Overlapping p orbitals

Conjugation (Chapter 14 introduction) A series of overlapping p orbitals, usually in alternating single and multiple bonds. For example, 1,3-butadiene is a conjugated diene, 3-buten-2-one is a conjugated enone, and benzene is a cyclic conjugated triene. 

FIGURE 1.5 Overlapping p orbitals form a Jt and iC orbital. The orbitals are shown in (a). They are still there in the states represented in (b), but have been removed from the picture for clarity. 

This is very different from the case with single bonds, which are freely rotating aU of the time. But a double bond is the result of overlapping p orbitals, and double bonds cannot freely rotate at room temperature (if you had trouble with this concept when you first learned it, you should review the bonding structure of a double bond in your textbook or notes). So there are two ways to arrange the atoms in space cis and trans. If you compare which atoms are connected to each other in each of the two possibilities, yon will notice that all of the atoms are connected in the same order. The difference is how they are connected in 3D space. This is why they are called stereoisomers (this type of isomerism stems from a difference of orientation in space— stereo ). 

Notice that the tt system in methyl methacrylate is nearly the same as that in butadiene. The only difference is the replacement of one C atom by an O atom. Both systems have four overlapping p orbitals, giving rise to two bonding MOs and two antibonding MOs, so it is reasonable that the tt systems are quite similar. 

Removal of one of the methylene protons generates a carbanionic center, but the corresponding single Lewis formula is a poor description of the electronic structure. More mesomeric forms of 2A may be written to give a more adequate fomu-lation. Alternatively, a circle may be drawn to symbolize the 3c2e n bond in 2A, resulting from overlapping p orbitals perpendicular to the plane of atoms involved. 

The six overlapping p orbitals overlap to form a set of six p molecular orbitals. Six tt electrons are completely delocalized around the ring, and... 

Figure 21-16 Calculated 77-molecular-orbital energies for N overlapping p orbitals in Mobius cyclic n systems...

To get an accurate picture for the formate anion, it is necessary to use delocalized MOs that involve all three of the overlapping p orbitals (one from each O and one from C). These three AOs overlap to form three pi MOs.There are four electrons (shown in the Lewis structures in parts 0 and as the pi electrons and one unshared pair on oxygen) in these three MOs.This drawing does not attempt to show the shapes of these three MOs, only the orbitals that overlap to form them. More on their energies and shapes will be presented in later chapters. 

To explain this reaction, consider the fact that, due to the overlapping p orbitals, double bonds are electron rich. This property allows olefins, under certain conditions, to act as nucleophiles. In the case of a double bond reacting with molecular bromine, the result is formation of a three-membered ring containing a positively charged bromine atom. This three-membered ring is known as a bridged bromonium ion. Concurrent to formation... 

All four carbon atoms of buta-1,3-diene are sp2 hybridized, and (in the planar conformation) they all have overlapping p orbitals. Let s review how we constructed the pi molecular orbitals (MOs) of ethylene from the p atomic orbitals of the two carbon atoms (Figurel5-3). Each p orbital consists of two lobes, with opposite phases of the wave function in the two lobes. The plus and minus signs used in drawing these orbitals indicate the phase of the wave function, not electrical charges. To minimize confusion, we will... 

The six overlapping p orbitals create a cyclic system of molecular orbitals. Cyclic systems of molecular orbitals differ from linear systems such as buta-1,3-diene and the allyl system. A two-dimensional cyclic system requires two-dimensional MOs, with the possibility of two distinct MOs having the same energy. We can still follow the same principles in developing a molecular orbital representation for benzene, however. 

Cyclobutadiene meets the first three criteria for a continuous ring of overlapping p orbitals, but delocalization of the pi electrons increases the electronic energy. Cyclobutadiene is less stable than its open-chain counterpart (buta-1,3-diene), and it is antiaromatic. 

Benzene is [6]annulene, cyclic, with a continuous ring of overlapping p orbitals. There are six pi electrons in benzene (three double bonds in the classical structure), so it is a (41V+2) system, with N = 1. Hiickel s rule predicts benzene to be aromatic. 

Cyclooctatetraene would be antiaromatic if Htickel s rule applied, so the conjugation of its double bonds is energetically unfavorable. Remember that Htickel s rule applies to a compound only if there is a continuous ring of overlapping p orbitals, usually in a planar system. Cyclooctatetraene is more flexible than cyclobutadiene, and it assumes a nonplanar tub conformation that avoids most of the overlap between adjacent pi bonds. Hiickel s rule simply does not apply. 

Huckel s rule is commonly used to determine aromaticity and antiaromaticity. A continuous, planar ring of overlapping p orbitals is required for the rule to apply. Otherwise, the system is. nonaromatic. 

A cyclic molecule or ion that has a continuous ring of overlapping p orbitals will be... 

Neither aromatic nor antiaromatic lacking the continuous ring of overlapping p orbitals required for aromaticity or antiaromaticity, (p. 722)... 

This arrangement accounts for the extra stability or aromaticity of benzene. The six overlapping p orbitals can be pictured as forming a delocalized 7i-electron cloud comprising of two rings (think of them as doughnuts ), one above and one below the molecular plane as shown in Figure 1.4. There are no localized C=C bonds as there are in alkenes. 

Chapter 16 is the first of three chapters that discuss the chemistiy of conjugated molecules—molecules with overlapping p orbitals on three or more adjacent atoms. Chapter 16 focuses mainly on acyclic conjugated compounds, whereas Chapters 17 and 18 discuss the chemistiy of benzene and related compounds that have a p orbital on every atom in a ring. 

Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms. As a result, the jc electrons are not localized between the carbon atoms of the double bonds, but rather delcx alized over four atoms. This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present. This shortens the bond. Drawing resonance structures illustrates this delcx alization. 

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. 

The selection rules can be applied to charged species as well as to neutral molecules. The only requirement is that the reaction be a concerted process involving electrons in overlapping p orbitals. For example, the conversion of a cyclopropyl cation to the allyl cation can be considered as a tt -electrocyclic process. For this process, the selection rules predict a disrotatory process. 

Figure 6.5. Ally radical. The p orbital of the middle carbon overlaps p orbitals on both sides to permit delocalization of electrons.

Pyrrole, furan, and thiophene give electrophilic substitution products because they re aromatic (Section 15.7). Each has six -ir electrons in a cyclic conjugated system of overlapping p orbitals. Taking pyrrole as an example, each of the four carbon atoms of pyrrole contributes one n electron, and the sp -hybridized nitrogen atom contributes two (its lone pair). The six ir electrons occupy p orbitals, with lobes above and below the plane of the ring, as shown in Figure 28.1. Overlap of the five p orbitals forms aromatic molecular orbitals just as in benzene. 

An extra amount of stability or instability is associated with a compound that has a cyclic array of continuously overlapping p orbitals. Such a compound may have a ring with alternating single and multiple bonds, or the ring may contain both alternating 7r bonds and one atom with a lone pair or an empty orbital. If there is an odd number of electron pairs in the cyclic array of orbitals, then the compound is especially stable (as compared with the corresponding acyclic system with two additional H atoms), and it is said to be aromatic. If there is an even number of... 

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