Alkanes

Alkanes are open-chain (acyclic) hydrocarbons comprising the homologous series with the general formula, CnH2n+2, where n is an integer. They have only single bonds and therefore are said to be saturated

Free Rotation about the Carbon-Carbon Single Bond: Conformations

Electron diffraction and spectroscopic studies have verified this structure in all respects, giving the following measurements for the molecule : bond angles, 109.5o; C-H length, 1.10Å; C-C length, 1.53 Å. Similar studies have shown that with only slight variations, these values are quite characteristic of C-H and C-C bonds and of carbon bond angles in alkanes.

This set of bond angles and bond lengths still does not limit the molecule of ethane to a single arrangement of atoms, since the relationship between the hydrogens of one carbon and the hydrogens of the other carbon is not specified. Different arrangement of atoms that can be converted into one another by rotation about single bonds are called Conformations. Arrangement I is called the eclipsed conformation and arrangement II is called its staggered conformation

Exercise 1: i) Write down the order of energy of eclipsed and staggered conformation for ethane molecule.

ii) The number of conformation for a single bond are
a) 1
b) 2
c) 6
d) infinite

The IUPAC system of alkane nomenclature is based on the simple fundamental principle of considering all compounds to be derivatives of the longest single carbon chain present in the compound. The chain is then numbered from one end to the other, the end chosen as number 1 is that which gives the smaller number at the first point of difference.

When there are two or more identical appendages – the modifying prefixes di-, tri-, tetra-, penta-, hexa-, and so on are used, but every appendage group still gets its own number.

When two or more appendage locants are employed, the longest chain is numbered from the end which produces the lowest series of locants. When comparing one series of locants with another, that series is lower which contains the lower number at the first point of difference.

Several common groups have special names that must be memorized by the student.

A more complex appendage group is named as a derivative of the longest carbon chain in the group starting from the carbon that is attached to the principal chain. The description of the appendage is distinguished from that of the principal chain by enclosing it in parentheses

When two or more appendages of different nature are present, they are cited as prefixes in alphabetical order. Prefixes specifying the number of identical appendages (di, tri, tetra and so on) and hyphenated prefixes (tert-or t, sec-) are ignored in alphabetizing except when part of a complex substituent. The prefixes cyclo-, iso-, and neo-count as a part of the group name for the purposes of alphabetizing.

When chains of equal length compete for selection as the main chain for purposes of numbering, that chain is selected which has the greatest number of appendage attached to it.

When two or more appendages are in equivalent positions, the lower number is assigned to the one that is cited first in the name (that is one that comes first in the alphabetic listing).

[The complete IUPAC rules actually allow a choice regarding the order in which appendage groups may be cited. One may cite the appendages alphabetically, as above, or in order of increasing complexity].

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