Structural isomers

The idea

Compounds with identical molecular formula but different structures are called structural isomers.

Same atoms, other arrangement

Take four carbon atoms and ten hydrogen atoms. The carbon atoms can be linked in more than one way.

How many different complete molecules with the formula C₄H₁₀ do you think can be drawn?

• Only one
• Two
• Ten

Two ways to link four carbons

Press Next step.

One formula, two structures

Counting the atoms in each molecule gives C₄H₁₀ both times, so the formula alone does not tell which structure it is.

Compounds with identical molecular formula but different structures are called structural isomers.

Notes

Compounds with identical molecular formula but different structures are called structural isomers.

Check yourself

Two compounds both have the formula C₄H₁₀ but different structures. What are they called?

How many hydrogen atoms does each of the two C₄H₁₀ molecules have?

Answer: 10

Both have the formula C₄H₁₀, so each has 10 hydrogen atoms.

Why can there be two molecules with the formula C₄H₁₀?

• Structural isomers — correct. Yes!
• Saturated compounds. Both are saturated, but the name for the same formula with different structures is structural isomers.
• Unsaturated compounds. C₄H₁₀ has only single bonds between carbon atoms, so these compounds are saturated, not unsaturated.
• The two molecules have different numbers of hydrogen atoms. Both have 10 hydrogen atoms. Their formulae are identical.
• The four carbon atoms can be linked in two different skeletons — correct. Yes!
• One of them has five carbon atoms. Both have four carbon atoms, as the formula C₄H₁₀ shows.