You would already be familiar with the prefix iso- from the isotopes you studied in the unit on atomic structure. Isotopes have the same number of protons but different numbers of neutrons. (Remember?) Iso- means same.

In this vein, what might be ‘the same’ for isomers? Isomers have the same molecular formula, but different structural formula. Pentane, methylbutane and dimethylpropane all have the molecular formula C₅H₁₂.

Problem 7. Draw structural formulae for pentane, methylbutane and dimethyl-propane.  (Recall Rule 6 on redundancy.  None of these molecules require numbering, eg 2-methylbutane.  Can you explain why?)

Problem 8. Draw the structural formulae for the four compounds listed in Table 3. Can you find one more isomer to complete the set?

If you have trouble telling whether two structures are equivalent or not, work out their names. If they have the same name, they are the same compound, no matter how twisted they may be.

Problem 9. Are these two structures the same or isomers? The hydrogens have been left out for clarity. You may assume that every carbon makes four bonds and all bonds not shown are to hydrogen atoms.

Problem 10. How many isomers can you find for C₃H₈ ? C₄H₁₀ ? C₇H₁₆ ? C₄H₈ ?    There are 75 isomers for C₁₀H₂₂, including 2,3,3,5-tetramethyl-hexane and 3,3-diethyl-2-methyl-pentane!

Problem 11. Structures (e) and (f) above are isomers of two of structures (a) through (d) on the Naming page.  Which of those structures (a-d) is the same as (not "is an isomer of") (e)?  Which is the same as (f)?  Naming them systematically should help.