Isomers Help

By — McGraw-Hill Professional
Updated on Aug 28, 2011

Introduction to Isomers

Berzelius who first used symbols to name elements, coined the word isomer through his work with chemical atomic weights. Not to be confused with isotopes, chemical element isomers have the same molecular formula, but different structural configurations.

Isomers have the same molecular formulas, but different three-dimensional structures.

Think of yourself as a chief baker and molecules as your dough. The recipe for the dough is the same, the isomers are the different shapes the dough can take. It can be made into loaves, ropes, twists, rolls, or a dozen other shapes depending on what you want. The dough is the same, but the shape may cause it to react differently. If the dough is made into a long, thin rope, and cooked on a metal baking sheet that easily conducts heat, it will bake much faster than a round, solid core of dough baked in a thick-walled clay baker. If the same dough is used and both shapes are cooked at the same temperature for the same length of time, the thin rope will cook in half the time and probably burn, in the time it takes for the solid round loaf to bake. The shape makes a lot of difference in how the same formula reacts.

Structural Isomers

There are basically two types of isomers, structural isomers and stereoisomers .

Isomers of pentane (C 5 H 12 ) are shown in Figure 13.4 . Though the groups are arranged differently structurally, the formula is the same. Chain isomers can be a few carbons long to many dozens of carbons in length.

Chemical Bonding Isomers Structural Isomers

Fig. 13.4. Pentane isomer groups are arranged differently, but the formula is the same.


Isomers that have the same formula, but their groups arranged differently in space are called stereoisomers . These groups can be located in many different places depending on the size and type of bonding. Stereoisomers are not structural isomers, they have their atoms combined in the same order, but are in different spacial positions. These isomers have the same functional groups, but again the placement is slightly different. Figure 13.5 shows the different placement of the chlorine group in stereoisomers of 1- and 2-chloropropane (C 3 H 7 Cl).

Chemical Bonding Isomers Stereoisomers

Fig. 13.5. Stereoisomers of 1- and 2-chloropropane (C 3 H 7 Cl) have chlorine at different locations.

When ring structures like those found in benzene are considered, functional groups can be placed in several locations all around the ring. Remember, when compounds contain single and double bonds alternating around a ring structure, they are known as aromatic compounds as in Figure 13.6 . gives examples of the different locations the functional groups can have around a ring structure.

Chemical Bonding Isomers Stereoisomers

Fig. 13.6. Aromatic compounds vary according to the functional groups bonded around the rings.

Cis/trans Isomerism

Stereoisomers that have the same molecular formula, but different arrangements of their groups in space are called cis/trans isomers . These isomers are also sometimes called geometric isomers . The groups located on the same side of the molecule are in the cis formation, while the groups located on the opposite sides of the molecule put it into the trans formation. Figure 13.7 shows the cis/trans structures of dichloroethene.

Chemical Bonding Isomers Cis/trans Isomerism

Fig. 13.7. Cis/trans bonding of dichloroethene shows the placement of chlorine atoms.

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