Cycle II Materials Science
John Dewey High School
Basic Principles of Organic Chemistry II:
The Benzene Ring and Distinguishing
Between the sigma-Bond and the pi-Bond
Aromatic Hydrocarbons and the Benzene Ring
Benzene is the simplest member of the class of organic compounds referred to as aromatic hydrocarbons. Aromatic hydrocarbons are those pleasantly-odored organic compounds which emerge from mixtures of hydrocarbons from natural sources, such as petroleum and coal that are separated (or fractionated) during a process known as distillation. Aromatic hydrocarbons include such familiar scents as wintergreen, cinnamon and vanillin. Each member of the aromatic hydrocarbons additionally possess a single common feature: a six-membered ring of carbon atoms called the benzene ring. Benzene possesses the formula C6H6 and a planar (flat) architecture in which each of the internal bond angles equals 120 degrees.
Distinguishing Between the sigma-Bond and pi-Bond
By definition, the first covalent (electron-sharing) bond formed between two nuclei is a sigma-bond. Each of the bonds that link every carbon atom to every other carbon atom and every carbon atom to a hydrogen atom in an alkane, for example, is a sigma bond. A pi-bond on the other hand is a double bond, consisting of two shared electron pairs. The bond which joins the two carbon atoms in a molecule of ethene, for example, is a pi-bond. A pi-bond can also occur with a sigma bond, such as in the triple bond which is characteristic of the alkynes.
Based on the information above and your knowledge of the pair-share molecular model, construct a simple model of the benzene molecule in the space below and answer the questions at the bottom of the page. Additionally, label each of the bonds in your model as either sigma or pi.
1. The atomic mass of benzene is ________________ amu.
2. The number of sigma-bonds in a benzene molecule is ______________.
3. The number of pi-bonds in a benzene molecule is _____________.
4. Benzene is stable in the liquid phase at room temperature and 1 x 10exp5 Pa pressure (1 atmosphere). If the density of benzene is 0.880 grams/liter, calculate the number of benzene molecules there are in one liter given the fact that there are 6.02 x 10exp23 benzene molecules in 78 grams of benzene.