carbonate, which is heated with the monochlor-acetic acid.
Monochlor-acetic acid is formed when chlorine acts upon
acetic acid :
CH,CO 9 H + CL = CH 9 C1 . CO 9 H + HC1.
(2) The second method, somewhat more complicated, starts from the
aldehydes. These are able to combine with ammonium cyanide to form
compounds, such as CH 3 . CH(NH 2 )(CN) :
CH 3 . CHO + NH 4 CN = CH 3 . CH<^ 2 +H 2 O.
These when treated with dilute acids (see p. 69) yield amido-acids by con-
version of the CN group into carboxyl, CO 2 H.
Amido-Acetic Acid, NH 2 . CH 2 CO 2 H (G-lycocoll), can
be extracted from glue (therefore from bones) by treatment with
sulphuric acid ; it is best prepared synthetically from mono-
chlor-acetic acid by the action of ammonia. It forms a dark-
blue copper salt (NH 2 CH 2 CO 2 ) 2 Cu, and also a chloride,
xii LEUCIN 91
HC1. NH 2 CH.,CO 2 H. Glycocoll itself is a white crystalline
solid, having a sweetish taste, and readily soluble in water.
Of the more complicated amido-acids we may mention
Leucin, C 4 H . CHNH . CO H, therefore amido-caproic
acid, which is one of "the most important products of the
decomposition of albumen, either by putrefaction or by boiling
with acids or alkalies ; it is also present ready-formed in
various glands of the body as the pancreas, spleen, etc.
QUESTIONS ON CHAPTER XII
1. What substances are formed by the action of ammonia upon (a)
monochlor-acetic acid, (/>) methyl chloride, (r) acetyl chloride? Give
equations.
2. What is the action of dilute hydrochloric acid upon (a) acetamide,
(!>} glycocoll ?
3. Describe the preparation of acetamide and of glycocoll from acetic
acid.
CHAPTER XIII
ALKYL COMPOUNDS OF PHOSPHORUS, ARSENIC,
SILICON, AND THE METALS
COMPOUNDS OF PHOSPHORUS
Phosphines. Phosphine, PH 3 , is much less basic in char-
acter than ammonia, but is yet capable of combining with
hydrogen iodide to form the compound phosphonium iodide,
PH 4 I. The organic phosphines, obtained by substituting the
hydrogen in PH 3 by alkyl groups, are stronger bases in pro-
portion to the number of alkyl groups introduced.
By the action of alkyl iodides on PH 3 only tertiary phos-
phines, PR 3 , and phosphonium compounds, PR 4 I, are obtained.
The primary and secondary bases can, however, be prepared
by employing, instead of PH. S , a mixture of PH 4 I with oxide of
zinc. The separation of these bodies is not then difficult, and
depends on the gradual increase in. their basic character
from primary phosphine to phosphonium compounds :
Tetra-methyl phosphonium iodide, P(CH 3 ) 4 I, is not decom-
posed by KOH.
Tri-methyl phosphonium iodide, P(CH 3 ) 3 . HI, is decom-
posed by KOH.
Dimethyl phosphonium iodide, HP(CH 3 ) . HI, is not decom-
posed by water.
Methyl phosphonium iodide, H 9 PCH 3 . HI, is decomposed
by water.
The phosphines are gases or volatile liquids, very inflam-
CHAP, xin THE PHOSPHINES 93
mable, and possessed of very strong unpleasant odours. Char-
acteristic of the tertiary phosphines is the readiness with which
they combine with O, S, C1. 2 , etc., to form compounds such as
P(C H 5 ) 3 O, triethyl-phosphine oxide, in which the radicle
P(C H 5 ) ;j plays the part of a divalent metal.
Methyl-phosphine, PH 9 (CH 3 ), is obtained along with
diethyl-phosphine, PH(CH 3 ) 2 , by heating a mixture of
phosphonium iodide with methyl iodide and zinc oxide in
sealed tubes to a temperature of 150 C. :
2PH 4 I + 2CH 3 I + ZnO = 2PH 2 (CH 3 ) . HI + ZnI 2 + H 2 O,
Phosphonium Methyl-phosphonium
iodide. iodide.
PH 4 I + 2CH 3 I + ZnO = PH(CH 3 ) 2 . H I + ZnI 2 + H 2 O.
Dimethyl-phosphonium iodide.
On heating the resulting product with water, the methyl-
phosphine, PH CH S , is liberated, while the dimethyl-phosphine,
PH(CH 3 ) 2 , remains combined with the hydriodic acid. On
subsequently boiling with sodium hydrate, the secondary
phosphine is in turn set free.
Trimethyl-phosphine, P(CH 3 ) 3 , is prepared by heating
phosphonium iodide with methyl iodide without the addition
of zinc oxide :
PH 4 I + 3CH 3 I = P(CH 3 ) 3 . HI + 3HI.
Trimethyl-phosphonium iodide.
The phosphine is obtained when the product is decomposed
by boiling with potash or soda.
All three of these methyl-phosphines are very inflammable,
and exhibit intolerable odours ; they readily combine with
chlorine, bromine, oxygen, or sulphur to form compounds
such as P(CH 3 ) 3 or PH(CH 3 ) 2 CI 2 .
COMPOUNDS OF ARSENIC
The Arsines are connected with AsH. 3 as the amines with
NH 3 . No primary or secondary arsines (such as AsH CH 3 )
are known, but tertiary compounds, As(CH 3 ) 3 , etc., have been
94 CACODYL COMPOUNDS CHAP.
prepared. These, like the parent substance AsH 3 , are incapable
of forming salts.
The most important organic compounds of arsenic form a
series in which the radicle As(CH 3 ) corresponds to an atom
of a monovalent metal, and it has been found convenient to
give a special name cacodyl to this radicle.
Cacodyl Oxide, {As(CH 3 ) 2 } 2 O or Cdl 2 O, can be obtained
by distilling As 2 O 3 with potassium acetate : .
4 CH 3 . C0 2 K + As 2 3 =
Potassium acetate. Cacodyl oxide.
and from this oxide, by the action of acids, various salts can be
made, amongst them cacodyl chloride, As(CH 3 ) 2 Cl, which
when reduced with zinc furnishes free cacodyl, As 9 (CH 3 ) 4 .
All of these compounds are liquids of disgusting odour and
intensely poisonous properties. They are also very inflammable,
and their investigation, which was carried out by Bunsen, was
a matter of great danger and difficulty.
Of the organic compounds of arsenic, other than the cacodyl
derivatives, we will mention only one.
Arsenic Trimethyl, As(CH 3 ) s , which is obtained by the
action of methyl iodide upon an alloy of arsenic and sodium.
It is a colourless volatile liquid of unpleasant smell, readily
combining with one atom of oxygen or two of chlorine to form
compounds in which the arsenic is pentavalent, e.g. As(CH 3 ) 3 O,
As(CH 3 ) 3 Cl 2 . It is not basic in character, and has no
tendency to combine with acids to form salts in the same
way as N(CH 3 ) 3 and P(CH 3 ) 3 .
COMPOUNDS OF SILICON
The organic derivatives of the tetravalent element silicon
may be put side by side with other organic compounds in
which the place of the silicon is taken by carbon, itself a
tetravalent element ; thus
Silicon Tetramethyl, Si(CH 3 ) 4 , may be compared with
the pentarje C(CH 3 ) 4 ; it is obtained by treating silicon
tetrachloride with zinc methyl (see p. 95) :
SiCl 4 + 2Zn(CH 3 ) 2 = Si(CH 3 ) 4 + 2ZnCl 2 ,
XIII ZINC METHYL 95
and is a volatile liquid unaltered by water. The analogy with
the pentane, C(CH S ) 4 , is not merely one of formulae, but is also
seen to some extent in the chemical behaviour of the two com-
pounds. This is not surprising in view of the fact that both
are of similar type, and each contains four methyl groups ; but
that there exists no complete analogy between the silicon
derivatives and those of carbon is evident from the great
differences between the simple corresponding derivatives of the
two elements; thus silico-chloroform, SiHCl s , is a liquid fuming
in the air and decomposed by water, therefore quite unlike
chloroform itself, CHC1 3 .
COMPOUNDS OF THE METALS WITH ALKYL RADICLES
Many of the metals form alkyl compounds, usually volatile
liquids which oxidise rapidly or even ignite spontaneously in
the air ; they are obtained by the action of alkyl iodides upon
the metals, or upon their alloys with zinc or sodium.
A second method is to act with zinc methyl (or zinc ethyl,
etc.) upon the chloride of the metal which is to be converted
into an alkyl derivative.
Zinc Methyl, Zn(CH 3 ) 2 , is obtained by the action of
methyl iodide upon zinc and subsequent distillation. In the
first place, direct combination of the zinc and methyl iodide
occurs :
/~-TT
Zn + CH 3 I = Zn< j 3.
Methyl iodide. Zinc methyl-iodide.
The compound thus formed is decomposed by further heating :
/-TT
2Zn< j 3 = Zn(CH 3 ) 2 + ZnI 2 .
Zinc methyl-iodide. Zinc methyl.
The first reaction takes place more readily when the zinc-
copper couple of Gladstone and Tribe is used instead of zinc
filings. This couple is merely an intimate mixture of finely
divided copper and zinc, and can be most readily prepared by
mixing zinc filings with one-ninth of their weight of copper-dust,
obtained by reducing powdered copper oxide in a current of
9 6
ZINC ETHYL
hydrogen. The couple only requires to be heated for a few
minutes in a flask to make it ready for use.
EXPT. 18. Mix 90 grams zinc filings with 10 grams of reduced copper.
Place the mixture in a flask fitted with cork and capillary tube, and heat
for a few minutes over the bare flame of a Bunsen burner. When cool,
FIG. 29. Preparation of Zinc Ethyl.
add 50 grams methyl iodide, and fit the flask with an inverted condenser
and a tube for introducing coal-gas (see fig. 29). Heat on the water bath
for ten hours ; then arrange the condenser for distillation and distil off
the zinc methyl in a slow current of coal-gas.
Zinc Ethyl, Zn(C.,H-). ( , is similarly prepared. Both are
important laboratory reagents for the purpose of introducing
methyl or ethyl groups in the place of chlorine or other
xiii LEAD TETRA-ETHYL 97
element, e.g. a ketone can be made by the action of zinc ethyl
on acetyl chloride :
. CO. Cl + Zn(C.,H 5 ), = 2CH 3 . CO. C. 2 H 5
Acetyl chloride. Methyl-ethyl ketone.
Zinc methyl and ethyl fume in the air and very readily take
fire, often spontaneously. They are decomposed by water :
Zn(CH 3 ) 2 + 2H 2 O = Zn(OH) 2 + 2CH 4 ,
Zinc methyl. Methane.
and by the halogens :
Zn(C 2 H.), + -Br, = ZnBr, + 2C. 2 H 5 Br.
Zinc ethyl. Ethyl iodide.
Mercury Methyl, Hg(CH 3 ).,, and Mercury Ethyl,
Hg(C.,H.) , are colourless liquids, whose vapours have only
feeble odour, but are very poisonous ; they are prepared from
sodium amalgam by the action of methyl or ethyl iodide :
NaHg + 2CH 3 I = 2NaI + Hg(CH 3 ).,.
Methyl iodide. Mercury methyl.
They are more stable than the zinc compounds, and neither
take fire in the air nor are decomposed by water. Their
general chemical behaviour is otherwise similar to that of their
zinc analogues.
Alkyl Compounds of other Metals. Many other
metals also form similar compounds, the most important cases
being perhaps those of lead and tin.
By acting on lead chloride, PbCl. 2 , with zinc ethyl the sub-
stance lead tetra-ethyl, Pb(C.,H 6 ) 4 , is obtained :
2PbCl, + 2Zn(C,H 5 ) 2 = Pb(C 2 H 5 ) 4 + Pb + 2ZnCl 2 ,
a fact which proves that lead is really a tetravalent element,
and is thus in complete agreement with the recent discovery
of the existence of a lead tetrachloride, PbCl 4 . Lead tetra-
ethyl is an oily liquid which takes fire when heated in contact
with air.
In the same way, by acting on stannous chloride, SnCl.,, with
H
98 TIN ETHYL CHAP, xin
zinc ethyl, we obtain tin tetra-ethyl, Sn(C 9 H 5 ) 4 , in which the
maximum valency of the metal is exerted :
2SnCl 2 + 2Zn(C 2 H 5 ) 2 = Sn(C 2 H 5 ) 4 + Sn+2ZnCl 2 ;
Zinc ethyl. Tin tetra-ethyl.
but tin di-ethyl, Sn(C 2 H 5 ) 2 , can be got by treating an alloy
of tin and sodium with ethyl iodide :
SnNa + 2C 2 H 6 I = Sn(C 2 H 5 ) 2 + 2NaI.
Ethyl iodide. Tin di-ethyl.
Sn(C. 2 H 5 ) 4 is a liquid which can be distilled without decom-
position, whereas Sn(C.,H 5 ) 2 decomposes into the tetra-ethyl
compound and metallic tin.
QUESTIONS ON CHAPTER XIII
1. Give the preparation of (a) phosphorium iodide, (b) tetra-methyl
phosphonium iodide. What is the action of water and of potassium
hydrate solution upon these two compounds ?
2. How is free cacodyl obtained ? Give the formulae of cacodyl oxide
and cacodyl chloride.
3. Give the preparation of zinc ethyl. What is the action upon it of
(a) water, (6) chlorine, (c ) acetyl chloride ?
4. What reasons have we for considering lead to be a tetra-valent
metal ?
CHAPTER XIV
GLYCOL AND ITS DERIVATIVES. SUCCINIC,
MALIC, AND TARTARIC ACIDS
Glycol, C.,H 4 (OH) <( , is a substance containing two
hydroxyls combined with the divalent radicle, C 2 H 4 , ethylene.
Each of these hydroxyls behaves similarly to the hydroxyl in
an alcohol, so that glycol may be termed a dihydric alcohol.
It is obtained from ethylene bromide, C H 4 Br.,, by replace-
ment of the bromine :
C 2 H 4 Br 2 + 2HOH = C 2 H 4 (OH) 2
Ethylene bromide. Glycol.
just as the ethyl alcohol can be got from ethyl bromide :
C 2 H 5 Br + HOH = C 2 H 5 OH + HBr.
It is necessary when preparing glycol from ethylene bromide
in this way to heat with a large quantity of water to temper-
atures of 150 C. or thereabouts ; the reaction takes place
more readily when sodium carbonate is added to the water.
This method of preparation indicates the constitutional
CH 2 OH
formula i for glycol, according to which it may be re-
CH 2 OH
garded as a dihydric primary alcohol, and this view is strength-
ened by consideration of the substance's general chemical
behaviour.
GLYCOL A DIHYDRIC ALCOHOL
As a dihydric alcohol glycol reacts with sodium or potas-
sium to form glycolates analogous to the alcoholates (p. 44) :
OT-T
C 2 H 4 (OH) 2 + Na = C,H 4 < + H)
and
C 2 H 4 (OH) 2 + 2Na = C 2 H 4 (ONa) 2 + H 2 ;
and ethereal salts of glycol can be obtained by the action on
it of acids :
C 2 H 4 (OH) 2 + 2CH 3 C0 2 H = C 2 H 4 (OC0 2 CH 3 ) 2 + 2H 2 O ;
Glycol. Acetic acid. Ethylene acetate.
just as ethyl acetate is got from ethyl alcohol and acetic acid,
so here ethylene acetate is obtained.
As a dihydric primary alcohol glycol furnishes, when treated
with oxidising agents, bodies in which the groups CH.,OH are
successively oxidised to aldehyde groups CHO, and finally to
carboxyl groups CO H. The substances thus obtained are
presently to be considered.
Glycol is a thick colourless liquid with a sweetish taste.
The isomeric glycol, ethylidene glycol, CH 3 . CH(OH) 2 , does not seem
able to exist unless in dilute solution ; instead of this we obtain aldehyde
CH 3 . CHO when the ethylidene glycol might be expected, e.g. in action
of water on CH 3 . CHC1 2 .
Glyoxal, (CHO) , is the di-aldehyde of glycol, and is
formed along with other substances in the oxidation of glycol :
CH 2 OH CHO
I +O= i + H 2 O.
CH 2 OH CHO
Glycol. Glyoxal.
Like other aldehydes, it readily reduces Fehling's solution or
ammoniacal silver solution (see p. 62).
Glycolic Acid, CH./OH). CO 2 H, is also formed in the
oxidation of glycol :
CH 2 OH CH 2 OH
I +0 2 = I +H,0,
CH..OH CO 2 H
Glycol. Glycolic acid.
OXALIC ACID
but is better prepared from mono-chloracetic acid by boiling
with water, to which calcium carbonate in fine powder has
been added (to combine with the HC1) :
CH,C1 . CO..H + H 2 O = CH 2 (OH) . CO 2 H + HC1.
Monochlor-acetic acid. Glycolic acid.
Glycolic acid forms white crystals. As an acid it yields well-
defined salts, such as silver glycolate, CH 2 OH . CO 2 Ag, while
as an alcohol it combines with acids to produce ethereal salts.
Oxalic Acid, (CO.,H).,, is a more completely oxidised pro-
duct of glycol :
CH 2 . OH CO.,H
I + 2O 2 = i +2H 2 O.
CH., . OH CXX.H
Glycol. Oxalic acid.
It is an important acid, the starting-point of a series of organic
dibasic acids. Oxalic acid is found in many plants, especially
the varieties of Oxalis, and can be prepared artificially in several
ways, of which three further ones (in addition to the oxidation
of glycol) may be mentioned :
1 i ) Carbon dioxide when passed over heated metallic sodium
combines with it to form sodium oxalate :
2CO 2 + 2Na = C 2 O 4 Na 2 .
Sodium oxalate.
(2) Sodium formate when strongly heated evolves hydrogen
and yields sodium oxalate :
2H.CO 2 Na = H 2 + C 2 O 4 Na 2
Sodium formate. Sodium oxalate.
(3) An important practical method for the manufacture of
oxalic acid is the action of caustic alkalies upon cellulose.
Sawdust (the form of cellulose generally used) is mixed into a
paste with a strong solution of potash, and then heated on iron
plates. The product is extracted with water, and the oxalic
acid separated by precipitation as calcium oxalate.
Oxalic acid forms crystals which contain two molecules of
SUCCINIC ACID
water of crystallisation. When heated the crystals lose water,
and then decompose into formic acid and carbon dioxide :
Oxalic acid. Formic acid.
Oxalic acid when heated with strong sulphuric acid does not
blacken, but is decomposed with evolution of the two oxides
of carbon in equal volumes :
Oxalic acid is a stronger acid than acetic, and being a dibasic
acid forms two series of stable salts.
Potassium Oxalate, C O 4 K 2 , is used in preparing the
" ferrous oxalate developer," fargely employed in photography.
Potassium Hydrogen Oxalate, C 2 O 4 KH, along with
free oxalic acid, composes the "salts of lemon" used for re-
moving ink-stains from cloth.
Ammonium Oxalate, C O 4 (NH 4 ) , is used as a reagent
in the laboratory.
SUCCINIC, MALIC, AND TARTARIC ACIDS
Succinic Acid, C 4 H 6 O 4 , was first obtained by distillation
of amber, and this is still the way prescribed for its preparation
in the British Pharmacopoeia. It is also present in some other
resins and in lignite. The artificial methods for making the
acid and its reactions are best represented by the constitutional
formula given below ; the chief of these methods are :
(1) Ethylene cyanide (from ethylene bromide and AgCN),
when boiled with dilute acids or alkalies, yields succinic acid :
CH 2 . CN CH 2 . C0 2 H
l + 4H 2 = i +2NH 3 .
CH 2 .CN CH 2 .CO 2 H
Ethylene cyanide. Succinic acid.
(2) Succinic acid is also obtained by the reduction of malic
acid, which can itself be similarly obtained from tartaric acid :
C 4 H 6 6 O = C 4 H fi 5 .
Tartaric acid. Malic acid.
MALIC ACID 103
C 4 H,.0 5 O = C 4 H ? 0,
Malic acid. Succinic acid.
The reduction can be effected by heating with hydriodic acid in sealed
tubes.
Succinic acid forms colourless crystals, soluble in water,
and possessing an unpleasant taste.
Malic Acid, C 4 H 6 O 5 , occurs in the juice of apples and of
many other fruits. Its close relation to succinic acid is indi-
cated by the reaction, above referred to, by which that acid is
obtained by the reduction of malic acid, and the exact char-
acter of the relation is made clear by the following method of
preparation :
(1) Malic acid is produced when monobrom-succinic acid
is treated with silver oxide and water :
CHBr.CO H CH(OH) . CO..H
I +AgOH= | +AgBr.
CH 2 .CO.,H CH 2 .CO 2 H
Monobrom-succinic Malic acid,
acid.
Malic acid is therefore monohydroxy-succinic acid.
(2) Malic acid is formed by the partial reduction of tartaric
acid :
C tf H 4 6 - O - C H 4 5 .
Tartaric acid. Malic acid.
Malic acid forms deliquescent needles. It is a somewhat
stronger acid than succinic, and forms several well-crystallised
salts. Very important is the existence of three isomeric forms
of malic acid which differ chiefly in their action upon polarised
light. One form, the ordinary one obtained from berries,
rotates the plane of polarisation to the left ; a second form,
prepared from dextro-tartaric acid, rotates the plane of polar-
isation to the right ; while the third form, obtained synthetic-
ally, is inactive. The fuller consideration of this case of
isomerism is deferred until Part II. of this book.
Tartaric Acid, C 4 H ( .O (3 , is present in the juice of many
fruits, especially in that of grapes ; practically the only source
of the acid is the " argol," an impure potassium tartrate, de-
104 TARTARIC ACID
posited during the fermentation of grape-juice. The constitu-
tional formula of the acid is evident from its relation to malic
and succinic acids (into which it is in turn converted by re-
duction), and from the following synthetical methods of pre-
paration :
(i) Dibrom-succinic acid when boiled with water and silver
oxide yields tartaric acid :
CHBr.CO 2 H CH(OH)CO 2 H
I +2AgOH= | +2AgBr;
CHBr.CO 2 H CH(OH)CO 2 H
Dibrom-succinic Tartaric acid,
acid.
tartaric acid is accordingly dihydroxy-succinic acid.
Tartaric acid furnishes another instance of the existence of
isomers inexplicable by the theory hitherto alone employed for
the explanation of cases of isomerism. The isomers again
differ, just as was the case with the malic acids, chiefly in their
action upon polarised light. Tartaric acid furnishes four such
isomers, of which one is dextro-rotatory (rotates the plane of
polarisation to the right), another is laevo-rotatory, while the
other two are inactive. We shall here consider only the com-
mon variety, dextro-tartaric acid, leaving the others to be dis-
cussed in Part II.
Dextro-tartaric acid is the tartaric acid of the shops. It is
prepared from argol by conversion into calcium tartrate (treat-
ment with milk of lime), and subsequent liberation of the free
acid by addition of sulphuric acid. It is purified by recrystal-
lisation, and forms large prismatic crystals which are readily
soluble in water. The solution rotates the plane of polarisa-
tion of light to the right. It is a dibasic acid, and the follow-
ing salts formed by it are of importance :
Potassium Hydrogen Tartrate, C 4 O 6 H 5 K, is the
"cream of tartar" of the druggist, and is obtained by purify-
ing the " argol " deposited in the fermentation of grape-juice.
It is only slightly soluble in water, and hence sodium hydrogen
tartrate will precipitate it from solutions of potassium salts,
unless very dilute ; this reaction is sometimes used as a test
for the presence of potassium in place of the more expensive
method by means of platinic chloride.
xiv THE TARTRATES 105
Potassium Sodium Tartrate, C 4 O H 4 KNa, is known
as " Rochelle salt," and is prepared by mixing solutions of
sodium hydrate and cream of tartar.
Tartar Emetic is the name of a substance which is ob-
tained by boiling cream of tartar and oxide of antimony with
water. Its constitution is generally supposed to be repre-
sented by the formula C 4 O 6 H 4 (SbO)K, according to which
one hydrogen atom of the tartaric acid is replaced by the
monovalent group (Sb'"O), antimonyl. Tartar emetic is then
to be termed potassium antimonyl tartrate.
The same group, SbO, exists in the compound which is obtained as a
white precipitate when water is added to a solution of antimony chloride.
This precipitate has the composition SbOCl, and is produced according to
the equation
SbCl 3 + H 2 O = SbOCl + aHCl.
QUESTIONS ON CHAPTER XIV
i. By what reactions would you proceed to prepare glycol from ethyl
alcohol ?
z. Show by its reactions that glycol behaves as a dihydric primary
alcohol.
3. Give two ways by which oxalic acid can be synthesised from its
elements. Describe the commercial process for the manufacture of the
acid.
4. What is the relation between succinic, malic, and tartaric acids ?
How can you pass from each of them to the others ?
5. Write down the formulae of (a) salts of lemon, () tartar emetic, (c)
cream of tartar, (</) succinic acid.
CHAPTER XV
LACTIC AND CITRIC ACIDS
Lactic Acid is a substance present in sour milk which, when
isolated and examined as to its chemical relationship, is found
to be predominantly an acid, but also to possess some of the
properties of alcohols. Its empirical formula is CH O as
determined by analysis, and as the acid cannot be vaporised
without decomposition, we are unable to ascertain its mole-
cular weight by a vapour density determination. It has
recently become possible to employ other means for finding
the molecular weight of the acid itself, but a little study of the
compounds of lactic acid enables us to discover its molecular,
and then its constitutional formula.
Lactic acid forms only one sodium salt, sodium lactate,
whose analysis indicates the formula C 3 H 5 NaO,,, and therefore
the molecular formula C 3 H g O 3 for the acid (this agrees with
the vapour density of ethyl lactate C 8 H 5 O 3 . C 9 H 5 ). Lactic
acid is therefore a monobasic-acid, and contains one carboxyl
group, CO 2 H. But in this sodium salt there is yet left a
hydrogen atom which can with some little difficulty be replaced
by sodium, and behaves like the hydrogen atom of an alcoholic
hydroxyl. Lactic acid is therefore seen to contain the group
OH also.
Lactic acid, Cj^H^O.^, may therefore be written C 2 H 4 (OH)
(CO 2 H), and the only question left to solve is whether the OH
and the CO..H are connected to the same or to different
carbon atoms, whether it is
CH 2 (OH) CH 3