Alkanes

Introduction to hydrocarbons

Hydrocarbons refer to organic compounds that contain only carbon and hydrogen. Alkanes are the simplest hydrocarbons containing only a single covalent bond in between. The 2 major classes of hydrocarbons are the aliphatic and aromatic hydrocarbons. Natural gas and petroleum are the major sources of aliphatic hydrocarbons while coal is the source of aromatic hydrocarbons. The flow chart below shows how hydrocarbons are classified.

 

Alkanes

According to IUPAC  alkanes are defined as "acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms". Still, some definitions state that an alkane is a saturated hydrocarbon, including the monocyclic (i.e. the cycloalkanes) or polycyclic, despite they are indicated by CnH2n. The simplest alkane is methane; one of the major component of natural gas. Methane gas is sometimes known as marsh gas because it is mainly formed by the action of bacteria on decaying vegetation in swampy and marshy areas. The methane molecule consists of four hydrogen atoms and one carbon atom. It is an example of C-H bonding. The carbon has four valence electrons and therefore four hydrogen atoms, each with one valence electron, form four covalent C-H bonds. The combination yields a methane molecule as shown below. The C-H bonding in methane illustrates an important principle: because a carbon atom is made up of four valence electrons, it always forms four covalent bonds.

 

Nomenclature 

To draw a structural formula for a straight alkane, always write the symbol for carbon as many times as necessary to obtain the proper chain length. Use the line to represent the covalent bonds and fill the carbon with hydrogen atoms. The names of alkanes end with –ane. A homologous series is only formed by compounds if there is a constant increase in the molecular structure from one compound in the series to the next. The –CH2- group is the increment of change in straight-chain alkanes.

From the table above, as the number of carbon atoms increase, so does the boiling point. This is also true for the melting point. Generally, the structural formulas show all the atoms and bonds in a molecule. However, we can use condensed formulas to represent these alkanes. The root part of an alkane's name shows the number of carbon atoms it contains. For example, meth implies that it contains only one carbon atom, eth means it has two carbon atoms, prop means it has three carbon atoms, dec means it has ten carbon atoms, etc.

Isomerism

Hydrogen atoms are not the only atoms that can bond to the carbon atoms in a hydrocarbon. The halogens and groups of atoms such as Sulphur, nitrogen, phosphorus, and oxygen can replace hydrogen in the structure. An atom or group of atoms that can replace a hydrogen atom on a parent hydrocarbon molecule is referred to us a substituent.

 

An alkyl group refers to a hydrocarbon substituent in a compound. In most cases, an alkyl group can either be one or more carbons. There are three main alkyl groups that are added to the hydrocarbons. These include the methyl group, ethyl group, and the propyl group. In simple words, an alkyl is an alkane in which a hydrogen atom has been removed. The alkyl groups are also known as radicals. They are named by deleting the –ane ending from the parent hydrocarbon name and adding –yl. For example, in CH3CH2CH2CH3, removing a hydrogen atom in this compound will form a butyl, CH3CH2CH2CH2 -.  When a substituent group is added to a straight-chain hydrocarbon, branches are formed. An alkane with one or more alkyl groups is known as a branched-chain alkane.

Preparation of Alkanes

Alkanes are prepared by the following methods.

1. Hydrogenation of unsaturated hydrocarbons

Hydrogenation refers to the addition of hydrogen to a hydrocarbon, and it takes place in the presence of a finely divided nickel catalyst at temperatures of between 200℃-250℃

The reaction is referred to as Sabatier and Sanderson’s reaction.

CH2= CH2 + H2  → CH3 –CH3

CH≡CH + H2 → CH3- CH3

2. Reduction of alkyl halides

Alkyl halide can be used to prepare alkanes. In this process, the alkyl halide is reduced with zinc or aluminium amalgam in either ethanol or LiAlH4.

R-X + 2H → R-H + HX

Alkyl halides can be reduced by heating them with hydrogen iodide and red phosphorous at 150℃.

R-I + HI → RH+I2

3.The action of water on aluminium carbide or beryllium carbide

Al4C3 + 12H2O →CH4 + 4Al((OH)3

Be2C + 4H2O → 2Be(OH)2+ CH4

Physical properties

State: The first four alkanes are colourless and odourless gases. The next thirteen alkanes are colourless and odourless liquids. Heavy alkanes such as C-18 and above are colourless and odourless solids.

Density : The density of alkanes increases very slowly due to the increase in the molecular mass up to appoint when their density becomes a constant, 0.8.

 Solubility: The solubility of alkanes in solvents vary from one member to another. Alkanes are insoluble in polar solvents as water but soluble in a non-polar solvent like ether.

Boiling points: The boiling point of straight-chain alkanes increases with an increase in the number of carbon atoms.

Melting points: The melting points of alkanes do not follow a very smooth gradation with an increase of molecular size.

Chemical properties 

The presence of the C-C and C-H bonds makes alkanes stable and inert compounds. They are saturated compounds with strong sigma bonds which can't be easily be broken. At high temperatures, alkanes, therefore, react by a free radical mechanism.

Free radical substitution/halogenation

Halogens such as chlorine and bromine react with alkanes in the presence of sunlight or in darkness at high temperatures to yield substituent compounds.

  1. CH4 +Cl2 → CH3Cl +HCl

  2. CH3Cl +Cl2 → CH2Cl2 +HCl

  3. CH2Cl +Cl2 → CHCl3 +HCl

  4. CHCl3 +Cl2 → CCl4 +HCl

Nitration

Nitration process is only possible to alkanes that contain three or more carbon atoms. For example, nitration of butane produces a mixture of compounds as illustrated below.

C4H9-H+HO-NO2→ C4H9NO2 +H2O

Sulphonation

Alkanes with more than six carbon atoms undergo sulphonation when treated with concentrated sulphuric acid.

C6H13+ HOSO3H→C6H13-SO3H + H2O

Combustion

Alkanes burn in the presence of oxygen gas to produce carbon (IV) oxide and water with the production of heat. The reaction is exothermic, and that is why alkanes are used as fuel.

Read More

  1. Alkynes

  2. Alkenes

  3. Introduction to Organic chemistry  

  4. Homologous series

Review questions

  1. Name the major source of aromatic hydrocarbons.

  2. Name the compound which has 3 carbon atoms and 8 covalent bonds.

  3. When does a homologous series is formed? 

  4. Explain the chemical properties of alkanes.

  5. Explain the solubility of alkanes in different solvents.

 



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