Difunctional molecules

Before we start with a systematic discussion of the syntheses of difunctional molecules, we have to point out a formal difficulty. A carbonmultiple bond is, of course, considered as one functional group. With these groups, however, it is not clear, which of the two carbon atoms has to be named as the functional one. A 1,3-diene, for example, could be considered as a 1,2-, 1,3-, or 1,4-difunctional compound. An a, -unsaturated ketone has a 1.2- as well as a 1,3-difunctional structure. We adhere to useful, although arbitrary conventions. Dienes and polyenes are separated out as a special case. a, -Unsaturated alcohols, ketones, etc. are considered as 1,3-difunctional. We call a carbon compound 1,2-difunctional only, if two neighbouring carbon atoms bear hetero atoms. 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

From the above discussion it should be obvious that antithetical analysis of difunctional molecules simply is a reversal of the synthesis scheme already described in chapter 1. 

Synthesis of large heterocycles usually involves condensation reactions of two difunctional molecules. Such molecules tend to polymerize. So far two special techniques have been described above to avoid this important side-reaaion , namely high dilution and use of templates. The general procedure to avoid polymerizations in reactions between difunctional molecules is, of course, the application of protecting groups as described in sections 4.1.2 and 2.6. 

The parameter r continues to measure the ratio of the number of A and B groups the factor 2 enters since the monofunctional reagent has the same effect on the degree of polymerization as a difunctional molecule with two B groups and, hence, is doubly effective compared to the latter. With this modification taken into account, Eq. (5.40) enables us to quantitatively evaluate the effect of stoichiometric imbalance or monofunctional reagents, whether these are intentionally introduced to regulate or whether they arise from impurities or side reactions. 

Step-growth polymers, such as polyamides and polyesters, are prepared by reactions between difunctional molecules. Polyamides (nylons) are formed by reaction between a diacid and a diamine polyesters are formed from a diacid and a diol. 

Table I contains the results, some of which have been derived previously (11, 12). These equations are appropriate for the case of equal reactivity of both ends of a difunctional molecule and allow for unequal rate constants for the A-B and A-C reactions. These results are presented here in terms of reaction probabilities, p, (the probability that reactant I has reacted with reactant J) where I,J = A, B or C, and p j = Pj - These should be distinguished from the sequential probabilities of...

Ketones are oxidatively cleaved by Cr(VI) or Mn(VII) reagents. The reaction is sometimes of utility in the synthesis of difunctional molecules by ring cleavage. The mechanism for both reagents is believed to involve an enol intermediate.206 A study involving both kinetic data and quantitative product studies has permitted a fairly complete description of the Cr(VI) oxidation of benzyl phenyl ketone.207 The products include both oxidative-cleavage products and benzil, 7, which results from oxidation a to the carbonyl. In addition, the dimeric product 8, which is suggestive of radical intermediates, is formed under some conditions. 

The same effect has been observed for the monoselective protection/modifica-tion of equivalently difunctional molecules [28]. The binding is only temporary for the duration of the reaction but permits the other pendant functional group to react in preference. An example of this was shown by Leznoff for the preparation of monoethers of symmetrical dihydroxy aromatic compounds [29]. Under conventional conditions such reactions are problematic, providing mixtures of mono and... 

Pranger, L., Goldstein, A. and Tannenbaum, R. (2005) Competitive self-assembly of symmetrical, difunctional molecules on ambient copper surfaces. Langmuir, 21, 5396-404. 

Remember that difunctional molecules give a linear polymer. Trifunctional molecules yield a cross-linked polymer. 

Effects of Curing Agent Type. Epoxide-Cured Propellant. Carboxyl-terminated polybutadiene is a linear, difunctional molecule that requires the use of a polyfunctional crosslinker to achieve a gel. The crosslinkers used in most epoxide-cured propellants are summarized in Table IV and consist of Epon X-801, ERLA-0510, or Epotuf. DER-332, a high-purity diepoxide that exhibits a minimum of side reactions in the presence of the ammonium perchlorate oxidizer, can be used to provide chain extension for further modification of the mechanical properties. A typical study to adjust and optimize the crosslinker level and compensate for side reactions and achieve the best balance of uniaxial tensile properties for a CTPB propellant is shown in Table V. These results are characteristic of epoxide-cured propellants at this solids level and show the effects of curing agent type and plasticizer level on the mechanical properties of propellants. 

Polymers are large molecules formed when many smaller monomers bond together. Alkene polymers such as polyethylene result from the polymerization of simple alkenes. Nylons and polyesters result from the sequential reaction of two difunctional molecules. 

Tonic ketals (6), more strictly acetal cations, can be formed in the gas phase8 by reaction of acylium ions R—C=0 with diols or other difunctional molecules HO(CH2) CH2X (n = 1-3, X = OH, OMe, NH2). Identified by MS, the method has applications in the detection of functional groups that give rise to acylium ions, or in the protection or elimination of such ions. 

Based on the classical definitions given in organic chemistry, the step-growth polymerization process can involve either condensation steps or addition steps. The former proceeds with elimination of by-products while the latter takes place without elimination of by-products. This is illustrated by Eqs (2.1) and (2.2), for the particular case of difunctional molecules ... 

Again, copolymers can likewise be synthesized by using prepolymers and another organic difunctional molecule (eq. 8).33... 

These studies of reduction of benzenoid aromatics reveal that the solvent, the electrolyte cation, the current density and the water content are all important variables. In general it is important to have a rather negative potential (large TAA+) and a proton source (water) present under conditions where hydrogen evolution or attack on the solvent does not occur. Under such conditions difunctional molecules can be selectively reduced by control over the number of Faradays/mole which are passed. This kind of predictable selectivity should give the electrochemical method real advantage over alkali metal reductions and the possibility to use materials other than liquid ammonia and alkali metal is quite attractive. 

B-Aminoborazines are of particular interest for fundamental studies. In these compounds, boron is bonded to three nitrogen atoms with two different types of environment. B-Aminoborazines are also useful precursors for the synthesis of thermally stable polymers. Quite a few polycondensates of aminoborazines and copolymerisates with organic difunctional molecules have been described 4>. Of major interest are difunctional borazines yielding linear polycondensates. The condensation of l,3,5-tris(2,6-dimethylphenyl)-2,4-dichloroborazine (cf. Section II.2.5) with aliphatic, aromatic, and heterocyclic diamines, as well as the preparation of the same linear polyborazines by transamination of 1,3,5-tris(2,6-dimethylphenyl)2,4-bis(diethyl-amino)borazine with diamines was studied 139). 

Special Difunctional Molecules Obtained by Direct Synthesis. . . 144... 

We have said several times that monomers are usually difunctional molecules. Molecules that undergo step-growth polymerization are clearly difunctional—they contain two functional groups, either the same or different. When we think of chain-growth polymerization, however the first compounds that come to mind are those containing one carbon-carbon double... 

Condensation polymers result from formation of ester or amide linkages between difunctional molecules. Condensation polymerization usually proceeds by step-growth polymerization, in which any two monomer molecules may react to form a dimer, and dimers may condense to give tetramers, and so on. Each condensation is an individual step in the growth of the polymer, and there is no chain reaction. Many kinds of condensation polymers are known. We discuss the four most common types polyamides, polyesters, polycarbonates, and polyurethanes. 

Step-growth polymers, the second inajon class of polymers, are prepared by re actiona between difunctional molecules the individual bonds in the polymer an formed independently of one another Ri/ycorbo a(e)i are formed from a diestor and a diet, and f yuftthanes are forraad lirom a diiao-cyanate and a diol. 

The formation of type II aziridines can also be carried out via the formation of two bonds simultaneously. One of the most common is the formation of two C-N bonds (bonds c and e ) to generate the aziridine ring. This quite often takes the form of a nitrene or nitrene equivalent adding to an alkene. Another highly common route that forms two bonds simultaneously is the reaction of a monocyclic azirine with a difunctional molecule such as a diene or dipole to form bonds d and e . A significantly less common route is the formation of bonds c and b through the addition of carbene or carbene equivalent to a cyclic imine. 

Another approach is to start with two difunctional molecules. 

An excess (5 1 mole ratio) of ethyl diazoacetate is used in these reactions to suppress cyclobutadimerization or Diels-Alder cyclodimerization. In difunctional molecules which have non-equivalent ionizable functionalities, cyclopropanation is highly selective for the more easily oxidized functionality. The latter selectivity is perhaps the most attractive aspect of the reaction. In contrast to transition metal (e.g. rhodium) catalyzed cyclopropanations, cation radical additions to electron deficient alkene moieties do not occur at all. The reaction is relatively sensitive to... 

In this case zinc happens to do a good job. Although this particular method works well only for the preparation of cyclopropane, it illustrates an important principle the carrying out of what is normally an intermolecular (between-molecules) reaction under such circumstances that it becomes an intramolecular (within-a-molecule) reaction. As we can see, it involves tying together the ends of a difunctional molecule. 

Step g wth polymers, the second major class of polymers, are pre pared by reactions between difunctional molecules the individual bunds in the polymer are formed independently of one another. Polycarbonates are formed from a diester and a diol, and polyurethanes are formed from a diisocyanate and a dio). 

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