Aim # 38: How do we describe alkanes? H.W. # 38 Study pp. 635-640 Ans. ques. p. 677 # 5-7,11

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.

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

“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!

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

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.

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

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?

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!

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. CH3 CH3 an unbranched alkane CH3 CH3 CH3 a branched alkane

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

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.

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

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