10.2: Alkanes and isomers
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Harper College Chemistry Department
Alkanes
Alkanes, or saturated hydrocarbons, contain only single covalent bonds between carbon atoms. Each of the carbon atoms in an alkane has sp 3 hybrid orbitals and is bonded to four other atoms, each of which is either carbon or hydrogen. The Lewis structures and models of methane, ethane, and pentane are illustrated in Figure 1 . Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. Because of the sp 3 hybridization, the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape.
The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas ). Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. Condensed structural formulas for ethane and pentane are shown at the bottom of Figure 1 , and several additional examples are provided in the exercises at the end of this chapter.
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. Figure 2 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):
Draw the skeletal structures for these two molecules:
Answer:
Interpreting Skeletal Structures
Identify the chemical formula (empirical 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 to obey the octet rule. 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:
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 ( Table 1 ) usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.
| Properties of Some Alkanes 3 | |||||
|---|---|---|---|---|---|
| Alkane | Molecular Formula | Melting Point (°C) | Boiling Point (°C) | Phase at STP 4 | 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:
The compounds n -butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different bonding arrangements of the atoms in their molecules. The n -butane molecule contains an unbranched chain , meaning that no carbon atom is bonded to more than two other carbon atoms. We use the term normal , or the prefix n , to refer to a chain of carbon atoms without branching. The compound 2–methylpropane has a branched chain (the carbon atom in the center of the Lewis structure is bonded to three other carbon atoms)
Identifying isomers from Lewis structures is not as easy as it looks. Lewis structures that look different may actually represent the same isomers. For example, the three structures in Figure 3 all represent the same molecule, n -butane, and hence are not different isomers. They are identical because each contains an unbranched chain of four carbon atoms.