Nomenclature of alkanes

Alkanes can be named in two ways, with a common name or an lUPAC name. 

Methane, ethane, propane, n-butane, n-pentane, isopentane, etc. are common names that have little systematic basis and cause confusion in nomenclature. To prevent this, lUPAC developed a standard for naming organic compounds. According to the lUPAC system, there is only one name for a compound. 

To name the alkanes according to the lUPAC system the following rules can be used. 

Determine the chain with the longest continuous number of carbon atoms. This gives the starting name (often called the parent name) of the alkane. 

There are six carbon atoms in the longest chain in the example. So the parent name of the compound is hexane. 

The letter n (for normal), as in -butane, denotes an unbranched chain of C atoms. The prefix iso-(/-) indicates a CH, branch on the second C from the end e.g., isopentane is 

Alkyl groups, such as methyl (CHj) and ethyl (CH CH ), are derived by removing one H from alkanes. 

The prefixes sec- and ten- before the name of the group indicate that the H was removed from a secondary or tertiary C, respectively. A secondary C has bonds to two other C s, a tertiary to three other C s, and a primary either to three H s or to two H s and one C. 

The H s attached to these types of carbon atoms are also called primary, secondary and tertiary (1 , 2 and 3°). respectively. A quaternary C is bonded to four other C s. 

The letter R is often used to represent any alkyl group. 

The names methane, ethane, propane, and butane have historical roots. From pentane on, alkanes are named using the Greek word for the number of carbon atoms, plus the suffix -ane to identify the molecule as an alkane. Table 3-2 gives the names and physical properties of the n-alkanes up to 20 carbon atoms. 

If all alkanes had unbranched (straight-chain) structures, their nomenclature would be simple. Most alkanes have structural isomers, however, and we need a way of naming all the different isomers. For example, there are two isomers of formula C4H10. The unbranched isomer is simply called butane (or n-butane, meaning normal butane), and the branched isomer is called isobutane, meaning an isomer of butane.  

The three isomers of C5H12 are called pentane (or n-pentane), isopentane, and neopentane. 

Isobutane, isopentane, and neopentane are common names or trivial names, meaning historical names arising from common usage. Common names cannot easily describe the larger, more complicated molecules having many isomers, however. The number of isomers for any molecular formula grows rapidly as the number of carbon atoms increases. For example, there are 5 structural isomers of hexane, 18 isomers of octane, and 75 isomers of decane We need a system of nomenclature that enables us to name complicated molecules without having to memorize hundreds of these historical common names. 

The lUPAC system works consistently to name many different families of compounds. We will consider the naming of alkanes in detail, and later extend these rules to other kinds of compounds as we encounter them. The lUPAC system uses the longest chain of carbon atoms as the main chain, which is numbered to give the locations of side chains. Four rules govern this process. 

If necessary, review the suggested sections to prepare for this chapter. 

PLUS Visit www.wileyplus.com to check your understanding and for valuable practice. 

Recall that hydrocarbons are compounds comprised of just C and H for example  

Ethane, is unlike the other examples in that it has no it bonds. Hydrocarbons that lack Tt bonds are called saturated hydrocarbons, or alkanes. The names of these compounds usually end 

This chapter will focus on alkanes, beginning with a procedure for naming them. The system of naming chemical compounds, or nomenclature, will be developed and refined throughout the remaining chapters of this book. 

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In the nomenclature of alkanes, we started to number the carbon atoms starting from the end nearest to the substituent. Note that when we are numbering the carbon atoms in alkenes, the double bond is more important. 

Nomenclature of Alkanes Section 2.6 Names are included with structures in Problem 2.I9. 

The nomenclature of alkanes and all other organic compounds is based on the recommendations of the International Union of Pure and Applied Chemistry (lUPAC). The first four alkanes (methane, ethane, propane, and butane) have nonsystematic names. As Table 24.1 shows, the number of carbon atoms is reflected in the Greek prefixes for the alkanes containing five to ten carbons. We now apply the lUPAC rules to the following examples ... 

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