SUBSTITUTION. One of the most important methods of forming chemical substances, and of ascertaining the constitution of com pounds, is that known as substitution. The term substitution, as thus chemically applied, simply means the replacement of a body already in combination by another body not in combination, and of course includes all processes of single or double decomposition. Thus, in a compound A B, substitution of B for c may be effected by simply bringing c into contact with AB, the new compound A C being pro duced : this case of substitution is one very frequently performed iu chemical research and manufacture, and is known as single decom position or single elective affinity. Again, in a compound A B, substi tution of B by c may be effected by bringing a compound c n, contain ing c, into contact with A B, thus producing the required substance A C ; this method of substitution is fully as important as the other just mentioned, and is known as double decomposition or double elective affinity, because there are, so to speak, two elections or choices going on, namely, A for c, and B for n, whereas in the former illustra tion only one occurs. By using varying amounts of a substituting body, c and bringing forces other than the chemical to his aid, the chemist can sometimes produce several distinct compounds of the same bodies, thus :—AB, + = A or A 13, = A + Frequently the body substituted unites with one half of the sub stituting body, thus + C.
Although, strictly speaking, single and double decomposition are processes of substitution, it is found convenient to restrict the term substitution to cases in which organic compounds are being operated on, while the phrases single decomposition and double decom position are conventionally applied to inorganic transformations only.
[CIIEXICAL AFFINITY.] The simplest examples of substitution occur when chlorine, or either of its analogues, bromine and iodine, is made to act upon an organio compound. Thus chloride of ethyl (C.II„ C1), by the action of chlorine suffers replacement of its hydrogen by chlorine ; hydrochloric acid being formed and the following bodies successively produced :— The constitution or construction of the original compound from which such a series of derivatives is obtained seems to alter but little. As might be expected from the nature of the elements, the boiling point of a body containing chlorine in the place of hydrogen is higher in direct proportion to the amount of hydrogen replaced. Specific gravity also undergoes relative increase ; while vapour volume remains constant. Only when complete substitution of one body by another has taken place does the molecular arrangement of the original com pound appear to give way.
The substitution of chlorine for oxygen, and of oxygen by hydrogen, is frequently performed by chemists. Peroxide of nitrogen (NO,), also, may be introduced into organic compounds by acting upon them, under certain conditions, with nitric acid; in these cases hydrogen is displaced and forms water with the fifth equivalent of oxygen, while peroxide of nitrogen takes the place of the hydrogen.