Bernhard Wunderlich Rensselaer Polytechnic Institute Troy, New York 12181
The Place of Macromolecules in Freshman Chemistry
During 1971-72 a pilot group of about 100 freshmen, randomly chosen from our incoming class of about 900, was taught a first chemistry course which among other objectives tested the inclusion of knowledge on macromolecules to a degree approaching its importance. This report was written to point out where this knowledge fits into a first course in chemistry. It turned out that many of the subject matters became considerably easier to teach by taking the broader outlook possible by dropping the restriction to small molecules. The student response was tested by asking after completion of the course for the importance of the topics (see Table 1 below) for general education and for their chosen field of interest (engineers, scientists and premedical students). For general educational reasons the most important topics were thought to be (in order): organic chemistry; polymer chemistry and biochemistry; atoms, chemical thermodynamics, and inorganic chemistry. For their chosen field they thought that (in order) chemical thermodynamics; states of matter; organic chemistry, bonding, polymer chemistry and biochemistry were most important. Before looking at the details of the course content it may be useful to delineate the small molecules from macromolecules. Obviously, there is no sharp dividing line between large and small. In the case of linear macromolecules, it has become customary to use Staudinger's condition that a macromolecule has more than 1500 atoms ( I ) . At this level an increase in molecular length by one more repeating unit of, let us say, 5-50 atoms does not have much influence on the physical properties of the molecule. Similar sizes expressed in number of atoms may be useful for nonlinear macromolecules, although molecular properties are more difficult to assess. The Chemistry of Large and Small Molecules To treat a large subject such as "Chemistry" one has two avenues, the historical development or the synthetic development as one sees i t today. Although the historic development is of great interest since it shows the enormous struggle of the human mind to understand nature, the synthetic development is more efficient since with it one can eliminate many often erroneous and/or difficult avenues. The course usually followed a t Rensselaer is thus the synthetic development with historical information included only as side-issues ( 2 ) . The course consists of 60 lectures of 50-min duration given in two semesters. During the first semester the basic theory of chemistry is treated, while during the second semester the lectures deal with the application of chemistry. The revised lecture schedule based on the experience of the pilot section is shown in Table 1. Quite clearly, most topics do not reveal special emphasis on macromolecules. The chemistry taught through the macromolecule is dispersed throughout the course and brought to a focus in the four lectures on polymer chemistry. The details are putlined below. The Microscopic and Macroscopic Description of Matter The first discussion of macromolecules is connected with the treatment of bonding (lectures 6-10, Table 1). The unique position of covalent bonding in being direc736
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Journal of Chemical Education
tional leads to the possibility of formation of 0-, I-, 2- and 3-dimensional complete bonding (see Table 2). Ionic and metallic bonds, in contrast, are non-directional and can only be completed when connected to other atoms in all Table 1. Sixty Lectures of Freshman Chemistry. Semester 1. TheTheoryof Chemistry Part 1: The Microscopicond M n c r n c o p ~Deserip&"nolMafI~r ~ Atoms 1. The Atoms and Light 2. Tho Nuclei and Eleelrons 9. e he Structure of Hydrogen 4. Tho Other ElemenW 5. The Periodic Tshie Bonding 6 . The Ionic Bond 7. ~ h Covalent o Bond 8. The PoiarBond 9. Tho Metallic Bond 10. The WeakBonds Statesof Matter 11. ThsMicmscopicPictum 12. The Maomcopb Pictwe 13. The Ideal Gas 14. The Real Gas andLiwid 15. The Solid Stsfe part*: Thermody~mlcsand Kinefiii Chemical Thsrmodv".,mic8
U4a"ic ChPrnbtry 4.The Elements COHNS 41. TheScructureofCarhonCompounds 42. The Functional Groups 43. The Organic Reactiansl 4 4 The organic Reactionnn 45. The Organic Reactiansof Harm fo the Environment
Bimhm~btv 50. The Proteins, Csrhohydrafea, m d Nuclaic Acids 51. TheBiological Reactions I 52. TheBiological Reactions 11 53. The Cycles ofBiologieal Matter 54. The Chemical OriginafLifo 55. ThesynthetieBioehemistry Part 5: Nuclear Chsmlstv 56. The Nuclear Chemistry 57. TheRadioCh~misfry 58. TheRadiationChemiary 59. Thelndusirial Applications M. General Review. Summary. Conclusion, end Outlaok
three directions of space. It is obvious that most atoms bonded strongly must be described as macromolecules. The linear macromolecules form the special connection between the molecules with zero-dimensionally extended bonds, such as all gases and most common molecules treated in organic chemistry, and the molecules with three-dimensionally extended bonds such as most salts, minerals, and metals. Because of the flexibility of linear macromolecules due to rotation around covalent bonds, the liquid or rubbery state can often he reached without losine the identitv of the molecules. All two-and threedimensional macromolecules break up on melting, which lareelv limits their chemical multiolicitv. There are more lines; macromolecules than mo1ec;les of any other type. In fact, there are more different linear macromolecules possible than could ever be made of all the atoms capable of covalent bonding present in the known universe. One can easily think of thousands of re'peating units which can be linked linearly to make linear macromolecules of the homonolvmer . , tvne. .. Mixine these within a sinele molecule in the mrm oicopolymer.; according to the many possihle sequence distributions leads to thr exolosive multinlicitv. . . &stricting ourselves to a rather smali protein molecule of only 100 repeating units, which according to the above definition is barely a macromolecule, and allowing only the 20 naturally occurring amino acids to make up the molecule, leads to as many as 20100 different moleculesl. This multiplicity lies a t the root of the variation found in bioloeical svstems. and it will also form the base of the future d&elopment of chemistry. The states of matter (lectures 11-15, Table 1) are similarly closely dependent on the size of the molecules. Gases must be small molecules. Liquids increase enormously in viscosity as their molecular weight increases. Solid glasses will form depending on molecular size (and shape) and intermolecular forces. The understanding of the crystalline solid state is closely linked to the understanding of macromolecules. Again, the linear macromolecule brings the connection between the "molecular solids" and the metals, oxides, and salts. The general principle of crystallography involves the achievement of dense packing through a maximum coordination number. The simplest group of crystals to understand are those which can be described as a close packing of spheres. Crystals of noble gases and approximately spherical small molecules like CHa and C10H16 (adamantane) as well as metals have coordination numbers of 12. Ions bring the first additional principle which overrides close packing, namely the saturation of opposite charges. This can never he achieved with coordination number 12. Depending on size ratio of the oppositely charged ions, the coordination number drops to 8, 6, 4, and finally to 3. The lower limits of compounds with these coordination numbers can be estimated from the radii for which cation-anion and anion-anion or cation-cation contact is reached simultaneously (double repulsion, ion radius ratios 0.732. 0.414. 0.240. and 0.155 for the above coordination numbers, ~espectively) (3). Irregularly shaped molecules still pack closely with coordination number 12; the possible crystal structures, however, have much lower symmetry (space groups Pi, B 1 , B1/c, Pca21, Pna21, and B12121 only for no symmetry (4)). Another restriction occurs with linear macromolecules which have mostly a distinct low energy conformation which may be a planar zig-zag conformation or a helix. Packing of helices allows, however, at best a coordination number 4, since, for close packing, left-handed and right-handed helices must pack side by side for intermeshing. Indeed, a check of all
-
'This number is truly large beyond comparison. Assuming a different molecule of this type could be made every 10-lo see, the time it takes for a single molecular vibration; one could only camplete 1.6 X loz7molecules in the time span of the age of the earth (5 x lo9 yr = 1.6 X 10" sec).
Table 2. Types of Molecules
often gaseous
Oz.Hx0, CHI. CsH.,
or liquid liquid, rubbery
CsHis iCHx-)=, l S c l s (CHR-CO--NH-I,
"0,.
Ionie
directional
Metallic
directional
LP.NaCI, A d
"0".
Fe. Al. Na
3
'Note that weak bonds of the dispersion type are alway. present in all three direction8 tortheso~idificationof~iqvids and gaPesst3ufficient~y lowtempor.ofapsee turn. Dipole interaction and hydrogen bond8 need to be considered when applicable aa parbitis11y and fully directionel weak bonds. respeetiw1y.
Table 3. Change of Entropy of an ldeal Gas and an ldeal Rubber Molecule at Constant Temperaturea
d E = TdS - p d V
I
i e o m p ~ i o n l lsfrotehingl
pdv =0 dE = TdS + Id1
P = TldSldV = RTIV
p
*E = Energy. T = temperature. p = pressure. V = volume, i = force, i = length, and r = and-@-end distancp.
known crystallizing macromolecules has shown that zigzag chains crystallize with coordination number 6, while all helices of sufficient depth crystallize with coordination number 3 if the helix symmetry is compatible with trigonal symmetry, or coordination number 4 if not. Exceptions are only some shallow helices like S , Se, and polypropylene (5). A full understanding of crystals can thus be reached by progressing from simple close packing considerations to overriding considerations of ion interactions and low energy molecular conformations. The remaining twoand three-dimensionally strongly bonded covalent structures must obey bond geometry for crystallization. Thermodynamics and Kinetics Thermodynamics (lectures 16-25, Table 1) deals with the macrosco~icdescriotion of matter. The key functions introduced are energy ienthalpy), entropy, and free energy. Little difficulty is usually experienced with the energy function and the first law of thermodynamics. Teaching the meaning of entropy, on the other hand, is one of the more difficult tasks. The macromolecule as such can help considerably in this connection in clarifying the concepts. An ideal macromolecule can be treated aloneside an ideal gas. Schematically the processes of gas compression and exnansion of a rubber molecule are shown in Table 3. The greater order (smaller entropy) in the two final states is obvious, and the driving force to greater disorder in opposite directions (expansion for the gas, contraction for the rubber) is a simple matter of experiment. The accompanying flow of heat (dS = 6Q/T) can be checked easily by extending and contracting a rubber band between one's lins. Bv carmine out the nrocess fast. equilibrium is lost and a temperature differential indicates heat flow. On extension, the rubber band heats (6Q negative, since f and T are always positive, d S is negativi as i s expected from the equation in Table 3); on contraction, it cools ( 6 6 positive). Rubber elasticity is a simple application of the secVolume 50, Number 11. November 1973 / 737
ond law of thermodynamics easily~. experimentally verified qualitatively without equipment. The shape of a flexible molecule governs the behavior of macromol&ules, distinguishing them from energy elastic metal springs. The effect of temperature increase can also be derived from such discussion. In the same way as increasing temperature increases the pressure of a gas, it must increase the retracting force of a rubber molecule. The kinetics lectures (lectures 26-30) can benefit from the incorporation of the polymerization reaction in the discussion of chain reactions. The major benefit is the actual proof of the chain reaction and length of the chain by analysis of the finished molecule. A typical reaction scheme is shown in Table 4. It is capable of many variations, and the results are immediately obvious for the produced polymer. The initiation reaction as shown in Table 4 would lead to a rate of initiation
ecules. The second semester goes into the application of chemistry and has thus a broad choice of topics. The ones selected as a result of the pilot section a t Rensselaer are described next. Inorganic Chemistry Inorganic chemistry (lectures 31-39, Table 1) has for a long time focused on the small molecules or ions into which matter can he broken down under appropriate conditions. Even solids are usually not looked upon as the macromolecules they actually are. The most common linear macromolecules of inorganic nature are found as allotropic modifications of sulfur and selenium. Particularly sulfur allows an easy demonstration of the increase in viscosity on polymerization and the resulting rubber elasticity on quenching which allows bypassing of the depolymerization to Ss-rings x S,(liquid, yellow)
where [I] is the initiator concentration R-R. The factor f accounts for the deactivation of some once formed free radicals due to recombination (usually 0-5070). It is, however, also possible to have a constant number of initiator molecules starting a t time zero (particularly common for anionic initiation). The rate of propagation is where [MI is the monomer concentration and [M,.] the concentration of all free radical species. The termination agaid may have several alternatives. Case (a) is called combination, case (b) disproportionation, and case (c) chain transfer. I t is also possible (for anionic polymerization) to have no termination. For disproportionation the rate of termination is R,
=
2k,[~;j'
=
-d[Milldt
(3)
Steady state is reached when Rt = R,. During this period of time the free radical concentration is practically constant [Mil
=
(fk,[Illk,)"Z
(4)
(5)
The chain length varies thus even under steady state conditions with concentration and can be checked by discussion of molecular weight distributions. In the first semester it was thus possible to show that the basis of understanding of chemistry must naturally include small and large molecules. For more advanced courses additional material could be the detailed treatment of rotational isomerism leading to low energy helices ( 5 ) , the effect of molecular shape on the chemical potentials ( 6 ) . and the liquid and glassy structure of macromol-
Sa,(viscous liquid, brown)
Carbon Chemistry The lectures on carbon chemistry are the key area of display of knowledge on macromolecules. The treatment of oreanic chemistrv. (lectures 40-45. Table 1) can easilv . be aGauged to include topics like nbmenclatire, description of carbon skeletons, functional groups, and reactions, all information needed for the discussion of topics dealing with macromolecules. The nomenclature most easily adopted for macromolecules is the naming according to the monomer. The macromolecule should be called poly(name of monomer)
R-R
R+ (21 h p ~ g p ~ g f i 0 "
R--CH2-CHX
3
CHz=CHX-
2R' R-CHs-CHX'
+ CH1=CHX-
X = H, polyefhylem: X = CI, poly(vinylchlor1dd:X = CsHs, polystyrene: 'Rmuld be for example C s H s C H a o that R-Ris benzoyl proride.
0
738
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Journal of Chemical Education
R+CH*--CHX-KHz-CHX
etc.
(7)
as for example in polyethylene, poly(viny1 chloride), and poly(ethy1ene terephthalate). The treating of repeating units, structural, geometrical, and optical isomers, and also copolymers broadens the concept of organic chemistry to include the newer fields of polymer chemistry and biochemistry without running the risk of the student not making the connection between the fields. The detailed lectures on polymer chemistry (lectures 4G49, Table 1) are thus only an extension of the organic chemistry lectures in the area of reactions and structure. The polymerization reactions treated include: (1) the addition reaction already used for the discussion of reaction
Table 4. Chain Reaction of a Vinyl Monomer to a Macromoleculea (lllnitrotion'
quenched
Selenium also polymerizes similarly and behaves quite analogous to organic macromolecules. A selection of inorganic linear macromolecules of two molecules showing varying degrees of chain flexibility are SOa, SeOz, Asz03, SiSz, and PtCI2. Several phosphates, silicates, and related compounds are additional examples of linear macromolecules. Overall, it can easily he demonstrated that there are many linear macromolecules among the inorganic substances (7). Many of them are, however, only poorly understood.
The chain length is given under steady state conditions and the conditions of termination (b) by v = RJR, = k,2[MlZ/(2k,R,)
160° -+
kinetics (Tahle 4); (2) the step reaction which illustrates the techniques necessary to reach high molecular weight through reactions which go to a high degree of completion; and (3) the matrix reactions which bridge polymer chemistry to organic chemistry and biochemistry. The lecture on amorphous molecules includes a detailed analysis of the random flight problem and its use to model polymer chains. Molecular weight distributions, glasses, and the connection of the rubber elasticity of a single molecule (Tahle 3) to a network are also included here. The lecture on crystalline polymers deals with the problem of how to order a random assembly of strings. Chain folding, fringed micelles, and amorphous portions are the key results of this topic. The influence on application of plastics and the need to relearn for designers are stressed. The final lecture, as several others in previous sections, is geared to the current questions of interest to society. The examples are almost unlimited. A few suggestions are that trashhags solve the collection problem, hut hinder the decomposition if the hag does not decompose itself. Glass bottles may be recycled, hut plastic hottles may take less energy for their one-way production than recycling of glass bottles. Using oil reserves to make plastic implements which can be burned after use to give energy may he better than burning oil directly, hut needs coordination. All these and many more present day problems are linked to the most intricate cycles of changes of matter on earth which must be understood and directed wisely to gain the most benefit for mankind. Any isolated discussion is apt to he incomplete and fail. The section on biochemistry (lectures 50-55, Tahle 1) follows the information on large molecules derived before. The macromolecules are shown to he the basic materials for life. Polycarhohydrates and fibrous proteins make up structural materials, and globular proteins and nucleic acids make up the molecules governing biological reactions. The discussion of the chemical and ~ h v s i c a lstructure of all these molecules follows from the &V~OUS chapters. The new aspects are the multitude of co~olvmers and the coupling of detailed structure with chekich activity. The details of enzymatic hydrolysis of a polycarbohydrate (the action of lysozyme) is shown to he no more than adsorption on a well-constructed active macromolecule, followed by a S N reaction ~ in a well-coordinated sequence, and, finally, desorption of the products. The other example treated is the svuthesis of uroteins bv a matrix reaction, an important -reaction in biological systems which has not found its equivalence in making of smaller molecules or even synthetic macromolecules. The biological cycle treated is the assimilation of CO2 which has biological as well as environmental ramifications, since the part of the cycle outside the biological system can be influenced by man. The final two lectures in this sequence combine a global history and outlook of chemistry. The stages of develop-
ment are divided into inorganic evolution (loss of Hz and other light molecules and accumulation of COz, N2, HzO, and CH4 by outgassing), organic evolution (formation of polymerizahle monomers), polymeric evolution (polymerization to inactive macromolecules), biologic evolution (random evolution of macromolecules to life and finally man), and the step to he taken in the future which one might call designed evolution (the further development based on the design by man). Nuclear Chemistry
The final set of lectures in the freshman course (lectures 56-59, Table 1) deals with nuclear chemistry. Even in this topic frequent reference to macromolecules is necessary to bring the discussion of chemistry up to date. Radiation and radio chemistry played a vital part in evolution. The application to medical and materials prohlems are many. Conclusions The above brief review of a pilot program on freshman chemistry emphasizing macromolecules should show that it is possible to include this material in the regular course. Obviously, when the total number of lectures is kept constant any new material included requires cutting in other areas. A comparison with other courses presently given is possible through Tahle 1. The aim of this pilot was to show that only by inclusion of macromolecular science in freshman chemistry (and later in the other subjects of the undergraduate curriculum) will it be possible to keep education in chemistry up to date. In this way chemistry can remain of key importance for all professions dealing with chemistrv and serve as a general educational course as well. separating polymer science into an individual course does a disservice to all students not suecializin~in this field, and lets chemistry as a subject slip to lowerlmportance. General References Billmeyer. F. W. Jr., "Textbook of Polymer Seimee." (2nd o d i Wiley-lnter8cience. NewYork, 1371. Flory, P. J.. "Principles of Polymei Seienee." Cornell Univernify Press, Ithaca, N.Y.. 1952. Lenz. R. W.. "Organic Chemistry of Synthetic High Polymers."~Wiley-lnferscipnce, Uow York. 1967. Mark. H. F.. Gaylard. N. G., and Rikales. N. M. (Edironl. "Eneydopsodia of Polymer York. 1961-1911. Scien~eandTeehnology,"Wiley~lnUr~eien~e.Inc..New
Literature Cited I11 Staudinger. H..andFrifschi, J.. H d u . Chim. A r l o . 5.7R811922). (21 Basrett. L. G.. Bunce. S. C., Carter. A. E. Clark. H. M.. and Hollinger. H. B.. "Principlesof Chemisfry."PrenticeHall.Engl~uoodClifls.N.J.. 1996. 13) Pauling. L.. "The Nature of the Chsmicsl Bond," (3rd ed.). Cornell University Press. Ithaea, N.Y.. 1990. (41 Kitaiprdrkii. A. I.. "Oiganicheskaya Krisfsllokhimiya," Preps of the Aead. Sci.. Moreor, 1955. Rovired Eng1i.h Lrsmilstion. Conrultants Bureau. New York. N.Y.. 1961. (51 Wunderlich. B.. ''Macromoleeular Physics. Vol I.." Academic Press. New York. 1971. (6) Mnrawelz. H.. "Macromolecules in Solution." Wiley-Interscience, New York, 196%. London, 1968. 17) Gimblen. R.G.R.,"lnorganicPoiymerCheminlry."Buffewonh~,
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