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Monday 29 April 2013

Alkanes

Hydrocarbons having no double or triple bond functional groups are classified as alkanes or cycloalkanes, depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings. Although these hydrocarbons have no functional groups, they constitute the framework on which functional groups are located in other classes of compounds, and provide an ideal starting point for studying and naming organic compounds. The alkanes and cycloalkanes are also members of a larger class of compounds referred to as aliphatic. Simply put, aliphatic compounds are compounds that do not incorporate any aromatic rings in their molecular structure.
The following table lists the IUPAC names assigned to simple continuous-chain alkanes from C-1 to C-10. A common "ane" suffix identifies these compounds as alkanes. Longer chain alkanes are well known, and their names may be found in many reference and text books. The names methane through decane should be memorized, since they constitute the root of many IUPAC names. Fortunately, common numerical prefixes are used in naming chains of five or more carbon atoms.

Examples of Simple Unbranched Alkanes

NameMolecular
Formula
Structural
Formula
Isomers
NameMolecular
Formula
Structural
Formula
Isomers
methaneCH4CH41
hexaneC6H14CH3(CH2)4CH35
ethaneC2H6CH3CH31
heptaneC7H16CH3(CH2)5CH39
propaneC3H8CH3CH2CH31
octaneC8H18CH3(CH2)6CH318
butaneC4H10CH3CH2CH2CH32
nonaneC9H20CH3(CH2)7CH335
pentaneC5H12CH3(CH2)3CH33
decaneC10H22CH3(CH2)8CH375
Some important behavior trends and terminologies:
  (i)   The formulas and structures of these alkanes increase uniformly by a CH2 increment.
 (ii)   A uniform variation of this kind in a series of compounds is called homologous.
(iii)   These formulas all fit the CnH2n+2 rule. This is also the highest possible H/C ratio for a stable hydrocarbon.
(iv)   Since the H/C ratio in these compounds is at a maximum, we call them saturated (with hydrogen).
Beginning with butane (C4H10), and becoming more numerous with larger alkanes, we note the existence of alkane isomers. For example, there are five C6H14 isomers, shown below as abbreviated line formulas (A through E):
Although these distinct compounds all have the same molecular formula, only one (A) can be called hexane. How then are we to name the others?
The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table. Note that the "ane" suffix is replaced by "yl" in naming groups. The symbol R is used to designate a generic (unspecified) alkyl group.
Group  CH3–   C2H5–   CH3CH2CH2–   (CH3)2CH–   CH3CH2CH2CH2–   (CH3)2CHCH2–   CH3CH2CH(CH3)–   (CH3)3C–   R– 
Name  Methyl  Ethyl  Propyl  Isopropyl  Butyl  Isobutyl  sec-Butyl  tert-Butyl    Alkyl  

IUPAC Rules for Alkane Nomenclature

 1.   Find and name the longest continuous carbon chain.
 2.   Identify and name groups attached to this chain.
 3.   Number the chain consecutively, starting at the end nearest a substituent group.
 4.   Designate the location of each substituent group by an appropriate number and name.
 5.   Assemble the name, listing groups in alphabetical order using the full name (e.g. cyclopropyl before isobutyl).
    The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing.
For the above isomers of hexane the IUPAC names are:   B  2-methylpentane    C  3-methylpentane    D  2,2-dimethylbutane    E  2,3-dimethylbutane
Halogen substituents are easily accommodated, using the names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-). For example, (CH3)2CHCH2CH2Br would be named 1-bromo-3-methylbutane. If the halogen is bonded to a simple alkyl group an alternative "alkyl halide" name may be used. Thus, C2H5Cl may be named chloroethane (no locator number is needed for a two carbon chain) or ethyl chloride. Halogenated alkyl substituents such as bromomethyl, BrCH2–, and trichloromethyl, CCl3–, may be listed and are alphabetized according to their full name

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