10.1 Condensed Structure and Line Structure

Shirley Wacowich-Sgarbi; Langara Chemistry Department

Chapter 10. Organic Chemistry

10.1 Condensed Structure and Line Structure

Learning Objectives

By the end of this section, you will be able to:

Interpret condense and line structures.
Draw the condensed structure of a given Lewis Structure or line structure.
Draw the line structure of a given Lewis Structure or condense structure.

If you look ahead in this chapter and in other resources at the way organic compounds are drawn, you will see that the figures are somewhat different from the Lewis structures you are used to seeing in your general chemistry book. In some sources, you will see condensed structures for smaller molecules instead of full Lewis structures:

**Figure 1.** Comparison between Lewis structures and condensed structures.

Example 1

Determine the Lewis Structure of the following condensed structure of oleic acid, a fatty acid that is found naturally in various animal and vegetable fats and oils.

CH₃(CH₂)₇CH=CH(CH₂)₇COOH

Solution

Start by drawing the CH₃. The (CH₂)₇ represents a repeating unit, meaning you must draw seven CH₂‘s one after another, which are bonded to a CH which is bonded to a CH, and then another seven CH₂‘s. The COOH represent a carboxylic acid, which means you have a C=O connected to an O-H. Always double check your structure to ensure every carbon is making four bonds. When you do this, you will see the two CH must be double bonded.

Test Yourself
Common organic compounds that you likely have at home are: acetone (CH₃COCH₃) found in nail polish remover, acetic acid (CH₃COOH) found in vinegar, and isopropanol ((CH₃)₂CHOH) found in rubbing alcohol. Determine the Lewis Structure for each of these household chemicals.

Answer

More commonly, organic and biological chemists use an abbreviated drawing convention called line structures, also known as skeletal structures or line bond structures. The convention is quite simple and makes it easier to draw molecules, but line structures do take a little bit of getting used to. Carbon atoms are depicted not by a capital C, but by a ‘corner’ between two bonds, or a free end of a bond. Open-chain molecules are usually drawn out in a ‘zig-zig’ shape. Hydrogens attached to carbons are generally not shown: rather, like lone pairs, they are simply implied (unless a positive formal charge is shown, all carbons are assumed to have a full octet of valence electrons). Hydrogens bonded to nitrogen, oxygen, sulfur, or anything other than carbon are shown, but are usually drawn without showing the bond. The following examples illustrate the convention.

**Figure 2.** Comparison between Lewis structure and line structure.

As you can see, the ‘pared down’ line structure makes it much easier to see the basic structure of the molecule and the locations where there is something other than C-C and C-H single bonds. For larger, more complex biological molecules, it becomes impractical to use full Lewis structures. Conversely, very small molecules such as ethane should be drawn with their full Lewis or condensed structures.

Sometimes, one or more carbon atoms in a line structure will be depicted with a capital C, if doing so makes an explanation easier to follow. If you label a carbon with a C, you also must draw in the hydrogens for that carbon.

Example 2

Draw the line structures for these two molecules:

Solution
Each carbon atom is converted into the end of a line or the place where lines intersect. All hydrogen atoms attached to the carbon atoms are left out of the structure (although we still need to recognize they are there):
Figure a shows a branched skeleton structure that looks like a plus sign with line segments extending up and to the right and down and to the left of the rightmost point of the plus sign. Figure b appears in a zig zag pattern made with six line segments. The segments rise, fall, rise, fall, rise, and fall moving left to right across the figure.

Test Yourself
Draw the line structures for these two molecules:
Figure a shows five C H subscript 2 groups and one C H group bonded in a hexagonal ring. A C H subscript 3 group appears above and to the right of the ring, bonded to the ring on the C H group appearing at the upper right portion of the ring. In b, a straight chain molecule composed of C H subscript 3 C H subscript 2 C H subscript 2 C H subscript 2 C H subscript 3 is shown.

Answers

Example 3

Identify the chemical formula of the molecule represented here:
This figure shows a pentagon with a vertex pointing right, from which a line segment extends that has two line segments attached at its right end, one extending up and to the right, and the other extending down and to the right.

Solution
There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule. Since we know that carbon atoms tend to make four bonds, each carbon atom will have the number of hydrogen atoms that are required for four bonds. This compound contains 16 hydrogen atoms for a molecular formula of C₈H₁₆.

Location of the hydrogen atoms:
In this figure a ring composed of four C H subscript 2 groups and one C H group in a pentagonal shape is shown. From the C H group, which is at the right side of the pentagon, a C H is bonded. From this C H, a C H subscript 3 group is bonded above and to the right and a second is bonded below and to the right.

Test Yourself
Identify the chemical formula of the molecule represented here:
A skeleton model is shown with a zig zag pattern that rises, falls, rises, and falls again left to right through the center of the molecule. From the two risen points, line segments extend both up and down, creating four branches.