Reactivity of Alkanes

Reactivity of Alkanes
Alknanes are among the least reactive types of organic compounds. They do not react with common acids or bases, nor do they react with common oxidizing or reducing agents. Alkanes, however, like other organic compounds, are flammable; that is they react rapidly with oxygen to give carbon dioxide and water, provided the reaction is initiated by a suitable heat source such as a flame of the spark from a spark plug. This reaction is called combustion. The combustion of methane is as following:

CH4 + O2 --------CO2 +H2O

The above reaction is example of complete combustion: combustion in which carbon dioxide and water are the only combustion products. Under conditions of oxygen deficiency, incomplete combustion may also occur with the formation of such byproducts as carbon monoxide, CO. Carbon monoxide is deadly poison because it bonds to, and displaces oxygen from, hemoglobin, the protein in red blood cells that transports oxygen to tissues. It is also odorless, and therefore difficult to detect without special equipment.Combustion of tremendous commercial importance because it liberates energy that can be used to keep us warm, generates energy, or move vehicles.Combustion is also used in the quantitative determination of elemental composition, called elemental analysis. Combustion has been used for this purpose since the beginning of the modern era of chemistry. Elemental analysis is useful for determining the formula of a compound at the two levels of molecular formula and empirical formula.
Occurrence and uses of alkanes
Most alkanes come from petroleum, or crude oil. Petroleum is a dark, viscous mixture of composed mostly of alkanes and aromatic hydrocarbons that are separated by technique called fractional distillation. In fractional distillation, a mixture of compounds is slowly boiled; the vapour is collected, cooled, and recondensed to a liquid. Because the compounds with the lowest boiling points vaporize most readily, the condensate from a fractional distillation is richest in the more volatile components of the mixture. As distillation continues, components of progressively higher boiling points appear in the condensate.Another important alkane source is natural gas, which is mostly methane. Natural gas comes from wells of different types. There are also significant biological sources of methane that could be exploited commercially. Methane is produce by the action of certain bacteria (methanogens) on decaying matter.
Naming Substituents
Carbon substituents bonded to a long carbon chain are called alkyl groups. An alkyl group is formed by removing one hydrogen from an alkane.An alkyl group is a part of a molecule that is now able to bond to another atom or a functional group. To name an alkyl group, change the -ane ending of the parent alkane to -yl. Thus,methane (CH4) becomes methyl (CH3 – ) and ethane (CH3CH3) becomes ethyl (CH3CH2 – ). As we learned in Section 3.1, R denotes a general carbon group bonded to a functional group. Rthus denotes any alkyl group.Naming three- and four-carbon alkyl groups is more complicated because the parent hydrocarbons have more than one type of hydrogen atom.
Functional groups
The structural features that make it possible to classify compounds by reactivity are called functional groups. A functional group is a part of a molecule composed of an atom or group of atoms that have a characteristic behavior. Chemically a given functional group behaves in nearly the same in every molecule it’s part of. For example, ethylene (ethane) the simplest compound with a carbon-carbon double bond, undergoes reactions that are remarkably similar to those of cholesterol, a much more complicated molecule that also contains a double bond. It can therefore be said that, the chemistry of every organic molecule, regardless of size and complexity, is determined by the functional groups it contains.

Functional groups are many but can be grouped into three categories:
1. Carbon-carbon multiple bonds
Alkenes, alkynes and arenas (aromatic compounds) all contain carbon-carbon multiple bonds. Because of their structural similarities, these compounds also have chemical similarities.

2. Carbon singly bonded to an electronegative atom
Alkyl halides, alcohols, ethers, amines, thiols, and sulfides all have a carbon atom bonded to an electronegative atom. In all cases, the bonds are polar (a polar covalent bond) with the carbon atom bearing a partial positive charge (+) and the electronegative atom bearing a partial negative charge (-).

3. Caron-oxygen double bond (carbonyl groups)
Carbon-oxygen double bonds are present in some of the most important compounds in organic chemistry. These compounds are similar in many respects but differ depending on the identity of the atoms bonded to the carbonyl-group carbon.

Physical Properties of Alkanes
Alkanes contain only nonpolar C –C and C – H bonds, and as a result they exhibit only weak
van der Waals forces. Table 4.2 summarizes how these intermolecular forces affect the physical
properties of alkanes.
The gasoline industry exploits the dependence of boiling point and melting point on alkane size
by seasonally changing the composition of gasoline in locations where it gets very hot in the
summer and very cold in the winter. Gasoline is refi ned to contain a larger fraction of higher boiling
hydrocarbons in warmer weather, so it evaporates less readily. In colder weather, it is refi ned
to contain more lower boiling hydrocarbons, so it freezes less readily.
Because nonpolar alkanes are not water soluble, crude petroleum that leaks into the sea from an
oil tanker creates an insoluble oil slick on the surface. The insoluble hydrocarbon oil poses a special
threat to birds whose feathers are coated with natural nonpolar oils for insulation. Because
these hydrophobic oils dissolve in the crude petroleum, birds lose their layer of natural protection
and many die.

Variation of physical properties of alkanes

REFERENCES

Crocker,Ernest C (1992).Application of the Octet Theory To Single-Ring Aromatic Compounds

Macmurry,John(2007).Organic Chemistry(7th edition).Brooks-Cole

  Organic Chemistry. Marc Loudon, 3^rd edition The Benjamin/Cummings Publishing Company,Inc., California, 1995.

  Organic Chemistry. John MucMurry. 3^rd edition. Brooks/Cole Publishing Company, 1992