1. Aim # 38: How do we describe alkanes?
H.W. # 38
Study pp
Ans. ques. p. 677 # 5-7,11

2. Organic Structure Review
Expanded Structure – shows all the atoms and bonds in the molecule.
Condensed Structure – eliminates C-H bonds and O-H bonds to save time/space.
Line Structure – represents each carbon atom with a point in a line. Hydrogen atoms are not shown.

3. Structural Isomers
Remember, sometimes one molecular formula can have more than one possible structure.
For example, I could write the formula C3H6O the following ways: OH
H3C CH2 O
H3C
CH3 H3C
O H2C

4. “Catching” all the isomers
In order to draw structural isomers, follow these steps:
1. Arrange the atoms with single bonds in a way
that makes sense (hydrogen cannot be
bonded to two atoms!)
2. Add double or triple bonds, as necessary, until
all atoms have their # of allowable bonds.
3. Experiment, experiment, experiment! – until
you’re sure you’ve caught them all!

5. Isomer Practice
What are the possible isomers for:
C4H8?
C2H4O?

6. Hydrocarbons – The Simplest Isomers
In terms of organic molecules, you can’t get much simpler than hydrocarbons.
Hydrocarbons are molecules made of hydrogen and carbon.
Oils, waxes- even petroleum- are made of various hydrocarbons.

7. Types of Hydrocarbons Hydrocarbons come in three basic varieties:
Chains - contain carbons linked H3C CH3
… in a chain pentane
Rings – contain carbons linked
… in a ring.
cyclopentane
Aromatics – contain that crazy
benzene molecule. We will not
discuss these in detail benzene

8. Saturation!!!
When you hear the word ‘saturated’, what do you think of?
‘Saturated’ means ‘filled to the maximum with something.
A saturated hydrocarbon has the most hydrogen atoms it can have.
What kind of bonds do you think a saturated hydrocarbon has?

9. Saturated and Unsaturated Hydrocarbons
Saturated Hydrocarbons have the maximum number of hydrogens per carbon- there are only single bonds.
Example: ethane (C2H6) – make this with your model kit!
Unsaturated Hydrocarbons have fewer than the maximum number of hydrogens – there are multiple bonds.
Example: ethene (C2H4) – make this with your model kit!

10. Alkanes – What are they? The main kind of hydrocarbon
you’ll see on the RegentsTM is
the alkane.
An alkane is a saturated
hydrocarbon chain.
Two types:
Unbranched- one simple chain
Branched- a chain with carbon ‘branches’ coming off the main chain.
CH CH3
an unbranched alkane
CH3
CH CH3
a branched alkane

11. Alkane Names Alkane Formula Alkane Name CH4 Methane CH3CH3 Ethane
CH3CH2CH3
Propane
CH3CH2CH2CH3
n-butane
CH3CH2CH2CH2CH3
n-pentane

12. Patterns abound! What is the pattern here?
When you name an unbranched alkane, there are two parts:
1. A prefix (for the number of carbons); and
2. –ane.
If there are more than 3 carbons, there are isomers that aren’t chains → add n- before the prefix.

13. Number of Carbons
Prefix 1
meth- 2
eth- 3
prop- 4
but- 5
pent- 6
hex- 7
hept- 8
oct- 9
non- 10
dec-

14. Two general alkane rules
The larger the alkane, the more London dispersion forces it has.
What does this do to the alkane’s boiling point?
All alkanes have the general formula CnH(2n+2), where n = number of carbons
Methane: n=1, (2n+2) = 4 → CH4
Ethane: n=2, (2n+2) = 6 → C2H6
Pentane: n=5, (2n+2) = 12 → C5H12