meTh^d 4161 ' 5 "'" by V ' Meyer ' s has almost entirely supplanted the
older ones of Hofmann and Dumas ;
the apparatus employed is shown in Fig. I 2. A cylindrical bulb
A, provided with a long and narrower neck, is heated to a steady
temperature by some suitable means, usually by the vapour
of a substance kept boiling in the jacketing tube. When
ii VICTOR MEYER'S METHOD 15
the temperature has become quite steady, the cork is removed,
and a small glass tube or thin bulb, containing about half a
decigram of the body to be examined, is allowed to fall into
the bulb, the cork being quickly replaced. In order that the
experiment may succeed, it is necessary that the temperature
of the bulb should be at least 20 or 30 C. above the boiling
point of the compound under investigation, when this latter
rapidly evaporates, and in doing so fills the lower part of the
bulb with vapour, driving out through the side tube a
corresponding volume of air, which is collected in E and
It calculating the result it is unnecessary to know the
temperature of A (it must, however, be steady). The vapour
given off at the bottom of the bulb displaces its own volume
of air, but this, before being measured, is cooled down to the
temperature of the water over which it is collected. What we
really obtain is, therefore, the volume which the vapour of the
amount of substance used would occupy at the temperature
and pressure in E. If, then, we divide the weight of sub-
stance used by the weight of the volume V of hydrogen (at
the temperature and pressure in E), we have at once the
vapour density of the compound examined.
The results, though not very accurate, are practically quite
sufficient, as the question is usually not one of determining
the exact molecular weight, but merely of the ratio of the
molecular to the empirical formula.
Example. .0623 gram of alcohol gave by Victor Meyer's method
31.5 c.c. of air, measured at 15 C. and 750 mm. pressure.
270 7 CQ
This volume would become 31. 5 x -^= x ^- c.c. at o C. and 760 mm. ;
and this volume of hydrogen (29.5 c.c.) would weigh .0896 x ' gram.
The vapour density is therefore *O ^i^f-^^Zt^f^ftrt^c.C. Q^o* 3. / .
.0623 1000 weight of substance
T , X
.0896 29.5 weight of gas obtained reckoned as hydrogen
Hofmann's Method is still occasionally made use of
for substances which cannot readily be vapourised without de-
composition under the ordinary pressure, though modifications
16 HOFMANN'S METHOD CHAP.
of V. Meyer's method have also been made for this purpose.
A long graduated tube, closed at one end, is filled with mercury,
and inverted in a mercury trough, while round the upper
portion of the tube a wider jacketing tube is placed, through
which can be blown the vapour of some liquid of suitable
boiling point. A small glass tube, containing about a fifth of
a decigram of the substance, is introduced into the inner tube,
and allowed to float up to the top of the mercury, where its
contents are then vapourised on passing a current of steam or
other vapour through the outside jacket.
On this method we require to notice the volume occupied
by the vapour, and the height of the mercury in the inner
tube above its level in the trough, besides knowing the
temperature of the jacketing vapour. Its advantage is that the
substance evaporates under a pressure considerably less than
that of the atmosphere, in consequence of its partial compensa-
tion by the column of mercury in the inner tube.
Example. .0243 gram of substance vapourised at the temperature of
boiling aniline (183 C. ) gave 54.5 c. c. ; the height of the mercury
column was 420 mm., that of the barometer 765 mm.
Now 54.5 c.c. of hydrogen at 345 mm. pressure (765 -420) and 183 C.
.0896 345 273
54.5 x X<2 Z^ X -^ = .00133 gram ;
1000 760 456
hence the vapour density of the substance is
Molecular "Weight of Non-volatile Substances.
There are many substances of which it is quite impossible to
determine the vapour density, as they are not volatile without
decomposition. In such cases we can obtain assistance by
applying methods, first established experimentally by Raoult,
depending upon certain properties of solutions. Van't Hoff
has brought forward a theory by which these various facts are
connected together, but for the purpose in view the experi-
mental data of Raoult are sufficient.
If we take 100 grams of water or any other solvent, and
dissolve in it i gram of any substance, it is found that (a) the
freezing point of the solution is lower, and (b) the boiling point
is higher than that of the pure solvent. The amount of change
is in each case dependent upon the molecular weight of the
dissolved body. For the same solvent the change is proportional
to the number of molecules dissolved in a given quantity of
The apparatus devised by Beckmann for applying the first
method, depending on the de-
pression of the freezing point, is
shown in Fig. 13. About 20
grams of the solvent are intro-
duced into the central tube
A of the apparatus, and the tem-
perature being slowly brought
down below the melting point,
the exact temperature at which
the solvent freezes is noticed
on the thermometer. A small
accurately weighed quantity
of the substance to be exam-
ined is now introduced into
the tube A, and made to
dissolve by vigorous stirring
and gentle warmth ; then the
temperature is again lowered,
and when freezing occurs the
freezing point of the solution
is observed on the thermo-
FIG. 13. Beckmann's apparatus for
determining molecular weights.
Let iu = weight of substance used ;
W= ,, solvent
/ = difference between freezing point of the solution
and freezing point of solvent ;
;// = molecular weight of the substance examined ;
then the number of molecules of the substance dissolved in
IV grams of the solvent is , and therefore in 100 grams of
the solvent there would be, for a solution of the same strength,
1 8 RAOULT'S METHOD
I OO IV
777 molecules dissolved. According to Raoult's results the
depression of the freezing point for the solution is proportional
to this number, and we have
.. I OO IV
or *=iooA ,
where AT is a constant depending on the nature of the solvent.
The values of K for the most important solvents are as
Water . . . . .19
Benzene . . . . . 49
Naphthalene ..... 74
Acetic Acid ..... 39
For further particulars of this method, and of the similar one
depending on the elevation of the boiling point, the student
may advantageously consult Outlines of General Chemistry,
by Ostwald, p. 137, or Quantitative Analysis, by Clowes and
Coleman, p. 432.
QUESTIONS ON CHAPTER II
1. What reasons have we for writing the formula of acetic acid as
C 2 H 4 Oo instead of the simpler one CH._.O ?
2. Describe Victor Meyer's method of determining vapour density.
Calculate the vapour density of a substance from the following data :
.0582 gram of the substance was used, and 23.5 c.c. of air were expelled
(measured at 18 C. and 755 mm. pressure).
3. What methods can be used to determine the molecular weight of a
substance such as sugar, which cannot be converted into vapour without
4. Butyric acid has the empirical formula CjHjO, and silver butyrate
is found to contain 55.4 per cent of silver ; what do you conclude from
these facts as to the molecular formula of the acid ?
5. Calculate the molecular weight of a substance from the following
results obtained by Raoult's method :
Weight of acetic acid taken . . 20. 5 grams
Freezing point of acetic acid . . 16.435 C.
Weight of substance dissolved . . .153 5 gram
Freezing point of solution . . . 16.305 C.
HYDROCARBONS OF THE METHANE SERIES
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 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,,S with vapour of CS. 2
over red-hot copper :
2H 2 S + CS 9 + 8Cu = CII 4 +X;u 2 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) :
NaC.,H,O., + NaOH = NaXO, + CH,
31 ma <
Sodium acetate. Methane.
If a glass vessel be used, it will soon be attacked by the melted
caustic soda ; and though this action can be lessened by using
an admixture of quicklime (soda-lime is best), it is more con-
venient when possible to employ a copper retort.
FIG. 14. Apparatus for the preparation of CH 4 from sodium acetate and
EXPT. 3. Prepare marsh gas by heating some dry anhydrous (not
crystallised) sodium acetate with about four parts of powdered soda-lime
in a small glass flask fitted with cork and delivery tube. Collect two jars
of the gas and examine its behaviour, (a) when a lighted taper is brought
near, (b) when allowed to mix with bromine vapour contained in a second
3. Pure methane is best prepared by the action on methyl
iodide, CH 3 I, of the zinc-copper couple in presence of alcohol.
The couple is merely zinc covered with a deposit of copper by
treatment with a solution of copper sulphate, and acts in pres-
ence of either water or alcohol as an excellent reducing agent :
+ H., = CH, +
Methyl iodide. Methane.
A more complete representation is given by the equation :
CH 3 I + Zn + C 2 H 6 = Zn
+ CH 4 .
Methane is a colourless gas, without taste or smell, only
slightly soluble in water, and very difficult to condense to a
liquid. It burns in the air with a nearly non-luminous flame,
which becomes much brighter if both the air and the methane
are strongly heated before combustion (regenerative burners),
and the products of the burning are water and carbon dioxide :
CH 4 + 2O 2 = CO 2 + 2H 2 O.
A mixture of i vol. CH 4 with 2 vols. O 9 explodes violently
when ignited. When strongly heated alone, methane is
decomposed with formation of carbon, hydrogen, and smaller
quantities of other products.
Methane is a very stable substance, and is. not readily
attacked even by the most active reagents. Nitric acid is
almost without action upon it ; chlorine and bromine attack it
slowly (more quickly in sunlight than in the dark) with forma-
tion of " substitution products," in which one or more hydrogen
atoms of the methane have been expelled (in combination with
Cl or Br as HC1 or HBr) and their place taken by halogen
CH 4 + Br 2 = CH 3 Br + HBr,
or CH 4 + 2Br. 2 == CH 2 Br 2 + 2 HBr, etc.
Homologry. Methane is the lowest member of a series of
hydrocarbons, all of which can (in general) be prepared by
similar reactions, and strongly resemble one another in their
chemical behaviour. Each member differs from the one below
it in the series by the replacement in its formula of an H atom
by the group CH 3 , to which the name methyl is given ; the nett
difference between any two successive members is therefore
CH . Such a series is called a homologous series, and the
study of organic chemistry is much simplified by the possi-
bility of classifying in this way the immense number of known
compounds into groups of similar bodies.
Starting from methane, CH 4 , we have as the formula of
the next member of the series CH 4 +CH., or C.jH^, for the
third C. t H s , and so on up to C ( . (0 H 100 , the highest which has
yet been prepared. The generic formula is C ;( H 0w+0 .
22 ETHANE CHAP.
Ethane, C.,H g , stands next to methane, and can be prepared
by similar reactions. In the first, we start not from sodium
acetate (as for methane), but from the sodium salt of the acid
next above acetic in the very important series of homologous
acids, of which acetic forms the second and propionic the third
member. Acetic acid is C 2 H 4 O 2 and propionic C,H 6 O . We
proceed then as follows :
. I. Sodium propionate is heated with sodium hydrate,
NaC H r O 9
9 9 f
C 2 H 6
when ethane is evolved and sodium carbonate remains.
FIG. 15. Apparatus for preparing C-jHg from ethyl iodide by the action of the
2. In the second method for preparing ethane, ethyl iodide,
/ C.,H r( I (homologous with methyl iodide, CH.,1), is reduced with
the zinc-copper couple :
in PROPANE AND BUTANE 23
C 2 H 5 I + H 2 .* C 2 H 6 + HI.
Ethyl iodide. Ethane.
3. A third method is of a type applicable only to the pre-
paration of those members of the series which contain an even
number of carbon atoms in the molecule. In this case we
start from methyl iodide, CH 3 I, and by treating it with metallic
sodium, abstract the iodine and cause two methyl residues
CH 3 to unite :
2CH ;J I + 2Na = 2NaI+C 2 H 6 .
Methyl iodide. Ethane.
In accordance with this method of preparation, the formula of
ethane may be written CH 3 . CH 3 .
Ethane is a combustible gas, and burns with a more lumin-
ous flame than methane. It resembles that gas very greatly
in chemical behaviour, and reacts in the same way with the
halogens, forming substitution products, such as
.,, . 7 = C H.C1 + HC1.
JO - Aw
Ethane. Ethyl chloride.
Propane, C.,H a ( = C..H.. + CHA stands next above ethane.
o o ^ & V &*
It may be prepared by methods corresponding to the first two
of those given for ethane. The best is the following :
i. Propyl iodide, C 3 H-I ( = C 2 H 5 I + CHA. is reduced with
the zinc-copper couple :
C 3 H 7 I + H 2 = C S H 8 + HI.
Propyl iodide. Propane.
Butane is the name given to the next hydrocarbon of this
series with the formula C 4 H 1Q . We here encounter for the first
time a fact of very great importance : that there may be, and
often are, more substances than one corresponding to a parti-
cular molecular formula. This experimental fact we interpret to
mean that two molecules, each containing the same atoms in
the same number, may yet be distinct both chemically and
physically ; and this difference we explain as being due to the
different arrangement of the atoms in the molecule. The
name isomerism is given to this phenomenon, and substances
which possess identical molecular composition, and yet differ
from one another in the way described, are said to be isomeric.
In the majority of such cases it is found possible to give a
reasonable representation of the different chemical behaviour
of the isomeric bodies by structural formulce, which are also
considered to represent more or less exactly the actual grouping
of the atoms inside the molecule. Let us now consider more
fully this particular case of the butanes.
In the first three members of this series no isomerism has
been found to exist. Their formulae, CH 4 , C H 6 , C 3 H g , may be
expanded into CH 4 , CH 3 .CH 3 , CH 3 .CH 2 .CH 3 , which are in
agreement with the valency hypothesis, and represent, more
completely than the simple formulas do, the modes of formation
and general chemical behaviour of the three substances. In
each case the formula is obtained from that of the next lower
compound by substituting methyl, CH 3 , for hydrogen. In
methane there are four hydrogen atoms in the molecule, but
these are all similarly circumstanced, and whichever of them
be replaced we obtain the same ethane, CH 3 . CH y Similarly,
when we proceed from this to propane ; the six hydrogen atoms
in the ethane molecule are all of equal value, and we get
always the same propane when any one of them is substituted by
a methyl group. But at the next step this is no longer the case,
for the eight hydrogen atoms in propane are not all similarly
placed ; while six of them are alike, and the other two also
like one another in position, there is a difference between the
atoms attached to the two terminal carbon atoms and those
which are connected to the carbon atom in the centre of the
chain. Hence two formulae for butane may be deduced from
that of propane by substituting CH 3 for H atoms of different
CH 3 . CH 2 . CH 3 gives (i) CH 3 . CH^ . CH 2 . CH 3
and (2) CH 3 . CH". (CH 3 ). r
So far by paper work. Experimental investigation has proved
that there are two butanes, each with the formula C 4 H 10 , and
each rightly placed, according to its general behaviour, in the
Of these two butanes one is prepared from ethyl iodide by
abstracting the iodine with sodium :
2CH 3 . CH 2 I
CH 2 . CH 3 ,
and this mode of formation is well represented by the formula
given in the above equation. This particular butane is called
normal butane, or simply butane. For the other butane the
formula CH 3 . CH . (CH 3 ). ( remains, and the name given to it is
Pentane, C.Hj.,, is the generic name of the isomeric hydro-
carbons corresponding to the formula given. If \ve attempt to
work out the number of isomers which may in accordance with
the valency theory be obtained, we find that three are possible ;
and experimental work has enabled us actually to prepare
isomeric pentanes, and to assign to each, one of the three
formulas indicated by theory.
The most important is normal pentane, CH 3 .CH .CH 2 .CH .
CH 3 , which is contained in crude petroleum, and can be isolated
from it as a volatile inflammable liquid boiling at 37. This
has been used as a means of obtaining a reliable standard of
illumination for photometric purposes.
The following table illustrates the isomerism of the butanes
and pentanes :
CH 3 .CH 2 .CH 2 .CH 3
From ethyl iodide and zinc dust :
CH 3 .CH 2 :(CH 3 ),
From isobutyl iodide by reduction :
CH 3 .CHo.CH,.CH 2 .CH 3
CH 3 .CH 2 .CH :(CH 3 ) 3
C(CH 3 ) 4
Separated from petroleum.
Petroleum and Paraffin. In various parts of the world,
more especially in Pennsylvania and in Baku, a province of
Southern Russia, oil-bearing strata occur from which an in-
flammable oil can be obtained. Wells are drilled through
the overlying layers of earth until the oil is struck at a depth
varying from 50 to 2000 feet and over. In many cases the
newly-opened well " spouts " oil, frequently with uncontrollable
violence, but as the original gas-pressure declines, it becomes
necessary to have recourse to pumping. The crude oil requires
to be refined, and both in America and in Russia this process is
carried on, not at the wells themselves, but at large refineries
conveniently situated for export. The oil is transported to the
refineries by means of long lines of pipes, through which it is
forced by powerful pumps.
Investigation has shown that American and Russian petro-
leums differ essentially in chemical composition. American
petroleum is almost entirely a mixture of various hydrocarbons
of the methane series from CH 4 itself up to solid hydrocarbons
of very high molecular weight. The refining of the crude oil
has for its chief purpose the separation of this complex mixture
into a number of fractions, and is accomplished by distillation.
The more volatile portions are the first to come over, and are
followed by others of higher and higher boiling points. The
most important fractions are :
(1) Gasoline, B.P. 3o-ioo, used for making "oil-gas,"
which is simply air saturated with vapour of gasoline.
(2) Petroleum proper, B.P. I5o-3oo, used in lamps.
(3) Higher boiling portions from which lubricating oils and
vaseline are obtained.
Russian petroleum contains only a very small percentage of
hydrocarbons of the methane series, the chief bulk being
"naphthenes " of the generic formula C w H 2>t . These are dis-
tinct from the defines of the same formula, and will not be
considered until the second part of this book in connection
with the benzene hydrocarbons, from which they are derived.
The products obtained by refining the Russian crude oil are
very similar to those from American petroleum, but a larger
yield of oils suitable for lubricating machinery is got, and the
residue is not usually worked up for a vaseline-like product, but
is generally employed as fuel.
Another important source of hydrocarbons of the methane
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,,H 2;;+2 . All the
hydrocarbons of this homologous series, from marsh gas itself up
to the highest member yet obtained, present an 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.
QUESTIONS ON CHAPTER III
1. Describe the preparation and properties of methane.
2. Methane and ethane are members of a homologous series ; show
the bearing of this statement upon the properties of the gas and the
methods used for their preparation.
3. What is meant by isomerism ? Deduce the formula of the two
isomeric butanes from that of propane.
4. What substances are contained in crude petroleum ? What com-
mercial products are obtained from it, and by what processes ?
OLEFINES AND ACETYLENE
THE defines form a second series of hydrocarbons, of-which
the starting-point isethylene, C 2 H 4 . The succeeding homologues
differ always by CH 2 , and it thus follows that every member
of the series has the same percentage composition. They differ,
of course, in molecular weight, and therefore in vapour density.
The generic formula is C n H 2 ,,, and while we again find the
same similarity in general chemical behaviour between all the
defines, as between all the hydrocarbons of the C w H 2><+2
series, there are important differences between the two separate
series. The chief of these are summed up in the contrast of
the two terms saturated and unsaturated, applied respectively
to the methane and to the olefine series.
Ethylene, C 2 H 4 , is the lowest known member of the series,
and as in every reaction where we should expect a substance,
CH 2 , to be produced we obtain instead C 2 H 4 , it seems estab-
lished that no compound of the formula CH 2 can exist. The
most convenient way of preparing ethylene is by the action of
concentrated sulphuric acid upon ethyl alcohol, C 2 H g O, a
reaction which may very concisely be represented thus :
C 2 H 6 .- H 2 - C 2 H 4 .
Ethyl alcohol. Ethylene.
What really happens is that ethyl-sulphuric acid, C 2 H. . HSO 4 ,
is first produced, and this, when heated, decomposes into
ethylene and sulphuric acid ;
C 2 H 5 OH
H 2 O
H.,SO 4 = C 2 H 5 .HSO 4
Ethyl alcohol. Ethyl-sulphuric acid.