ALKENE - Physical properties of alkane (melting point, boiling point and density), combustion and halogenation of alkanes

ALKENE

Alkanes are open-chain (acyclic) hydrocarbons comprising the homologous series with the general formula,

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

                                                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.

                                Sawhorse Representation

                          

            Newman

 

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.

                                      2-methylpentane

  

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.

 

          2,2-dimethylpentane

 

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.