Naming compounds alkane isomers

The number of constitutional isomers increases rapidly as the number of carbons in an alkane increases. For example, there are 75 alkanes with molecular formula C10H22 and 4347 alkanes with molecular formula C15H32. To avoid having to memorize the names of thousands of stmctural units, chemists have devised rules that name compounds on the basis of their structures. That way, only the rules have to be learned. Because the name is based on the structure, these rules make it possible to deduce the structure of a compound from its name. 

They have a different structure, although they both have the same molecular formula. One is called normal butane, abbreviated to -butane the other may be called isobutane or 2-methylpropane - the latter name describes the structure of the compound, as you will see later. These compounds are isomers of one another. They are not the same compounds and have different melting points, boiling points and solubilities. Isomers are compounds which have the same moiecuiar formuia, but different moiecuiar structures. After butane, the longer the carbon chain of an alkane, the more structurai isomers are possible for a particular molecular formula. For example, there are 75 decanes (C10H22) and over three-hundred-thousand eicosanes (C20H42) ... 

This common naming system has limitations. Pentane has three isomers, and hexane has five. The more complicated the compound, the greater the number of isomers, and the greater the number of special prefixes needed to name all the isomers. It would be extremely difficult and time-consuming to try to identify each of the 75 isomeric alkanes containing 10 carbon atoms by a unique prefix or name. 

We have just seen that the three C5H12 isomers all incorporate pentane in their names and are differentiated by the prefixes n- iso , and neo. Extending this approach to alkanes beyond C5H12 fails because we run out of descriptive prefixes before all the isomers have unique names. As difficult as it would be to invent different names for the 18 constitutional isomers of CgHjg, for example, it would be even harder to remember which structure corresponded to which name. For this and other reasons, organic chemists have developed systematic ways to name compounds based on their structure. The most widely used approach is called the lUPAC rules lUPAC stands for the International Union of Pure and Applied Chemistry. (See the boxed essay. What s in a Name Organic Nomenclature,)... 

As these two examples demonstrate, the names of alkanes end with -ene and nnmbers are nsed to indicate the position of the double bond. The parent chain is nnmbered so that the lowest nnmber possible is given to one of the carbon atoms in the donble bond, regardless of any other substitnents present in the compound (for example, alkyl gronps or halides). The numbers in the names of alkenes refer to the lowest numbered carbon atom in the chain that is part of the C=C bond of the alkene. The name butene means that there are four carbon atoms in the longest chain. Because of restricted rotation abont the carbon-carbon double bond, alkenes can form geometric isomers (Section 4.4). In this case, the name of an alkene must also specify whether the isomer is cis or trans ... 

Chapter 11, Introduction to Organic Chemistry Hydrocarbons, combines Chapters 10 and 11 of GOB, eleventh edition. This new chapter compares inorganic and organic compounds, and describes the structures and naming of alkanes, alkenes including cis-trans isomers, alkynes, and aromatic compounds. 

Alkanes are a class of saturated hydrocarbons with the general formula C H2n. -2- They contain no functional groups, are relatively inert, and can be either straight-chain (normal) or branched. Alkanes are named by a series of IUPAC rules of nomenclature. Compounds that have the same chemical formula but different structures are called isomers. More specifically, compounds such as butane and isobutane, which differ in their connections between atoms, are called constitutional isomers. 

The chain and branched chain saturated hydrocarbons make up a family called the alkanes. Some saturated hydrocarbons with five carbon atoms are shown in Figure 18-11. The first example, containing no branches, is called normal-pentane or, briefly, n-pentane. The second example has a single branch at the end of the chain. Such a structural type is commonly identified by the prefix iso- . Hence this isomer is called /50-pentane. The third example in Figure 18-11 also contains five carbon atoms but it contains the distinctive feature of a cyclic carbon structure. Such a compound is identified by the prefix cyclo in its name—in the case shown, cyclopentane. 

The naming rules for straight-chain alkanes can, with a few additions, help you recognize and name other organic compounds. You now know that the name of a straight-chain alkane is composed of a root (such as meth-) plus a suffix (-ane). Earlier in the chapter, you saw the isomers of C6Hi4. Figure 13.13 shows one of them, called 2-methylpentane. 

When naming alcohols, take the name of the parent alkane and drop the final e from the parent name. Then add the suffix -ol to the end of the parent name. For example, the first alcohol in Figure 11.8 has one carbon atom and four single bonds. It looks like methane, but it is an alcohol. The name of this compound will be methanol. The second compound has three carbon atoms and is an alcohol. This compound is called 1-propanol, the l indicating that the functional group is on the first carbon atom. The alcohol on the right is called 2-propanol. Because it differs from 1-propanol only by the location of the —OH group, it can be considered to be an isomer of 1 -propanol. 

In molecules containing chains longer than three carbon atoms it is often necessary to ve the location on a chain of any attached group. For example, in butane (the first molecule where this occiu s in alkanes) there are two possible structures, namely, butane or 2-methylpropane. Both have the same general formula but the shapes of the molecules are different. We call such compounds isomers of each other. (This is discussed in detail in Unit 6.4.)... 

Two or more compounds with the same molecular formula but different properties are called isomers. Isomers with different arrangements of bonded atoms are constitutional (or structural) isomers alkanes with the same number of C atoms but different skeletons are examples. The smallest alkane to exhibit constitutional isomerism has four C atoms two different compounds have the formula C4H10, as shown in Table 15.3 on the next page. The unbranched one is butane (common name, /t-butane n- stands for normal, or having a straight chain), and the other is 2-methylpropane (common name, i obutane). Similarly, three compounds... 

Compounds such as butane and isobutane that have the same molecular formula but differ in the order in which the atoms are connected are called constitutional isomers—their molecules have different constitutions. In fact, isobutane got its name because it is an iso mer of butane. The stmctural unit—a carbon bonded to a hydrogen and two CH3 groups—that occurs in isobutane has come to be called iso. Thus, the name isobutane tells you that the compound is a four-carbon alkane with an iso structural unit. 

Many alkanes and other organic compounds have both common names and systematic names. Why are both common and systematic names used Usually the common name came first and is widely known. Many consumer products are labeled with the common name, and when only a few isomers are possible, the common name adequately identifies the product for the consumer. For example, isobutane, the common name for methylpropane, is sufficient because there is only one branched-chain isomer possible for C Hjq. However, a system of common names quickly fails when several structural isomers are possible. 

The suffix labels the functional group whose presence places the molecule into the appropriate class of chemical compounds. In this scheme the saturated hydrocarbons, the alkanes have the suffix -ane. For naming isomers, the system is more complicated and includes additional rules. Since the molecules of isomers are branched, the root name must correspond to the longest chain. The sidechains are treated as additional groups called substituents. In the final name of the structure, the substituents are introduced as prefixes to the root. The names of substituents are formed following the same rules as in the case of simple alkanes, i.e. the number of C-atoms followed by the suffix -yl. 

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. 

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