Functional Groups and Nomenclature

Organic compounds containing only carbon and hydrogen 

Because of their nonpolar nature, alkanes are insoluble in water. They are hydrophobic (water-hating) compounds. They are composed of atoms of lower mass (C and H) than water (O and H), so liquid alkanes are less dense than water. A mixture of water and a liquid alkane forms two layers, with the alkane as the upper layer. Many of the solvents that are used in the organic laboratory are compounds composed mainly of carbon and hydrogen, and so they tend to be less dense than water. Solvents whose molecules contain a significant fraction of more massive elements, such as chlorine, are more dense than water. 

Alkanes are rather unreactive compounds and are sometimes called paraffins (from the Latin for Tittle affinity ). They have strong C—C and C—H bonds, which are hard to break. All their electron pairs are in relatively stable sigma bonding MOs. They have no polar sites or unshared electrons to attract Lewis acid or base reagents. They are extremely weak proton acids. Therefore, only a few of the reactions covered in this text involve alkanes. 

Section 2.7 showed that it is often convenient to view a molecule as being composed of two parts the functional group, where reactions occur, and the alkyl group, sometimes called the backbone of the compound. The alkyl group is just the alkane part of the molecule and, like alkanes, does not tend to enter into reactions. In most cases it does not matter what the exact structure of the alkyl group is, because the reactions occur at the functional group. 

Alkanes occur in nature in deposits of natural gas and oil. Natural gas is primarily methane, containing progressively smaller amounts of ethane, propane, and butanes. Petroleum is a complex mixture of alkanes and other hydrocarbons. Petroleum is refined by distillation. The various distillation cuts, based on boiling point, are shown in Table 5.2. 

1 Follow steps 1 through 5 outlined in Section 5.3 in the text for naming these alkanes. 

Step 1 Find the longest, continuous carbon chain in the compound. The number of carbons in this backbone chain determines the root. If there is more than one chain of equal length, choose the one with the more branches. For example, in (e) there are two different 6 carbon chains, one with two branches and the other with three branches. 

Choose the one with the three branches as the root chain. 

Step 2 Attach the suffix (-ane for alkanes). In the above example, the root chain has six carbons, so this compound is named as a hexane. 

Step 3 Number the root carbon chain starting from the end that will give the lower number to the first branch. If the first branch occurs at an equal distance from either end, then choose the end that will give the lower number to the second branch. In the above example, starting at either end of the root chain, the first branch occurs at position 2. In the correct numbering, shown above, the second branch occurs at position 3. If the other end were chosen as position 1, the second branch would be at 

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