Outlines of organic chemistry, a book designed especially for the general student

Front matter

CcpigteM

CQEXRiGHT DEPOSIXl

NOTE

Preface

During the past few years, it has been one of the author's most agreeable duties at the Massachusetts Institute of Technology, to deliver a course of about thirty lectures upon the underlying principles of Organic Chemistry. Those attending this course have been, almost exclusively, candidates for the bachelor's degree in Physics, Biology and Sanitary Engineering. The experience thus gained has produced the conviction that, for such students, a selection of topics is desirable which is somewhat different from that given to those who are fitting themselves to become organic chemists, and consequently different from that outlined in most text-books designed for both classes of students.

The material which has been found suitable for presentation in these lectures is here put in book form, in the hope that it may prove useful to others who study Organic Chemistry from the non-professional point of view.

In selecting from the vast number of organic substances those best adapted for study in a work of this kind, compounds have, as a rule, been included, either because they are themselves of practical importance, or because they serve as the most convenient examples for illustrating fundamental principles which elucidate the chemical character of substances which are of practical importance. The word "practical" has been abused in so many ways, that some definition is desirable. For the purposes of this book those substances have been considered of practical importance which have a wide technical application, like acetylene or linseed oil; those which are important factors in vital

iii iv PREFACE

processes, like glycogen or urea; those which are familiar in the operations of daily life, like sugar or starch; or finally those which throw light upon some important theory. Theories and processes have been selected upon a similar basis. The attempt has been made to present these in a thoroughly scientific manner, no important subject being omitted merely on account of inherent difficulties of presentation or comprehension. Numerous subjects of the greatest importance to the professional organic chemist have, however, been omitted without scruple, if their inclusion did not seem called for on the basis of the principle already laid down. The professional will look in vain for any mention of a subject as important to him as the aceto-acetic ester synthesis or the Grignard reaction, and he will find no mention of the broad subject of cis-trans isomerism. Optical isomerism has, however, been dealt with in considerable detail, because so many important natural products are optically active. It has been the same in the matter of constitutional proof. All will agree that it serves no good purpose to present a constitutional formula unless it is at the same time made clear how such a formula serves as an epitome of the whole general chemical character and behavior of a compound. To give a rigid proof in the case of every substance mentioned would, on the other hand, lead far beyond the limits of a book of this character. The attempt has been made to meet these difficulties in the following way: For simple and important compounds, well adapted for purposes of illustration, proof has been presented in considerable detail, in order that the student may clearly understand just what is meant by this important element in a chemist's work. With more complicated substances and derivatives, the chemistry of the compounds has usually been discussed with sufficient fullness to make the formula plausible. In some isolated cases the author must plead guilty to having presented

PREFACE V

formulae without any evidence at all. Where this has been done, it has been done as a choice of evils. In the case of such compounds as pinene, camphor, or uric acid, for example, it seems at least equally undesirable either to omit formulae altogether, or, on the other hand, to marshal for their support a host of unfamiliar and, for every other purpose, useless facts, each of which is as difficult to remember as the formula it is intended to establish. The semiphilosophical question of the limit of information conveyed by a graphic formula and the validity of the valence theory are discussed in the last chapter as critically as seems appropriate in addressing non-professional students. The chapter which touches upon Physiological Chemistry stands, scientifically, upon a somewhat different footing from the rest of the book, but this difference has been perhaps sufficiently emphasized in the text.

F. J. MOORE.

MASSACHUSETTS INSTITUTE OF TECHNOLOGY,

Chapter I. Introductory

The division of the subject of Chemistry into two great fields called respectively Organic and Inorganic Chemistry , like all such classifications in science, is merely a matter of practical convenience. The distinction grew up historically from the tendency to classify substances according to their origin; thus those which were obtained directly or indirectly from plant or animal sources were ranked as organic, all others as inorganic. In the first class were placed such substances as sugar, alcohol, and acetic acid; in the second, those like salt, sulphur, or lead. In earlier times the distinction between these classes seemed more fundamental than at present, because as yet no process was known by which an organic compound could be prepared from an inorganic one, or built up from its constituent elements in the laboratory. Since such processes have been devised, and the synthetic method has assumed so much importance in the study of organic compounds, the arbitrary nature of the old classification has become more apparent. The old names have, however, been retained with a somewhat altered significance. We now define Organic Chemistry as that portion of the science which treats of the compounds of carbon. This element occurs so universally in substances of animal or vegetable origin that most compounds ineluded in the old classification are retained in the new, the subject-content of Organic Chemistry remaining much the same.

Not only does the great number of the carbon compounds- — probably exceeding those formed by all the other elements combined — justify the separate treatment of this important branch, but the methods of study show well-defined differences, both in mechanical technique and in theoretical point of view. It is, of course, true that the same laws govern chemical action in the one field as in the other. On the other hand, the every-day processes carried out in laboratory practice involve the habitual use of different operations, and the theoretical considerations which govern the work are, as a general rule, quite different. For example, the fundamental theoretical conception which is constantly in the mind of the organic chemist in planning his researches is the valence theory. This has proved a far less serviceable guide in the inorganic field.

Among those properties of organic compounds which are most likely to impress the beginner as differing from inorganic ones, the following may be mentioned as typical.

1. Instability at High Temperature. While many inorganic compounds can be heated to incandescence without undergoing chemical change, nothing of the kind is to be observed among organic compounds. These, almost without exception, undergo decomposition by the time the temperature has been raised to dull redness, while a large proportion decompose at a much lower temperature. This decomposition by heat is usually accompanied by deposition of free carbon, and this phenomenon, known as "charring," is therefore one of the most common analytical tests for the presence of "organic matter. 5 '

2. Solubility. We seldom have occasion to work with the more common inorganic compounds in any other than aqueous solution; the majority of organic compounds, however, are practically insoluble in water, most of these being soluble to a greater or less extent in such media as alcohol, ether, acetone, chloroform, carbon bisulphide, and the like. Most of the reactions carried on in the organic laboratory take place to best advantage in such solvents. The. division of substances between two non-miscible solvents is also an operation frequently employed.

3. Velocity of Reaction. Most of the inorganic compounds with which we are most familiar are electrolytes, that is, they are dissociated in aqueous solution into ions; hence the reactions which take place in such solutions are usually those between ions. As a result these reactions are practically instantaneous in character. The greater number of organic compounds, on the contrary, are not electrolytes, and the reactions between them, though often rapid, are frequently extremely slow, requiring hours, and sometimes even days or weeks, for completion. As a result, the long digestion of reaction-mixtures at elevated temperatures is quite the rule in organic operations.

It must be understood that no one of the distinctions which have been mentioned is in any sense absolute. Many organic compounds are electrolytes, many inorganic compounds are soluble in non-aqueous solvents, many reactions between organic substances take place with great velocity, and many inorganic substances decompose at no very high temperature. What has been said is only intended to show enough of the broad differences between the two subjects to justify an independent treatment. The name " Organic Chemistry" is also well justified, for although no one any longer believes that any mysterious "vital force" makes the operations of chemical affinity something different in the one class of compounds from what it is in the other, yet two things do remain true : first that, in practice, most organic compounds are still obtained directly or indirectly from organized bodies (animal or vegetable) and second, that the vital processes themselves are, as we shall see in more detail later, reactions between organic compounds.

THE PURIFICATION OF ORGANIC COMPOUNDS.

The fact that small amounts of impurities have a marked effect upon the physical properties, and sometimes upon the chemical behavior, of organic substances makes it imperative, in most cases, that compounds be subjected to processes of purification before they can be analyzed or employed for synthetic operations. This makes it necessary to discuss those tests which may be looked upon as furnishing criteria of purity for an organic substance. Those most commonly employed are the determination of the melting point in the case of solids, and the boiling point in that of liquids. The knowledge of other physical constants such as the crystalline form, the specific gravity, the electric conductivity, and the refractive index is also extremely useful either as means of identification or as furnishing tests for purity. These latter determinations are, however, much less frequently made in laboratory practice.

To return to those first mentioned, a liquid is usually regarded as pure when its boiling point is constant, that is, when the whole of it will distill at a constant temperature. It should be stated at once that such a conclusion is not justifiable in all cases. Certain mixtures of liquids of definite composition also distill at a constant temperature for a given pressure. In such a case, changing the pressure reveals the presence of a mixture of this kind, for then the boiling point will no longer remain constant.

The boiling point of a liquid is usually obtained by distilling it ; — a process also very useful for purposes of purifiGation. The apparatus employed consists of a flask provided with a side neck which passes to a condenser. (Fig. 1.) Through the stopper of the flask is introduced a thermometer in such a way that the bulb comes just opposite the

Fig. 1.

opening of the side neck. It thus measures the temperature of the vapors passing over. This gives the boiling point of the distillate at any given moment. It is frequently desirable to carry out a distillation under diminished pressure — "distillation in vacuum" the operation is commonly called. For this purpose all joints in the apparatus are made air-tight, and the receiver is attached to some form of vacuum pump. A manometer is usually connected to the apparatus to indicate the pressure.

The melting point of a solid is a characteristic property. A small amount of an impurity will frequently depress the melting point many degrees, so that in practice, when dealing with an unknown substance, processes of purification are repeated until the melting point of the material has reached a maximum.

The melting point of a solid is determined as follows: — a little of the finely powdered substance is introduced into a thin- walled tube closed at one end. (Fig. 2.) The tube

Fig. 2.

used should be about five centimeters long and one millimeter in diameter. It is attached to a thermometer in such a way that the substance is just beside the bulb of the latter. Both are now heated slowly in a bath of some transparent liquid — usually concentrated sulphuric acid — until the substance is seen to melt. The temperature is then read off on the thermometer.

Solids are most frequently purified by crystallization. This operation is so constantly carried out in an organic laboratory that it should be described in some detail. The substance to be recrystallized is heated with such a quantity of an appropriate solvent that the boiling solution is nearly saturated. The hot solution is rapidly filtered, in order to remove any insoluble impurities, and the filtrate is allowed to cool slowly. When cold the crystals which have been deposited are filtered off and washed with a little of the pure solvent. The adhering solvent is then removed as far as possible by suction, and the product finally dried, either by heat or by leaving it in a vacuum-desiccator (Fig. 3) which also contains some substance capable of absorbing the vapors of the solvent. For this purpose concentrated sulphuric acid is generally used, as it will absorb the vapors of water, alcohol, ether, and many other liquids. In carrying out the above operation, one of the most important things is the selection of a proper solvent. One should be used from which the solid will be deposited in wellformeds crystal easily washed.

Fig. 3.

In order to secure a good yield, it is also extremely desirable that the substance be but sparingly soluble in the cold solvent, but quite freely soluble at a higher temperature. In many cases, of course, these conditions can be but imperfectly realized.

A problem frequently met with is the separation of two or more liquids which are miscible in all proportions, but which have different boiling points. If it does not so happen that the liquids form a mixture boiling at a constant temperature, it is possible to separate them by fractional distillation. When a mixture of water and methyl alcohol, for example, is boiled, the portion which first volatilizes is relatively richer in alcohol than the liquid from which it distills. As the distillation proceeds, the percentage composition of the distillate progressively changes, so that the later portions contain relatively more water than the original mixture. If now, instead of collecting the whole distillate in one receiver, the receivers be changed repeatedly, a series of fractions will be obtained of varying concentrations. If each of these fractions be again distilled, further changes of concentration in the same sense take place, so that, by repeated distillations, it

Fig. 4.

is possible to effect a practically complete separation of the two components.

Another operation of constant application is distillation with steam. (Fig. 4.) Many substances insoluble in water distill readily in a current of steam, although their own boiling points may be a hundred degrees or more higher than that of water. The process is chiefly employed for the purpose of removing from a reaction-mixture some one component which alone is volatile with steam.

Another operation which serves the same purpose depends upon the differing solubilities of substances in two liquids which are not miscible. If some substance, for example, is formed in an aqueous solution which is more soluble in ether than in water, its separation and purification may be effected by shaking the reaction mixture with ether. Later when the two layers have been allowed to separate, the substance sought is found in the ethereal layer, whence it may be readily obtained by distilling off the ether. This operation of " shaking out with ether" is of constant application, and is usually performed in a piece of apparatus called a "separatory funnel" (Fig. 5) which permits the lower layer of liquid to be drawn off through a stopcock at the bottom. Other liquids which do not mix with water may, of course, be substituted for the ether in special cases. Were any substance absolutely insoluble in one liquid and very soluble in another, it is clear that' all of the material might be obtained by one operation. As a rule, however, several treatments are necessary, because the conditions of solubility hinted at above are never realized in practice. The rule is that the concentrations of the Fig * 5 -

solute in two non-miscible solvents is proportional to the solubilities of the dissolved substance in each solvent considered separately. This ratio of solubilities is accordingly known as the " distribution coefficient."

QUALITATIVE ANALYSIS.

A complete system of analysis, such as we find in use for the study of inorganic substances, is at the present time lacking in Organic Chemistry. We have as yet no standard methods which can be relied upon to reveal all the components in any mixture of organic substances, and owing to the close resemblance which exists between the members of the various classes of organic compounds, and the multitude of classes which exist, it is probable that such a general system can only be developed in the distant future. It follows that for the present, when dealing with mixtures, special methods must be employed which are suitable to particular kinds of mixtures. When these are unfamiliar, new methods must be worked out by the experimenter. Such work requires a wide knowledge of Organic Chemistry, and therefore cannot be discussed at all here. A practical problem which comes up constantly in organic work is the following: — Given a pure homogeneous organic compound, to detect the elements which it contains, and to determine the percentage of each which is present. The processes involved in the solution of this problem are, in theory at least, extremely simple, and may well be taken up at this point. The opeiation, as a whole, is spoken of as the " elementary," or more frequently as the " ultimate," analysis of an organic compound.

The elements which usually occur in organic compounds are comparatively few in number. The only ones which need to be considered here are carbon, hydrogen, oxygen, nitrogen, chlorine, bromine, iodine and sulphur. The vast majority of compounds contain no element outside of the first four in the list.

To test for carbon, the substance is merely heated in a tube closed at one end. The presence of carbon is revealed by charring, with the simultaneous evolution of what is commonly called a " burnt odor." A few organic substances are so volatile that they do not show this test well. In such cases it is necessary to completely oxidize the substance. This can usually be best effected by heating with copper oxide. If carbon is present the gases evolved will then contain carbon dioxide, and this can be readily detected by the turbidity which it produces in lime water.

A test for hydrogen is usually superfluous after having tested for carbon, since practically all organic compounds contain hydrogen. If any test is desired, the formation of water on heating or by combustion is sufficient.

For combined oxygen, Chemistry still lacks any universally applicable qualitative test, or any good general method for its quantitative determination.

The other elements mentioned — that is, nitrogen, the halogens, and sulphur — are most conveniently detected after total decomposition of the organic material. For this purpose the substance is heated with metallic sodium in a small tube closed at one end, until decomposition has taken place, as witnessed by abundant charring. The tube is then broken under water, and the solution boiled and filtered.

The chemical results of this operation may be summarized as follows : — Most of the carbon the substance contains is deposited directly as such. If nitrogen is also present, this combines with some of the carbon and some of the sodium to form sodium cyanide. Similarly, if the halogens and sulphur are present, they unite with the sodium to form the corresponding binary compounds. Hence a solution prepared as above indicated would contain sodium cyanide if the substance analyzed contained nitrogen, sodium sulphide if it contained sulphur, and sodium chloride, bromide, or iodide, if these halogens were present in the original material. In this solution, then, which now contains no organic matter, the above anions may be tested for according to any of the methods commonly employed in the qualitative analysis of inorganic compounds. In order to test for the presence of a cyanide, for example, the alkaline solution is treated with a few drops of a solution containing iron in both the ferrous and ferric condition, and the mixture digested for a few minutes. Then upon acidification, a precipitate of Prussian blue results if nitrogen was originally present in the organic substance.

6NaCN + FeS0 4 =Na 4 Fe(CN 6 ) + Na 2 S0 4 . FeCl 3 + 3NaOH = Fe(OH) 3 + 3NaCl. 4Fe(OH) 3 + 3Na 4 Fe(CN) 6 + 12 HC1

= 12NaCl + Fe 4 [Fe(CN) 6 ] 3 + 12H 2 0. Chlorides, bromides, and iodides may be recognized by the precipitates which they give with silver nitrate in dilute nitric acid solution. They may be further distinguished by the fact that chlorine-water liberates free bromine from bromides,

Cl 2 + 2NaBr = 2NaCl + Br 2 ,

and free iodine from iodides. The former imparts a bloodred color to carbon bisulphide, the latter gives a violet color.

If sulphides are present, they may be recognized by the black precipitates which they give with neutral silver nitrate or lead acetate:

A more sensitive test, however, is that effected by adding a dilute solution of sodium nitroprusside, Na 2 (NO)Fe(CN) 5 . This reagent gives a violet color with soluble sulphides.

When two or more of the elements just mentioned are present at the same time, the analysis is a little more complicated. Such cases are, however, rather the exception in practice. For dealing with them, the student is referred to any work upon Qualitative Analysis.

QUANTITATIVE ANALYSIS.

The principles involved in the quantitative determination of the elements in an organic compound are also extremely simple. The determination of carbon and hydrogen is

Now ^ of 0.12 is 0.0133, which is the weight of hydrogen in the water

formed in the combustion. This hydrogen is, however, the same

hydrogen which was originally in the substance used for the analysis.

0133 The percentage of hydrogen in the latter must therefore be ' or

6.66.

Similarly, the weight of carbon dioxide formed in the combustion

is 0.2933 gram. The formula CO. shows that in carbon dioxide there

are twelve parts by weight of carbon to thirty-two parts of oxygen,

that is, the carbon constitutes \\ or T 3 r of the whole. T 8 r of 0.2933 is

0799 0.07999 gram carbon. Hence — — - — or 39.99 is the percentage of car-

bon in the substance.

The method most generally used for the determination of nitrogen in an organic compound is based upon principles entirely similar to those underlying that employed for the determination of carbon and hydrogen. In this case a difference of application is made necessary. The carbon as before is burned to carbon dioxide, and the hydrogen to water, while the nitrogen is liberated as such. This gas is collected and measured. It is therefore necessary that no other gases should be present which cannot be conveniently separated from nitrogen. In practice, the combustion is accomplished entirely by means of copper oxide in an atmosphere of carbon dioxide. A hard-glass tube is employed, sealed at one end. This has the same dimensions as that used for the determination of carbon and hydrogen. Next the closed end is first introduced some powdered magnesite or manganese carbonate, which will yield a steady stream of carbon dioxide when gently heated, MgC0 3 = MgO + C0 2 . Next to this comes some coarse copper oxide, then the substance itself intimately mixed with fine copper oxide, then a long layer of coarse copper oxide and finally a spiral of freshly reduced metallic copper. The use of the latter will be explained a little farther om From the combustion tube, a small tube leads to a piece of apparatus called an azotometer. This is shown in Fig. 7. It consists of an upright tube with a stopcock at the top, and, near the bottom, two side arms, by means of one of which it is connected to the

Fig. 7.

combustion tube. From the other, a long piece of rubber tubing leads to a leveling bulb. This serves to fill the vertical tube with a concentrated solution of potassium hydroxide as needed. At the beginning of the operation the upright tube is left nearly empty, the stopcock at the top being open. The combustion tube is now heated near the closed end until the air in the apparatus has been replaced by carbon dioxide. The leveling bulb is then raised till the vertical tube of the azotometer has been filled with potassium hydroxide solution, and the stopcock at the top is closed. The copper oxide at both ends of the combustion tube is next raised to a red heat, and finally the substance itself is subjected to this temperature. The nitrogen evolved passes over into the azotometer, the accompanying carbon dioxide being absorbed by the solution of potassium hydroxide. It sometimes happens that substances containing nitrogen produce, on combustion, not only this gas, but also oxides of nitrogen. These are soluble in potassium hydroxide and some nitrogen would be lost in consequence, were it not for the presence of the hot spiral of reduced copper mentioned above. When oxides of nitrogen pass over this, the copper takes the oxygen, forming copper oxide while free nitrogen is liberated,

2Cu + 2NO = 2CuO + N.

It should be added that such a spiral of reduced copper must also be used in the determination of carbon and hydrogen whenever the substance to be analyzed contains nitrogen. The gas in the azotometer may be measured directly in that instrument or, better, transferred to a measuring tube over water. Its volume is reduced to standard conditions of temperature and pressure and the result multiplied by the weight of one cubic centimeter of nitrogen under those conditions. This gives the weight of nitrogen in the sample, from which the percentage may be calculated.

To take a specific case, let it be supposed that 2 gram of an organic substance when treated as above yields 25.6 cubic centimeters of nitrogen at 20° and 782 millimeters pressure as observed by the barometer. This observed volume is reduced to standard conditions by

means of the familar formula V = +0*00367 in which V o

represents the volume the gas would occupy at 0° and 760 millimeters, v is the observed volume, t the observed temperature, and p the pressure. Here it has to be remembered that since the gas has been measured over water, the actual pressure under which it stands is equivalent to the atmospheric pressure as registered by the barometer minus the vapor tension of water at the prevailing temperature, in this case 782 minus 17 or 765 millimeters. Having substituted these values in the

IS OUTLINES OF ORGANIC CHEMISTRY

formula, the result shows that the nitrogen present would occupy 24 cubic centimeters at 0° and 760 millimeters. Under these conditions one cubic centimeter of nitrogen gas weighs 0.001254 gram. Twentyfour cubic centimeters will weigh 0.0301. Hence the percentage of nitrogen in the substance is 0.0301 -*- 0.2 or 15.05 %.

When an organic compound contains one of the halogens or sulphur, their determination is effected by a method which also involves the complete oxidation of the carbon and hydrogen. In brief, this method consists in heating the substance in a sealed tube for several hours with fuming nitric acid at a temperature rather over 300°. By this treatment, complete oxidation of the organic material is effected, and any sulphur present is oxidized to sulphuric acid. The latter can now be determined in the usual way, by precipitation with barium chloride, forming insoluble barium sulphate. If the substance contains halogen it is customary to seal up with it in the tube an excess of solid silver nitrate. The chlorine, bromine, or iodine is then found as the corresponding insoluble silver halide when the tube is opened. This is filtered off and weighed, subject to the ordinary precautions observed in analytical work. As the percentage of halogen in the silver compounds, as well as that of sulphur in barium sulphate, is readily derived from their formula?, the calculation of the analyses offers no new difficulties and need not be further discussed here. For the complications which arise when two or more of the above elements are present at the same time, the student is referred to any work upon Quantitative Analysis.

A determination of the oxygen in an organic compound is never made in practice, on account of the lack of any satisfactory method. Instead the percentage of oxygen is obtained by difference, the percentages of the other elements being added together and the sum deducted from 100 %. This practice has the serious objection, for one thing, that all the errors of experiment made in the analysis are thrown upon the oxygen. It has also happened in serious scientific investigation that the presence of an element has been entirely overlooked because its atomic weight happened to be an even multiple of that of oxygen.

CALCULATION OF THE EMPIRICAL FORMULA.

Having obtained the percentages of the different elements in an organic compound, the next step is to determine from these data the empirical formula. This resembles in principle a problem in elementary arithmetic which may be stated thus : "In a pile of cannon balls weighing a hundred pounds, there are thirty pounds of ten-pounders, forty of five-pounders and thirty of two-pounders. How many of each kind of ball are there in the pile?" In this case the total weight of each kind of cannon ball is divided by the weight of one ball of that kind. Similarly, in the chemical problem, the weight of each element present is divided by the atomic weight of that element. There is this difference, however, in the two problems: the weights of the cannon balls are actual weights, while the atomic weights of the elements are relative quantities. The resulting quotients, therefore, in the latter case cannot be the actual number of atoms present in the molecule of the compound but are simply quantities which stand to each other in the same ratio as those numbers. On this account it is just as well to divide the percentages of the elements by the atomic weights as to employ the actual weights, and in practice this is usually done.