12.1 Lecture

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Intro to Organic Chemistry - Structures and Naming

Lecture Outline

  1. Organic Chemistry
    • Structural Formula, Condensed Structural Formula, Line Angle Drawings
    • Representations of Methane and Butane
    • Branching and Isomers
    • Isomers – worksheet
    • Ring Structures, Aromatics, Benzene
    • Line-Angle Drawings
  2. Alkanes
    • Prefix name = number of carbons
    • Representations of Organic Molecules - worksheet
  3. IUPAC Naming of Organic Molecules
    • Organic Molecules: Structure to Chemical Name – 2 worksheets
    • Organic Molecules: Chemical Name to Structure - worksheet

Summary

(adapted from Chemistry: Atoms First 2e)

The simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. 

Alkanes, or saturated hydrocarbons, contain only single covalent bonds between carbon atoms. The Lewis structures and models of methane, ethane, and pentane are illustrated in the figure below. Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. 

The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas). Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. 

The figure illustrates four ways to represent molecules for molecules of methane, ethane, and pentane.
Lewis structures, ball-and-stick models, and space-filling models for molecules of methane, ethane, and pentane.

A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure). In this type of structure, carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon. Other atoms besides carbon and hydrogen are represented by their elemental symbols. The figure below shows three different ways to draw the same structure.

In this figure, a hydrocarbon molecule is shown in three ways.

Supplemental Reading

from Chemistry: Atoms First 2e/OpenStax - Intro to Organic Chemistry Links to an external site.

from Chemistry: Atoms First 2e/OpenStax - Organic Molecule Structures Links to an external site.

from Chemistry for Allied Health/LibreTexts - Representing Structures Links to an external site.

from Introductory Chemistry/LibreTexts - Isomers Links to an external site.

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