Conjugation is the term used to describe an arrangement of chemical bonds in which two double bonds are separated by one single bond. Chemical structures with this configuration are very stable and can show unusual behavior during the course of a chemical reactions. This effect results from the ability of the electrons in conjugated bonds to delocalize, or spread their effect throughout the molecule. Two important examples of a conjugated system are isoprene and butadiene, both of which are important components used in the manufacture of synthetic rubber.
In molecules with conjugated bonds, saturated bonds (carbon atoms with four attached atoms) alternate with unsaturated bonds (carbon atoms with less then four attached atoms.) This bond arrangement is possible between atoms whose p-orbital electons are capable of interacting. (The term "p-orbital" describes how the electrons orbit the nucleus of the atom.) For example in 1,3-butadiene (Figure 1), the unsaturated bonds can interact or overlap with one another. This interaction is maximized when the electron's bonds overlap in a single plane. This overlap results in stabilization of the system and lowers the reactivity of the molecule. For this reason, conjugated double bonds in 1,3-butadiene are less reactive than the isolated one in 1-butene. Not only is the conjugated system less reactive, but the chemical transformations it undergoes are somewhat different than nonconjugated systems.
Conjugated systems also interact with light waves in a characteristic manner. The overlap of the p orbitals results in a narrowing of the energy gap between the highest energy filled electronic orbital and lowest unfilled electronic orbital. Consequently, light absorption occurs at lower energy in conjugated systems than in nonconjugated systems. This absorption corresponds to electronic excitation from the highest filled molecular orbital to lowest unfilled orbital. The more conjugated p-bonds present in a molecule, the lower the energy of the lowest energy transition. When eight double bonds are in conjugation, the molecule absorbs visible light and is colored.
Conjugation can also occur between other p-systems such as triple bonds between carbon atoms as well as bonds between carbon and other atoms. It is important to note that overlap can occur between like or unlike p-systems and it is not restricted to organic systems (i.e., molecules containing carbon atoms.) When conjugation occurs in planar monocyclic carbon rings, the total number of p-electrons fits the formula 4n+2 (where n = a nonnegative integer), and conjugated orbitals form an uninterrupted cyclic array, the system has a special stability called aromaticity. Aromatic systems are an important topic in their own right and have a dramatically different chemical reactivity than nonaromatic systems. A representative aromatic species is benzene (Figure 2). Benzene is planar and has an uninterrupted cyclic array of three conjugated double bonds arranged in a monocyclic six membered ring. The stability of benzene is even greater than nonaromatic conjugated species, which have 6 p-electrons. An example of a similar nonaromatic system is 1,3,5-hexatriene. The chemical reactivity of benzene is also rather different than that of 1,3,5-hexatriene.
While conjugation is most common for systems with alternating single bonds and unsaturated bonds, it can occur whenever the p-bonded systems have a spatial arrangement that allows the orbitals to overlap. Thus conjugation can occur in molecules in which the unsaturated sites are close in space but are separated by more than a single covalent bond.
Unsaturated systems are also able to overlap or conjugate with unfilled or partially filled orbitals. For example, the allyl cation has a cationic (i.e., positively charged) center conjugated with a double bond and the species is more stable than nonconjugated cations (Figure 3). Likewise, a radical center can be conjugated with a p-system resulting in a specially stabilized radical such as the allyl radical.
While conjugation is most often associated with p systems, other bond orbitals can also exhibit this phenomenon. For instance, electrons in the s orbital can be conjugated with p- orbitals when in the proper spatial relationship. For carbon systems, this is known as hyperconjugation. A typical form is the overlap or conjugation of the electrons of a s-bond with an adjacent radical or cationic center (Figure 4). The more of these interactions there are, the more stable the radical or cation. Hyperconjugation also accounts for why the more alkyl substituents an alkene bears, the more stable it is upon comparison to similar less substituted alkenes. The stability associated with hyperconjugation is generally not as great as that associated with normal conjugation.
Another form of conjugation is exhibited by heavy group 14 elements such as silicon, germanium, and lead. When linked together in a chain (see Figure 4) the atoms in these atoms can form delocalized systems through conjugation of neighboring s-bonds. This arrangement is known as s-conjugation. One consequence of s- conjugation is that certain species of silicon and germanium strongly absorb energy in the near ultraviolet range. These absorption bands correspond to electronic transitions from s-bonding to s-antibonding orbitals. Bicyclic tin compounds are known that also exhibit s- conjugation; as a consequence, some of these tin compounds are even colored.
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