In our previous topic on hydrocarbons here, we gave some of the physical properties of alkanes, alkenes, and alkynes however, we did not give the chemical reactions of these hydrocarbons. Hydrocarbons have both physical and chemical properties and it is also very vital to know the chemical reactions of these compounds we would start with the alkanes first.
Chemical Reactions Of Alkanes
Alkanes are generally reactive towards polar reagents such as water, acid, alkalis, and redox agents. This is because they are saturated hydrocarbons hence the reason why they are called paraffin which is a greek word meaning little affinity. Alkanes undergo combustion and substitution reactions only.
Combustion Of Methane
Methane burns with a pale-blue flame in excess air, to produce carbon(IV) oxide and water vapour.
In a limited supply of air, Carbon(II) oxide is formed:
c. In an insufficient supply of air, lampblack is deposited:
d. When a mixture of methane and air is ignited, a violent explosion occurs. This reaction is responsible for most explosions that occur in coal mines.
Alkanes burn with pale blue flames in excess air, to produce carbon(IV) oxide, water vapour, and large amounts of heat.
General equation
Substitution reactions: Halogenation
Methane undergoes successive substitution reactions with chlorine, bromine, or iodine at room temperature in the presence of sunlight(or ultraviolet light) as the catalyst. In the substitution reaction with chlorine, a chlorine atom replaces one hydrogen atom in methane to produce hydrogen chloride and dichloromethane.
The substitution reaction goes on until all the hydrogen atoms in methane have been replaced.
General Methods Of Preparation Of Alkanes
Alkanes can be prepared by the following methods
From anhydrous Sodium Aklanoate
Alkanes are prepared by heating a mixture of anhydrous Sodium alkanoate and soda lime. Soda lime is an anhydrous mixture of NaOH and Cao. It is non-deliquescent and does not attack glass. NaOH alone cannot be used, because it is deliquescent, and attacks glass.
General Equation
From an Alkene
An alkane is obtained from the corresponding alkene for example, propane can be obtained from propene.
Chemical Reactions Of Alkenes
Alkenes have carbon-carbon double covalent bonds(unsaturated) meaning they are highly reactive. They undergo addition, combustion, oxidation, and polymerization reaction. The general representation of ethene and propane represents the general reactions of the alkenes.
Combustion
Alkenes burn in Oxygen with sooty(smoky) luminous flames unlike the alkanes due to higher carbon contents in the alkenes than in the corresponding alkanes. The products are Carbon(IV) oxide, water vapour, and heat:
The general equation for the complete combustion of alkenes in Oxygen is of the form:
Addition Reactions
An alkene undergoes addition reaction at the carbon-carbon double bonds to produce a saturated compound. Examples of addition reaction include hydrogenation, halogenation(the addition of halogens Chlorine, Bromine, or Iodine), the addition of bromine water, hydration, and hydrohalogenation(the addition of hydrogen halides like HCl, HBr, or HI).
General Methods For The Preparation Of Alkenes
Alkenes can be prepared by the following methods:
Dehydration Of Alkanols(Elimination Reaction)
Alkenes are prepared in the laboratory by the dehydration of alkanols(saturated alkanols) using excess of concentrated tetraoxosulphate(VI) acid. Dehydration involves the elimination of a water molecule in the form of -H and -OH from two adjacent carbon atoms; it leads to the formation of a carbon-carbon double bond and an example can be seen in the preparation of ethene which is prepared by the action of heat on a mixture of ethanol and excess of concentrated H2SO4 at about 170oC to 180oC.
By The Dehalogenation Of Haloalkanes(Alkyl Halides, RX) – Elimination Reaction
Dehydrohalogenation is the removal of hydrogen halide, HX(where X=F, Cl, Br, or I) from a haloalkane(alkyl halide, RX). When a haloalkane is heated with an alcoholic solution of potassium hydroxide KOH, the corresponding alkene is obtained. The dehydrohalogenation of chloroethane gives ethene.
Cracking Of Higeher Alkanes
Alkenes can be produced industrially by breaking down large, alkane molecules in petroleum fractions by heating, in the presence of a catalyst.
Chemical Reactions Of Alkynes
The alkynes are highly unsaturated because of the triple carbon-carbon bonds that exist. Reactions they undergo include addition and combustion reactions.
Combustion
The alkynes like the alkenes burn in oxygen with sooty luminous flames due to high carbon contents, to produce Carbon(IV) oxide and steam. When ethyne gas is passed through a jet and lighted, the resultant hot flame is called oxyethyne(or oxyacetylene) flame.
The General Equation for the complete combustion is outlined below
Addition Reaction
Alkynes undergo addition reaction in two stages and the mode of addition is identical. Examples of addition reactions that alkynes undergo include hydrogenation, halogenation, hydrohalogenation, and hydration.
Reaction Of Terminal Alkynes With Metals
Examples of these reactions include substitution reactions in alkynes(reaction with sodium metal, reaction with ammonical AgNO3 solution, and Reaction with ammonical Cu2Cl2 solution).