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What Are Alkanes?

Methane (CH4) and ethane (C2H6) are the first two members of the alkane family. Figure 1 shows molecular formulas, Lewis structures, and ball-and-stick models for these molecules. The shape of methane is tetrahedral, and all H-C-H bond angles are 109.5°. Each carbon atom in ethane is also tetrahedral, and all bond angles are approximately 109.5°.


Although the three-dimensional shapes of larger alkanes are more complex than those of methane and ethane, the four bonds about each carbon atom are still arranged in a tetrahedral manner, and all bond angles are still approximately 109.5°.


The next members of the alkane family are propane, butane, and pentane. In the representations that follow, these hydrocarbons are drawn first as condensed structural formulas that show all carbons and hydrogens. They are then drawn in an even more abbreviated form called a line-angle formula. In this type of representation, a line represents a carbon–carbon bond, and an angle represents a carbon atom.

A line ending represents a  -CH3 group. Although hydrogen atoms are not shown in line-angle formulas, they are assumed to be there in sufficient numbers to give each carbon four bonds.

Alkanes: Formula, Isomers FIGURE 1: Methane and ethane.

FIGURE 1: Methane and ethane.

 


Line-angle formula: an abbreviated way to draw structural formulas in which each vertex and each line ending represents a carbon atom and a line represents a bond.

Line-angle formula: an abbreviated way to draw structural formulas in which each vertex and each line ending represents a carbon atom and a line represents a bond.

We can write structural formulas for alkanes in still another abbreviated form. The structural formula of pentane, for example, contains three CH2 (methylene) groups in the middle of the chain. We can collect these groups together and write the structural formula as CH3(CH2)3CH3. Table 1 gives the names and molecular formulas of the first 20 alkanes. Note that the names of all these alkanes end in -ane. We will have more to say about naming alkanes.

 

 

How to Interpret Line-Angle Formulas?

Use the mnemonic “a carbon at every bend and at every end”

How to Interpret Line-Angle Formulas?


Because carbon requires four bonds to satisfy its valency, count the number of visible bonds to each carbon and then subtract this number from 4 to determine the number of hydrogens also bonded to that carbon (but not shown).

How to Interpret Line-Angle Formulas?


 

Table 1: Names, Molecular Formulas, and Condensed Structural Formulas for the First 20 Alkanes with Unbranched Chains

Table 1: Names, Molecular Formulas, and Condensed Structural Formulas for the First 20 Alkanes with Unbranched Chains


Alkanes have the general molecular formula CnH2n+2. Thus, given the number of carbon atoms in an alkane, it is easy to determine the number of hydrogens in the molecule and also its molecular formula. For example, decane, with 10 carbon atoms, must have (2X10)+2=22 hydrogens and the molecular formula C10H22.

 

 

What Is Constitutional Isomerism in Alkanes?

Constitutional isomers are compounds that have the same molecular formula, but different structural formulas. By “different structural formulas,” we mean that these compounds dif- fer in the kinds of bonds they have (single, double, or triple) or in their connectivity (the order of attachment among their atoms).

For the molecular formulas CH4, C2H6, and C3H8, only one order of attachment of atoms is possible. For the molecular formula C4H10, two orders of attachment of atoms are possible. In one of these, named butane, the four carbons are bonded in a chain; in the other, named 2-methylpropane, three carbons are bonded in a chain, with the fourth carbon as a branch on the middle carbon of the chain.

What Is Constitutional Isomerism in Alkanes?


Butane and 2-methylpropane are constitutional isomers; they are different compounds and have different physical and chemical properties. Their boiling points, for example, differ by approximately 11°C. We will discuss how to name alkanes in the next section.

 

To find out whether two or more structural formulas represent constitutional isomers, write the molecular formula of each and then compare them. All compounds that have the same molecular formula, but different structural formulas, are constitutional isomers.

 

 

Example 1

Do the structural formulas in each pair represent the same compound or constitutional isomers?


Do the structural formulas in each pair represent the same compound or constitutional isomers?


Strategy

To determine whether these structural formulas represent the same compound or constitutional isomers, first find the longest chain of carbon atoms in each. Note that it makes no difference whether the chain is drawn straight or bent. Second, number the longest chain from the end nearest the first branch. Third, compare the lengths of each chain and the sizes and locations of any branches. Structural formulas that have the same connectivity of atoms represent the same compound; those that have a different connectivity of atoms represent constitutional isomers.

 

Solution

(a) Each structural formula has an unbranched chain of six carbons. The two structures are identical and represent the same compound:


(a) Each structural formula has an unbranched chain of six carbons. The two structures are identical and represent the same compound:


(b) Each structural formula has a chain of five carbons with two CH3 branches. Although the branches are identical, they are at different locations on the chains. Therefore, these structural formulas represent constitutional isomers:


(b) Each structural formula has a chain of five carbons with two CH3 branches. Although the branches are identical, they are at different locations on the chains. Therefore, these structural formulas represent constitutional isomers:


 

 

Example 2

Draw structural formulas for the five constitutional isomers with the molecular formula C6H14 .

 

Strategy

In solving problems of this type, you should devise a strategy and then follow it. Here is one such strategy: First, draw a line-angle formula for the constitutional isomer with all six carbons in an unbranched chain. Then, draw line-angle formulas for all constitutional isomers with five carbons in a chain and one carbon as a branch on the chain. Finally, draw line-angle formulas for all constitutional isomers with four carbons in a chain and two carbons as branches.

 

Solution

Draw structural formulas for the five constitutional isomers with the molecular formula C6H14 .


 The ability of carbon atoms to form strong, stable bonds with other carbon atoms results in a staggering number of constitutional isomers. As the following table shows, there are 3 constitutional isomers with the molecular formula C5H12 , 75 constitutional isomers with the molecular formula C10H22 , and almost 37 million constitutional isomers with the molecular formula C25H52:


constitutional isomers with the molecular formula C10H22 , and almost 37 million constitutional isomers with the molecular formula C25H52:

Thus, for even a small number of carbon and hydrogen atoms, a very large number of constitutional isomers is possible. In fact, the potential for structural and functional group individuality among organic molecules made from just the basic building blocks of carbon, hydrogen, nitrogen, and oxygen is practically limitless.


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