MOLECULAR MODELS IN THE ELEMENTARY ORGANIC LABORATORY. I WALLACE R. BRODE AND C B CE. ~ Boom Tm Omo STATEUNNERSITY, COLWUS, OHIO
The present paper deals with the use of structural models in a laboratory course of elementary organic chemistry to dewelop the fundamentel concepts of homology and isomerism. This i s made possible by the fact that suitable model sets have now been made available at a price easily within the reach of every student. A few examples are given of simple laboratory exercises designed to enable the beginning student to darelop for himself the basic principles of structural organic chemistry.
. . . . . .
The usual laboratory course in elementary organic chemistry begins with a series of exercises designed to acquaint the student with the operations used to separate and purify organic compounds and to test their purity. These are followed by a few exercises adapting the principles of qualitative analysis to the detection of the elements. Sometimes this latter group is extended to include the quantitative determination of a few elements, particularly carbon and hydrogen. The major portion of the course is normally made up of a series of preparations of typical organic compounds and a study of their chemical characteristics. In recent years many instructors, in order to drive home the characteristic class reactions, are also including a greater or lessq amount of qualitative organic analysis. Such a program is devoted almost exclusively to a study of the reactions of I)' functional groups. The framework formed by the union of carbon to carbon by single, double, and triple bonds, the general tetravalent and tetrahedral character of the carbon atom, with the resultant phenomena of homology and isomerism, are left wholly dependent on the clarity of the text, the ingenuity of the instructor and the imagination of the student. The engineering student with his background of analytical geometry probably has little difficulty in visualizing the plane diagrams of structural organic chemistry in three-dimensional space. It is not so with the non-engineering student. To the students in pharmacy, arts-chemistry, premedicine, and hope economics, the non-existence of two dichloro derivatives of methane or the equivalence of the six hydrogen atoms of ethane are not immediately apparent. The desire to place a set of structural models in the hands of each of their beginning students has doubtless originated in the minds of many instructors. The possibility of providing such a set at a suitable price was a subject of general informal discussion among several members of the Organic Division a t the Indianapolis Meeting of the American Chemical Society. During the following summer a set of models was designed and 1774
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constructed. These models have been successfully used in all classes in elementary organic chemistry a t The Ohio State University during the past academic year. The usual list of synthetic preparations was closely scrutinized and a few of the more expensive and least desirable experiments omitted. It was thus found possible to provide each student with an individual set of the models a t an actual saving. A number of laboratory exercises were then devised with the view of evolving, developing, and illustrating the principles of homology, isomerism, and nomenclature. These exercises were inserted into the laboratory program a t those points which seemed most suitable. The exercises developing homology, simple chemical isomerism, and nomenclature naturally came quite early in the course, while the ones dealing with geometrical isomerism and asymmetric carbon were introduced after the dibasic and bydroxy acids had been reached. It was also found feasible to group several exercises, requiring the models, together so that the whole laboratory period was taken up by this type of experimentation. A total of four periods was given over to the work. The opinion of the instructional staff was unanimous that the innovation was highly successful. A number of these exercises are given in detail below. Every laboratory teacher will no doubt desire to delete, alter, or extend parts of the directions as given. It was found desirable to recede this work with a brief laboratory lecture during which the instructor in charge stated the purpose of the models, demonstrated their use, and outlined briefly the work to be covered. It was also found advantageous, particularly in the larger sections, to have a full st& of laboratory assistants on hand to facilitate the correcting, checking, and approval of the structures presented. Preliminary Exercise.
The Use of Structural Models
Prepare samples of the following, write their structural formulas in your notebook, and have your instructor approve the completed set of models. A. Combmations between two atoms.
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1. Carbon to Hydrogen. (C-H) (insert small wooden pegs in all holes so as to indicate unsatisfied bonds) 2. Carbon to Carbon (single bond) 3. Carbon to Carbon (two bonds, use springs) 4. Carbon to Carbon (three bonds, use springs) :i. Carbon to Halogen 6. Carbon to Oxygen (single bond) 7. Carbon to Oxygen (double bond) After you have obtained your instructor's approval on part A, part B should be prepared and approved. B. Combinations between three atoms. 1. Carbon to Carbon to Carbon 2. Carbon to Oxygen to Carbon 3. Carbon to Oxygen to Hydrogen 4. Oxygen to Carbon (double bond) to Oxygen (single bond)
n Exercise 1.
Homology and Isomerism
Erect as many models of the methane molecule as possible from your set. Place aside one of these models for reference. From a second methane molecule remove one of the hydrogen atoms. What is the name of the radical formed? What is its valence? 2. Using a third molecule of methane, replace one of the hydrogen atoms by the C H s group obtained above. What is the formula for the hydrocarbon which this new structure represents? What is its name? Are all the hydrogen atoms of the new hydrocarbon equivalent? Place aside this molecular model and from the remaining methane molecules assemble a second one like it. From the new model remove one of the hydrogen atoms. Fzcun~2.-BUTANEAND What is the name of the radical formed? I~OBUTANE What is its valence? Are all the hydrogen atoms of tbis radical equivalent? 3. Replace the hydrogen atom removedby amethyl group. What is the molecular formula of the hydrocarbon represented by the new structure? What is its name? Are all the hydrogens in this hydrocarbon equivalent? Place this structure aside and assemble a second one like it. Remove one of the hydrogen atoms from one of the end carbon atoms of this structure. What is the name of the radical formed? What is its valence? By removing the hydrogen atom from the middle carbon atom would a different radical have been obtained? 4. Unite tbis new radical with a methyl group. What is the molecular formula of the hydrocarbon represented by the new structure? What is its name? What is the difference in carbon and hydrogen content of the two hydrocarbons formulated in parts 1 and 2? In parts 2 and 3? In parts 3 and 4? A series of compounds having this constant difference 1.
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is called what kind of a series? m a t is the name and general formula for this series? Record the answers to each of the above questions in your notebook and tabulate the data indicated by the form given below: Numb" Carbon Aloms
Name of Xydrocarbon
Molccvlor Pormdo
Difference C ond H
MoC.
W1.
Siruciurnl Farmulo
A l k d Group Fa~mdo Nome
5. Erect two models of the propane molecule. On one of these models replace a hydrogen from one of the end carbon atoms by a chlorine atom. On the second propane model replace a hydrogen from the middle carbon atom by chlorine. Write the structural formulas of these two compounds. Also write their molecular formulas. Are the two compounds identical? Do they contain the same carbon skeleton? If they are not identical, in what respects do they differ? (Figure 3.) What term is used to describe this condition when it exists between two compounds? 6. In each of the two models for chloro substituted derivatives of propane obtained above, replace the chlorine atoms by methyl groups. Write the molecular formulas for the two compounds write their strut- F1cmg.3-T~~FoURISOMERIC BuTn tural formulas. Are these two ALCOHOLS compounds identical? Do they contain the same carbon skeleton? Does this condition resemble in any way that found between the two chloro propanes? In what respects do the two cases differ? Devise terms which will suitably describe these two forms of the phenomenon.
Exercise 2. Structure and Classification of Alcohols 1. Erect a model of a molecule of methane. Replace one of the hydrogen atoms by an OH group. Compounds bearing this relationship to a saturated hydrocarbon form what class of organic com~ o u n d s ? What is the characteristic endins for the names of the compounds of this class? 2. Set the model aside for reference and erect a second one like it. Replace one of the hydrogen atoms, attached to carbon, by a methyl group. What is the name of the compound represented by the new structure? Write its structural formula. It may be regarded as a derivative of what hydrocarbon? The hydroxyl derivative of methane
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is frequently called methanol. In the same system of nomenclature what would the new compound be called? In an older system of nomenclature hydroxy-methane is called carbinol. Considering the way in which the new structure was derived from carbinol what would the new compound he called in the same system? 3. Place this model aside and erect another one l i e it. Replace a second hydrogen atom of the carbinol groufi by a methyl group. What is the common name of the compound here represented? Write its structural formula? Of what hydrocarbon is it a derivative? What is its carbinol name? 4. Place this model aside and erect a second one like it. Replace the third and last hydrogen atom on the original carhinol residue by a methyl group. Write the structural formula of this new structure. What is the carbiuol name of the compound it I represents? 5 . We have before us now the models of four alcohols. Each contains a single hydroxyl group bound to carbon. Note how mauy hydrogen atoms are bound directly to this same carbon atom in each case. Disregarding the original carbimol (hydroxy methane) of which there is only one representative, how many classes of alcohols will we have, based on the - nature of the carbinol residue or eroun? Consult your text and give the typeforinula and FIGnRE 4.-1s0name for each of these classes. Tabulate the AND PRENE~ACETYLENE, data for each of the alcohols formulated above METAYLACPITYLENE as indicated in the foxm eiven below and have your instructor approve both your tablz and mGdels. Do these alcohols form an homologous series? If so what is the general formula? s
" m e -
a *
-
Numbn Carbon Atoms
Noms o Alcohof
Mdaculnr Parmulo
Diffnsncc C and H
Shvclurol Pamulo
Clorsificalior
Carbinol Nomr
6. By dismembering the above models set up the structures for the two isomeric butanes. How mauy different alcohols may be obtained by replacing a single hydrogen atom in normal butane by an hydroxyl group? Record the structural formulas, names, and classification of each. How many diierent alcohols may be derived from isobutane? Record the stmcturalformula, names, andclassification of each. What is the relationship existing between all of these alcohols? (See Figure 4.) Without using the models, work out the structural formulas, names, and classification of the alcohols in this series containing five carbon atoms.
Exercise 3. Oxidation Products of the Alcohols Let us suppose each of the alcohols formulated in the preceding exercise were submitted to oxidation and that a hydrogen atom (bound to
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carbon) of the carbinol group were changed to hydroxyl. This carbon atom would now be in combination with two hydroxyl groups. Long experience has shown that such compounds are very unstable, the two hydroxyl groups reacting to split off water, the remaining oxygen atom becoming united with the carbon atom by a double bond. Reassemble each of the alcohols indicated above on the models and make the transformations indicated. Record in your notebook the molecular formulas of both the alcohol and its oxidation product. How do they differ in composition? Compounds having the general formula R-C=O are called aldehydes and those having the general formula
I
H
R
>LO
R --
are called ketones. After your instructor has approved each
of the models for the oxidation products, tabulate the-data indicated by the following form in your notebook. Slrurlurol Formulo Alcohol
CIorr of Alcohol
SI~uclvrnl Formvlo of Oxidoiion
Praducl
Common Namc
Derioorion Name
Geneaa
Nome
What conclusions may one draw as to the oxidation products of pri. mary, secondary, and tertiary alcohols? Exercise 4. Organic, Nomenclature Many organic compounds can be named by more than one method. The details of the several methods are fully discussed in the text. The student should strive to become sufliciently familiar with each of these systems of nomenclature so that he can formulate the structure of the compound from its name or name the compound from its structure. The instructor will indicate a number of compounds in the following lists. The student will then erect a model of each of the compounds indicated. He should tabulate the name of the compound, its plane projection formula and its name according to other systems of nomenclature in his laboratory notebook, in the form given below, and report to his instructor for the approval of both the model and table.
After a given structure has been approved by the instructor the model may be dismembered and used in assembling the next compound in the list. The student should be prepared to define and to use correctly in his discussions with his instructor each of the following terms: bomologous series, isomerism, substitution, addition, alkyl group, saturated com-
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pound, unsaturated compound, paraffin, olefin, acetylene hydrocarbon, double bond, radical, substitution product, addition product, and carbinol group. HYDROCARBONS
1 Common Names n-pentane propylene propane isobutylene n-butane neopentane methane 2 Derivation Names metbylacetylene trimethylmethane ethylacetylene tetramethylethylene methylethylene sym-dimethylethylene methylmethane 3 Genwa Names ethane ethene ethine propane propene propine butane 2-methylbutane 2-metbylpentane 3-ethvloentane 3-meth$l-l-butene l,3-butadiene 1,4-pentadiene
acetylene isobutane ethylene ethane butylene isopentane isohexane ethylethylene dimethylmethane unsym-dimethylethylene tetramethylmethane dimethylacetylene dimethylethylmethane triethylmethane pentane l-pentene 2-pentene 2-bexene 2-pentine l-hexene 3Jlexene 3-methylhexane 2-methylpropane 3-methvlnentane 2;4-dimeth$lpentane 2-methyl-1,3-butadiene 2,3-dimetbyl-1,3-butadiene
HYDROCARBON RADICALS-ALKYL
ethyl n-propyl isobutyl isopropyl methyl isoamyl
GROWS
n-butyl n-amyl n-hexyl ethylene methylene ethylidene
HALOGEN SUBSTITUTION AND ADDITION PRODUCTS
trichloromethane ethyl bromide l-bromopropane 1,l-dichloroethane 1.2,3,4-tetracblorobutane
2-bromopropane 2,3-dibromobutane methyl chloride methylene chloride carbon tetrachloride
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dichloromethane 2-bromo-l-butene
ethylene bromide ethvlidene chloride
t&achloromethane chloroform isopropyl bromide
n-propyi iodide 1,Pdibromobutane 2-bromopropylene
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ALCOHOLS
1 Common Names methyl alcohol n-butyl alcohol isopropyl alcohol n-amyl alcohol see.-butyl alcohol 2 Carbinol Names carbinol ethyl carbinol dimethyl carbinol methylethyl carbinol methylisopropyl carbinol isobutyl carbinol 3 Geneva Names ethanol I -propano1 2-butanol 2-methyl-2-propanol .'3-hexanol :%methyl-l-pentan01 2-methyl-l-propanol 2-hexanol
ethyl alcohol n-propyl alcohol isobutyl alcohol isoamyl alcohol tert.-butyl alcohol methyl carbinol trimethyl carbinol ethyl-n-propyl carbinol diethyl carbinol see.-butyl carbinol kt.-butyl carbinol methanol 2-propanol l-butanol . 2,2-dimethyl-l-propanol 3-pentanol 2-ntkthyl-2-butanol 3-methyl-l-butanol l-hexanol
Discussion The preliminary exercise serves the useful purpose of making the student familiar with this new form of laboratory experimentation. In the exercise on homology and isomerism the student is able to build up for himself, unit by unit, the first four members of the paraffin series. He sees the structure of these hydrocarbons in their true perspective. It becomes immediately apparent to him that in such compounds carbon is united to carbon forming a continuous chain or frame-work of carbon atoms. He has by this structural process evolved for himself the true relationship between the members of a homologous series. In section 5 of this exercise he discovers that there are two monochloro derivatives of propane. They contain the same carbon skeleton and differ only in the position of the chlorine atoms. These two compounds are not identical, although they contain the same kinds of atoms and the same number of each kind, for the atoms are differently arranged. The compounds are isomeriefiosition isomers. In section 6 the idea of isomerism is extended.
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When the chlorine atoms are replaced by methyl groups the two new hydrocarbons still contain the same kmd, and the same number of each kind of atoms but they no longer contain the same carbon skeleton. The isomerism has projected itself more deeply into the molecular structure; the new compounds are nuclear isomers. The same evolutionary process is continued in exercise 2 in the development of the structure and classification of alcohols. Beginning with methanol, the student is led to develop systematically the structures for methyl carbinol, dimethyl carbinol, and trimethyl carbinol. By examining the molecular formulas for these compounds he finds that they form a homologous series. He finds, too, that these alcohols differ from the corresponding hydrocarbon in each case only in that a hydrogen atom has been replaced by a hydroxyl group. He may now be led to formulate correctly his own definition for the term alcohol. Incidentally, he also has discovered that alcohols may be classified into three classes or types depending upon the nature of the carbiuol residue or group. And he should be able to name these compounds in the carbinol system from their method of derivation. In section 6 of this exercise the student, by use of the models, is enabled to derive the structure of the four carbon atom alcohols from the correspondinghydrocarbons. Here heis required to use the iuformatiou previously gained, in the classificationand name of these derivatives. In exercise 3 a well-recognized mechanism is used in evolving the structure of the aldehyde and ketone by the oxidation of the primary and secondary alcohol respectively. By the application of the models to this oxidation process not only are the underlydg principles of an experimental method for Merentiating primary, secondary, and tertiary alcohols made clear but the student is also enabled to visualize wherein aldehydes and ketones are alike and wherein they differ in structure. The exercise in organic nomenclature is intended to supplement rather than replace the usual discussion of nomenclature in the text. The models furnish a convenient medium for training the student in the art of formulating a compound from its name or naming a compound from its structure. In a relatively short time the students become unusually proficient in the use of both the carbinol and the Geneva systems of nomenclature. Psychologists tell us some students are memory minded and some are motor minded. Some remember best what they read in books or hear in lectures. Others remember what they do with their own hands. Aside from the training which they acquire in the use of the scientific method perhaps it is to the latter element we may attribute the value of laboratory work as a pedagogical process. The student soon learns to visualize the invisible molecule in terms of the visible model. With the molecule made real the idea of a carbon skeleton made up of quadrivalent carbon atoms and the resultant phenomena of homology and isomerism fix themselves more firmly because they are better understood.
