Question

Following is a structural formula of benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}\), which we study in Chapter 21 . (a) Using VSEPR, predict each \(\mathrm{H}-\mathrm{C}-\mathrm{C}\) and \(\mathrm{C}-\mathrm{C}-\mathrm{C}\) bond angle in benzene. (b) State the hybridization of each carbon in benzene. (c) Predict the shape of a benzene molecule. (d) Draw important resonance contributing structures.

Short answer

Question: Describe the key features of the benzene molecule, such as its bond angles, hybridization, shape, and resonance structures. Answer: The benzene molecule has sp2 hybridized carbon atoms with H-C-C and C-C-C bond angles of approximately 120°. Its shape is planar, symmetrical, and hexagonal. There are two main resonance structures, where pi electrons are delocalized within the carbon-carbon bonds, resulting in bond lengths and strengths that are intermediate between a single bond and a double bond.

Step-by-step solution

Determine bond angles in benzene using VSEPR theory

To predict the bond angles in benzene, we first need to understand its structure. Benzene is an aromatic compound with six carbons in a planar hexagonal ring, with each carbon carrying one hydrogen atom. Each carbon is bonded to two other carbons and one hydrogen. There are two types of bond angles we are interested in: H-C-C and C-C-C. The carbon atoms in benzene are sp2 hybridized, and all the bond angles around sp2 hybridized atoms are approximately 120 degrees. Therefore, both H-C-C and C-C-C bond angles in benzene are around 120°.

Determine the hybridization of carbon atoms in benzene

As mentioned in Step 1, the carbon atoms in benzene are sp2 hybridized. This means that one s and two p orbitals combine to form three sp2 hybrid orbitals in the plane of the benzene ring. The remaining p orbital is perpendicular to the plane of the ring and is responsible for the delocalized π bonding.

Predict the shape of benzene

With six carbon atoms connected in a planar hexagonal ring and sp2 hybridization, the benzene molecule has a planar, symmetrical, and hexagonal shape. Each hydrogen atom is located in the plane of the ring, bonded to a carbon atom.

Draw important resonance contributing structures

Benzene has two main resonance contributing structures, which are created by the delocalization of the pi electrons in the carbon-carbon double bonds: 1. The first structure has three alternating single and double C-C bonds in the ring. 2. The second structure is identical to the first, but the double bonds have shifted positions within the ring (rotated by one carbon). These two resonance structures contribute equally to the overall electronic structure of benzene, and as a result, the bond lengths and bond strengths of the carbon-carbon bonds in benzene are intermediate between a single bond and a double bond.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used by chemists to predict the geometry of molecules based on the number of electron pairs surrounding their central atoms. At its core, VSEPR theory suggests that electron pairs are negatively charged and, therefore, will repulse each other to the farthest possible extent. This repulsion affects the molecular shape by arranging the electron pairs as far from each other as possible to minimize electron pair repulsion, resulting in specific, predictable shapes.

For instance, in the case of benzene (C6H6), each carbon atom is bonded to two other carbons and one hydrogen atom. Applying VSEPR theory, we can predict that these carbon atoms will have a trigonal planar shape due to their sp2 hybridization, in which three electron pairs are spread as far apart as possible, creating bond angles of about 120 degrees. Hence, both the H-C-C and C-C-C bond angles in benzene are approximately 120°, leadings to its flat hexagonal structure.

sp2 Hybridization

Hybridization is a concept in molecular orbital theory that describes the mixing of atomic orbitals to create new hybrid orbitals. In the context of benzene, each of the six carbon atoms undergoes sp2 hybridization. This process involves the combination of one s orbital with two p orbitals from the same atom, forming three hybrid orbitals that are all planar and 120 degrees apart.

These sp2 hybridized orbitals are responsible for the formation of the sigma bonds in the molecule, which are the strong bonds between the carbon atoms and between carbon and hydrogen atoms. The unhybridized p orbital, which is perpendicular to the plane of the sp2 orbitals, overlaps with p orbitals on adjacent carbon atoms to form delocalized π bonds, which are essential for the resonance and stability of aromatic compounds like benzene.

Resonance Structures

Resonance structures are a way of representing the distribution of electrons in a molecule where the actual structure is an intermediate, or a hybrid, of those depicted by the individual resonance forms. Benzene is a classic example that demonstrates this concept; it is depicted by two different resonance structures. Each structure shows alternating double and single bonds between carbon atoms in the aromatic ring.

However, these structures are not static or separate; instead, the electrons are delocalized across the ring, creating a situation where the bond lengths and energies are identical for all carbon-carbon bonds. This characteristic is a cornerstone of aromaticity, endowing benzene with its unique chemical stability and uniformity. The true representation of benzene's electronic structure cannot be captured by a single structural formula but as a resonance hybrid of its contributing structures.

Aromatic Compounds

Aromatic compounds, such as benzene, possess a unique stability due to the delocalization of π electrons across a conjugated cycle. This electron delocalization leads to a closed loop of overlapping p orbitals, creating a fully conjugated π electron system. Aromaticity can be defined by a set of rules known as Hückel's criteria: a planar ring of continuously overlapping p orbitals, containing an odd number of electron pairs—this is often simplified to the '4n+2' rule, where 'n' is a non-negative integer.

Benzene is the simplest aromatic compound and is often used as a building block for more complex structures in organic chemistry. Its aromaticity imparts notable chemical properties such as resistance to addition reactions where non-aromatic compounds would react easily, instead favoring substitution reactions which preserve the aromatic electron system. Understanding the nature of aromatic compounds is essential for grasping many concepts in organic synthesis and the reactivity of organic molecules.