Anthracene resonance structures

Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the synthesis of dyes. The molecular formula of acridine is C13H9N, and its ring system is analogous to that of anthracene except that one CH group has been replaced by N. The two most stable resonance structures of acridine are equivalent to each other, and both contain a pyridine-like structural unit. Write a structural formula for acridine. 

There are four resonance structures for anthracene, one of which is shown. Draw the other three. 

Anthracene has the formula Cl4Hln. It is similar to benzene but has 3 six-membered rings that share common C—C bonds, as shown below. Complete the structure by drawing in multiple bonds to satisfy the octet rule at each carbon atom. Resonance structures are possible. Draw as many as you can find. 

Like phenanthrene, some PAHs have a unique Clar structure, whereas several alternative Clar structures are possible for other PAHs [21]. Thus, Clar s rule does not designate the resonance structure mainly responsible for the aromaticity of anthracene. Clar s model cannot differentiate between its outer and inner ring (Scheme 28.2). 

The configurations of the molecules are those expected for the resonating structures. Through resonance each bond acquires some doublebond character, which causes the adjacent bonds to strive to be co-planar. The molecules are thus brought into completely planar configurations, with 120° bond angles. This has been verified for naphthalene and anthracene and many larger aromatic hydrocarbons by careful x-ray studies. 

Show the three additional resonance structures for anthracene. Discuss whether the experimental bond lengths shown in the following structure are in accord with predictions based on these resonance structures ... 

The resonance theory says that if two compounds have the same set of atoms and bonds, then that compound for which more resonance structures can be drawn will be the more stable. Using this rule, which would be more stable, anthracene, phenanthrene, or benzazulene ... 

Phenanthrene is best represented as a hybrid of the five canonical forms 20-24. It has a resonance energy of 380 kJ mol and is more stable than anthracene. In four of the five resonance structures, the 9,10-bond is double and its length is about the same as an alkenic C=C bond. The numbering system for phenanthrene is shown in 20. Five different mono-substituted products are possible. 

Problem 17.12 Draw the four resonance structures for anthracene. 

There are four possible resonance structures one can draw for anthracene (the atoms have been numbered to aid you in seeing the differences between them) ... 

Bromine adds to the central ring of anthracene to give a 1,4-addition product. Write the structure of the product that would be formed if addition took place on one of the outer rings. By writing resonance structures for the product shown here and the one formed by addition to the outer ring, can you suggest why addition to the central ring is preferred ... 

The method just described is more difficult to apply to larger molecules. For naphthalene the HMO method leads to a 10 x 10 determinant because ten p orbitals are mixed to make the MO. For the resonance method, however, the size of the determinant is limited only by our willingness to include resonance structures. In general, there are many more resonance structures that should be considered than there are p orbitals, and a reasonable resonance treatment of naphthalene would require a 42 x 42 determinant. For anthracene and phenanthrene, the HMO determinant would be 14 x 14, but the resonance method would require solution of a 429 x 429 determinant in each case. Consideration of symmetry can reduce the phenanthrene... 

This procedure may be illustrated by the example of anthracene. The graph of the molecule shown on the left of Figure 4.69 depicts the cr skeleton of cyclotetradecaheptaene, where each vertex represents a carbon atom with a p orbital, Two resonance structures can be written for this structure, so the SC for this fragment is 2 ... 

Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures. 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives 56, 57, 104, 105, 106, 107, 108, 109. Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures 59, 60, 61, 62, 63, 64, 65, 85, 104, 106, 107, 108,109,110,113,114,118. 

By comparison, the radical species AH aheady exists in a non-aromatic 13 re form the resonance structure of two separated 6jre systems (Fig. 7.12) and a localised radical on an sp carbon is also relatively stabilised and therefore should predominate. The addition of a further electron to this relatively localised orbital does not significantly alter the aromaticity of the molecule or the general geometry and is therefore much less thermodynamically disfavoured than anthracene reduction, such that its formal potential is more positive. 

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