Conjugated it systems

An extension of the above method was developed for conjugated it-systems Partial Equalization of Pi-Electronegativity (PEPE)47,48). After calculation of the charge distribution in the a-skeleton, the various resonance structures of a it-system are generated. The 7t-charge distribution is obtained by assigning weights to these... 

Aromaticity was found to be a general property of many (but not all) cyclic, conjugated jt systems. Moreover, it was found diat aromaticity in molecules can be predicted by Huckel s rule. The structural requirements implicit in Huckel s rule are that there be An + 2 (n is an integer) n electrons in a cyclic, conjugated n system. Obviously benzene, which has six jt electrons (An + 2, n = 1) in a conjugated it system, is aromatic. However, Huckel s rule predicts that molecules such as cyclodecapentaene An + 2 = 10 (n = 2) and [18]-annulene An + 2 = 18 (n = A) should be aromatic, have equal bond lengths, and be planar—and they are. 

Finally, when eliminations which give conjugated systems are possible, they are favored significandy by the greater stability of the conjugated it system. 

Which of the following groups, when bonded to the terminal carbon of a conjugated it system, probably affects the value of Xmax the least ... 

Depending on the donor atoms, these can be designated N4, N202, N2S2, and so on. The heterocycles can be broadly classed into those without and those with conjugated it systems the latter are discussed later. 

A cycloaddition is a one-step ring-forming reaction between two conjugated it systems in which two new a bonds are formed, joining the two reagents at each end. The mechanism has one step with no intermediates, and all the arrows start on it bonds and go round in a ring. 

This problem involves the addition of HBr across a conjugated it system. We therefore expect the mechanism to exhibit two steps (1) proton transfer and (2) nucleophilic attack. In order to predict the products properly, we must explore the mechanism and identify the carbocation that is formed. In other words, we must carefully consider the regiochemistry of the protonation step. In this case, there are four distinct locations where the proton can be placed. 

In the previous section, we saw that conjugated it systems will undergo 1,2-addition as well as 1,4-addition. The exact ratio of products is highly dependent on the temperature at which the... 

In the previous section, we noted that a compound containing a conjugated it system will absorb UV-Vis light. The region of the molecule responsible for the absorption (the conjugated tt system) is called the chromophore, while the groups attached to the chromophore are called auxochromes. 

Other electrophiles, such as Br2, will also add across conjugated It systems to produce 1,2- and 1,4-adducts. 

In this chapter, we expand our discussion of conjugated it systems and explore compounds that exhibit conjugated it bonds enclosed in a ring. For example, benzene and its derivatives belong to a class of compounds called arenes. 

Annulenes are compounds consisting of a single ring containing a fully conjugated it system. Due to steric hindrance, [lOjannulene and [14]annulene do not meet the first criterion and are nonaromatic. 

The equilibrium concentrations of the diketone and the enol depend on the solvent that is used, but the enol generally dominates. Two factors contribute to the remarkable stability of the enol in this case (1) The enol has a conjugated it system, which is a stabilizing factor (see Section 17.2), and (2) the enol can form an intramolecular H-bonding interactions between the hydroxyl proton and the nearby carbonyl group (shown with a gray dotted line above). Both of these factors serve to stabilize the enol. 

In this case, the aldol addition product cannot be isolated. Even at moderate temperatures, only the condensation product is obtained, because the condensation reaction involves formation of a highly conjugated it system. Even in cases where the aldol addition product can be isolated (by performing the reaction at a low temperature), the yields for condensation reactions are often much greater than the yields for addition reaction. The following example iUusttates this point ... 

When the reaction is performed at low temperature, the aldol addition product is obtained, but the yield is very poor. As explained earlier, the starting material is a ketone, and the equilibrium does not favor formation of the aldol addition product. However, when the reaction is performed at an elevated temperature, the aldol condensation product is obtained in very good yield, because the equilibrium is driven by formation of a conjugated it system. 

This type of reaction is called a conjugate addition, or a 1,4-addition, because the nucleophile and the proton have added across the ends of a conjugated it system. Conjugate addition reactions were first discussed in Section 17.4 when we explored the reactivity of conjugated dienes. A major difference between conjugate addition across a diene and conjugate addition across an enone is that the latter produces an enol as the product, and the enol rapidly tautomerizes to form a carbonyl group. 

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