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  Chemistry of Alkanes, Chemistry tutorial

Introduction:

We are familiar that hydrocarbons as organic compound comprising of carbon and hydrogen only. We were as well identifying that hydrocarbons can simply be categorized into alkanes, alkenes, alkynes and aromatic (particularly benzene) compounds. The nomenclature of such classes of hydrocarbon was as well illustrated.

The alkanes, having a general formula CnH 2n+2 or RH is the simplest homologous series and comprise of saturated hydrocarbons.

Alkanes, being saturated hydrocarbons are somewhat inert and don't undergo any reaction other than combustion and substitution. They play a significant part in petroleum chemistry. Alkanes are as well termed to as paraffins from the Latin words meaning a little affinity. As alkanes have atoms which are linked by only single bonds (that is, saturation), the few chemical reactions they experience is mainly of substitution type reactions-one atom being replaced by the other.

Natural Sources of Alkanes:

Natural Gas:

This is one of the main sources of alkanes. The natural gas is found in the strata of earth in various parts of the universe, example - Kuwait, Saudi Arabia, Nigeria, USA, Iraq and so on, and it is at times found related with petroleum. Natural gas is the main source of methane altogether with smaller quantities of C2 - C6 alkanes (that is, ethane, propane, butane, pentane and hexane). The alkanes are separated through fractional distillation. Whenever sulphur is present, it is oxidized to sulphur (VI) oxide, the precursor to H2SO4.

Petroleum or crude oil:

The fractional distillation of petroleum gives a broad range of alkanes. The C1 - C5   alkanes are as well obtained like in natural gas and the fractions of higher boiling points include mostly higher alkanes.

Vegetable Origin:

Methane is obtained as the product of anaerobic action on organic matter found buried in the earth or in sewages or in marsh gas and fire damp. Coal gas is obtained from the destructive distillation of natural coal includes around 30% methane.

Laboratory preparation of alkanes:

1) Heating anhydrous salts of alkanoic (organic) acids. The -CO2- group of alkanoic acid can be eliminated by heating the sodium salts of the alkanoic acid with soda lime (that is, a mixture of sodium and calcium hydroxides).

For illustration:

RCOONa+ + NaOH → RH + Na2CO3

CH3CO2Na            +       NaOH        →     CH4 + Na2CO3

Sodium ethanoate    (from soda lime)      methane

The methane gas is collected over the water.

2) Wurtz coupling - alkyl halides example: iodoalkanes can be coupled in the presence of the sodium metal to form alkanes, example:

2RI + 2Na → R-R + 2NaI

2CH3I + 2Na → CH3-CH3 + 2NaI

                          Ethane

Properties of Alkanes:

Physical properties:

1) The boiling and melting points of simple alkanes increase steadily as the number of carbon atoms rises due to the increasing strength of the van der waals forces and increase in the molecular mass. There is a gradation in crate from gas to liquid to solid as you move from lower to the higher members. Example: methane (CH4), is a gas, hexane (C6H12) is a liquid, dodecane C12H24 is a waxy solid.

2) All the alkanes are almost insoluble in water, and being less dense, the liquid and solid alkanes float on the surface of water. Example: hexane and dodecane. That is why water can't be employed for putting out petrol and oil fires. The lower members of the alkane series are soluble in the organic solvents.

Chemical Properties:

1) Combustion:

Alkanes burn in the ample supply of oxygen to form carbon (IV) oxide and water. This is as well characteristic of other hydrocarbons - ethene, ethyne and benzene. In limited supply of oxygen, carbon (II) oxide and water is formed rather.

CH4 + 2O2 → CO2 + 2H2O

2CH4 + 3O2 → 2CO + 4H2O (limited supply of oxygen)

2C2H6 + 7O2 → 4CO2 + 6H2O

The ease of burning accounts for the use of many alkanes as fuels.

2) Substitution Reactions:

One or more of the hydrogen atoms in the alkane can be substituted by the halogens (Cl, Br or I). The alkane reacts with chlorine, bromine or iodine in the presence of ultra-violet light or a temperature of around 400°C.

The reaction is halogenation (that is, addition of halogens) reaction and it is a substitution (substituting the hydrogen atom with the other element) method.

281_Substitution Reaction.jpg

Fig: Substitution Reactions

(c) Cracking:

It is a significant industrial method used for breaking large hydrocarbon molecules to smaller ones. Alkanes experience industrially significant elimination reactions in which they lose hydrogen. The reaction needs a high temperature and, possibly, a catalyst. The products are generally unsaturated hydrocarbons (alkenes and alkynes) and at times soot (that is, carbon black) is formed.

C2H6 → 850oC → H2C = CH2 + H2

C3H8 → Cr2O3/Al2O3 catalyst → H3C - CH = CH2 + H2

2CH4 → 1500oC → HC ≡ CH + 3H2

Uses of Alkanes:

1) They are generally employed as fuels - butane gas and petrol.

2) Higher alkanes like paraffin wax is employed for candle manufacture, lubricants and so on.

3) Alkanes react by chlorine in the presence of light to give chloroalkanes example - chloroform, employed in hospitals as an 'Anaesthetic' agent, carbon tetrachloride employed as solvent for grease in dry cleaning and as well in the fire extinguishers.

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