Chapter 11 Introduction to Organic Chemistry
Organic Chemistry: The Chemistry of Carbon Compounds. • The element carbon has a special role in chemistry because it bonds with other carbon atoms to give a vast array of molecules. • The variety of molecules is so great that we find organic compounds in many common products such as gasoline, medicine, shampoos, plastic bottles, perfumes, etc. • The food we eat is also composed of different organic compounds that supply us with fuel for energy and the carbon atoms needed to build and repair the cells of our bodies. • Large organic molecules make up the proteins in hair and skin, the lipids in cell membranes and adipose • Although many organic compounds occur in nature, chemists have synthesized even more. The cotton, wool, or silk in your clothes contain naturally occurring organic compounds, whereas materials such as polyester, nylon, or plastic have been synthesized through organic reactions. • Sometimes, it is convenient to synthesize a molecule in the lab even though that molecule is also found in nature. Example: Vitamin C.
Study Goals • Identify the number of bonds for carbon and other atoms in organic compounds. • Describe the tetrahedral shape of carbon with single bonds in organic compounds. • Classify organic compounds as polar or nonpolar. • Describe the properties that are characteristic of organic compounds. • Identify the functional groups in organic compounds. • Write condensed structural formulas for organic compounds. • Write structural formulas for constitutional isomers.
11.1 Organic Compounds Review: Bonding: • Ionic Bonding—transfer of electrons. So:
Bonding (continued): • Covalent Bonding—sharing of electrons. The simplest way to symbolize the bonding of a covalent molecule is to use Lewis dot structures. In doing so, it is important to remember the octet rule.
Octet Rule: • Atoms will react (i.e. gain, lose or share electrons) in order to have the same number of valence electrons as the nearest noble gas. The sum of all the shared and unshared valence electrons about an atom must total 8. Note that this does not apply to all atoms (e.g. H, B, and sometimes Al, S.)
Lewis/Kekule Structures • dots = electrons • line = two shared electrons • all unshared electrons are shown, and are referred to as nonbonding electrons. Each pair of non-bonding electrons is also known as a “lone pair” Lewis Kekule
Multiple bonding in Lewis/Kekule structures: • Single Bond:sharing of one electron pair between two atoms. • Double Bond:sharing of two electron pairs. • Triple Bond:sharing of three electron pairs.
Solution H H H H H H H • —C—C—C—N—C—C—N— • —O—C—C—O—C—C—Cl H H H H H H H H H H H H H H H H Sample Problem: Complete the following structures by adding the correct number of hydrogen atoms: • C—C—C—N—C—C—N • O—C—C—O—C—C—Cl
a. 1 b. 2 c. 3 d. 4 Question How many bonds will oxygen make? Answer: “b”.
a. 1 b. 2 c. 3 d. 4 Question How many bonds will nitrogen make? Answer: “c”.
a. Atom 1 needs 3 hydrogens; atom 2 needs 3 hydrogens; atom 3 needs 1 hydrogen; atom 4 needs 3 hydrogens. b. Atom 1 needs 2 hydrogens; atom 2 needs 2 hydrogens; atom 3 needs 1 hydrogen; atom 4 needs 3 hydrogens. c. Atom 1 needs 3 hydrogens; atom 2 needs 2 hydrogens; atom 3 needs 1 hydrogen; atom 4 needs 3 hydrogens. d. Atom 1 needs 3 hydrogens; atom 2 needs 2 hydrogens; atom 3 needs 2 hydrogens; atom 4 needs 3 hydrogens. Question The atoms in the compound below are numbered 1-4. How many hydrogens are needed on each atom to complete the structure? Answer: “c”.
The electrons in the electron SHELLS or Main ENERGY LEVELS can be described in more detail. Within each shell, the electrons with identical energy are grouped as SUBSHELLS, which are identified by the letters s, p, d, and f. The s subshell has the lowest energy, followed by the p subshell, then the d subshell, and finally the highest energy subshell. Page 69, Figure 2.10.
The shapes of orbitals differ; s orbitals are spherical and p orbitals are dumbbell shaped. Each orbital holds a maximum of two electrons.
The bond is formed by overlapping of two p orbitals on adjacent carbon atoms.
In ethyne, C2H2, the overlap of two p orbitals forms two bonds required for the triple bond.
11.2 The Tetrahedral Structure of Carbon In most organic molecules, a carbon atom is bonded to four other atoms. The VSEPR (Valence-Shell Electron-Pair Repulsion) theory predicts that when four bonds are arranged as far apart as possible, they have a tetrahedral shape. VSEPR theory: A theory that predicts the shape of a molecule by moving the electron pairs on a central atom as far as possible to minimize the repulsion of the negative regions. (Review: what are the shapes of the molecules of CH4, CO2 and NH3)