Branched-chain alkanes naming

As we cover new functional groups in later chapters, the applicable IUPAC rules of nomenclature will be given. In addition, Appendix A at the back of this book gives an overall view of organic nomenclature and shows how compounds that contain more than one functional group are named. For the present, let s see how to name branched-chain alkanes and learn some general naming rules that are applicable to all compounds. 

All but the most complex branched-chain alkanes can be named by following four steps. For a very few compounds, a fifth step is needed. 

Haloalkanes can be regarded as substituted alkanes in which one or more of the hydrogen atoms is replaced by a halogen atom. They are named in a similar fashion to branched-chain alkanes with the halogen atoms treated like branches. For example, the anaesthetic halothane has the structure shown in the diagram and is called 2-bromo-2-chloro-l,l, 1-trifluoroethane. 

S Branched alkanes. Branched-chain alkanes can be considered to be constituted of a principal chain and side-chains. They are named by using a precise set of operations ... 

Branched-chain alkanes are named by following four steps ... 

For example, butane has a boiling point of -0.5°C, while 2-methylpropane s boiling point is -11.7°C. Only hydrocarbon chains with at least four carbon atoms are capable of forming a branch. Because there must be a way to distinguish isomers, the nomenclature must take these into account. The following are general rules for naming branched-chain alkanes ... 

In addition to branched chains, alkanes can also form rings. The two terminal carbons bond together to complete a ring. The nomenclature is very similar except the prefix cyclo- is added before the name of the alkane. For instance, when the two terminal carbon atoms in hexane connect together, a six-sided ring is formed, called cyclohexane. A five-sided ring would be called cyclopentane. Each of these molecules is shown in Figure 19.7 ... 

The names of branched-chain alkanes (and most other aliphatic compounds) have the same general format, as shown in Figure 13.14. This format will become clearer as you learn and practise the rules for naming hydrocarbons. To start, read the steps on the next page to see how 2-methylpentane gets its name. 

The structure of an alkane can be much more complex than the structure of 2-methylpentane. For instance, there can be many branches bonded to the main chain, and the branches can be quite long. As a result, you need to know several other IUPAC rules for naming branched-chain alkanes and other aliphatic compounds. 

If you think that the structures represent two different substances, you are right. The structure on the left represents butane, and the structure on the right represents a branched-chain alkane, known as isobutane, a substance whose chemical and physical properhes are different from those of butane. As you see, carbon atoms can bond to one, two, three, or even four other carbon atoms. This property makes possible a variety of branched-chain alkanes. How do you name isobutane using lUPAC rules ... 

You ve seen that both a straight-chain and a branched-chain alkane can have the same molecular formula. This fact illustrates a basic principle of organic chemistry the order and arrangement of atoms in an organic molecule determine its identity. Therefore, the name of an organic compound also must describe the molecular structure of the compound accurately. 

The naming process To name organic structures, chemists follow systematic rules approved by the International Union of Pure and Applied Chemistry (lUPAC). Here are the rules for naming branched-chain alkanes. 

Example Problem 22-1 and the Practice Problems that follow it will help you develop skill at naming branched-chain alkanes. 

For more practice with naming branched-chain alkanes go to Supplemental Practice Problems in Appendix A. 

Naming branched-chain alkenes When naming branched-chain alkenes, follow the lUPAC rules for naming branched-chain alkanes—with two differences. First, in alkenes the parent chain is always the longest chain that contains the double bond, whether it is the longest chain of carbon atoms or not. Second, the position of the double bond, not the branches, determines how the chain is numbered. Note that there are two 4-carbon chains in the molecule shown below, but only the one with the double bond is used as a basis for naming. This branched-chain alkene is 2-methylbutene. 

The rules for naming branched-chain alkanes are as follows ... 

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