Alkanes

In the previous two chapters we have studied in some detail the properties of the two simplest saturated hydrocarbons, methane and ethane, and have shown how their simple derivatives are named using the rules of the International Union of Pure and Applied Chemistry (IUPAC rules). In this chapter we shall examine the larger alkanes, including those that are cyclic (cycloalkanes). Open-chain, or acyclic, alkanes have the general formula CnHZn+, , whereas cycloalkanes have the formula CnHz,. It is essential that one be able to name compounds correctly and we will begin our discussion of alkanes with a survey of nomenclature.

nomenclature

The IUPAC rules for naming alkanes are simple and easy to apply. However, few people adhere strictly to these rules and it is necessary to be familiar with some other commonly used naming terms; these are often simple and convenient when applied to simple compounds but become cumbersome or ambiguous with more complex compounds. The IUPAC name for a compound is always acceptable; hence, when asked to supply a name to a compound whose structure is given, it is best to follow the IUPAC rules. You should become familiar enough with other common terms, however, so you can supply the correct structure to a compound whose name is given in nonIUPAC terminology.

The alkanes are classified as "continuous chain " (i.e., unbranched) if all the carbon atoms in the chain are linked to no more than two other carbons, or " branched chain" with one or more carbon atoms linked to three or four other carbons. Branching is only possible with alkanes C_4, and up.'

The first four continuous-chain hydrocarbons have nonsystematic names:

The name Zmethylpropane is appropriate because the longest continuous chain in the molecule is made up of three carbons; it is thus a derivative of propane. The second carbon (from either end) has one of its hydrogens replaced by a methyl group, hence the prefix 2-methyl. It should be remembered that the shape of this molecule is not as depicted in the formula shown. The tetrahedral arrangement about the central carbon atom makes all three methyl groups equivalent (Figure 3.1).

There are three known compounds with the formula C5HI2, and this is the number of isomers expected on the basis of carbon being tetravalent and hydrogen monovalent :

The prefix iso denotes a compound with two methyl groups at the end of a chain, and neo means three methyl groups at the end of a chain.

When we come to the C₆, alkanes we find that there are five isomeric compounds, C₆H_I4,. Clearly, trivial prefixes become less and less useful as the length of the chain grows and the number of possible isomers increases. Accordingly, hexane and its four branched-chain isomers are here only given IUPAC names. (The second compound in the list might be called isohexane and the fourth neohexane.)

The branched-chain compounds considered so far all contain the simplest group, methyl, as substituent. Alkyl groups such as methyl are obtained by writing the alkane minus one of its hydrogens. Thus, we have methyl (CH₃-) and ethyl (CH₃,-CH₂-). A problem arises when we come to the alkyl groups of propane, CH₃-CH₂,-CH₃, . The hydrogen atoms in this molecule are not all equivalent. Removing one of the six terminal hydrogens produces the n-propyl group, CH₃-CH₂-CH₂-; removing one of the two central hydrogens produces the isopropyl group, CH₃,-CH-CH₃, (usually written

Additional examples are listed in Table 3-2. These have been further classi

fied according to whether they are primary, secondary, or tertiary. An alkyl group is described as primary if the carbon at the point of attachment is bonded to only one other carbon, as secondary if bonded to two other carbons, and tertiary if bonded to three other carbons. The methyl group is a special case and is regarded as a primary group.

In a few cases, where there is a high degree of symmetry, hydrocarbons are conveniently named as derivatives of methane or ethane.

In naming complex compounds, you must pick out the longest consecutive chain of carbon atoms. The longest chain may not be obvious from the way in which the structure has been drawn on paper. Thus the hydrocarbon [I] is a pentane rather than a butane derivative, since the longest chain is one with five carbons.

The parent hydrocarbon is numbered starting from the end of the chain, and the substituent groups are assigned numbers corresponding to their positions on the chain. The direction of numbering is chosen to give the lowest sum for the numbers of the side-chain substituents. Thus, hydrocarbon [l] is 2,3-dimethylpentane rather than 3,4-dimethylpentane. Although the latter name would enable one to write the correct structure for this compound, it would not be found in any dictionary or compendium of organic compounds.

Where there are two identical substituents at one position, as in [2], numbers are supplied for each. Remember that the numerals represent positions and the prefixes di-, tri-, and so on represent the number of substituents. Note that there should always be as many numerals as there are substituents, that is, 2,2,3 (three numerals) and trimethyl (three substituents).

Branched-chain substituent groups are given appropriate names by a simple extension of the system used for branched-chain hydrocarbons. The longest chain of the substituent is numbered starting bith the carbon attached directly to the parent hydrocarbon chain. Parentheses are used to separate the numbering of the substituent and the main hydrocarbon chain. The IUPAC rules

permit use of the substituent group names in Table 3.2, so that s-butyl can be used in place of (I-methylpropyl) for this example.

When there are two or more different substituents present, the question arises as to what order they should be cited in naming the compound. Two systems are commonly used which cite the alkyl substituents (1) in order of increasing complexity or (2) in alphabetical order. We shall adhere to the latter system mainly because it is the practice of Chemical Abstracts.' Examples are given below.

Biweekly publication of the American Chemical Society: an index to, and a digest of, recent chemical publications throughout the world.

Derivatives of alkanes, such as haloalkanes (R-Cl) and nitroalkanes (R-NO₂,), where R denotes an alkyl group, are named similarly by the IUPAC system. Definite orders of precedence are assigned substituents of different types when two or more are attached to a hydrocarbon chain. Thus, alkanes with halogen and alkyl substituents are generally named as haloalkylalkanes (not as alkylhaloalkanes); alkanes with halogen and nitro substituents are named as halonitroalkanes (not as nitrohaloalkanes).

Note that I-chlorobutane is written as one word whereas the alternate name n-butyl chloride is written as two words. In the former the chloro group is substituted for one of the hydrogens of butane and the position of substitution is indicated by the numeral. The latter name is formed by simply combining names for the two parts of the compound just as one would do for NaCl, sodium chloride.

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