Alkanes
Alkanes, or saturated hydrocarbons, contain only single covalent bonds between carbon atoms. Each of the carbon atoms in an alkane has sp3 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 sp3 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.
![The figure illustrates four ways to represent molecules for molecules of methane, ethane, and pentane. In the first row of the figure, Lewis structural formulas show element symbols and bonds between atoms. Methane has a central C atom with four H atoms bonded to it. Ethane has a C atom with three H atoms bonded to it. The C atom is also bonded to another C atom with three H atoms bonded to it. Pentane has a C atom with three H atoms bonded to it. The C atom is bonded to another C atom with two H atoms bonded to it. The C atom is bonded to another C atom with two H atoms bonded to it. The C atom is bonded to another C atom with two H atoms bonded to it. The C atom is bonded to another C atom with three H atoms bonded to it. In the second row, ball-and-stick models are shown. In these representations, bonds are represented with sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds. The molecule names and structural formulas are provided in the fourth row. Methane is named and represented with a condensed structural formula as C H subscript 4. Ethane is named and represented with two structural formulas C H subscript 3 C H subscript 3 and C subscript 2 H subscript 6. Pentane is named and represented as both C H subscript 3 C H subscript 2 C H subscript 2 C H subscript 2 C H subscript 3 and C subscript 5 H subscript 12.](https://cnx.org/resources/a75f99f6a3472d708a31e32853ca94689a36fd38/CNX_Chem_20_01_alkanes.jpg)
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.
![In this figure, a hydrocarbon molecule is shown in three ways. First, an expanded formula shows all individual carbon atoms, hydrogen atoms, and bonds in a branched hydrocarbon molecule. An initial C atom is bonded to three H atoms. The C atom is bonded to another C atom in the chain. This second C atom is bonded to one H atom and another C atom above the chain. The C atom bonded above the second C atom in the chain is bonded to three H atoms. The second C atom in the chain is bonded to a third C atom in the chain. This third C atom is bonded to on H atom and another C atom below the chain. This C atom is bonded to two H atoms and another C atom below the chain. This second C atom below the chain is bonded to three H atoms. The third C atom in the chain is bonded to a fourth C atom in the chain. The fourth C atom is bonded to two H atoms and a fifth C atom. The fifth C atom is bonded to two H atoms and a sixth C atom. The sixth C atom is bonded to three H atoms. Second, a condensed formula shows each carbon atom of the molecule in clusters with the hydrogen atoms bonded to it, leaving C H, C H subscript 2, and C H subscript 3 groups with bonds between them. The structure shows a C H subscript 3 group bonded to a C H group. The C H group is bonded above to a C H subscript 3 group. The C H group is also bonded to another C H group. This C H group is bonded to a C H subscript 2 group below and a C H subscript 3 group below that. This C H group is also bonded to a C H subscript 2 group which is bonded to another C H subscript 2 group. This C H subscript 2 group is bonded to a final C H subscript 2 group. The final structure in the figure is a skeletal structure which includes only line segments arranged to indicate the structure of the molecule.](https://cnx.org/resources/f9d310a73fdcdef30851de14b05a4d2d799bb961/CNX_Chem_20_01_LineStruct1.jpg)
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:
![Figure a shows five C H subscript 2 groups and one C H group bonded in a hexagonal ring. A C H subscript 3 group appears above and to the right of the ring, bonded to the ring on the C H group appearing at the upper right portion of the ring. In b, a straight chain molecule composed of C H subscript 3 C H subscript 2 C H subscript 2 C H subscript 2 C H subscript 3 is shown.](https://cnx.org/resources/05dc78c56bffa92c252d84c587b1b6f2e3c9e0f5/CNX_Chem_20_01_LineStruct4_img.jpg)
ANSWER:
![In a, a hexagon with a vertex at the top is shown. The vertex just to the right has a line segment attached that extends up and to the right. In b, a zig zag pattern is shown in which line segments rise, fall, rise, fall, and rise moving left to right.](https://cnx.org/resources/5c435ddd1ee6e36994c37f0dd8eab482a22472ba/CNX_Chem_20_01_LineStruct5_img.jpg)
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 C8H16.
Location of the hydrogen atoms:
Check Your Learning
Identify the chemical formula of the molecule represented here:
C9H20
All alkanes are composed of carbon and hydrogen atoms and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of CnH2n+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 Alkanes3 | |||||
---|---|---|---|---|---|
Alkane | Molecular Formula | Melting Point (°C) | Boiling Point (°C) | Phase at STP4 | Number of Structural Isomers |
methane | CH4 | –182.5 | –161.5 | gas | 1 |
ethane | C2H6 | –183.3 | –88.6 | gas | 1 |
propane | C3H8 | –187.7 | –42.1 | gas | 1 |
butane | C4H10 | –138.3 | –0.5 | gas | 2 |
pentane | C5H12 | –129.7 | 36.1 | liquid | 3 |
hexane | C6H14 | –95.3 | 68.7 | liquid | 5 |
heptane | C7H16 | –90.6 | 98.4 | liquid | 9 |
octane | C8H18 | –56.8 | 125.7 | liquid | 18 |
nonane | C9H20 | –53.6 | 150.8 | liquid | 35 |
decane | C10H22 | –29.7 | 174.0 | liquid | 75 |
tetradecane | C14H30 | 5.9 | 253.5 | solid | 1858 |
octadecane | C18H38 | 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 C4H10: 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.
![The figure illustrates three ways to represent molecules of n dash butane. In the first row of the figure, Lewis structural formulas show carbon and hydrogen element symbols and bonds between the atoms. The first structure in this row shows three of the linked C atoms in a horizontal row with a single C atom bonded above the left-most carbon. The left-most C atom has two H atoms bonded to it. The C atom bonded above the left-most C atom has three H atoms bonded to it. The C atom bonded to the right of the left-most C atom has two H atoms bonded to it. The right-most C atom has three H atoms bonded to it. The C atoms and the bonds connecting all the C atoms are red. The second structure in the row similarly shows the row of three linked C atoms with a single C atom bonded below the C atom to the left. The left-most C atom has two H atoms bonded to it. The C atom bonded below the left-most C atom has three H atoms bonded to it. The C atom bonded to the right of the left-most C atom has two H atoms bonded to it. The right-most atom has three H atoms bonded to it. All the C atoms and the bonds between them are red. The third structure has two C atoms bonded in a row with a third C atom bonded above the left C atom and the fourth C atom bonded below the right C atom. The C atom bonded above the left C atom has three H atoms bonded to it. The left C atom has two H atoms bonded to it. The right C atom has two H atoms bonded to it. The C atom bonded below the right C atom has three H atoms bonded to it. All the C atoms and the bonds between them are red. In the second row, ball-and-stick models for the structures are shown. In these representations, bonds are represented with sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds.](https://cnx.org/resources/1ea01ba5dce1f40a84f16b08ff94cf1fd543b538/CNX_Chem_20_01_butane_img.jpg)