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JEE Important Chapter - Hydrocarbons

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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 CxHy. 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 CnH2n+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 CnH2n and that of alkynes is CnH2n-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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Last updated date: 26th Sep 2023
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FAQs on JEE Important Chapter - Hydrocarbons

1. What are the Uses of Hydrocarbons?

Answer- Hydrocarbons are widely used for various purposes-

  • They are used as fuels in households, in motor vehicles. Like LPG, CNG, etc.

  • Hydrocarbons are required to manufacture polymers that give polythene, polystyrene, etc.

  • They are also used as organic starting compounds in manufacturing drugs and dyes.

  • Due to their lubricating properties, they are used as lubricating oils and grease.

2. What are Some of the Important Properties of Hydrocarbons?

Answer- Some of the important properties of hydrocarbons are as follows.

  • Hydrocarbons can form complex molecules like cyclohexane and in rare instances aromatic hydrocarbons like benzene.

  • Alkanes with 10 C-atoms or less are generally gases at room temperatures more than 10 C-atoms, the molecules are gases or liquid. 

  • Alkanes have high molecular mass and high boiling points. For example, C2H6 has a higher boiling point than CH4.