these are "conjugated" double bonds, for example: CH=CH-CH=CH; in such a system, the two outer carbon atoms possess partial valences, and a hidden double bond is formed between the inner carbon atoms (denoted by an arc). The addition reaction takes place at the place of the partial valences, i.e., at the outer pair of carbon atoms, and the hidden double bond turns into a real double bond: x.CH=CH-CH=CH.x-*x.CH-CH=CH-CH.x. By Thiele's theory, this explains the peculiar course of addition reactions in compounds with conjugated double bonds. Some have put forward a theory of unsaturated carbon atoms, allowing the existence of a trivalent carbon in ethene compounds and a bivalent "ENTSKY" in acetylene compounds, without the assumption of double and triple bonds. Substances containing a double bond possess a strong odor (biological reaction) and a more pronounced pharmacological action than the corresponding saturated compounds. As an example, many alkaloids can serve; choline (see) is a saturated and non-toxic compound, and neurine (see), close to choline, is an unsaturated compound and IS A STRONG POISON.

^and" Ovaries."