Structural isomers of alkanes

Isomers are distinctly different compounds, with different properties, that have the same molecular formula. In Section 22.1, we considered structural isomers of alkanes. You will recall that butane and 2-methylpropane have the same molecular formula, C4H10, but different structural formulas. In these, as in all structural isomers, the order in which the atoms are bonded to each other differs. 

Isomers are compounds that have the same molecular formulas but different structural formulas. Review the writing of the various structural isomers of alkanes. Make sure that each carbon atom has four bonds. 

Molecular Formulas and Possible Structural Isomers of Alkanes... 

Table 8.1. 1 Number of structural isomers for alkanes of increasing carbon number ...

The following example deals with the number of structural isomers of an alkane. 

Ans. The structural formula need not be used for this equation since the reaction involves complete dissection of the alkane. The molecular formula is sufficient and the same equation can be used for the combustion of any of the structural isomers of CgHig. The unbalanced equation is... 

In structural Isomerism, first met in the discussion of alkanes and which exists between but-l-ene, but-2-ene and 2-methylpropene, the compounds have their atoms bonded to different groups of atoms. For example, consider the underlined carbon atoms in the structural isomers of butene (C4H8) ... 

Diazirines, which are structural isomers of diazoalkanes, similarly extrude nitrogen to give carbenes. Thus, the adamantylidene derivative (118) gives the spiro-alkane (119) on heating in cyclohexene. 

Figure 16.2 shows the structures of methane, ethane, propane, and butane, the first four alkanes ( = 1 to n = 4). Natural gas is a mixture of methane, ethane, and a small amount of propane. Recall from Section 4.3 that the carbon atom of methane is i/ -hybridized. In fact, the carbon atoms in all the alkanes are sp -hybridized. There is only one way to join the carbon atoms in ethane and propane, but there are two possible ways in butane, resulting in the structural isomers n-butane and isobutane. (See Section 4.4 to review structural isomers.) All alkanes with n > 4 can exist in a varie g i0t isWwtWtlUly gagifecr of possiblities increasing rap-... 

Table 2.1 shows that there are 75 structural isomers of the alkane C,oH22. How many such isomers do you think there might be if we double the number of carbons (C20H42) The answer is 366,319 And if we double the number of carbons again (C40H82) Exactly 62,481,801,147,341. Of course, no one sits down with pencil and paper or molecular models and determines these numbers by constructing all the possibilities it could take a lifetime. Complex mathematical formulas have been developed to compute these numbers. 

With the five-carbon alkane, pentane, there are three ways to draw the structural formula of this compound with five carbon atoms and twelve hydrogen atoms. The isomers of normal pentane are isopentane and neopentane. The structural formulas of these compounds are illustrated in Table 2, while typical properties are given in Table 1. 

As in the alkanes, it is possible for carbon atoms to align themselves in different orders to form isomers. Not only is it possible for the carbon atoms to form branches which produce isomers, but it is also possible for the double bond to be situated between different carbon atoms in different compounds. This different position of the double bond also results in different structural formulas, which, of course, are isomers. Just as in the alkanes, isomers of the alkenes have different properties. The unsaturated hydrocarbons and their derivatives are more active chemically than the saturated hydrocarbons and their derivatives. 

Isomers are substances having the same molecular formula and molecular weight, but differing in physical and chemical properties. Since branched and straight-chain alkanes with the same molecular formula can exist as distinct structures having different geometrical arrangement of the atoms, they are termed structural isomers. One example is C H,j (butane) which has two isomers ... 

C09-0075. Draw Lewis structures of all possible stmctural isomers of the alkanes with formula Cg H14. ... 

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