Hydrocarbons Chapter - Chemistry JEE Main

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.

JEE Main Chemistry Chapters 2024

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.

Alkanes

Conformations: Sawhorse and Newman Projections (of Ethane):

Alkanes are hydrocarbons with single covalent bonds. Ethane (C2H6) provides an excellent example for understanding conformational isomerism. Conformations are different spatial arrangements of the atoms in a molecule resulting from the rotation of sigma (σ) bonds. Ethane can exist in two conformations: eclipsed and staggered.

Eclipsed Conformation:

In the eclipsed conformation, the two methyl (CH3) groups are as close as possible, leading to steric hindrance and higher energy.

Staggered Conformation:

In the staggered conformation, the CH3 groups are as far apart as possible, minimizing steric hindrance and lowering energy.

To visualize the conformational changes, we use Sawhorse and Newman projections.

Sawhorse Projection:

In the Sawhorse projection, you view the molecule at an angle to see the 3D arrangement. It shows the dihedral angle between two groups.

Newman Projection:

The Newman projection looks down the carbon-carbon (C-C) bond. It shows the front carbon atom as a circle and the rear carbon atom as a dot. The dihedral angle is the angle between the C1-C2 bond and the C2-H bond.

Mechanism of Halogenation of Alkanes:

Halogenation is the process of adding halogens (Cl2, Br2) to alkanes. This reaction is initiated by UV light or heat and proceeds through a free radical mechanism.

Initiation: Ultraviolet (UV) light or heat breaks the diatomic halogen molecule (Cl2 or Br2) into two radicals: Cl∙ and Cl∙.

Propagation: The Cl∙ radical abstracts a hydrogen atom from the alkane, forming a hydrogen halide (HCl) and leaving a carbon radical. The newly formed carbon radical can react with another diatomic halogen molecule to repeat the process.

Termination: Two radicals can combine to form a stable molecule, terminating the chain reaction.

Alkenes

Geometrical Isomerism:

Alkenes exhibit geometrical isomerism when there are two different groups attached to each carbon of the C=C double bond. Geometrical isomers have the same molecular formula but differ in spatial arrangement.

For example, in 2-butene, there are two possible geometric isomers: cis-2-butene and trans-2-butene. In cis-isomer, the substituents are on the same side of the double bond, while in the trans-isomer, they are on opposite sides.

Mechanism of Electrophilic Addition:

The electrophilic addition reaction involves the addition of electrophiles to the carbon-carbon double bond in alkenes. This reaction follows the Markovnikov rule, where the electrophile adds to the carbon atom with more hydrogen atoms.

Addition of Hydrogen (Hydrogenation):

Hydrogenation is the addition of hydrogen (H2) to alkenes using a catalyst, typically a metal like palladium (Pd), platinum (Pt), or nickel (Ni).

Addition of Halogens (Halogenation):

Halogenation involves the addition of halogens (Cl2 or Br2) to the double bond. The reaction proceeds through electrophilic addition.

Addition of Water (Hydration):

Hydration is the addition of water (H2O) to alkenes in the presence of an acid catalyst. The reaction forms alcohols.

Addition of Hydrogen Halides:

In this reaction, hydrogen halides (HCl, HBr) add to alkenes to form alkyl halides. The Markovnikov rule dictates which carbon atom of the double bond receives the hydrogen and which the halogen.

Ozonolysis and Polymerization:

Ozonolysis:

Ozonolysis is a reaction where ozone (O3) is used to cleave carbon-carbon double bonds. The reaction breaks the double bond and forms carbonyl compounds.

Polymerization:

Polymerization is the process by which small monomers are linked together to form long-chain polymers. For alkenes, polymerization can yield important products like polyethylene and polypropylene.

Alkynes

Acidic Character:

Alkynes are more acidic than alkanes and alkenes due to the presence of a triple bond. The acidity is due to the ease with which the C≡C triple bond can release a proton (H+).

Addition Reactions:

Alkynes undergo addition reactions similar to alkenes. These reactions include the addition of hydrogen (H2), halogens (Cl2, Br2), water (H2O), and hydrogen halides (HCl, HBr).

Polymerization:

Alkynes can also undergo polymerization reactions to form important products, such as neoprene.

Aromatic Hydrocarbons

Nomenclature:

Aromatic hydrocarbons, like benzene, are named using IUPAC rules. They have a benzene ring as their structural unit. Substituted benzene compounds are named based on the location and type of substituent.

Benzene Structure and Aromaticity:

Benzene has a hexagonal ring structure with alternating single and double bonds. It exhibits a special stability due to its aromaticity, which arises from the delocalization of π electrons.

Electrophilic Substitution Reactions:

Aromatic compounds undergo electrophilic substitution reactions where an electrophile (electron-seeking species) replaces a hydrogen atom on the benzene ring. Common reactions include halogenation, nitration, Friedel-Crafts alkylation, and acylation.

Halogenation:

Halogenation of benzene involves the addition of a halogen (Cl2 or Br2) to the benzene ring. It follows electrophilic aromatic substitution.

Nitration:

Nitration is the addition of a nitro group (-NO2) to the benzene ring, typically using concentrated nitric acid (HNO3) and sulfuric acid (H2SO4).

Friedel-Crafts Alkylation:

Friedel-Crafts alkylation allows for the introduction of alkyl groups onto the benzene ring. It involves the reaction between an alkyl halide and an aluminum chloride (AlCl3) catalyst.

Friedel-Crafts Acylation:

Friedel-Crafts acylation is the process of adding an acyl group to the benzene ring. It involves the reaction between an acyl chloride and an aluminum chloride (AlCl3) catalyst.

To read more about Friedel-Crafts reactions, check out Vedantu’s page on the same.

Directive Influence of Functional Group:

In mono-substituted benzene compounds, the presence of substituents can influence the position of further substitution in subsequent reactions. This influence can be either ortho (next to the existing group), meta (two carbons away), or para (opposite position).

Understanding these reactions and concepts is essential for JEE Main students, as they form the foundation for organic chemistry and are critical for solving complex problems in the field.

JEE Main Hydrocarbons Study Materials

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