Alkanes and Alkane Isomers

CHAPTER 3 ORGANIC COMPOUNDS ALKANES AND THEIR STEREOCHEMISTRY 

Compounds like butane and pentane, whose carbons are all connected in a row, are called straight-chain alkanes, or normal alkanes. Compounds like 2-methylpropane (isobutane), 2-methylbutane, and 2,2-dimethylpropane, whose carbon chains branch, are called hranched-chain alkanes. The difference between the two is that you can draw a line connecting all the carbons of a straight-chain alkane without retracing your path or lifting your pencil from the paper. For a hranched-chain alkane, however, you either have to retrace your path or lift your pencil from the paper to draw a line connecting all the carbons. 

Compounds like the two C4H10 molecules and the three C5H12 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 

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isohutane 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. 

FIGURE 3.2 Some representations of butane, C Hiq. The molecule is the same regardless of how it s drawn. These structures imply only the connections between atoms they don t imply any specific geometry. 

Identify the functional groups in the following model of arecoline, a veterinary drug used to control worms in animals. Convert the drawing into a line-bond structure and a molecular formula (red = O, blue = N). 

ThomsonNOW Click Organic Interactive to learn to draw and recognize alkane isomers. 

Compounds like butane and pentane, whose carbons are all connected in a row, are called straight-chain alkanes, or normal alkanes. Compounds like 2-methylpropane (isobutane), 2-methylbutane, and 2,2-dimethylpropane, whose carbon chains branch, are called branched-chain alkanes. 

Different carbon skeletons C4H10 CH3 CH3CHCH3 and CH3CH2CH2CH3 

Different functional groups C2H6O CH3CH2OH Ethanol and CH3OCH3 Dimethyl ether 

We saw in Section 1.8 that the carbon-carbon single bond in ethane results from cr (head-on) overlap of carbon sp3 orbitals. If we imagine joining three, four, five, or even more carbon atoms by C-C single bonds, we can generate the large family of molecules called alkanes. 

Formula Number of isomers Formula Number of isomi 

Problem 3.1 Identify the functional groups in each of the following molecules  

Problem 3.2 Propose structures for simple molecules that contain the following functional groups (a) Alcohol (b) Aromatic ring (e) Carboxylic acid 

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Functional Groups 75 Alkanes and Alkane Isomers Alkyl Groups 84... 

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