Similar molecules

Consider first two substances which have very similar molecules. He, hydrogen fluoride and HCl. hydrogen chloride the first is a Weak acid in water, the second is a strong acid. To see the reason consider the enthalpy changes involved when each substance in water dissociates to form an acid ... 

Structurally similar molecules are expected to exhibit similar physical properties or, similar biological activities. ... 

Polymerisation involves the chemical combination of a number of identical or similar molecules to form a complex molecule. The resulting polymer has a high molecular weight. The term synthetic polymer is usually employed to denote these compounds of very high molecular weight. 

Molecular mechanics depends on the concept of atom types and parameters associated with these atom types. Since the number of atom types is very large for the universe of possible molecules, parameters will probably be missing for a random new molecule unless a force field has been developed for molecules similar to the new molecule. Molecular mechanics predicts how the new molecule will behave based upon the behavior of known, similar molecules. 

Extra.ctlon, The advantage of extraction is that a Hquid is purified rather than a vapor, allowing operation at lower temperatures and the removal of a series of similar molecules at the same time, even though these molecules differ widely in boiling point. An example is the extraction of aromatics from hydrocarbon streams (see Extraction, liquid—liquid). 

For a polar surface and molecules with permanent dipole moments, attraction is strong, as for water adsorption on a hydrophilic adsorbent. Similarly, for a polar surface, a molecule with a permanent quadrupole moment vidll be attracted more strongly than a similar molecule with a weaker moment for example, nitrogen is adsorbed more strongly than oxygen on zeolites (Sherman and Yon, gen. refs.). 

In addition polymerisation a simple, low molecular weight molecule, referred to in this context as a monomer, which possesses a double bond, is induced to break the double bond and the resulting free valences are able to join up to other similar molecules. For example poly(vinyl chloride) is produced by the double bonds of vinyl chloride molecules opening up and linking together (Figure 2.1). 

A chemical will be a solvent for another material if the molecules of the two materials are compatible, i.e. they can co-exist on the molecular scale and there is no tendency to separate. This statement does not indicate the speed at which solution may take place since this will depend on additional considerations such as the molecular size of the potential solvent and the temperature. Molecules of two different species will be able to co-exist if the force of attraction between different molecules is not less than the forces of attraction between two like molecules of either species. If the average force of attraction between dissimilar molecules A and B is and that between similar molecules of type B Fbb and between similar molecules of type A F a then for compatibility Fab - bb and AB - P/KA- This is shown schematically in Figure 5.5 (a). 

The three most common types of inhibitors in enzymatic reactions are competitive, non-competitive, and uncompetitive. Competitive inliibition occurs when tlie substrate and inhibitor have similar molecules that compete for the identical site on the enzyme. Non-competitive inhibition results in enzymes containing at least two different types of sites. The inhibitor attaches to only one type of site and the substrate only to the other. Uncompetitive inhibition occurs when the inhibitor deactivates the enzyme substrate complex. The effect of an inhibitor is determined by measuring the enzyme velocity at various... 

Double-bond polymerisation In this case the double bond in a small molecule opens up and allows it to join to another similar molecule, e.g. with ethylene ... 

The length of the axial bond would be expected on all theories to be important. The barrier height does decline from ethane to methyl silane to methyl germane, but of course the bonded atoms are different. Unfortunately reliable values are not available for dimethyl mercury, dimethyl acetylene, and similar molecules with still longer bonds. An apparent exception is provided by methyl mercaptan and methyl alcohol. The latter, with the shorter axial bond, has the lower barrier. 

Similarly, examples of barriers arising largely from simple steric hindrance can be found, as for instance in the hindered diphenyls.35 On the other hand there are many arguments suggesting that this is not the important force in ethane and similar molecules. It would be difficult to understand the relatively slow fall in barrier from ethane to methyl silane to methyl germane on a van der Waals repulsion basis. Furthermore, the small effect of substituting F, Cl, or Br on one end would also seem mysterious. The equilibrium orientation in propylene is opposite to the predictions of one of the quantitative van der Waals theories. Finally, the apparently small effect of bending back the C—H bonds is not in accord with either the electrostatic or van der Waals pictures. 

BOPs and charges should be used only in a relative sense, among groups of similar molecules and, especially, using the same MO method and the same basis set. 

Beta-carotene is used in foods to provide color (margarine would look as white as vegetable shortening without it). Another similar molecule, annatto, is used in cheeses. Another famous carotenoid dye, saffron, is used to color rice and other foods. 

Similar molecules can be made using other fatty acids, such as the shorter-chained lauric acid. 

With regard to molecular morphology, whether it is A- or B-amylose, there is no room for a water or similar molecule to enter the cavity in the interior of the... 

A solid will dissolve in a liquid if there is mutual compatibility this depends on the relative magnitudes of three forces of interaction. For a polymer, p, and a solvent, s, the forces of attraction between the similar molecules are and respectively, while the force of attraction between the dissimilar molecules is... 

In most cases with more than two chiral centers, the number of isomers can be calculated from the formula 2", where n is the number of chiral centers, although in some cases the actual number is less than this, owing to meso forms.An interesting case is that of 2,3,4-pentanetriol (or any similar molecule). The middle carbon is not asymmetric when the 2- and 4-carbon atoms are both (/ ) [or both (5)] but is asymmetric when one of them is (/ ) and the other (5). Such a carbon is called a pseudoasymmetric carbon. In these cases, there are four isomers two meso forms and one dl pair. The student should satisfy himself or herself, remembering the rules... 

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