Chemistry Reference
In-Depth Information
Example 1
Write the molecular formula and structural formula for a molecule
of propene.
The first thing that you want to do is determine the molecular formula
for the compound. The prefix “prop-” tells us (see Figure 4-4b) that we
are dealing with a compound that contains three carbon atoms. The suffix
“-ene” tells us that we are dealing with an alkene. Alkenes contain twice as
many hydrogen atoms as carbon atoms, so propene must have six hydrogen
atoms (2 × 3 = 6).
A.
Molecular formula: “prop-” = 3 carbon atoms, or C
3
“-ene” means the # of hydrogen atoms = 2n = 2 × 3 = 6 or H
6
so, the molecular formula for propene is C
3
H
6
As for the structural formula, we start by laying down the three carbon
atoms in a line, as shown here.
CCC
You will recall that the electron configuration for carbon is 1s
2
2s
2
2p
2
,
so each carbon atom, with four valence electrons, can make up to four
single bonds. The suffix “-ene” tells us that there is a double bond between
two of the carbon atoms. The carbon atoms involved in the double bond
will use up two out of their four bonding electrons with that one double
bond, and will only be able to make two additional single bonds. It doesn't
matter between which two carbon atoms we show the double bond.
CCC
Notice that each of the carbon atoms has been drawn with a total of four
lines coming out of it. These lines represent bonds or bonding sites. The
carbon atom on the left has made a double bond and still has two single
bonding sites available for bonding. The middle carbon atom has made one
double bond and one single bond and only has one single unpaired electron
available for bonding. The carbon atom on the right has only made one single
bond and has three unpaired electrons available for bonding.
How do we know that all of the unpaired electrons that haven't been
used will be used to form single bonds? Each hydrogen atom, with only
one valence electron, can only make a single covalent bond. If any of the
