Bonding in Benzene, 11(4)

Arrangement of orbitals to form structure of benzene.

1. Bonding in the Benzene Dr. K. Shahzad Baig Memorial University of Newfoundland (MUN ) Canada Petrucci , et al. 2011. General Chemistry: Principles and Modern Applications. Pearson Canada Inc., Toronto, Ontario. Tro , N.J. 2010. Principles of Chemistry. : a molecular approach. Pearson Education, Inc.

2. Delocalized Electrons: Bonding in the Benzene Molecule Benzene is built from: hydrogen atoms (1s 1 ) and carbon atoms (1s 2 2s 2 2p x 1 2p y 1 ). Each carbon atom is attached with 1 hydrogen and 2 carbons. Each carbon atom doesn't have enough unpaired electrons to form the required number of bonds , therefore, it needs to promote one of the 2s 2  pair into the empty 2p z  orbital. an electron is promoted from the 2s to the empty 2p to give 4 unpaired electrons C 6 H 6

3. In benzene each carbon is only joining to 3 other atoms , when the carbon atoms hybridise their outer orbitals before forming bonds, they only need to hybridise  three  of the orbitals rather than all four. [They use the 2s electron and two of the 2p electrons, but leave the other 2p electron unchanged] The new orbitals formed are   sp 2  hybrids . The three sp 2  hybrid orbitals arrange themselves as far apart as possible - which is at 120° to each other in a plane. End-to-end overlap of the orbitals produces σ - bonds. The π - bonds are not localized between specific carbon atoms but are spread out around the six-membered ring. Hence, called delocalized The six remaining orbitals construct the π - bonds

4. The π - bonds are not localized between specific carbon atoms but are spread out around the six-membered ring. Hence, called delocalized The six remaining orbitals construct the π - bonds

5. The three bonding molecular orbitals, distribute electron charge in the molecule in the form of two doughnut-shaped regions: one above and one below the plane of the C and H atoms. Because they are spread out among all six C atoms instead of being concentrated between pairs of C atoms, these molecular orbitals are called delocalized molecular orbitals

6. Structures with Delocalized MOs The VSEPR theory, gives a trigonal-planar electron-group geometry with a bond angle 117 o for O 3 . The sp 2 hybridization is chosen for the central O atom and terminal atoms, Thus, each O atom uses the orbital set sp 2 + p 2. Of the 18 valence electrons in O 3 , assign 14 to the hybrid orbitals of the σ -bond framework. 4 of these are bonding electrons and 10 are lone-pair electrons. 3 . The three unhybridized 2p orbitals combine to form 3 molecular orbitals of the π type. One of these orbitals is a bonding molecular orbital, and the second is antibonding. The third is a nonbonding molecular orbital. A nonbonding molecular orbital has the same energy as the atomic orbitals from which it is formed, and it neither adds to nor detracts from bond formation 4. The remaining 4 valence electrons are assigned to the π molecular orbitals is 2 go into the bonding orbital and 2 into the nonbonding orbital. The antibonding orbital remains empty 5. The bond order associated with the π molecular orbitals is (2-0) /2 = 1. This π bond is distributed between the two O-O bonds and amounts to ½ of a π bond for each.