Reading condensed formula of organic compounds

The most important thing to remember is the number of bonds carbon forms. How many bonds does carbon want to form in the organic compounds we have been studying?

Answer: 4 bonds

This is very important along with the fact that hydrogen only needs one bond. So lets look at a condensed formula and see how we would 'see' it.

Consider CH₃CH₂CHCHCH₂CH₃. Is this an alkane, an alkene or an alkyne?

Answer: We need to write the formula in the form C_whateverH_whatever. In this case writing the formula we get C₆H₁₂. This is the formula of an alkene. Remember alkenes have the general formula C_nH_2n.

So since this compound is an alkene we know it contains a double bond. Where is it located in this condensed formula? To answer this question we need draw a Lewis structure, either on paper or mentally and associate the hydrogens with the correct carbons as described in the formula. An important thing to remember is the the hydrogen(s) associated with a particular carbon are generally written to the right of the carbon atom. (Although sometimes the hydrogens are written to the left, but usually only for the left-most carbon in the formula.)

Using this information draw the Lewis structure showing the carbons and the hydrogens?

Answer.

In the condensed formula shown below I have colored the first three carbon atoms and the hydrogen atoms associated with those carbon atoms. Then I drew the lewis structure showing the relationship between the condensed formula and the Lewis structure.

Notice that two of the carbon atoms do not have four bonds! Yet we have accounted for all of the hydrogens shown in the condensed formula. What do we do?? The simplest solution is to recall that we had already identified this formula as a member of the alkenes and we know an alkene has a double bond. So this is where the double bond must be located.

Try these examples to see how well you understand.

Draw the Lewis structure for each of the following compounds;

a) CH₃CH₂CH₂CH₂CH₃ Answer

b) CH₃CH₂CH₂CH₂CCCH₃ Answer

c) CH₂CHCH₂CH₂CH₂CH₂CH₂CH₂CH₃ Answer

d) CH₃CHCHCH₂CCCH₂CH₃ Answer

Hopefully, you will now feel better about looking at condensed formulas and 'seeing' the Lewis structure and the locations of double or triple bonds.