20.2 Alkane Formulas

Types of Formulas to Represent Hydrocarbons

We use several kinds of formulas to describe organic compounds. A molecular formula shows only the kinds and numbers of atoms in a molecule. The general formula for alkanes: C_nH_{2n+ 2}. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. Table 20.2a. shows the molecular formulas of the first 10 straight-chain alkanes.

Table source: “12.2: Structures and Names of Alkanes” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

For example in Table 20.2a. above, the molecular formula C₄H₁₀ tells us there are 4 carbon atoms and 10 hydrogen atoms in a molecule, but it doesn’t distinguish between butane and isobutane. A structural formula shows all the carbon and hydrogen atoms and the bonds attaching them. Thus, structural formulas identify the specific isomers (learned in the next section) by showing the order of attachment of the various atoms.

Unfortunately, structural formulas are difficult to type/write and take up a lot of space. Chemists often use condensed structural formulas to alleviate these problems. The condensed structural formulas for the first 10 alkanes are demonstrated in Table 20.2a. Parentheses in condensed structural formulas indicate that the enclosed grouping of atoms is attached to the adjacent carbon atom. The condensed formulas show hydrogen atoms right next to the carbon atoms to which they are attached, as illustrated in Figure 20.2a. for butane:

The ultimate condensed formula is a line structure formula. 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. For example in Figure 20.2b., we can represent pentane (CH₃CH₂CH₂CH₂CH₃) and isopentane [(CH₃)₂CHCH₂CH₃] as follows:

In summary, the following Figure 20.2c. demonstrates the structural (expanded), condensed and line structure formulas.

For a summary chart of the formulae used in organic chemistry, infographic 20.2a. demonstrates the different ways to represent organic compounds. Molecular and empirical formulas are explained as the simplest followed by condensed, structural and skeletal (or line) formulae.

 

Example 20.2a

Draw the line 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):

Example & image source: General Chemistry 1 & 2 , CC BY 4.0.

Exercise 20.2a

Draw the line structures for these two molecules:

Check your answer^[1]

Example & image source: General Chemistry 1 & 2 , CC BY 4.0.

 

Example 20.2b

Identify the molecular 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 C₈H₁₆.

Location of the hydrogen atoms:

Example & image source: General Chemistry 1 & 2 , CC BY 4.0.

Exercise 20.2b

Identify the molecular formula of the molecule represented here:

Check your Answer^[2]

Example & image source: General Chemistry 1 & 2 , CC BY 4.0.