[Chemistry Class Notes] Hydrocarbons Pdf for Exam

What are Hydrocarbons?

Simplest organic compounds, hydrocarbons are majorly studied in organic chemistry and it is an organic compound that bears only hydrogen and carbon atoms. Those hydrocarbons from where one hydrogen atom is removed are called hydrocarbons and these represent the functional groups. Hydrocarbons are usually colourless, hydrophobic and have only weak odours.

We can find hydrocarbons in nature and besides fossil fuels, these are present in trees and plants. For example, in carrots and green leaves, these are available in the form of carotenoids that are known as pigments. Fuel in buses and autos, CNG, cooking gas and LPG all consist of hydrocarbons. A hydrocarbon polymer or hydrocarbon chain is a chain-like molecule that consists of multiple units which are linked together and an example of it is such that over 98 percent of natural crude rubber. The chemistry and structure of individual hydrocarbons is dependent on the types of chemical bonds linking the atoms of their constituent molecules.

Types of Hydrocarbons

Besides alcohols, aldehydes and carboxylic acids, hydrocarbons are the organic compounds which are simplest and can be divided into the following types:

1. Saturated Hydrocarbons

A saturated hydrocarbon compound contains only C-C single bonds and all the carbons are fully used in this. It is less reactive. Alkanes are the saturated hydrocarbons.

These have only single bonds, i.e. when n=1 (only one carbon atom), Methane, CH₄ is the example of an alkane. When n=2, 4 covalent bonds are formed for each carbon and C₂H₆ is formed. Similarly, C₃H₈ is formed; all of these (methane, ethane and propane respectively) are alkanes. Therefore, the general formula for alkanes is CnH(2n+2).

Number of carbon atoms derive the names of the organic compound where one carbon atom presence refers to meth, two is eth, 3 is prop, 4 is but, 5 is pent, 6 is hex, 7 is hept, 8 is oct, 9 is non and 10 carbon atoms presence means dec.  

2. Unsaturated Hydrocarbons

Here, at least two carbon atoms are connected by a double bond or a triple bond. Unsaturated hydrocarbons are more reactive because double and triple bonds can be broken easily and something can be added on it. Alkenes and alkynes are the two types of unsaturated hydrocarbons where alkenes contain atleast one double bond and alkynes contain atleast one triple bond. 

These compounds have double bonds. When n=1, we do not have a valid alkene. When n=2, C₂H₄ (ethene) is formed, when n=3, C₃H₆ (propene) is formed. Therefore, the general formula for alkenes is CnH(2n).

These are also unsaturated hydrocarbons having triple bonds. When n=1, it is not a valid alkyne, when n=2, C₂H₂ (ethyne) is formed, and when n=3, C₃H₄ (propyne) is formed. Therefore, the general formula for alkynes is CnH(2n-2). All alkynes will get ‘yne’ suffix and ‘ene’ is for alkene whereas ‘ane’ suffix is for alkanes.

3. Cyclic or Closed Chain Hydrocarbons

Besides open chain or aliphatic compounds (alkanes, alkenes and alkynes) as discussed above, there exist hydrocarbons that have at least one aromatic carbon ring and these are known as aromatic hydrocarbons. 

 

Examples of Hydrocarbons

Gaseous hydrocarbons are methane and propane, liquid hydrocarbons are hexane and benzene, low melting solids or waxes hydrocarbons are paraffin wax and naphthalene and polymeric chains of hydrocarbons include polystyrene, polypropylene and polyethylene.

 

Uses of Hydrocarbons

There’s not a single use of hydrocarbons, multiple uses of hydrocarbons are known. Let’s explore the same under the following points:

• These are used as a combustible fuel source, eg: Methane which is the component of natural gas. 

• Gasoline, jet fuel and naphtha are aliphatic hydrocarbons which are highly used in different industries.

• Roofing compounds bitumen or pavement composition and wood preservatives are varied forms of hydrocarbons.

• Their applications as large-scale non-fuels such as ethane and propane are obtained from petroleum and natural gas. These two gases can further be converted into ethylene and propylene.

• Special hydrocarbons such as mixture of benzene, toluene and xylene isomers are known and their consumption is very high.

• Naturally occurring hydrocarbons are found in stingless Brazilian bees which leave unique hydrocarbon scents and help in determination of kin from non-kin.

How Reactive are Hydrocarbons?

Hydrocarbons are well known for inertness, especially the saturated members of hydrocarbons are inert. However, others will undergo three major types of reactions mentioned as follows:

1. Substitution Reaction: This kind of reaction is undergone by aromatic compounds. For example: Benzene and ethylene react to form ethylbenzene.

2. Addition Reaction: This kind of reaction is undergone by alkenes and alkynes and a variety of reagents add across the pi-bonds. Such reagents include hydrogen chloride, chlorine and hydrogen. Alkenes and alkynes undergo alkyne metathesis, polymerization and alkene metathesis.

3. Combustion: When hydrocarbons are combusted, the energy produced are the heat sources and electrical energy. It can be used as home heaters that make use of petroleum or natural gas. In the presence of oxygen, hydrocarbons produce steam, carbon dioxide and heat during combustion.