Chemical Properties of Carbon
Compounds of Carbon with Hydrogen
HydrocarbonsCarbon forms with hydrogen a large number and variety of compounds known as hydrocarbons. The existence of these many compounds depends on the union of carbon atoms with each other, which may result in the formation of open-chain or cyclic hydrocarbons. Examples of the latter are:
Most of the hydrocarbons find a place only in the study of "organic" chemistry; and it will be sufficient here to notice the main types of open-chain hydrocarbons, and describe in detail the properties of a few typical examples.
The simplest hydrocarbon is methane or marsh-gas, CH4; it is the first representative of a series of saturated, open-chain hydrocarbons, which may be represented thus:
have the general formula CnH2n+2, are known as the paraffin hydrocarbons.
When four or more carbon atoms are present in the molecule, branched chains may occur, with the phenomenon of isomerism. For example, there are two hydrocarbons of the formula C4H10:
Normal butane – CH3-CH2-CH2-CH3;
and three of the formula C5H12:
Normal pentane - CH3-CH2-CH2-CH2-CH3
The normal paraffins constitute an homologous series in which is shown a gradation in physical properties from member to member, thus:
|B.P. (Atm. pressure)||Density|
|Normal pentane||C5H12||37° C.||0.627 at 14° C|
|Normal hexane||C6H14||69° C.||0.658 at 20° C|
|Normal heptane||C7H16||98° C.||0.683 at 20° C|
|Normal octane||C8H18||125° C.||0.702 at 20° C|
|Normal nonane||C9H20||150° C.||0.718 at 20° C|
|Normal decane||C10H22||173° C.||0.730 at 20° C|
The boiling-points of the normal paraffins are higher than those of their isomers, e.g.
|Normal butane||B.P.|| +1° C|
|Normal pentane||B.P.||+37° C|
These differences may be attributed to the greater compactness of the branched as compared with the unbranched carbon chains.
Since methylene, CH2, is unknown, the hydrocarbon ethylene, C2H4, is the first of the olefine series, whose general formula is cnh2n. These compounds contain two atoms of hydrogen fewer than the corresponding paraffin hydrocarbons. The two adjacent carbon atoms from each of which a hydrogen atom is lacking, are formulated as joined together by a double bond. In the case of olefine hydrocarbons containing four or more carbon atoms isomerism occurs owing to the double bond occupying different positions. The following are the constitutional formulae for the oiefines C2H4, C3H4, and C6H8:
Ethylene - HC=CH
Propylene - CH3-CH=CH2
α-Butyleno - CH3-CH2-CH=CH2
β-Butylene - CH3-CH=CH-CH3
The olefine hydrocarbons are unsaturated, i.e. they form additive compounds which are paraffin hydrocarbons or their derivatives. In this process the double bond is ruptured, thus:
H2C=CH2 + Cl2 = ClH2C-CH2Cl
or Ethylene → Ethylene dichloride or symmetrical dichlorethane.
Acetylene, C2H2, is the first of a third series of hydrocarbons, named after the first member, and having the general formula CnH2n-2. These contain a triple bond and unite with four hydrogen atoms or their equivalent, forming paraffin hydrocarbons or their derivatives:
Acetylene - HC≡CH
Allylene, Methyl-acetylene - CH3C≡CH
Crotonylene, Dimethyl-acetylene - CH3C≡CCH3, etc.
Isomeric with the acetylenes are the di-olefines, e.g.
Allene or symmetrical allylene: CH2=C=CH2.
Olefine-acetylenes are known, e.g. CH2=CH-C≡CH, and di-acetylenes, e.g. HC≡C-C≡CH.
Carbon and the Halogens
Carbon HalidesNumerous halides of carbon are theoretically capable of existence. Not only may successive hydrogen atoms of the many hydrocarbons be replaced by halogen atoms to form substitution derivatives, each series of which ends in a carbon halide, but mixed halide derivatives may also exist. The formation of halogen derivatives of methane by substitution, viz. the series of compounds CH3Cl, CH2Cl2, CHCl3, CCl4, has already been noticed, as well as that of additive compounds, such as C2H4Cl2 and C2H2Cl4, by ethylene and acetylene respectively.
The present study will be limited to halides containing a single carbon atom, i.e. to the four compounds CF4, CCl4, CBr4, CI4.
Carbon OxyhalidesJust as the carbon tetrahalides, CX4, may be regarded as the halides - or the halanhydrides - of orthocarbonic acid, C(OH)4, so the oxy-halides, COX2 are the halides of metacarbonic acid, CO(OH)2.
Carbon and the Oxygen Group
Oxides of CarbonThe two important oxides of carbon, carbon monoxide, CO, and carbon dioxide, CO2, were for many years the only known oxides. In 1873 the oxide C4O3 was obtained by Brodie by subjecting carbon monoxide to prolonged electric discharges; and in 1877 Berthelot produced C8O3 by heating Brodie's oxide to 300°-400° C. In 1906 Diels and Wolf obtained C3O2, a substance which has aroused much interest. Mellitic anhydride is C12O9; an unsuccessful attempt has been made to prepare dicarbon dioxide, CO:CO.
Amino-derivatives of Carbonic Acid
|When the hydroxyl groups of carbonic acid are replaced successively by amino- (NH2) groups there are produced respectively carbamic acid and carbamide. |
Carbonic acid: HO-CO-OH
Carbamic acid: HO-CO-NH2