what is a secondary carbon?

Question

Im not 100% sure of what is a secondary carbon or how i can identify one. the question was which of the following molecules are made up of only secondary carbons

• toluene
• 1,2 dimethylcyclopentane
• benzoic acid
• benzene
• propene

Answer 1

Correct answer:  None of the above

Probable expected answer:  benzene

As Daniel stated, a secondary carbon has two adjacent carbons (benzene is the only one that fits).

However, there is another aspect of the definition, in that the term is only used for tetrahedral (sp³) carbons.  Of the choices you were given, only the second has all tetrahedral atoms, but they are not all secondary.

Answer 2

A primary carbon is a carbon atom bound to only one other carbon atom. A secondary carbon is a carbon atom bound to two other carbon atoms.  

Ex of primary and secondary carbons in propane:  H3C-CH2-CH3

Both the 1st and 3rd carbons would be primary since they are bound to only one other carbon molecule.

The middle carbon would be secondary since it is bound to two different carbon atoms (the 1st and the 3rd).

Take a look at the diagram at http://en.wikipedia.org/wiki/Carbon–carbon_bond