C 2 H 5 . HS0 4 - C 2 H 4 + H 2 S0 4 .

EXPT. 4. Prepare ethylene by heating in a capacious flask (2 litres) a
mixture of 40 c.c. methylated spirit with 200 c.c. concentrated H2SO4
along with some sand, to prevent frothing. The gas can be purified
from S(>> and other impurities by washing with solution of caustic soda,
and can be collected over water.




FIG. 16. Preparation of Ethylene from alcohol and sulphuric acid.

Ethylene is a colourless gas with a faint sweetish smell, and
burns in the air with a bright yellow flame. Like all the
other members of the series, it is an unsatitrated compound,
being able to unite directly with several substances Cl, Br, I,
H, HI, etc., nothing being driven out from the ethylene
molecule to be replaced by the substituting atoms. This is
explained on the valency hypothesis as due to those carbon
valencies which are unsaturated in the molecule of ethylene.

Ethylene combines readily with Cl, Br, and I (in alcoholic
solution) at the ordinary temperature, with hydrogen when the
two mixed gases are passed over platinum black ; ethane or
a derivative of it is in each case the product. When passed
into concentrated H.,SO 4 , ethylene is absorbed and ethyl-
sulphuric acid is formed :

C 2 H 4 + H 2 SO 4 = C 2 H 5 . HSO 4 .



3 o PROPYLENE CHAP.

EXPT. 5. Fill two jars with ethylene and bromine vapour ; bring
them together mouth to mouth. The colour of the bromine is rapidly dis-
charged, and small oily drops of a liquid ethylene dibromide are formed:



Try a similar experiment with methane and bromine. In this case the
action takes place slowly, and the bromine makes its way into the methane
molecule only by expelling some of the hydrogen :

C H 4 + Br 2 = CH 3 Br + H Br.
Methane is termed a saturated compound, whereas ethylene is unsaturated.

Propylene, C 3 H g , is obtained most conveniently by a
reaction typical of a second general method of preparing the
defines. Isopropyl iodide, C 3 H 7 I, a halogen derivative of the
corresponding hydrocarbon of the methane series (in this case
C 3 H g ), is treated with an alcoholic solution of potash, whereby
one atom of hydrogen and one of the halogen are abstracted :

C 3 H 7 I + KOH = C 3 H 6 + KI + H 2 O.
Isopropyl iodide. Propylene.

The formula of the isopropyl iodide is CH g . CHI . CH 3 . The
C 3 H 6 obtained from this by abstraction of HI may be either
CH 3 . C . CH 3 or CH 3 . CH : CH 2 , but the first formula is nega-
tived by numerous facts, of which a very conclusive one is that
propylene furnishes three isomeric chloro-propylenes, C 3 H 5 C1.
This is readily explained by the second, but is quite inconsist-
ent with the first. The two dots in the correct formula indicate
that the two carbons, one on either side, are each capable of
further uniting with an additional atom, and are connected
together in a different manner from that prevailing between
carbon atoms which are exerting their maximum valency. Two
such carbon atoms as those we have been considering in the
propylene molecule are said to be united by an etliylene
linkage, or by a double bond; but it must not be imagined
from the latter expression that such atoms are more firmly
held together than those united in the ordinary way (single
bond). As a matter of fact, an unsaturated molecule when it
suffers decomposition usually breaks up most readily at the
so-called double bond. Thermo-chemical investigation also
shows that an ethylene linkage cannot be regarded as simply



IV VAN'T HOFF'S TETRAHEDRAL THEORY 31

the double of single linkage ; there is a real difference in kind
between those two modes in which carbon atoms may be
connected.

Great assistance in correlating a very large number of ex-
perimental facts is furnished by the tetrahedral theory of the
carbon atom, which was proposed by Van't Hoff in 1877, and
has since been extended by Wislicenus and other chemists.
According to this valuable hypothesis, the carbon atom is re-
garded as being similar in shape to a regular tetrahedron, a
solid figure bounded by four equilateral triangles, and two
carbon atoms may be connected together in the following
three ways :

a. Simple linkage : the two tetrahedra are in contact at a
corner of each.

b. Double linkage : the two tetrahedra are in contact along
an edge of each.

c. Triple linkage : the two tetrahedra have a whole face of
each in contact.

The first kind of linkage is exemplified in the case of ethane,
the second in ethylene, and the third in acetylene. Substances
which contain a double or triple linkage are unsaturated, and
the theory affords a clear representation of the way in which
an unsaturated body becomes saturated by addition of chlorine,
bromine, etc. The following diagrams will illustrate these
points better than any verbal explanations.

To return to propylene : the theory which we have been
considering embodies very conveniently a large number of ex-
perimental generalisations, among them this, that such a
formula as CH 3 .C.CH 3 , in which carbon acts as a divalent
element, represents an arrangement of atoms incapable of
permanent existence. We have already seen reason to reject it
in favour of the alternative CH 3 . CH : CH 2 .

There are a few exceptional cases in which carbon is divalent, and
where it is therefore necessary to suppose that two of the four corners of
the carbon tetrahedron are unemployed, e.g. CO.

Butylene, C 4 H g , the next member of the series, furnishes
an instance of isomerism. There are three isomeric butylenes,
all of which may be looked upon as derived from ethylene by



BUTYLENE



replacement of hydrogen by methyl or ethyl groups, and their
names are best chosen to represent the manner of this
derivation :

a. Symmetrical dimethyl-ethylene, CH 3 . CH : CH . CH g .

b. Unsymmetrical dimethyl-ethylene, CH., : C(CH 3 ) 2 .

c. Ethyl-ethylene, C 2 H 5 CH : CH 2 .




FIG. 17. Representation of carbon atoms linked together as in
(A) Ethane, (B) Ethylene, (C) Acetylene.

The names are somewhat cumbrous, but have the advantage
of telling as much about the substances as the formulae them-
selves ; they are, in fact, merely the formulae in words instead
of symbols. The methods by which these three isomers have
been prepared are too complicated for us to enter into ; the



ACETYLENE



33



important thing is that the theoretical number of isomers have

been obtained.

Acetylene, C^H,,, is the lowest member of another series

of unsaturated hydrocarbons. In this we have a triple linkage

between the carbons, so that only two hydrogen atoms can be

attached to them, one to each : HC ; CH.

Acetylene can be made by several different reactions :
(i) By heating ethylene dibromide, C 2 H 4 Br 2 , with an

alcoholic solution of potash :




FIG. 1 8. Preparation of Acetylene by the action of alcoholic potash on ethylene
bromide ; both flasks contain potash, and the bromide is allowed to drop
slowly from the tap-funnel into the heated potash in the first flask.

CH L ,Br. CH L ,Br-2HBr=HC;CH.
Ethylene bromide. Acetylene.

(2) By direct combination of carbon and hydrogen, when
the electric arc is passed between carbon poles in an atmo-
sphere of hydrogen.

(3) By the action of water upon barium carbide :

BaC 2 + 2H.OH = C 2 H 2 + Ba(OH).,.
Barium carbide. Acetylene.



34



ACETYLENE



CHAP.



This is a very convenient method of preparing the gas
when it is wanted in considerable quantity.

(4) By the degraded combustion of coal-gas, such as
occurs when the temperature of the flame is artificially
lowered by contact with a metal surface (a Bunsen burner in
which the gas is burning at the bottom of the brass tube) :



C 2 H + O., = C 2 H 2 H
Ethane. Acetylene.



2H 2 0.



Acetylene is a colourless gas with an unpleasant smell.
It is soluble in about its own volume of water, and burns in
the air with a bright smoky flame. The most remarkable
chemical characteristic of acetylene is its property of forming
explosive compounds containing copper or silver. By means
of these compounds acetylene can be readily detected and
isolated from its mixture with other gases.




FIG. 19. Preparation of Acetylene by the degraded combustion of coal-gas.

EXPT. 6. Prepare some cuprous chloride. CuCl, by passing SO 2 gas
into a solution of 90 grams NaCl and 200 grams crystallised CuSO4 until
the gas is no longer absorbed ; pour into about half a litre of water, and
filter. The white precipitate of CuCl is collected, and dissolved in some
strong ammonia solution.

Open wide the air-holes of a large Bunsen burner, and light the gas at
the bottom of the tube. Over the burner support a funnel, which is con-
nected with a gas-washing bottle containing the ammoniacal solution of
cuprous chloride. Join the other tube of the wash-bottle to an aspirator,
and draw a steady current of air through the apparatus.



IV



ACETYLENE



35



A dark red precipitate will form in the solution of cuprous chloride.
This has the composition C.^HCu, and when dry explodes on being
heated, or if struck between two metal surfaces. Acetylene can be
recovered from it by treatment with dilute hydrochloric acid.



Acetylene unites readily with hydrogen, when the two gases
are passed over platinum black, to form ethane :



CHiCH

Acetylene.



t s CH $ .CH y

Ethane.




+CL =






FIG. 20. Representation on the tetrahedral theory of the conversion of Acetylene
into dichlor-ethylene and tetrachlor-elhane by the action of chlorine.



Chlorine is without action upon the pure gas in the dark,
but in sunlight dichlor-ethylene and tetrachlor- ethane are
successively formed :



36 ACETYLENE CHAP, iv

(a) CH:CH + C1 2 =-CHC1:CHC1,
Acetylene. Dichlor-ethylene.

(6) CHC1:CHC1 + C1. 2 = CHC1 2 . CHC1 2 .

Dichlor-ethylene. Tetrachlor-ethane.

These changes are represented on the tetrahedral theory by
the diagrams in Fig. 20.

QUESTIONS ON CHAPTER IV

1. What is the chief difference in chemical behaviour between a satur-
ated and an unsaturated compound ?

2. Give the preparation of acetylene and the most important properties
of the gas.

3. Give an elementary account of the tetrahedral theory of Van't
Hoff as applied to explain the structure of the three hydrocarbons
C2Hg, CjH^ CgH2.

4. How is ethylene made? What is formed when it is passed into
concentrated sulphuric acid?

5. How could you prepare ethane from the elements carbon and
hydrogen ?



CHAPTER V
HALOID DERIVATIVES

IN any hydrocarbon it is generally possible to replace from
one up to the full number of hydrogen atoms present in the
molecule, by chlorine, bromine, or iodine.

Methyl Chloride, CH 3 C1, is the first to be considered of
all these haloid derivatives ; it may be prepared

1 . By the direct action of chlorine upon methane, according
to the equation :

CH 4 + C1 2 = CH 3 C1 + HC1,

a process which is favoured by the influence of sunlight.

2. From methyl alcohol, CH 4 O (a substance to be con-
sidered in the next chapter, when we shall learn that the
formula is conveniently written CH 3 . OH, in order to indicate
the way in which the alcohol most readily reacts), by the
action of various compounds containing chlorine. The equa-
tion is simplest in the case when HC1 gas is used :

CH 3 .OH + HC1 = CH 3 C1 + H 2 0,

Methyl alcohol. Methyl chloride.

a reaction which easily occurs when HC1 is passed into
boiling methyl alcohol, to which some zinc chloride (a very
hygroscopic substance) has been added.

Methyl chloride is a gas with a pleasant smell, fairly
soluble in water, and pretty easily condensed by cold or
pressure to a liquid. It is used commercially in the manu-
facture of certain aniline dyes, and for this purpose is prepared



CHLOROFORM



from a by-product of the beet-sugar industry, and sold com-
pressed in strong steel cylinders. It burns with a green
flame.

Methene Chloride, CH 2 C1 2 , can be obtained by the
further action of chlorine upon methyl chloride :

CH 3 C1 + C1 2 = CH 2 C1 2 + HC1.

This is the second step in a series of successive substitutions
of the hydrogen atoms by chloride ; the third step yields

Chloroform, CHC1 3 , which is, however, more readily
prepared by a complicated reaction where ordinary ethyl
alcohol, C 2 H 6 O, is treated with bleaching powder.




FIG. 21. Preparation of Chloroform from alcohol and bleaching powder.

EXPT. 7. Mix 50 grams of bleaching powder with 250 c.c. of water,
and put the mixture in a large retort, or large flask fitted to a Liebig's
condenser (see figure) ; add 250 c.c. of methylated spirit, and heat the
mixture until it begins to boil. Then remove the burner, and allow the
reaction to proceed by itself. Chloroform and water are condensed in the
flask B, the chloroform sinking to the bottom of the water.

The mechanism of this reaction may be explained now,
though it will scarcely be fully understood until further
acquaintance with the subject has been made. The bleaching
powder acts both as an oxidising and as a chlorinating agent.
In the first capacity it removes two hydrogen atoms from the
ethyl alcohol :



METHYL IODIDE 39



CH 3 . CH 2 . OH + Cag C1 = CH 3 . CHO + H 2 O + CaCl 2 ,
Ethyl alcohol. Ethyl aldehyde.

but the product CH^. CHO, ethyl aldehyde, is at the same
time chlorinated and converted into trichlorethyl aldehyde
or chloral :

CH 3 . CHO + 3C1 2 = CC1 3 . CHO + 3HC1,
Aldehyde. Chloral.

while the chloral, under the influence of the lime of the
bleaching powder, gives chloroform and calcium formate:

2CC1 3 . CHO + Ca(OH) 2 = 2CHC1 3 + (HCO 2 ) 2 Ca.
Chloral. Chloroform. Calcium formate.

Chloroform is a heavy liquid of pleasant ethereal smell, and is
much used in surgery on account of the property which its
vapour possesses of producing insensibility when inhaled.

Carbon Tetrachloride, CC1 4 , is the last product of the
substituting action of chlorine upon methane :

CHC1 3 + C1 2 =CC1 4 + HC1,

and is obtained as a pleasant smelling liquid when boiling
chloroform is subjected to the prolonged action of a stream of
chlorine gas.

Methyl Iodide, CH 3 I, is an important reagent often used
in the synthesis of organic compounds. It cannot well be
prepared by the direct action of iodine upon methane, but is
readily obtained by the action of iodine and phosphorus upon
methyl alcohol in the way described in detail for making
ethyl iodide. It is a volatile liquid which turns brown when
exposed to light ; it is much used in the organic laboratory.

lodoform, CHI 3 , is used in surgery on account of its
marked antiseptic properties. The method of preparation is
analogous to that of chloroform, but instead of bleaching
powder, we employ iodine together with some alkali, such as
sodium hydrate or carbonate.



40 ETHYL CHLORIDE



EXPT. 8. Dissolve 10 grams of soda crystals in 50 c.c. of water, and
add 8 c.c. of methylated spirit. Heat to about 70 C. , and then add
gradually 5 grams of iodine. lodoform separates out as a yellow
precipitate.

lodoform is a yellow solid with a characteristic smell, and
is slightly soluble in hot water, from which it crystallises in
lustrous plates.

Ethyl Chloride, C. 7 H 5 C1, is a volatile liquid boiling at
about 12 C., and can now be obtained sealed up in stout
glass tubes, in which it is sold for use as a local anaesthetic
in minor surgical operations. It acts in this way by virtue of
the intense cold produced by its rapid evaporation when the
liquid is allowed to spray from a fine opening upon the part
where the operation is to be done.

It is prepared by the action of HC1 gas upon ethyl alcohol
in the presence of zinc chloride :

C 2 H 5 . OH + HC1 = C 2 H 5 C1 + H 2 O,
Ethyl alcohol. Ethyl chloride.

and conversely when heated under pressure with water (better
with solution of an alkali) ethyl chloride yields ethyl alcohol :

C 2 H 5 C1 + H 2 O = C 2 H 5 . OH + HC1.

Dichlor-ethane, C 2 H 4 C1 2 , is the second in the series of
substitution products obtained by the action of chlorine upon
ethane :

C 2 H 6 +C1 2 = C 2 H 5 C1 +HC1,
C 2 H 5 C1 + C1 2 = C 2 H 4 C1 2 + HC1, etc.,

but we here meet with a further instance of isomerism, and
there are two distinct substances possessing the formula
C 2 H 4 C1 9 . In one of these, ethene dichloride, CH 3 . CHC1.,,
both chlorine atoms are connected with the same carbon atom,
while in ethylene dichloride, CH 2 C1 . CH C1, they are attached
one to each of the two carbons.

Ethene Dichloride, CH a . CHC1 2 , is obtained by the
action of chlorine upon ethyl chloride, as a rather volatile
liquid with a smell similar to that of chloroform.

Ethylene Dichloride, CH 2 C1 . CH 2 C1, is prepared by the



ETHYL BROMIDE 41



direct combination of ethylene and chlorine, and is the oily
liquid from whose formation the old name olefiant gas arose :

C 2 H 4 + C1 3 = C 2 H 4 C1 2 .
Ethylene Ethylene dichloride.

Ethyl Bromide, C.,H 5 Br, can be obtained by the action of
bromine upon ethane :

C 2 H tf + Br 2 = C 2 H 5 Br + HBr,
Ethane. Ethyl bromide.

but more readily by the action of phosphorus tribromide (or
phosphorus and bromine together) upon ethyl alcohol.
Phosphorus tribromide reacts with water thus :

PBr 3 + 3 H 2 = P(OH) 3 + 3 HBr,

that is to say, the three bromine atoms are exchanged for the
same number of hydroxyls. Ethyl (or any other) alcohol
behaves similarly to water :

PBr 3 + 3C 2 H 5 OH = P(OH) 3 + 3C 2 H 5 Br,

yielding phosphorous acid and ethyl bromide.

Ethyl bromide is a volatile liquid with a pleasant ethereal
smell.

Just as with dichlor-ethane, C 2 H 4 C1 9 , there are also two
isomeric substances of the formula C H 4 Br 2 , and their modes
of formation are precisely similar to those of the corresponding
chloro-derivatives.

Ethene Dibromide, CH s CHBr 9 , is obtained by the action
of bromine upon ethyl bromide :

C^H.Br+Br., = C H 4 Br 2 + HBr.
Ethene bromide.

Ethylene Dibromide, CH 2 Br . CH 2 Br, by passing a
stream of ethylene through bromine contained in a series of
gas washing cylinders :

2 = C,H 4 Br 2
Ethylene bromide.



ETHYL IODIDE



Both ethene and ethylene bromides are heavy liquids with
a smell similar to that of chloroform, but they differ markedly
in many respects.

Ethyl Iodide, C H 5 I, is an important reagent prepared in
a way precisely similar to that employed for ethyl bromide,
except that iodine is used instead of bromine.

EXPT. 9. 10 grams of red phosphorus and 60 c.c. of strong alcohol
("absolute" alcohol must be used ; rectified spirits of wine is useless)
are placed in a retort, and 100 grams of iodine added little by little. The
mixture is allowed to stand for several hours, and is then distilled from the
water-bath (see figure). If the alcohol employed has been weak, fumes of




FIG. 22. Preparation of Ethyl Iodide.

HI will be evolved in torrents, but from absolute alcohol only traces of
HI will be given off. The methyl iodide is condensed in the Liebig's
condenser, and collects in the flask. It is washed with caustic soda
solution and water, then dried by being left to stand in a highly-corked
flask over lumps of fused CaCl2, and then re-distilled.

The equation for the reaction is

3 C,H f OH + PI 3 = 3C 2 H 5 I + H 8 P0 3 .
Ethyl alcohol. Ethyl iodide.

Ethyl iodide is a colourless liquid with an ethereal smell.
It boils at 72, and is heavier than water, sinking to the bottom
like an oil. It gradually decomposes when exposed to light,
and the liberated iodine colours the liquid brown. Both
methyl and ethyl iodides are largely used in experimental
organic chemistry, their use depending on the great mobility



ETHYL IODIDE 43



of the iodine contained in them. This iodine is readily
exchanged for various atoms or radicles by appropriate
reactions, and many new compounds have been obtained by
this means.



QUESTIONS ON CHAPTER V

1. Describe the preparation of chloroform. In what way would you
attempt to prove the correctness of the formula CHCls, which is assigned
to it?

2. For what purposes are methyl and ethyl iodides employed, and
how are they made ?

3. Give an account of the two isomeric substances corresponding to
the formula C 2 H 4 Br 2 .



CHAPTER VI
THE ALCOHOLS

THE alcohols form a homologous series, of which the starting-
point is methyl alcohol, CH 4 O. Of the four hydrogens in this
molecule one is distinguished from the others by the greater
readiness with which it is exchanged for other atoms or radicals,
while the fact that methyl alcohol may easily be obtained from
or converted into methyl chloride, CH 3 C1, indicates to us that
the formula CH 4 O may better be written CH 3 .OH. This
leads us to consider water, H . OH, as the inorganic type of
the alcohols, and it will be useful to remember that there exist
many points of resemblance between water and the alcohols
in their chemical behaviour.

Methyl alcohol, CH 3 . OH, being the starting-point of the
series, the next member is ethyl alcohol, C 2 H 5 . OH, and so
on to the highest known member, myricyl alcohol, C 30 H 61 . OH ;
the generic formula is C,,H OH+1 . OH. Chemically, they are
characterised by the presence of the group OH, the hydrogen
of which may easily be replaced ( i ) by sodium or potassium :

C 2 H 5 .OH + Na = C 2 H 6 .ONa + H,

Sodium ethylate.
with which compare

H.OH + Na = H.ONa + H;
Sodium hydrate.

or (2) by an acid residue to form an ethereal salt or " ester,"
in which the alkyl group, C,,H 2 ,, +1 , takes the place of a mono-
valent metallic atom in an inorganic salt, as :



WOOD-SPIRIT 45



(a) C 2 H 5 O H + HO NO 2 = C 2 H 5 . ONO 2 + H 2 O,
Ethyl nitrate.

(/*) CH 3 . OH + CH 3 . CO 2 H = CH 3 . CO 2 CH 3 + H 2 O,

Methyl acetate,
with which compare

(a) NaOH + HO . NO 2 = NaNO 3 + H 2 O,
Sodium nitrate.

and (*) KOH + CH 3 .CO 2 H CH 3 .CO 2 K + H 2 O.

Potassium acetate.

On the other hand, the whole group, OH, in any alcohol is
readily driven out by the action either of phosphorus penta-
chloride or of HC1 (in the presence of some hygroscopic sub-
stance such as ZnCl 4> ), and its place taken by a chlorine atom :

CH 3 OH + HC1 = CH 3 C1 + H 2 O
Methyl alcohol. Methyl chloride.

C 2 H 5 OH + PC1 5 = C 2 H 5 C1 + HC1 + POC1 3 .
Ethyl alcohol. Ethyl chloride.

Methyl Alcohol, CH 3 . OH, is contained in wood-spirit, a
product of the destructive distillation of wood. In this process,
now largely carried on in scientifically- constructed retorts,
there are obtained, besides the charcoal left in the retorts, the
following : (a) non- condensable gases, chiefly CO, H 2 , and
CH,, which, after admixture of hydrocarbon vapour, may be
used for illuminating purposes ; (6) a watery liquid containing
acetic acid, methyl alcohol, and many other substances ; and
(c) tar. The watery distillate (b) is distilled anew after ad-
dition of enough lime to retain the acetic acid, and the crude
wood -spirit thus obtained, after some further treatment, is
saturated with CaCl 2 and heated by steam to 100 C. The
impurities are thus driven off, and a residue is left, consisting of
a compound of CaCl 2 with methyl alcohol. This is mixed
with water, when the methyl alcohol is liberated and can be
recovered by distillation, but the distillate requires to be again
rectified over quicklime in order to free it from water.

Methyl alcohol is a light colourless mobile liquid with a



46 METHYL ALCOHOL CHAP.

spirituous odour. When ignited it burns with a pale blue
flame. The pure alcohol is used in the manufacture of certain
aniline dyes, and for this purpose it should be as free as pos-
sible from acetone, a substance largely present in crude wood-
spirit ; but for other purposes, such as for dissolving resins in
the manufacture of varnish, a wood-spirit rich in acetone is
desirable, on account of its greater solvent power.

The presence of acetone can be detected and its amount estimated by
means of its property of yielding iodoform when treated with iodine and
potash. Pure methyl alcohol itself does not produce iodoform, whereas
one molecule is obtained from each molecule of acetone present.

Methyl alcohol boils at 66. It is considerably lighter than
water, but mixes with it readily.

Chemically, methyl alcohol is the type of a primary alcohol,
that is, of one containing the group CH 2 . OH. Every primary
alcohol when oxidised loses first two atoms of hydrogen, and
gives an aldehyde characterised by the group CHO, which
can be further oxidised to the group COOH, so yielding an
acid. In this particular case the alcohol, HCH 2 . OH, is first
oxidised to formaldehyde, HCHO, and this to formic acid,
HCOOH, by treatment with appropriate oxidising agents.

As in all other alcohols, whether primary or other, the