Constitutional isomers of alkanes

For an alkane, the number of possible constitutional isomers increases with increasing molecular size. Table 4.4 illustrates this trend. 

TABLE 4.4 NUMBER OF CONSTTTUnONAL ISOMERS FOR VARIOUS ALKANES 

When drawing the constitutional isomers of an alkane, make sure to avoid drawing the same isomer twice. As an example, consider CgHi4 for which there are five constitutional isomers. When drawing these isomers, it might be tempting to draw more than five structures. For example  

At first glance, the two highlighted compounds seem to be different. But upon further inspection, it becomes apparent that they are actually the same compound. To avoid drawing the same compound twice, it is helpful to use lUPAC rules to name each compound. If there are duph-cates, it will become apparent  

These two drawings generate the same name, and therefore, they must be the same compound. Even without formally naming these compounds, it is helpful to simply look at molecules from an lUPAC point of view. In other words, each of these compounds should be viewed as a parent chain of five carbon atoms, with a methyl group at C3. Viewing molecules in this way (firom an lUPAC point of view) will prove to be helpful in some cases. 

---

TABLE 27-4 Numbers of Possible Constitutional Isomers of Alkanes... 

Robinson, Harary and Balaban [54], by plying Pdlya s theorem and Otter s dissimilarity characteristic equation [85], presented for the first time recurrence formulas for counting the achiral isomers of alkyl radicals and alkanes. If the constitutional isomers of alkanes are denoted by Vn (quartic trees), and if their number, including stereoisomers, is denoted hy fn (steric trees), the latter number may be decomposed into achiral (tn) chiral (cn) ismners. The numbers fcH n = 1-14 are presented in Table 3 ... 

Common examples include the separation of positional isomers of substituted aryl- or naphthyl compounds, constitutional isomers of alkanes, geometric isomers of alkenes or alkynes, or isomers of polynuclear aromatic hydrocarbons. Other studies in-... 

In the course of doing this problem, you will write and name the 17 alkanes that, in addition to octane, CH3(CH2)6CH3, comprise the 18 constitutional isomers of C8H18. 

Counting the constitutional isomers of trees, even with the restriction that no vertex should have degree higher than four, is a problem which was solved long ago by Cayley [35,36], followed by Henze and Blair [42], and then P61ya [47]. However, alkanes can be chiral if they have at least seven carbon atoms. Counting stereoisomeric alkanes is a more... 

C4H9 Br is correct as it fits the general formula for an alkane where the total of H and Br is given by (2n + 2). It involves the replacement of H by Br in 411,0. One constitutional isomer is 1-bromobutane, CH3CH2CH2CH2Br. The other constitutional isomers of C4H9Br are 2-bromobu-tane, l-bromo-2-methylpropane, and 2-bromo-2-methylpropane. 

Ans. Alkenes and cycloalkanes have the same general formula and are constitutional isomers of each other. Knowing that the molecular formula of some unknown such as C5H10 fits the general formula C H2 tells us that it is not an alkane and that it is either an alkene or a cycloalkane. The molecular formula alone does not allow us to distinguish an alkene from a cycloalkane. However, alkenes and cycloalkanes have very different chemical properties. Cycloalkanes have similar chemical behavior as alkanes since both have the same kinds of bonds (C — C and C — H single bonds). The chemistry of alkenes is very different due to the presence of the carbon-carbon double bond (Sec. 12.6). 

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,)... 

For the C5 alkanes, the atoms can be arranged in three possible orders. These three different C5H12 alkanes are thus also constitutional isomers. There are five constitutional isomers of C6H14 flable 23-3). 

Imagine that you have an unknown compound with a molecular formula of C6H12O. The molecular formula by itself does not provide enough information to draw the structure of the compound. There are many constitutional isomers of CgHi20. Nevertheless, a careful analysis of the molecular formula can often provide helpful clues about the structure of the compound. To see how this works, let s begin by analyzing the molecular formula of several alkanes. 

Branched alkanes are constitutional isomers of straight-chain alkanes... 

Methane is the only alkane of molecular formula CH4 ethane the only one that is C2H6 and propane the only one that is C3Hj Beginning with C4H10 however constitutional isomers (Section 1 8) are possible two alkanes have this particular molecular formula In one called n butane, four carbons are joined m a continuous chain The nmn butane stands for normal and means that the carbon chain is unbranched The second isomer has a branched carbon chain and is called isobutane... 

Compounds like the two C4M [() molecules and the three C3I I 12 molecules, which have the same formula but different structures, are called isomers, from the Greek isos + meros, meaning "made of the same parts." Isomers are compounds that have the same numbers and kinds of atoms but differ in the way the atoms are arranged. Compounds like butane and isobutane, whose atoms are connected differently, are called constitutional isomers. We ll see shortly that other kinds of isomers are also possible, even among compounds whose atoms are connected in the same order. As Table 3.2 shows, the number of possible alkane isomers increases dramatically as the number of carbon atoms increases. 

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethanol and dimethyl ether), or different locations of a functional group along the chain (as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties, but with the same formula. 

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. 

They have the same molecular mass, but differ in their manner of linkage, their branching, and in the order (sequence) of their atoms (constitutional isomers). The number of possible isomers increases rapidly in higher generations - by analogy with the classical case of the alkanes. 

Constitutional isomers that differ only in the position of a double bond. Double-bond isomers hydrogenate to give the same alkane, (p. 298)... 

Constitutional isomers like butane and isobutane belong to the same family of compounds they are both alkanes. In contrast, constitutional isomers like CH3CH2OH and CH3OCH3 have different functional groups and belong to different families CH3CH2OH is an alcohol and CH3OCH3 is an ether. 

The maximum number of possible constitutional isomers increases dramatically as the number of carbon atoms in the alkane increases, as shown in Table 4.1. For example, there are 75 possible isomers for an alkane having 10 carbon atoms, but 366,319 possible isomers for one having 20 carbons. 

Problem 15.4 Draw all constitutional isomers formed by monochlorination of each alkane. 

---