# Hydrocarbons   Share

## What is Hydrocarbons

Hydrocarbons are the chemical compounds composed of Carbon and Hydrogen. Everything mostly that we use in our daily life is hydrocarbons such as LPG, petrol, insecticides, soaps, etc. There is a large number of organic compounds exist in nature because of two reasons:

(i) Tetravalency: The valency of carbon is four due to which it is able to form covalent bonds with other atoms at a time.
(ii) Catenation: The ability of carbon atom to bind with other carbon atoms to form long chains.
Because of the above properties, hydrocarbons are able to exist in various forms like linear structure, branched-chain structure, and ring structure.

The given below are the major applications of hydrocarbons:

(i) Natural gas: This is naturally occurring hydrocarbon gas, consisting mainly of methane and some other higher alkanes as well. This gas in the form of LPG which majorly contains butane is mainly used in our kitchens.

(ii) Soaps and detergents: Hydrocarbons are used in the manufacturing of soaps and detergents.

(iii) Insecticides and pesticides: Hydrocarbons are majorly used in insecticides and pesticides. Some common examples of these containing hydrocarbons are DDT, Malathion, carbofuran, etc.

## Overview of Hydrocarbons

The hydrocarbons can be classified in the following three categories, on the basis of the bonds present between carbon-carbon atoms.

(i) Saturated hydrocarbons: These are the types of hydrocarbons in which all carbon-carbon and carbon-hydrogen are singly bonded with each other. These hydrocarbons either exist as an open chain or circular ring structured and they are known as alkanes and cycloalkanes respectively. Some common examples of saturated hydrocarbons are:

(ii) Unsaturated hydrocarbon: These are the hydrocarbons that have double or triple bonds between the carbon atoms. These unsaturated hydrocarbons are of two types, i.e, Alkenes (containing double bonds) and Alkynes (containing triple bonds). The general formula for alkenes and alkynes are CnH2n and CnH2n-2, respectively.

(iii) Aromatic hydrocarbon: These hydrocarbons are circular rings with delocalized pi electrons between carbon atoms. The simplest aromatic compound known is benzene, with chemical formula C6H6.

This classification of hydrocarbons can be tabulated as follows:

 Hydrocarbon Characteristic Example Saturated (Alkanes) Only a single bond Butane Unsaturated (Alkenes, Alkynes) Double and Triple bonds Ethene, Butyne Aromatic Localized pi electrons Benzene

Alkanes
Alkanes are the most basic class of organic compounds. They are also known as saturated compounds. All the carbon-carbon bonds in these molecules are single bonds.

• Nomenclature and Isomerism
The IUPAC rules for nomenclature and isomerism we have already studied in the previous chapter - 'Some basic principles of organic chemistry'. Here we will see them again with a few examples
The molecular formula of the compound is C4H10. Let's try to draw its different structures with their scientific names.

(I)                                            (II)

The (I) compound is named as simply 'butane' and (II) compound is named as '2-Methylpropane'. Now since two molecules differ in their structures thus, they are known as structural isomers.

• Preparation
There are various methods to prepare alkanes. Some of them are listed below.
(i) From unsaturated hydrocarbons: In this reaction, alkenes are reacted with hydrogen in the presence of catalysts like palladium or nickel and form alkanes.
$\mathrm{CH_{2}=CH_{2}\, +\, H_{2}\overset{Pt/Pd/Ni}{\rightarrow}CH_{3}-CH_{3}}$
(ii) From alkyl halides: Alkyl halides on reaction with zinc and dilute hydrochloric acid give alkanes.
$\mathrm{CH_{3}-Cl\, +\, H_{2}\overset{Zn, H^{+}}{\rightarrow}CH_{4}}$

• Chemical Reactions
There are various important chemical reactions that are done by alkanes.
In this reaction, alkanes react with halogens and form alkyl halide as the product.

(i) Halogenation:

$\mathrm{CH_{4}\, +\, Cl_{2}\overset{hv}{\rightarrow}CH_{3}Cl\, +\, HCl}$

ii) Combustion: Alkanes undergo oxidation in the presence of air and form carbon dioxide and water.
$\mathrm{CH_{4}(g)\, +\, 2O_{2}(g)\overset{hv}{\rightarrow}CO_{2}(g)\, +\, 2H_{2}O(l)}$

Alkenes
Alkenes are another class of organic compounds. These compounds are also known as unsaturated hydrocarbons. In these compounds, one or more than one carbon-carbon double bond is always present.

• Nomenclature
The IUPAC rules for the nomenclature of alkenes have already been discussed in an earlier chapter. Some of the important alkenes and their IUPAC names are mentioned below.
$\mathrm{(i)CH_{3}-CH_{2}=CH_{2}\:\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \:\: \: \: \: \: \: \: \: \: Propene}$
$\mathrm{(i)CH_{3}-CH_{2}-CH=CH_{2}\:\: \: \: \: \: \: \: \: But-1-ene}$

• Isomerism
Alkenes both show structural and geometrical isomerism.

(i) Structural isomerism: The alkene compound with molecular formula C2H4 exists in various structures as shown below.

(ii) Geometrical isomerism: Many of the alkenes show different geometries of the same molecule and isomers are known as cis and trans.

• Preparation
Alkenes are prepared in several ways. Some of the important methods to prepare alkenes are discussed below.

(i) From alkynes: In this reaction, alkynes are reacted with hydrogen in the presence of palladised charcoal and form alkenes.
$\mathrm{CH\equiv CH\, +\, H_{2}\overset{Pd/C}{\rightarrow}CH_{2}=CH_{2}}$
(ii) From alkyl halides: Alkyl halides on heating with alcoholic KOH forms alkenes.
$\mathrm{CH_{3}-CH_{2}Cl\, +\, H_{2}\overset{alc.\, KOH}{\rightarrow}CH_{2}=CH_{2}}$

• Chemical Reactions
There are some important chemical reactions that alkenes do and form different products.

(i) Addition of halogens: In this reaction, halogens are added to alkenes and form vicinal halides as shown below.
$\mathrm{CH_{2}=CH_{2}\, +\, Br-Br\,\overset{CCl_{4}}{\rightarrow}CH_{2}Cl=CH_{2}Cl}$
(ii) Addition of alkyl halides: In this reaction, hydrogen halide is reacted with an alkene and form alkyl halide.
$\mathrm{CH_{2}=CH_{2}\, +\, HBr\,\rightarrow CH_{3}CH_{2}Br}$

## How to prepare for hydrocarbons

• This chapter is the beginning of the organic chemistry and possesses a very high weightage of marks in boards exams and other competitive exams like JEE and NEET. It is completely theory-based. You are not supposed to memorize any formula. Practice a lot of numerical for getting a good hold on this chapter.

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• For preparing this chapter, firstly, you must have a basic understanding of chapter - "Organic Chemistry- Some Basic Principles and Techniques". After this, you must read this chapter, "Hydrocarbons" from NCERT  thoroughly with each and every example solved on your own.

• Practice each reaction carefully, specially, Grignard reagent, Wittig's reaction, Cope's reaction, etc. a lot of times, so that you would be able to understand the concepts at your fingertips.

• In the nutshell, it can be said that although this chapter is lengthier than other chapters, it is a very simple and straightforward one. So always say a "Big YES" to this chapter.

## Chemistry Chapter- wise Notes for Engineering and Medical Exams

 Chapters No. Chapters Name Chapter 1 Some basic concepts in chemistry Chapter 2 States of matter Chapter 3 Atomic Structure Chapter 4 Solutions Chapter 5 Chemical Thermodynamics Chapter 6 Equilibrium Chapter 7 Redox Reaction and Electrochemistry Chapter 8 Chemical kinetics Chapter 9 Surface Chemistry Chapter 10 General Principle and processes of Isolation of metals Chapter 11 Classification of Elements and Periodic table Chapter 12 Hydrogen Chapter 13 p- block Elements Chapter 14 s-block Elements (Alkali and Alkaline Earth Metals) Chapter 15 Chemical Bonding and Molecular Structure Chapter 16 d- and f- BLOCK ELEMENTS Chapter 17 Coordination Compounds Chapter  18 Environmental Chemistry Chapter 19 Purification and Characterisation of Organic Compounds Chapter 20 Some Basic Principles of Organic Chemistry Chapter 22 Organic Compounds containing Halogens Chapter 23 Organic Compounds containing Oxygen Chapter 24 Organic Compounds Containing Nitrogen Chapter 25 Polymers Chapter 26 Biomolecules Chapter 27 Chemistry in Everyday Life Chapter 28 Principles Related to Practical Chemistry

### Topics from Hydrocarbons

• general methods of preparation, properties and reactions. ( JEE Main, GUJCET Pharmacy ) (9 concepts)
• Mechanism of electrophilic substitution: halogenation, nitration, Friedel - Craft alkylation and acylation ( JEE Main, GUJCET Pharmacy ) (1 concepts)
• Alkanes - Conformations: Sawhorse and Newman projections (of ethane); Mechanism of halogenation of alkanes ( JEE Main, GUJCET Pharmacy ) (6 concepts)
• Aromatic hydrocarbons - Nomenclature, benzene -structure and aromaticity ( JEE Main, GUJCET Pharmacy ) (1 concepts)
• Alkenes - Geometrical isomerism; Mechanism of clcctrophilic addition: addition of hydrogen, halogens, water, hydrogen halides ( JEE Main, GUJCET Pharmacy ) (13 concepts)
• Alkynes - Acidic character; Addition of hydrogen, halogens, water and hydrogen halides; Polymerization ( JEE Main, GUJCET Pharmacy ) (5 concepts)
• ALKANES ( JEE Main, GUJCET Pharmacy ) (11 concepts)
• ALKENES ( JEE Main, GUJCET Pharmacy ) (14 concepts)
• ALKYNES ( JEE Main, GUJCET Pharmacy ) (5 concepts)
• AROMATIC HYDROCARBON ( JEE Main, GUJCET Pharmacy ) (11 concepts)

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