Benzene free valency

Fig, H8. (a) Partial rale factors of free radical phenylation, relative to benzene (397). (b) Free valence calculated by HMO method (117). (c) Radical localization energy (in units) calculated by HMO method (117). 

Radicals derived from monocyclic substituted aromatic hydrocarbons and having the free valence at a ring atom (numbered 1) are named phenyl (for benzene as parent, since benzyl is used for the radical C5H5CH2—), cumenyl, mesityl, tolyl, and xylyl. All other radicals are named as substituted phenyl radicals. For radicals having a single free valence in the side chain, these trivial names are retained ... 

The name phenylene o-, m-, or p-) is retained for the radical —C5H4—. Bivalent radicals formed from substituted benzene derivatives and having the free valences at ring atoms are named as substituted phenylene radicals, with the carbon atoms having the free valences being numbered 1,2-, 1,3-, or 1,4-, as appropriate. 

It is however in the reactions of substituted aromatic molecules that patterns emerge which clearly indicate the relevance of the free valence in radical reactions. The benzene derivatives provide a good example. 

Orientation effects in benzene derivatives operate in two ways. If the substituent is inductive there are large first order charge displacements at the ortho and para positions, and these can be estimated approximately using the atom polarizabilities (which is very small at the meta position). The changes of bond order, however, and consequently of free valence, vanish in first order and hence depend on Sa. The charge g g at position s therefore increases or decreases from the value unity in the... 

Equation (39) therefore remains unchanged since the term in a cancels. An exactly similar argument holds for nucleophilic substitution we therefore conclude that (1) the relative reactivities of different hydrocarbons, and the relative reactivities of different positions in the same hydrocarbon, should be the same for substitution by reagents of all kinds, and (2) there should be in each case a linear relation between log k and The first relationship holds qualitatively in all cases except where steric effects are involved thus naphthalene is more reactive than benzene to reagents of all three types (e.g. nitric acid, phenyl radicals, sodamide), and in each case the a-position in naphthalene is more reactive than the / the corresponding free valence numbers are ... 

Figure 4 - Pauling bond orders and free valences for benzene, naphthalene, and trimethylene-methyl.

The principal assumption was that nitration of aromatic compounds in the medium under consideration depends on the saturation of the free valences of the sulphuric acid with nitrous acid and water. To make use of the most N204 the ratio of free sulphuric acid to the amount of water (n-1) (m+1), should by the end of the reaction be not less than a certain minimum value, characteristic for the compound being nitrated. For benzene the value is 4 1, for chlorobenzene 5 1, for toluene 1.8 1. 

Partial rate factors relative to benzene for free radical phenylation Free valence calculated by HMO method ... 

In addition one can also speak of a free valence (index) or indice de liaison fibre of 0.07 for each corner of the benzene molecule since the para-para bond in the DEWAR-configurations is not a real bond but rather represents two free electrons (with opposite spins) at opposite corners. 

Table III gives some values of k jkB measured (where kB is the constant for benzene) and the values of the free valence for atoms at the position of reaction.

Fig. 8. Relation between free valence values and reaction speed on CC1. 1, Benzene 2, diphenyl 3, phenanthrene 4, naphthalene 5, chrysene, 6, pyrene 7, stilbene, 8, 1,2—5,6-dibenzantbracene 9, anthracene 10, naphthacene 11, 1,2-benzauthracene.

Free valence can be used for reactions which involve substituted hydrocarbons. Fierens, Hannaert, Van Rysselberge, and Martin35 have studied the exchange of chlorine by iodine in the chloro-methylated derivatives of aromatic hydrocarbons using potassium iodide in acetone solution. This is an SN2 reaction. Figure 10 gives the reaction path which is assumed for a benzene derivative. 

In going from benzene to pentacene the largest sum of the free valence of a pair of atoms in a para position increases as does the ease of the diene synthesis in this series. It is well known that maleic anhydride hardly reacts with benzene or naphthalene. A reaction takes place with anthracene and even more readily with naphthacene and pentacene.12... 

Table 4-4 illustrates some other examples of free valences, at the atoms indicated, in various molecules. It is quite clear that these values are consistent with chemical experience. Benzene is stable, with a small free-valence, ethylene is more reactive benzyl is a radical and hence able to react very freely, and the quinodimethane species behaves almost like a diradical. This type of analysis is therefore qualitatively sensible and has the further merit of being a semi-quantitative measure of reactivity. For some reactions, it is possible to obtain a very close correlation between the rate at which the reaction takes place and the free valence of particular centres in the reactants. 

The dependence of the chemical shifts on the 7t-electron charge density in aromatic molecules has frequently been observed. Karplus and Pople showed that the magnetic shielding (p.p.m. from benzene) of a nucleus (A) in a conjugated molecule is a function of the local charge density, Pa, the free valence, Fa, and the polarity of the C—H bonds, Ah, and is given by the expression—... 

Free valence of aromatic cations of nitro-benzene 96... 

Tf it is assumed that the six carbon atoms, in consequence of the attraction of the free valencies, so rotate about the axes depicted by the sides of the hexagon that their direction is always in the plane of the ring, the six points of attack (Angriffspunkte) lie inside the ring in completely symmetrical positions, and can there so reciprocally paralyse one another that they are ordinarily not effective (fiir gewohnlich nicht zur Geltung kommen), which is identical with the expression the carbon in benzene is tervalent. ... 

The continuous reduction in size of a solid finally leads to a situation where the original solid state properties can be only partially observed or may be even completely lost, as these properties are exclusively the result of the cooperation between an infinite number of building blocks. Further reduction of size finally leads to typical molecular behavior. On the other hand, even here are structural relations to the bulk occasionally detectable. For instance, the arrangements of the sp hybridized carbon atoms in cyclohexane or in adamantane can easily be traced back to the diamond lattice, whereas benzene or phenanthrene represent derivatives of the graphite lattice. However, neither cyclohexane, benzene, nor phenanthrene have chemical properties which are comparable with those of the carbon modifications they originate from. The existence of the above mentioned Q, C]o or Ci4 units is otUy made possible by the saturation of the free valencies by hydrogen atoms. Comparable well known examples for other elements are numerous, for instance the elements boron, silicon, and phosphorous. Figure 1-1 illustrates some of the relations between elementary and molecular structures. 

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