4.2: Isomers
- Page ID
- 430688
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We can write the structure of butane (C4H10) by stringing four carbon atoms in a row,
–C–C–C–C–
and then adding enough hydrogen atoms to give each carbon atom four bonds:
The compound butane has this structure, but there is another way to put 4 carbon atoms and 10 hydrogen atoms together. Place 3 of the carbon atoms in a row and then branch the fourth one off the middle carbon atom to make an isomer of butane called isobutane.
Now we add enough hydrogen atoms to give each carbon four bonds.
The structure of isobutane shows a continuous chain of three carbon atoms only, with the fourth attached as a branch off the middle carbon atom of the continuous chain, which is different from the structures of butane (compare the two structures in Figure \(\PageIndex{1}\).
Butane and Isobutane are isomers. Isomers are compounds that have the same molecular formula, but different structural formula. Both butane and isobutane have the same molecular formula C4H10. However they differ in the connectivity of the atoms. The line bond formula of the two isomers are different and hence they are different compounds.
Unlike C4H10, the compounds methane (CH4), ethane (C2H6), and propane (C3H8) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds.
Pentane has three isomers. Each compound has the same molecular formula: C5H12. The compound on the left is pentane. It has all five carbon atoms in a continuous chain. The compound in the middle is isopentane; like isobutane, it has a one CH3 branch off the second carbon atom of the continuous chain. The compound on the right, discovered after the other two, was named neopentane (from the Greek neos, meaning “new”). Although all three have the same molecular formula, they have different properties, including boiling points: pentane, 36.1°C; isopentane, 27.7°C; and neopentane, 9.5°C.
A continuous (unbranched) chain of carbon atoms is often called a straight chain even though the tetrahedral arrangement about each carbon gives it a zigzag shape.
Draw an isomer of the following compounds.
a. 
b. 
- Answer
-
a. Move the chlorine to the second carbon atom to obtain an isomer. This is the only isomer possible for this compound.
b. Move the oxygen to a different carbon atom to obtain an isomer. There are 2 other isomers possible.
