Preparation of Alkanes & Alkanes Reactivity

Preparation of Alkanes

the alkanes are generally prepared by petroleum or crude oil and natural resources. Various methods are used to prepare alkanes but the following two methods that is widely used is as follow

Hydrogenation of alkenes and alkynes

the world hydrogenation means addition of hydrogens. As we know that both the alkenes and alkyne are unsaturated therefore they can add further atoms into their chain. So, when hydrogen is added to the alkenes or alkynes with the presence of a specific catalyst like nickel (Ni), palladium (Pd), and platinum (Pt) etc. at temperature 250 degree to 300 degree. Then the corresponding alkanes are obtained.

Hydrogenation of alkenes

When the hydrogen is added to the alkene that is ethene in presence of catalyst at 250 to 300 temperature then the alkanes is formed. Such as

Hydrogenation of Alkynes

as we know that alkynes are having triple covalent bond. So, when we add hydrogen to alkynes then the alkene is formed and when again we add hydrogen to this alkene then the alkanes is formed in presence of catalyst at temperature of 250 to 300 degree. That’s why this step completes in two steps. Such as

Reduction of alkyl halides

The chain of hydrocarbons containing halogens are called halides. Halogens are so reactive so they do not require any heat or energy for reaction as room temperature is enough for them. But only suitable reducing agent like Zn is needed for them to react. Such as

Oxidizing agents

It is defined as

The substance which accepts electrons.
The substance which is reduced. Such as F, Cl, Br etc.
The substance whose oxidation state or number is decreased.

Explanation: It is also called oxidants and oxidizer. It is usually

The chemical substance having high oxygen number. Such as H₂O₂, H₂SO₄, MnO₄ etc.
The chemical substances having high electronegativity to gain one or more electrons. Such as all halogens and oxygen etc.

Reducing agents

It is defined as

The substance which donate/gives electrons.
The substance which is oxidize.
The substance whose oxidation state or number is increased.

Explanation: It is also called reductants and reducers. It is usually the chemical substance having high electropositive and less electronegativity to donate/loss/gives electrons. Such as Li, Na, Mg, Fe, Zn etc.

Physical properties of Alkanes

The physical properties are as follow

Alkanes are non – polar covalent compound.
Alkanes are not soluble in polar solvents.
Alkanes are soluble in non – polar solvent like petrol and benzene etc.
Alkanes are non – polar in character.
Alkane’s melting point, boiling point and density increases with increase in the number of carbon atoms. Such as first four member of alkanes are gases at normal pressure and temperature. And alkanes having more than five carbon atoms are liquid at normal pressure and temperature and alkanes having more than 20 carbon atoms are waxy solid at normal pressure and temperature.
Alkanes are used as a good non – polar solvents.
Alkanes are used as water repellents.
A mixture of alkanes that is paraffin wax coating is used to give shine to the apples.
Alkanes are used in making of many important compounds.
Alkanes are also used as a fuel.

The Chemical properties and reactivity of alkanes

The alkanes ate less react organic compounds that’s why they are called paraffins where parum means little and affins means attraction or to react. But it’s not mean that they are totally inert. Some important reaction of alkanes are as follow;

Halogenation

the elements like F, Cl, Br, I and At which are present in 7^th or 17^th group in periodic table is called halogens. The reaction of halogens with other atom molecules are called halogenation. As reactivity of halogens decreases down the group. So, during reaction the require some source of energy and this energy is provided by the sun light. Thus the process in which the reaction takes place in presence of sun light ( sun light as a catalyst ) is called photochemical reaction. Such as when chlorine reacts with methane in presence of sun light it forms chloro ethane and HCl. That is

Here the CCL₄ is the organic compound as there is no hydrogen but it is the derivatives of organic compound due to the fact of chloroniation of methane.

Combustion

The Process of burning is called combustion and always takes place in presence of oxygen. The combustion or complete oxidation is vital for human Because during this process, a lot of heat is produced which we use as a chief source of energy. As most of the fuel in petroleum product is in the form of alkanes and we used these as a chief source of energy for our needs. And alkanes burn in air to form carbon dioxide, water and heat. As alkanes is less reactive that’s why alkanes go under oxidation. Formula for such type of reaction is

Such reaction is

CH₄+ 2O₂ ====è CO₂ + 2H₂O + heat ΔH = – 890 KJ/mole

C₃H₈+ 5O₂ ====è 3CO₂ + 4H₂O + heat ΔH = – 2219 KJ/mole

Here negative sign shows that the energy or heat has been released.

16 February 2015