CHAPTER TWO Alkanes CH3 Neopentane: CH3CCH (CH3)C he number of C,H2n+z iso. Table 2.3 presents the number of possible alkane isomers as a function of the num- mers has been calculated for ber of carbon atoms they contain. As the table shows, the number of isomers increases alues of n from 1 to 40 enormously with the number of carbon atoms and raises two important questions and the comment ma l. How can we tell when we have written all the possible isomers corresponding to C157H33s exceeds the number a particular molecular formula? niverse(10). These obser 2. How can we name alkanes so that each one has a unique name? ations and the historical The answer to the first question is that you cannot easily calculate the number of April 1989 issue of isomers. The data in Table 2.3 were determined by a mathematician who concluded that the Journal of Chemical Edu- there was no simple expression from which to calculate the number of isomers. The best cation(pp. 278-281). way to ensure that you have written all the isomers of a particular molecular formula is to work systematically, beginning with the unbranched chain and then shortening it while dding branches one by one. It is essential that you be able to recognize when two different looking structural formulas are actually the same molecule written in different ways. The key point is the connectivity of the carbon chain. For example, the following group of struc tural formulas do not represent different compounds; they are just a portion of the many ways we could write a structural formula for isopentane. Each one has a continuous chain of four carbons with a methyl branch located one carbon from the end of the chain CH The fact that all of these CH3,CI CH3CHCH2CH3 CH3 CHCHCH3 ame substance can be clearly CH3 CH3CH,CHCH3 CHCH, CH3 H3 TABLE 2. 3 The Number of Constitutionally Isomeric Alkanes of Particular Molecular formulas Molecular formula Number of constitutional isomers CHa CaH 1235 C6H14 C7H16 cC 855 15n32 4,347 366,319 62491,178805831 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website60 CHAPTER TWO Alkanes Table 2.3 presents the number of possible alkane isomers as a function of the num￾ber of carbon atoms they contain. As the table shows, the number of isomers increases enormously with the number of carbon atoms and raises two important questions: 1. How can we tell when we have written all the possible isomers corresponding to a particular molecular formula? 2. How can we name alkanes so that each one has a unique name? The answer to the first question is that you cannot easily calculate the number of isomers. The data in Table 2.3 were determined by a mathematician who concluded that there was no simple expression from which to calculate the number of isomers. The best way to ensure that you have written all the isomers of a particular molecular formula is to work systematically, beginning with the unbranched chain and then shortening it while adding branches one by one. It is essential that you be able to recognize when two different￾looking structural formulas are actually the same molecule written in different ways. The key point is the connectivity of the carbon chain. For example, the following group of struc￾tural formulas do not represent different compounds; they are just a portion of the many ways we could write a structural formula for isopentane. Each one has a continuous chain of four carbons with a methyl branch located one carbon from the end of the chain. CH3CHCH2CH3 W CH3 CH3CHCH2CH3 W CH3 CH3CH2CHCH3 W CH3 CH3CH2CHCH3 W CH3 CHCH2CH3 W W CH3 CH3 CH3 CH3CCH3 CH3 Neopentane: or (CH3)4C or TABLE 2.3 The Number of Constitutionally Isomeric Alkanes of Particular Molecular Formulas Molecular formula CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 C15H32 C20H42 C40H82 Number of constitutional isomers 1 1 1 2 3 5 9 18 35 75 4,347 366,319 62,491,178,805,831 The number of CnH2n2 iso￾mers has been calculated for values of n from 1 to 400 and the comment made that the number of isomers of C167H336 exceeds the number of particles in the known universe (1080 ). These obser￾vations and the historical background of isomer calcu￾lation are described in a pa￾per in the April 1989 issue of the Journal of Chemical Edu￾cation (pp. 278–281). The fact that all of these structural formulas represent the same substance can be clearly seen by making molecular models. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website