Reaction between like molecules

The rate of reaction can be calculated in a similar way, making sure that each A molecule is not counted twice - each A molecule can be considered as both hitting and being hit by another A molecule. 

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Association was first thought of as a reversible reaction between like molecules that distinguished it from polymerization, which is not reversible. Association is characterized by reversibility or ease of disassocialion, low energy of formation (usually about 5 and not more than 10 kcal per mole), and the coordinate covalent bond which Lewis called the acid-basc bond. Association takes place between like and unlike species. The most common type of this phenomenon is hydrogen bonding. Association of like species is demonstrable by one or more of the several molecular weight methods. Association between unlike species is demonstrable by deviation of the system from Raoult s law. 

The order parameter can be defined in two different ways. It can be either a function of atomic coordinates or just a parameter in the Hamiltonian. Examples of both types of order parameters are given in Sect. 2.8.1 in Chap. 2 and illustrated in Fig. 2.5. This distinction is theoretically important. In the first case, the order parameter is, in effect, a generalized coordinate, the evolution of which can be described by Newton s equations of motion. For example, in an association reaction between two molecules, we may choose as order parameter the distance between the two molecules. Ideally, we often would like to consider a reaction coordinate which measures the progress of a reaction. However, in many cases this coordinate is difficult to define, usually because it cannot be defined analytically and its numerical calculation is time consuming. This reaction coordinate is therefore often approximated by simpler order parameters. 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

This reaction is obviously not an elementary one. It involves reaction between three molecules as well as breaking and forming of a multiple chemical bonds. More likely the reaction between a hydrogen molecule and an oxygen molecule could result in two hydroxyl... 

The energy that all cells (including ours) require to live comes from chemical reactions between covalent molecules like sugars and oxygen. When carbon compounds react with oxygen, for example, they give out energy and produce carbon dioxide and water. They also make, as a side reaction, a very small quantity of carbon monoxide. 

For results like those presented in Schemes 4.82-4.85, the routine, matter-of-fact style of regular publications seems almost inappropriate. Here we are dealing with an outstanding discovery. The previously unimaginable creation of a molecular vessel to perform chemical reactions between isolated molecules was finally achieved and thus brought unprecedented opportunities for the future development of organic chemistry. "... 

Reactions between organic molecules are numerous and have been studied largely for their importance in organic synthesis, which has allowed chemists to create new organic molecules for use as pesticides, therapeutic drugs, plastics, and the like. 

Both flavanol-anthocyanin pigments and colorless A(-0-)F have been detected in wines 11). Mass signals do not allow to distinguish between F-A+ and A+-F derivatives. However, dimeric anthocyanins consisting of one unit under flavylium cation and the other one under hydrated hemiketal form (A+-AOH) were characterized by mass spectrometry in wine like solutions (72). The existence of such structures arising from direct reactions between anthocyanin molecules confirms that they react both as nucleophiles and as electrophiles in wine. 

Similarly, chemical reactions may be used to convert a lipophilic chemical into a hydrophilic, water-seeking species. Solute mass transfer then occurs from the oily solvent phase into the aqueous phase. Pure fluids or mixtures of species that form hydrogen bonds or contain polar moieties are usually highly nonideal. In pure fluids, strong attractive interactions between like molecules may cause molecular aggregation through dipole-dipole and other interactions. In mixtures, specific interactions can occur between molecules of the same species (self-association) or between molecules of different species (solvation). 

Typical examples of case 2 with different heights of the activation barrier of the surface include H-bridge between like molecules, the transition region of a degenerate Sn2 reaction, pyramidal molecules like NHs isymmetrically substituted cis-trans isomers and — with a very high barrier — a pair of optically isomers. 

For a bimolecular reaction, the rate depends on the number of collisions in unit time in Section 30.5 it was shown that the number of collisions between like molecules is... 

Despite their limited set of functional groups, ribozymes can accelerate complex organic transformations like Diels-Alder reactions between small molecules in a way similar to protein enzymes or supramolecular catalysts featuring multiple turnover, substrate specificity and stereoselectivity. The three-dimensional sbucture shows striking similarities with proteins evolved for similar reactions, and the catalytic strategies used appear to be similar as well. 

Summing up, then, we And that under the conditions of excitation and perturbation prevailing in aqueous solutions the resonance interaction could not cause a speciflc attraction between like molecules and therefore could not be effective in bringing about auto-catalytic reactions. 

Because reactions can occur in steps, the order of a reaction is often less than corresponds to the stoichiometry. A reactive collision between three molecules is much less likely than one between two, and one between four molecules is exceedingly unlikely indeed it is doubtful whether elementary reactions between four molecules ever occur. A reaction whose stoichiometric equation involves more than three molecules always proceeds more rapidly by two or more processes of lower molecularity. 

Recent results suggest that carbon dioxide polymerization does not occur via intermediate states where molecules gradually distort as pressure increases, but is most likely caused by solid-state chemical reactions between CO2 molecules [59]. Such studies are important to gain better insight into transformations of light element-based molecules under high pressure and temperature and may open up new horizons in solid state chemistry under extreme conditions. 

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride ... 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

Frontier orbital theory also provides the basic framework for analysis of the effect that the symmetiy of orbitals has upon reactivity. One of the basic tenets of MO theory is that the symmetries of two orbitals must match to permit a strong interaction between them. This symmetry requirement, when used in the context of frontier orbital theory, can be a very powerful tool for predicting reactivity. As an example, let us examine the approach of an allyl cation and an ethylene molecule and ask whether the following reaction is likely to occur. 

Esters, like aldehydes and ketones, are weakly acidic. When an ester with an a- hydrogen is treated with 1 equivalent of a base such as sodium ethoxide, a reversible carbonyl condensation reaction occurs to yield a /3-keto ester. For example, ethyl acetate yields ethyl acetoacetate on base treatment. This reaction between two ester molecules is known as the Claisen condensation reaction. (We ll use ethyl esters, abbreviated "Et," for consistency, but other esters will also work.)... 

The preferred kinetic model for the metathesis of acyclic alkenes is a Langmuir type model, with a rate-determining reaction between two adsorbed (complexed) molecules. For the metathesis of cycloalkenes, the kinetic model of Calderon as depicted in Fig. 4 agrees well with the experimental results. A scheme involving carbene complexes (Fig. 5) is less likely, which is consistent with the conclusion drawn from mechanistic considerations (Section III). However, Calderon s model might also fit the experimental data in the case of acyclic alkenes. If, for instance, the concentration of the dialkene complex is independent of the concentration of free alkene, the reaction will be first order with respect to the alkene. This has in fact been observed (Section IV.C.2) but, within certain limits, a first-order relationship can also be obtained from many hyperbolic models. Moreover, it seems unreasonable to assume that one single kinetic model could represent the experimental results of all systems under consideration. Clearly, further experimental work is needed to arrive at more definite conclusions. Especially, it is necessary to investigate whether conclusions derived for a particular system are valid for all catalyst systems. 

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