in the state of vapour. When the solution is evaporated or the
vapour cooled, a solid substance is obtained of the same com-
position as formaldehyde, but not of the same molecular weight ;
this is para-formaldehyde, and has the formula (CH 2 O). rt where
x is possibly equal to 3, but is not known with certainty. Para-
formaldehyde, (CH O) 3 (?), is said to be polymeric with form-
aldehyde, CH 2 O, as the two substances have the same composi-
tion, but different molecular weights. The opposite change is
easily accomplished by vapourising the solid para-formaldehyde
when a vapour whose density shows it to be made up of the
simple molecules, CH Q O, is obtained ; but on cooling, these
again gradually unite to the more complex molecules, (CH 2 O) 3 .
Beyond its tendency to polymerisation, the chief charac-
teristic of formaldehyde is the readiness with which it takes up
oxygen from other substances to effect the change into formic
acid :
H . CHO + O = H . COOH;
Formaldehyde. Formic acid.
accordingly, formaldehyde is a strong reducing agent ; it
reduces in the cold both Fehling's solution and solutions of
silver salts.
EXPT. 14. Prepare a quantity of Fehling's solution by dissolving 100
grams of Rochelle salt (sodium potassium tartrate) in a little water, add-
ing 30 grams of NaOH in 300 c. c. of water, and then 20 grams of crystal-
lised CuSO 4 , dissolved in about 100 c.c. of water ; mix and keep in a
stoppered bottle.
Place some of the formaldehyde solution prepared in Expt. 13 in a
vin ACETALDEHYDE 63
beaker, and add Fehling's solution drop by drop. Notice that its dark
blue colour is discharged and a light red precipitate produced. This is
Cu.jO, formed by reduction of the CuSO 4 :
2CuSO 4 + 4KHO+H. CHO= HCO 2 H +Cu2O + 2K 2 SO 4 + 2H 2 O.
EXPT. 15. Dissolve 3 grams AgNO 3 in a mixture of 20 c.c. strongest
ammonia solution (sp. gr. .88) with its own volume of water, and add a
solution of 3 grams NaOH in 25 c. c. of water. Keep in a small stoppered
bottle in a dark place.
Take some of this silver solution in a test tube, and add a few drops of
the formaldehyde solution ; allow to stand in the cold. In a few minutes
a brilliant mirror of metallic silver will be deposited on the sides of the
test tube.
2Ag:NO 3 + H 2 O + HCHO = HCOOH + 2 HNO 3 + 2Ag.
Acetaldehyde, CH 3 . CHO, is prepared by the oxidation
of ethyl alcohol by distillation with a mixture of potassium
bichromate and dilute sulphuric acid.
EXPT. 1 6. Place in a flask 30 grams K 2 Cr 2 O 7 (in small lumps) and
1 20 c.c. water. Mix in a beaker 40 c.c. methylated spirit and 25 c.c.
strong sulphuric acid, and allow to cool. Then add this mixture gradu-
ally to the bichromate, taking care to keep cool by running water over the
outside of the flask.
Heat the mixture on the water-bath, and collect the distillate in a re-
ceiver kept cold by ice. The impure acetaldehyde collected can be purified
partly by fractional distillation, and finally by conversion into the solid
compound which it forms with ammonia.
Acetaldehyde is a colourless liquid, boiling at 21 C. and
possessing a characteristic smell. Like formaldehyde, it is a
strong reducing agent, as may be shown by experiments similar
to Nos. 14 and 15. It further resembles the lower member
of the aldehyde series in the readiness with which it poly-
merises to paraldehyde (C.,H 4 O) r Acetaldehyde itself is a
colourless very volatile liquid (B.P. 21 C.) with a pleasant
smell, but on standing in contact with even a trace of various
substances H 9 SO 4 , HC1, SO 2 , etc. it changes almost en-
tirely to parald'ehyde, which is a liquid at ordinary temper-
atures, but solidifies in a freezing mixture, boils at 124, and
gives a vapour whose density corresponds to the molecular
formula (C.,H 4 O) 3 . Paraldehyde, though so directly obtained
64 COMPOUNDS OF ACETALDEHYDE CHAP.
from acetaldehyde, is not itself a real aldehyde at all. This
is shown by its whole chemical behaviour, especially by the
fact that it does not reduce metallic silver from an ammonia-
cal silver solution, and leads us to conclude that in the
formula of paraldehyde the group CHO no longer occurs.
Paraldehyde is easily reconverted into ordinary acetaldehyde
by distillation with a little H 2 SO 4 .
Metaldehyde is another substance of the formula (C 2 H 4 O) ;j obtained by
polymerisation of acetaldehyde ; it is isomeric with paraldehyde.
Compounds of Acetaldehyde. In some respects acet-
aldehyde is more typical than its lower homologue of the group
of aldehydes, and we have therefore delayed till now the con-
sideration of certain reactions exhibited by aldehydes as a
class, in which certain substances, such as NH 3 , HCN, etc.,
are added to the aldehyde molecule.
(a) Acetaldehyde, like all the other aldehydes except
H . CHO, unites directly with ammonia to form a compound
of the type R . CH(OH)(NH 2 ) :
This particular one is called simply aldehyde-ammonia, and is formed
as a white crystalline solid when dry NH 3 is passed into an ethereal solu-
tion of aldehyde. It is decomposed by dilute acids into aldehyde and
ammonia.
EXPT. 17. Pass NH 3 , dried by quicklime, into a solution of alde-
hyde in ether ; collect on a filter the white precipitate produced, and
show that some of it when warmed with dilute HoSO 4 regenerates alde-
hyde.
(b) Addition compounds with HCN are also formed by the
aldehydes, thus
OTJ
CH... CHO-f HCN = CH... CH^^'
CJN
(c} Sodium hydrogen sulphite, NaHSO.^ also gives addition
products, which are often used as a means of separating and
purifying the aldehydes. The following equation represents
what happens in the case of acetaldehyde :
OTT
CH 3 . CHO + NaHSO, = CH 3 .
via CHLORAL 65
Compounds of this type are obtained when an aldehyde is shaken with
a saturated solution of NaHSO 3 . They are white crystalline solids, soluble
in water, and decomposed by dilute acids with regeneration of the alde-
hyde.
Chloral is a very important derivative of ordinary alde-
hyde ; its formula is CC1 3 . CHO, and its systematic name
trichlor-aldehyde.
Chloral is prepared by passing chlorine into alcohol and
decomposing the solid crystalline product (a compound of
chloral and alcohol) with sulphuric acid. The reaction may
be represented as occurring in two parts :
(a) The alcohol is oxidised to aldehyde,
C H ., . C H O H + C1.,= C H . CHO
83 mm
Ethyl alcohol. Acetaldehyde.
(b) The aldehyde is converted into trichlor-aldehyde or
chloral,
CH 3 . CHO + 3C1 2 = CC1 3 . CHO + sHCl.
Acetaldehyde. Chloral.
It is a liquid with a penetrating smell, and possesses most of
the properties (reducing power, etc.) characteristic of the alde-
hydes. It is decomposed by alkalies with production of chloro-
form :
CCl ;j . CHO + KOH = CHC1, + H . CO 2 K,
Chloral. Chloroform. Potassium
formate.
hence perhaps the well-known narcotic power of chloral.
Chloral Hydrate, CC1 3 . CHO + H.,O, is a compound of
chloral with water, produced by direct combination of the two
liquids. It is a crystalline solid, and is the form in which
chloral is usually administered.
KETONES
The ketones are a series of compounds resembling in many
respects the aldehydes, but differing in others ; and we attempt
to represent both resemblances and differences by giving to
the ketones the formula R . CO . R, closely allied to the alde-
hyde formula R . CO . H.
66 KETONES CHAP.
(i) Just as the aldehydes are obtained by carefully gradu-
ated oxidation of primary alcohols,
R . CH 2 OH + O = R . CO . H + H 2 O,
Primary alcohol. Aldehyde.
so the ketones are the first products formed by the oxidation
of secondary alcohols, that is, alcohols in which the group
CH(OH) is combined with two alkyl groups :
. OH + O = >CO + H0.
Secondary alcohol. Ketone.
(2) Another method of general application for the prepara-
tion of ketones is the dry distillation of the calcium salts of
fatty acids ; thus calcium acetate gives acetone or di-methyl
ketone :
^ >Ca = CH 3 > C + CaCO 3'
CH 3 ;COO
Calcium acetate. Acetone.
(3) A third method of considerable importance is the treatment of
acetyl chloride or similar compounds with zinc methyl, ethyl, etc. The
reaction may be represented thus :
2CH 3 COC1
Acetyl chloride. Zinc ethyl. Methyl-ethyl
ketone.
The ketones resemble the aldehydes in their power of form-
ing addition products with HCN, and with NaHSO 3 . They do
not possess the same energetic reducing power, nor do they
combine with ammonia in the same way as the aldehydes.
Acetone, (CH 3 ) 2 CO, or dimethyl ketone is the simplest
ketone. It can be prepared by any of the general methods
given above, the one generally adopted being the dry distilla-
tion of calcium acetate :
(CH 3 CO 2 ) 2 Ca = (CH 3 ) 2 CO + CaCO 3 .
Acetone is also found amongst the products of the dry distilla-
tion of wood (see p. 45), and is largely obtained from that
vni ACETONE 67
source. It is used as a solvent, and for the preparation of
iodoform and other substances. It is a volatile inflammable
liquid with a pleasant smell.
Oximes and Hydrazones. Special importance attaches to the com-
pounds which aldehydes and ketones form with hydroxylamine, NH 2 (OH),
and phenylhydrazine, C 6 H 5 NH . NH 2 . In these oximes and hydrazones
the oxygen of the CO group in the aldehyde or ketone is replaced by a
divalent residue, thus :
(CH 3 ) 2 CO + H 2 N . OH = (CH 3 ) 2 C : N . OH + H 2 O
Oxime.
CH 3 COH + H 2 N . NHC 6 H 5 = (CH 3 )CH : N . NHC 6 H 5 + H 2 O.
Hydrazone.
The importance of these oximes and hydrazones lies in their great utility
as a means of characterising the various aldehydes and ketones. The
hydrazones especially are usually crystalline solids, only slightly soluble in
the ordinary solvents, and are therefore much more easily identified than
the aldehydes or ketones from which they are prepared.
QUESTIONS ON CHAPTER VIII
1. How is acetaldehyde prepared ? Mention its chief properties.
2. How are the aldehydes as a class characterised by their reactions,
and how is their behaviour represented in the generic formula R . CHO ?
3. What is the relation of chloral to acetaldehyde ? Give its pre-
paration and properties.
4. What bodies are formed by the oxidation of (a) ethyl alcohol, (l>)
aldehyde, (c) chloral?
5. Illustrate the chief points of resemblance and difference in the
chemical behaviour of aldehyde and acetone.
The Fatty Acids form an important homologous series,
some higher members of which are contained in all natural
fats. The lower members are liquids of strongly acid char-
acter and sharp penetrating odour, but with increasing
molecular weight the members of the series lose their solu-
bility in water, and with it their acid taste and power of turn-
ing blue litmus red. The power of forming salts is, however,
unimpaired even in the highest member of the series yet
obtained.
The first acid of the series is formic acid, CH 2 O 2 , the second
acetic acid, C 2 H 4 O 2 . The general formula of the whole series
is C K H 2tt O 2 , but this is better written C w H 2w+1 . CO 2 H, to in-
dicate that every acid of the series contains the " carboxyl "
group, CO 2 H, combined with a hydrocarbon residue (or "alkyl "
group), such as methyl, CH 3 , ethyl, C 2 H 5 , etc. Formic acid is
then written H . CO 2 H, acetic acid CH 3 . CO 2 H, and so on.
The reasons for writing the formulae in this way will be
best understood if we consider in detail the case of acetic acid ;
this has the molecular formula C 2 H 4 O 2 . Of the four hydrogens
only one can be replaced by metals, i.e. the acid is monobasic,
and therefore one of the four hydrogen atoms is differently
related to the molecule from the other three. Again the action
of phosphorus pentachloride on acetic acid or on sodium
acetate yields a substance, acetyl chloride, of the formula
C 2 H g OCl ; that is, a Cl atom takes the place of an O atom and
an H atom. This could not happen unless that O and that H
were connected to form the monovalent hydroxyl group OH.
PREPARATION OF FATTY ACIDS
We have now arrived at the formula, C 2 H 3 O . OH, for acetic
acid. The next question is whether the three remaining hydro-
gen atoms are all connected to the same carbon atom or not.
Acetic acid treated with chlorine yields a derivative trichlor-
acetic acid of the formula C.,C1 3 O . OH, in which the hydroxyl
group is still present, and the other three hydrogens are re-
placed by chlorine. Now trichloracetic acid readily yields
chloroform when boiled with water :
C,C1 3 O.OH = CHC1 3 + C0 2 ,
Trichloracetic acid. Chloroform.
showing that all three Cl atoms, and therefore the three hydrogen
atoms, whose places they occupy, are connected to the same
carbon. Hence acetic acid contains the groups CH 3 and OH,
and the only formula in agreement with these experimental
results is CH 3 . COOH.
General Methods of Preparation. (i) The first
method is one which also furnishes valuable evidence in favour
of the formula C, ; H 2w+1 . CO 2 H for the series, inasmuch as we
start in each case from a substance, C ; ,H <W/+1 . CN, in order to
prepare the corresponding acid. Such an alkyl cyanide is
obtained by treating the iodide of the same radical with silver
cyanide, e.g. :
CH 3 I + AgCN = CH 3 .CN + Agl,
Methyl iodide. Methyl cyanide.
and when heated with water undergoes a reaction of the fol-
lowing type :
CH 3 .CN + 2H 2 O = CH 3 . CO 2 H + NH 3 .
Methyl cyanide. Acetic acid.
Such a reaction is spoken of as hydrolysis, and takes place
much more readily when a dilute mineral acid is used instead
of pure water ; or a solution of an alkali may be employed.
(2) By the action of carbon monoxide on the sodium com-
pound of an alcohol, e.g. from sodium methylate sodium acetate
is obtained :
CH 3 . ONa + CO = CH 3 . COONa.
70 FORMIC ACID
Similarly, sodium formate may be obtained from sodium hydrate:
H . ONa + CO = H . COONa.
This method is of theoretical interest only.
3. An important practical method is the oxidation of a
primary alcohol containing the same number of carbon atoms
as the acid to be prepared :
CH 3 . CH 2 OH + O 2 = CH 3 . COOH + H 2 O.
Ethyl alcohol. Acetic acid.
In the laboratory a mixture of potassium bichromate with
dilute sulphuric acid is usually employed as the oxidising agent ;
in the commercial manufacture of acetic acid (vinegar) the
oxygen of the air is utilised.
In this method the group CH 2 (OH) is oxidised to COOH.
When a less complete oxidation is effected, the product is an
aldehyde containing the group CHO :
R . CH 2 (OH) > R . CHO > R . COOH.
Primary alcohol. Aldehyde. Acid.
Poimic Acid, H . CO 2 H, may be prepared by any of the
three general methods, i.e. :
(i) From HCN, hydrocyanic acid, by heating with a dilute
mineral acid in sealed tubes :
HCN + 2H 9 O = H . CO 9 H + NH q .
L LA
Formic acid.
(2) From sodium or potassium hydrate, and carbon
monoxide :
NaOH + CO = H . COONa,
Sodium formate.
a reaction which occurs with great readiness when moist CO is
passed over porous soda-lime heated to about 200 C.
(3) By the oxidation of methyl alcohol :
v
HCH 2 . OH + O 2 = HCOOH + H 2 O.
Methyl alcohol. Formic acid.
FORMIC ACID 71
Another method of considerable interest in connection with the physio-
logical chemistry of plants by which formates can be obtained is by the
reduction of CO 3 in the presence of water. Thus, when thin slices of
metallic potassium are exposed to a moist atmosphere of CO 2 they are
gradually converted into potassium formate and carbonate :
2 K + 2 CO 2 + H. 2 O = H . CO 2 K + KHCO 3 .
Possibly this reduction to formic acid is the first step in the transformation
by plants of CO 2 into sugar and starch.
The most practically useful method for preparing formic
acid is by the decomposition of oxalic acid. This, when heated
alone, or better with glycerine, breaks up as follows :
C 2 H 2 4 = H . C0 2 H + C0 2 .
Oxalic acid. Formic acid.
A mixture of equal quantities of glycerine and crystallised oxalic acid is
heated in a retort until no more CO 2 is evolved. The distillate collected
during this period is a very weak formic acid. On adding more oxalic
acid, and again heating, a stronger acid will be obtained, but the acid got
in this way never contains less than about 40 per cent of water.
Anhydrous formic acid is prepared from lead formate by
the action of hydrogen sulphide. The lead salt is easily obtained
from any (weak) formic acid. It is dried and then exposed to
a stream of H 2 S gas in a tube kept warm by means of a
steam jacket. The anhydrous acid distils over, and is a
colourless liquid with an acrid odour and very caustic properties.
Formic acid differs from the other members of the series in
being a strong reducing agent. It reduces solutions of silver
and mercury salts with separation of the metals. When heated
with strong sulphuric acid it is decomposed into CO and water :
H . CO,H = H 2 O + CO.
The Formates of the alkali metals are fairly stable sub-
stances, which crystallise only with difficulty. Those of the
heavy metals, such as silver, are very easily decomposed with
separation of the metal.
Acetic Acid, CH 8 . CO 2 H, can be obtained by any of the
three general methods, i.e.:
ACETIC ACID
(l) From CH.^.CN, methyl cyanide, by heating with a
dilute mineral acid or a dilute alkali :
CH 3 .CN
Methyl cyanide.
. CO 2 H + NH 3 .
Acetic acid.
Methyl cyanide, CH 3 CN, can be made by acting with methyl iodide on
silver cyanide :
CH 3 I + AgCN = CH 3 CN + Agl.
(2) From sodium or potassium methylate and carbon
monoxide :
CH 3 .ONa + C(
Sodium methylate.
For sodium methylate, see p. 46.
= CH 3 . COONa.
Sodium acetate.
(3) By the oxidation of ethyl alcohol :
CH 3 CH 2 OH + 2 = CH 3 . COOH + H 2 O.
Ethyl alcohol. Acetic acid.
The only one of these methods employed on a large scale
is the third. In the preparation of vinegar (which is a dilute
acetic acid flavoured by
minute quantities of
other substances) the
alcoholic liquid, whether
wine, diluted brandy, or
merely potato-spirit and
water, is exposed to the
simultaneous action of
the air (which supplies
the oxygen), and of the
fermentative influence of
a particular organism,
the" mycoderma aceti.
The process is carried
on most rapidly by al-
lowing the alcoholic
liquor to trickle through
FIG. 28. The tnycoJerma aceti or "mother of
vinegar " seen under the microscope.
tubs filled with shavings,
on which the mycoderma has developed. New shavings are
THE ACETATES 73
at first almost inactive, but they soon become coated with the
organism, and the oxidation then takes place readily.
Large quantities of acetic acid are also obtained as one of
the products of the destructive distillation of wood. The acid is
separated from the other products, chiefly methyl alcohol and
acetone, by neutralising with lime and distilling the alcohol and
acetone from the calcium acetate. This last is then decomposed
by addition of sulphuric acid, and the acetic acid recovered by
distillation.
Acetic acid, when perfectly free from water, is a crystalline
solid which melts at 17 C. The strongest acetic acid of
commerce is termed " glacial acetic acid," from the fact that it
is solid in moderately cold weather. It has a penetrating acid
smell, and acts like a caustic on the skin.
The salts of acetic acid, the acetates, are prepared by
acting with the acid upon the oxide or carbonate of the metal
whose acetate is required. They are all more or less readily
soluble in water, and crystallise well.
Sodium Acetate, NaC 9 H. J O , crystallises with three mole-
cules of water. When heated above 1 00 C. the water of crys-
tallisation is driven off, and the anhydrous sodium acetate is
left as an amorphous mass. The anhydrous salt is used in
organic synthesis as a dehydrating agent, and in the prepara-
tion of methane :
NaC 2 H 3 O 2 + NaOH = CH 4 + Na 2 CO 3 .
Ammonium Acetate, (NH 4 )C 2 H 3 O 2 , is a deliquescent
solid. Its solution is made use of in qualitative analysis for
dissolving lead sulphate, and so separating it from mercuric
sulphide, with which it may be mixed in the course of working
through the second group of metals.
When strongly heated, ammonium acetate yields acetamide
and water :
CH 3 COONH 4 ==CH 3 CONH 2 + H 2 O.
Calcium Acetate, Ca(C.,H 3 O 2 ) 9 , is used for preparing
acetone :
CHXOO'
74 PROPIONIC ACID
Lead Acetate, Pb(C.,H 3 O.,) , is the " sugar of lead " of
commerce, and is made by dissolving litharge in acetic acid.
It is largely used in the manufacture of white-lead (basic lead
carbonate) and chrome-yellow (PbCrO 4 ).
Aluminium Acetate is obtained in solution when calcium
acetate is mixed in the presence of water with aluminium
sulphate. The solution decomposes on evaporation into acetic
acid (which escapes as vapour) and alumina ; hence the ex-
tensive use of aluminium acetate as a mordant, the alumina
combining with the dye to form an insoluble lake, which adheres
firmly to the fibre of the cloth.
As tests for the presence of acetic acid may be utilised
either the dark red colour of the solution of ferric acetate
(destroyed on boiling, with separation of a basic acetate of iron)
formed when ferric chloride is added to a solution of an acetate,
or the formation of ethyl acetate with its characteristic pleasant
smell when an acetate is heated with alcohol and concentrated
sulphuric acid.
It is, however, to be noted that these tests give almost identical results
with any of the acids of this series. To distinguish acetic acid from its
higher homologues, the surest plan is to prepare the silver salt and
determine the percentage of silver which it contains.
Propionic Acid, C 2 H 5 . CO 2 H, may be prepared by any of
the three general methods, but most conveniently by the third,
starting from normal propyl alcohol :
C,H 5 . CH 2 OH + 2 = C 2 H 5 . COOH + H. 2 O.
Propyl alcohol. Propionic acid.
The constitution of propionic acid is shown by this method
of preparation, as also by that from ethyl cyanide by hydrolysis:
C 2 H 5 CN + 2H 2 = C 2 H 5 C0 2 H + NH 3 .
Ethyl cyanide. Propionic acid.
Butyric Acid, C 3 H 7 . CO 2 H, is the lowest member of the
series for which isomeric forms are theoretically possible or
have been actually obtained. These are two in number, viz. :
(i) Normal butyric acid, CH y CH 2 CH 2 CO 2 H.
/ TT
(ii) Isobutyric acid,
BUTYRIC ACIDS 75
Their constitution is made clear by their synthetical prepara-
tion from normal propyl iodide and isopropyl iodide respect-
ively through the intermediary of the cyanides :
(i)CH 3 CH 2 CH 2 I ^CH 3 CH. 3 CH 2 CN ^CH 3 CH 2 CH 2 CO 2 H
Normal propyl iodide. Normal propyl cyanide. Normal butyric acid.
(ii) (CH 3 ) 2 CHI > (CH 3 ) 2 CHCN > (CH 3 ) 2 CHCO 2 H
Isopropyl iodide. Isopropyl cyanide. Isobutyric acid.
Normal Butyric Acid, C 3 H 7 . CO 2 H, is present in butter
in the form of glycerine butyrate, the ethereal salt of glycerine
and butyric acid. There are, however, many other similar
compounds of glycerine contained in butter, and the isolation
of the butyric acid in a state of purity is a matter of difficulty,
and the acid is more cheaply and easily obtained by the fer-
mentation (under the influence of the bacillus subtilis) of sugar
or starch. This process is carried out on a fairly large scale,
the butyric acid being converted into its ethyl salt, which is
used as a flavouring under the name of essence of pine-apples.
The same fermentation of starch and sugar occurs in the
human stomach in certain cases of deranged digestion. Start-
ing from glucose (see Chapter XVIII), the change produced by
the fermentation may be represented by the equation :
C H 12 6 = C 4 H S 0, + 2C0 2 + 2H 2 .
Glucose. Butyric acid.
Butyric acid is an oily liquid with an unpleasant rancid smell.
The change which butter undergoes in turning rancid may be
represented (so far as the glycerine butyrate in it is concerned)
as follows :
C 3 H.(OCOC 3 H 7 ) 3 + 3 H 2 = C 3 H 5 (OH) 3 + 3 C 3 H 7 . CO 2 H,
Glycerine butyrate. Glycerine. Butyric acid.
and it is to the presence of free butyric acid in rancid butter
that its characteristic taste and smell are due.
Isobutyric Acid, C 3 H 7 CO H, can be prepared from iso-
propyl cyanide (see above), and resembles the normal acid in
smell and taste, though differing considerably from it in some
other physical and chemical properties.
76 STEARIC ACID
The fifth acid of the series is valerianic acid, C^g . CO^H, and theory