JEE Important Chapter - Hydrocarbons

What are Hydrocarbons?

Hydrocarbons are organic compounds that are composed of only two elements – carbon and hydrogen. In general, hydrocarbons are colourless gases with quite weak odours. They may have simple or complex structures depending upon the type of hydrocarbons. There are 4 types of hydrocarbons, namely, alkanes, alkenes, alkynes, and aromatics. By studying hydrocarbons, we can have an insight into the chemical properties of various other important functional groups, organic compounds, and their preparation. 

Hydrocarbons such as propane and butane are used commercially as fuels in the form of Liquefied Petroleum Gas (LPG). Also, benzene, one of the simplest aromatic hydrocarbons, serves as raw material for the synthesis of many synthetic drugs. A commonly used molecular formula of a hydrocarbon is C_xH_y. Hydrocarbons are more often found in trees and plants. Carotenes are organic pigments found in carrots are a type of hydrocarbon.

Classification and Types

Earlier, hydrocarbons were classified as either aliphatic or aromatic compounds. They were classified on the basis of their source and properties. It is because it was found that Aliphatic hydrocarbons were derived from the chemical degradation of fats or oils whereas aromatic hydrocarbons contained substances that were a result of the chemical degradation of certain plant extracts.

Today, hydrocarbons are classified on the basis of their structure. 

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Types of Hydrocarbons

• Saturated Hydrocarbons: These compounds have carbon-carbon atoms and carbon-hydrogen atoms held together by single bonds. They are considered as the simplest hydrocarbons. In terms of hybridization, they show the presence of sp³ hybridized carbon atoms with no sp² or sp hybridized carbon atoms. They are called alkanes with the general formula C_nH_2n+2.

• Unsaturated Hydrocarbons: These compounds may have single, double, or triple bonds between carbon-carbon atoms. The double-bonded compounds are classified as alkenes and the triple bonded compounds are alkynes. The general formula for alkenes is C_nH_2n and that of alkynes is C_nH_2n-2.

• Cycloalkanes: These hydrocarbons possess one or multiple carbon rings with the hydrogen atom attached to the carbon ring.

• Aromatic Hydrocarbons: These hydrocarbons are also identified as arenes. These are compounds consisting of at least one aromatic ring.

• Aliphatic Hydrocarbons: These hydrocarbons are mostly straight-chain structures having no rings in them.

• Alicyclic Hydrocarbons: These hydrocarbons are present in a ring structure. The carbon atoms can be sp, sp², or sp³ hybridized in such compounds.

Properties of Hydrocarbons

Owing to their different molecular structures, the empirical formula of hydrocarbons is also different from each other. 

For example, the amount of bonded hydrogen decreases in alkenes and alkynes. This phenomenon occurs due to the property of catenation of carbon that prevents the complete saturation of the hydrocarbon by the formation of double or triple bonds. Catenation is the ability of hydrocarbons to bond to themselves. Due to this property, they can form more complex molecules like cyclohexane and in rare instances aromatic hydrocarbons like benzene.

Cracking Of Hydrocarbons

Cracking of hydrocarbons is a process in which heavy organic molecules are broken down into lighter molecules. It is done by supplying an adequate amount of heat and pressure. Catalysts are also used in some reactions to speed up the process. This process is widely used in the commercial production of diesel fuel and gasoline.

Physical Properties

Alkanes with 10 C-atoms or less are generally gases at room temperatures. For more than 10 C-atoms, the molecules are gases or liquid. Alkanes generally have low boiling and melting points owing to their weak Van Der Waals interaction.

The boiling point depends on the following factors:

• Molecular mass

• Branching

Alkanes have high molecular mass and high boiling points. Eg: C₂H₆ has more boiling point than CH₄

Alkanes having the same molecular mass but having a different number of branches: the one with less branching has more boiling point this is because the Van Der Waals force becomes weak as the area increases.

For example, CH₃-CH₂-CH₂-CH₃ has more boiling points. Alkanes are very feebly soluble in water but they are soluble in non-polar solvents such as Benzene, CCl₄, etc.

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