Benzene equivalent hydrogen atoms

Hi) Isomer number According to molecular orbital theory, since all the six carbon atoms of benzene are completely equivalent, hydrogen atoms attached to these are also equivalent. Hence benzene forms only one mono - substituted and three disubstituted products. 

There are two sets of equivalent hydrogen atoms those located at the two equivalent tertiary carbon atoms and those located at the four equivalent vinyl carbon atoms. Thus, two monobromo derivatives would result from replacing a hydrogen atom at either of these two sites. However, only one monobromobenzene exists. Therefore, the bicychc structure for benzene cannot be correct. 

Furthennore, only one monobromination product of benzene was ever obtained, which suggests that all the hydrogen atoms of benzene are equivalent. Substitution of one hydrogen by bromine gives the sane product as substitution of any of the other hydrogens. 

This view was given further support by the discovery of "tautomerism," that is, that some compounds behave as if they have two different structures simultaneously. Peter Laar suggested in 1886 that this can best be explained as the result of continual oscillation of a hydrogen atom between two positions within a single molecule, 109 a hypothesis influenced by Kekule s suggestion that the peculiarities of benzene are the result of an oscillation of atoms in benzene and that "equivalence" or valence is "the relative number of contacts which occur in a unit of time between atoms." 110... 

The < vo-methylene structure of the 6-methylenedibenzo[<7,g][l,3,6]trithiocine 11 was determined by X-ray crystallographic analysis. Any axis, plane, or point of symmetry does not exist in the molecule in the crystalline state, although the two benzene rings and two < vo-methylene hydrogen atoms are equivalent in solution as is evident from the H NMR spectrum <1999T10057>. 

We will illustrate this procedure by considering the bonding in benzene, an important industrial chemical that must be handled carefully because it is a known carcinogen. The benzene molecule (C6H6) consists of a planar hexagon of carbon atoms with one hydrogen atom bound to each carbon atom, as shown in Fig. 14.48(a). In the molecule all six C—C bonds are known to be equivalent. To explain this fact, the LE model must invoke resonance [see Fig. 14.48(b)]. 

Assume the five hydrogen atoms in the benzene ring are equivalent with no spin-spin coupling. 

In 1867, Dewar proposed several possible structures for benzene, one of which was 5. However, in 1874, Ladenburg proved experimentally that all the hydrogen atoms of benzene were equivalent and suggested the prismatic structure 6. 

Symmetry of Benzene.—If then benzene is acarbo-cyclic compound, as is so strongly indicated by its relationship to cyclo-hexane, the structural formula should express first of all this equivalence or likeness of the hydrogen atoms. In any geometric representation of such a condition we would naturally indicate it by a symmetrical figure. 

Benzene yields only one monosubstitution product C HsY. Only one bromobenzene, C HsBr, is obtained when one hydrogen atom is replaced by bromine similarly, only one chlorobenzene, C6H5CI, or one nitrobenzene, QH5NO2, etc., has ever been made. This fact places a severe limitation on the structure of benzene each hydrogen must be exactly equivalent to every other hydrogen, since the replacement of any one of them yields the same product. 

There is formed not as in the case of benzene, in which the six hydrogen atoms are equivalent, a single sulphonic add, but a mixture of two isomeric sulphonic adds ... 

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