Structure and Bonding:  The Arrangement of Bonds

Examine the three structures given below.  Count the number of carbons and hydrogen on each.

n-pentane                    2-methyl-butane             2,2-dimethyl-propane

C ___ H ___                    C ___ H ___                    C ___ H ___

How do the molar masses compare for the three compounds?

How do the three structures compare or differ?

These three compounds are isomers, as they have the same number of each atom in the formula, but are connected in different fashions by branching side chains. 

Since the three structures have the same molar mass, will they have the same boiling points?

The table below gives the boiling points and melting points for the three compounds.  How do the boiling points compare?  How do the melting points compare?

Property n-pentane 2-methyl-butane 2,2-dimethyl-propane
boiling point 36oC 28oC 9oC
melting point -130oC -160oC -17oC

CRC Handbook of Chemistry and Physics, 1997

View the two structures given below and decide if they are isomers.  How do they differ?

  

These are the cis-2-butene (on the left) and trans-2-butene (on the right) isomers.  The difference is position about the double bond in the 2-butene.  

How would you define the difference between the cis and trans isomers?  

Can cis and trans isomers occur about a single bond?  Explain why or why not.

Examine the structures of hexane and cyclohexane given below.  Are they isomers?  Explain why or why not.

hexane                             cyclohexane

  

From a structural point of view, isomers are nonsuperimposable on one another, such as your left-hand on your right-hand.  Are the two structures of CHBrClI below isomers?

The hexane and cyclohexane structures are not isomers since the formulas are C6H14 for hexane and C6H12 for the cyclohexane.  The cyclohexane loses two hydrogen atoms when the ring closes.  The CHBrClI structures are isomers, since they have the same formula, but are not superimposable on one another.  These two molecules are mirror images just like your two hands.  Go check it out in a mirror!

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