Chemical Properties of Carbon

Hydrocarbons

Carbon 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:

Benzene C6H6	Naphthalene C10H8	Anthracene C14H10

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:

Methane CH3-H	Ethane CH3-CH3	Propane CH3-CH2-CH3

have the general formula C_nH_2n+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;
Isobutane -
and three of the formula C₅H₁₂:
Normal pentane - CH₃-CH₂-CH₂-CH₂-CH₃
Isopentane -
Tetramethylmethane -

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
Isobutane	B.P.	-17° C
Normal pentane	B.P.	+37° C
Isopentane	B.P.	+31° C
Tetramethylmethane	B.P.	+10° 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 c_nh_2n. 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
γ-Butylene -

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 C_nH_2n-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 Halides

Numerous 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 Oxyhalides

Just 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.

Oxides of Carbon

The 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.

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
Carbamide: NH₂-CO-NH₂