Electron delocalization conjugated dienes

Section 10 7 Conjugated dienes are stabilized by electron delocalization to the extent of 12-16 kJ/mol (3 kcal/mol) Overlap of the p orbitals of four adja cent sp hybridized carbons in a conjugated diene gives an extended tt system through which the electrons are delocalized... 

Conjugated diene (Section 10 5) System of the type C=C—C=C in which two pairs of doubly bonded carbons are joined by a single bond The tt electrons are delocalized over the unit of four consecutive sp hybridized carbons Connectivity (Section 1 6) Order in which a molecule s atoms are connected Synonymous with constitution Constitution (Section 1 6) Order of atomic connections that defines a molecule... 

In part from the additional delocalization of the n electrons that occurs in conjugated dienes. 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

Chemical separation of conjugated dienes and other polyunsaturated hydrocarbons is based on the availability of tt delocalized electrons. The use of a strong dienophile (e.g. tetracyanoethylene, TCNE) will derivatize only conjugated dienes, thus separating the polyunsaturated compounds into two groups. However, such derivatization is not always reversible since a retro-Diels-Alder reaction may require a high temperature. Hence, the retrieved compounds may be the thermostable ones and not those present in the initially analysed mixture. 

Roth and coworkers23 reported NMR data of the orthogonal butadiene (Z,Z)-3,4-dimethylhexa-2,4-diene. (Z,Z)-13 having the planes of the double bonds at a dihedral angle not far from 90°. This diene serves as the model for conjugated diene lacking rr-electron delocalization and for the transition state for interconversion of antiperiplanar (trans) and synperiplanar (cis or gauche) butadiene. 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

Isoprene can be polymerized in the laboratory by a radical chain mechanism. As shown in the following equations, the odd electron of the initially produced radical is delocalized onto both C-2 and C-4 by resonance. Either of these carbons may add to another isoprene monomer to continue the chain reaction. If C-2 adds, the process is called 1,2-addition if C-4 adds, the process is called 1,4-addition. (This is similar to the addition of electrophiles to conjugated dienes discussed in Section 11.13 and the addition of nucleophiles to a,/8-unsaturated carbonyl compounds described in Section 18.10.)... 

Why is a conjugated diene more stable than an isolated diene Because a conjugated diene has overlapping p orbitals on four adjacent atoms, its 71 electrons are delocalized over four atoms. This delocalization, which cannot occur in an isolated diene, is illustrated by drawing resonance structures. 

Conjugated diene (Section 16.1 A) A compound that contains two carbon-carbon double bonds joined by a single G bond. Pi (jt) electrons are delocalized over both double bonds. Conjugated dienes are also called 1,3-dienes. 

In the next three sections we shall see how two different factors have been invoked to account for the relative stabilities of conjugated dienes, and of simple alkenes as well (a) delocalization of tt electrons, and (b) strengthening of a bonds through changes in hybridization of carbon. 

In a similar way, the unusual stability of conjugated dienes is attributed, not to delocalization of the n electrons, but to the fact that sp"-sp- hybridization makes the C2 -C3 bond short (1.48 A) and strong. 

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.2). Interaction between the ir electrons of the C=C bond and the ir electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows a partial delocalization of the it electrons over all four atomic centers (Figure 23.3, p. 944). 

Dienes have two or more carbon-carbon double bonds which may be either isolated (R -C=C-R-C=C-R ), cumulated (R-C=C=C-R), or conjugated (R-C=C-C=C-R). Each state represents a different stability due to electron delocalization, and the conjugated form is generally favored in organic molecules. 

Conjugated dienes are particularly stable due to the delocalization of the pi electrons along sp hybridized orbitals, and they also tend to undergo reactions atypical of double bond chemistry. For instance, chlorine can add to 1,3-butadiene (CH2=CH-CH=CH2) to yield a mixture of 3,4-dichloro-1-butene (CICH2-CI 1C1-( H=CH ) and 2,4-dichloro-2-butene (C1CH2-CH=CH-CH2C1). These are known as 1,2 addition and 1,4 addition, respectively. 1,2-addition is favored in mild reaction (irreversible) conditions (the kinetically preferred product) and 1,4-addition is favored in harsher reaction (reversible) conditions (which results in the themiodynamically preferred product). 

---