March 15, 2018

Download PDF by Muriel Tomlinson: An Introduction to the Chemistry of Benzenoid Compounds

By Muriel Tomlinson

ISBN-10: 0080156592

ISBN-13: 9780080156590

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A true compound, capable in principle of isolation, it lies in an energy trough at W: transition states, on the other hand, exist at energy maxima, ΤΛ and T29 and do not represent even potentially isolable species. Not much is known about the precise geometry or energy of the transition states so the graph is merely to show the various energy troughs and peaks and has no quantitative significance. Figure 5 represents a substitution in which the slower, and therefore ratecontrolling, stage of the process is the formation of the intermediate 25 BENZENE A N D ITS HOMOLOGUES Potential energy qft+XY Reactants Products Reaction co-ordinate FIG.

2 moments are called positive when the positive end of the dipole is remote from the hydrocarbon residue : those in the opposite direction are called negative (Sutton, 1955). -substitutions predominate. DISUBSTITUTION IN BENZENOID COMPOUNDS 37 When the group attracts the π-electrons from the benzene ring (mArX -raAlkX) is negative and substitution occurs mainly in the ra-position. FURTHER CONSIDERATION OF THE MECHANISM OF AROMATIC SUBSTITUTION Substitution into a benzene nucleus which already carries a sub­ stituent must proceed by a path related to that already described for benzene itself, but the presence of the substituent will clearly affect the stability of the transition states and the Wheland intermediate, and from what has been said above, it is obvious that the effect of a substituent will not be the same at all positions on the nucleus.

O or o~ o cr ô ό-ό-ό-ό-ό ^ Ν ^ V N ^ V N ^ V N '^ ^Ν^ ^ Ν ^ + (6) (6a) (6b) (6c) (6d) (6e) positive charges: a clue to the reduced reactivity of such nuclei to electrophilic reagents emerges. DIPOLE MOMENTS OF ARYL-X COMPARED WITH ALKYL-X Mesomeric and inductive effects produce dipole moments. In tertiary butyl compounds no mesomeric effect is possible and so by measuring the difference between the dipole moments of a benzenoid compound, Ar-X, and a tertiary butyl compound, Alk-X, Sutton (1931) concluded that it was possible to estimate the degree of meso­ meric interaction between an aromatic nucleus and a substituent group attached to it.

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An Introduction to the Chemistry of Benzenoid Compounds by Muriel Tomlinson


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