A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure). In this type of structure, carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon. Other atoms besides carbon and hydrogen are represented by their elemental symbols. [link] shows three different ways to draw the same structure.
Drawing skeletal structures
Draw the skeletal structures for these two molecules:
Solution
Each carbon atom is converted into the end of a line or the place where lines intersect. All hydrogen atoms attached to the carbon atoms are left out of the structure (although we still need to recognize they are there):
Check your learning
Draw the skeletal structures for these two molecules:Answer:
Interpreting skeletal structures
Identify the chemical formula of the molecule represented here:
Solution
There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule. Since we know that carbon atoms tend to make four bonds, each carbon atom will have the number of hydrogen atoms that are required for four bonds. This compound contains 16 hydrogen atoms for a molecular formula of C 8 H 16 .Location of the hydrogen atoms:
Check your learning
Identify the chemical formula of the molecule represented here:Answer:
C 9 H 20
All alkanes are composed of carbon and hydrogen atoms, and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of C n H 2n+2 . The number of carbon atoms present in an alkane has no limit. Greater numbers of atoms in the molecules will lead to stronger intermolecular attractions (dispersion forces) and correspondingly different physical properties of the molecules. Properties such as melting point and boiling point ( [link] ) usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.
| Properties of Some Alkanes Physical properties for C 4 H 10 and heavier molecules are those of the normal isomer , n -butane, n -pentane, etc. | |||||
|---|---|---|---|---|---|
| Alkane | Molecular Formula | Melting Point (°C) | Boiling Point (°C) | Phase at STP STP indicates a temperature of 0 °C and a pressure of 1 atm. | Number of Structural Isomers |
| methane | CH 4 | –182.5 | –161.5 | gas | 1 |
| ethane | C 2 H 6 | –183.3 | –88.6 | gas | 1 |
| propane | C 3 H 8 | –187.7 | –42.1 | gas | 1 |
| butane | C 4 H 10 | –138.3 | –0.5 | gas | 2 |
| pentane | C 5 H 12 | –129.7 | 36.1 | liquid | 3 |
| hexane | C 6 H 14 | –95.3 | 68.7 | liquid | 5 |
| heptane | C 7 H 16 | –90.6 | 98.4 | liquid | 9 |
| octane | C 8 H 18 | –56.8 | 125.7 | liquid | 18 |
| nonane | C 9 H 20 | –53.6 | 150.8 | liquid | 35 |
| decane | C 10 H 22 | –29.7 | 174.0 | liquid | 75 |
| tetradecane | C 14 H 30 | 5.9 | 253.5 | solid | 1858 |
| octadecane | C 18 H 38 | 28.2 | 316.1 | solid | 60,523 |
Hydrocarbons with the same formula, including alkanes, can have different structures. For example, two alkanes have the formula C 4 H 10 : They are called n -butane and 2-methylpropane (or isobutane), and have the following Lewis structures: