Organic chemistry in our life. Part III
Sulphur and Phosphorous
The qualitative recognition of sulphur or phosphorus in an
organic body may be effected by heating the dry substance
with a little metallic sodium. If sulphur is present, sodium
sulphide will be formed, and may be detected by the evolution
of H 9 S on addition of water and an acid, or by the use of
sodium nitro-prusside, which gives an intense violet colouration
with a trace of soluble sulphide. In the case of phosphorus,
sodium phosphide (or if, as is advantageous, aluminium filings
be employed, aluminium phosphide) is formed, from which
the dampness of the breath is sufficient to evoke the character-
istic smell of hydrogen phosphide.
Oxygen.
There is no convenient method known for the
detection or estimation of oxygen in a compound. Its amount
is determined by difference, i.e., by subtracting the percentages
of all the other elements present from 100, and taking the
remainder to represent the percentage of oxygen.
Methane or marsh gas is theoretically the simplest of all
the compounds of carbon and hydrogen. Analysis shows that its
empirical formula is CH 4 , and the fact that the gas is eight times
heavier than hydrogen indicates the molecular weight sixteen,
and shows that this simplest formula is also the molecular one.
It occurs naturally in the gas which occasionally comes off
in bubbles from the bottom of stagnant ponds ; in the “ natural
gas “ escaping from fissures in the earth in certain oil-bearing
districts, and constitutes the fire-damp of the coal miner ;
while ordinary coal-gas contains about one-third of its volume
of methane.
Of methods used in the laboratory the three following are
important, the first from the theoretical standpoint, and the
two latter from that of practical work :
1. Methane can be synthesised, i.e. built up from inorganic
materials, by passing a mixture of H 2 S with vapour of CS 2
over red-hot copper: 2H 2 S + CS 2 + 8Cu - CH 4 + Cu,S.
2. A convenient laboratory method, yielding, however, a
somewhat impure methane, is to heat cautiously a mixture of
sodium acetate with sodium hydrate (barium hydrate gives a
less impure gas): N,iC 2 H 3 2 + NaOH = Na 2 C0 3 + CH 4
Sodium acetate, Methane.
Parafin series is the destructive distillation of bituminous shale or
other material of similar composition. This process is largely
carried on in the south-west of Scotland, and from the products
various valuable mixtures of hydrocarbons are separated by
refining. One of these is ‘paraffin oil’; another is the white
solid ‘paraffin wax’, and both are made up almost exclusively
of hydrocarbons of the methane series.
Properties of the Hydrocarbons, C fl H 2n+iy. All the
hydrocarbons of this homologous series, from marsh gas itself up
to the highest member yet obtained, present at almost complete
resemblance in chemical behaviour. They are all very inert
substances, not attacked by nitric acid, and only gradually
acted upon by chlorine or bromine. The products formed by
the action of the halogens are substitution products, in which
some of the hydrogen of the hydrocarbons has been replaced
by chlorine or bromine. In no case are addition products
formed by the members of this series.
The physical properties of the members change gradually
as we pass from one end of the series to the other. The lowest
members are gases requiring great pressure or cold to convert
them into liquids; the pentanes are volatile liquids, and,
ascending the series, we come to liquids of higher and higher
boiling point; while still farther up the series we meet with
hydrocarbons which are solid at the ordinary temperature.