Hydrocarbons
Hydrocarbons are organic compounds composed of only hydrogen and carbon atoms. They are the building blocks for all other organic molecules.
Hydrocarbons are organic compounds composed of carbon and hydrogen atoms. Generally colorless and possessing a weak odor, hydrocarbons can range from simple to complex structures and can be classified into four subcategories: alkanes, alkenes, alkynes, and aromatic hydrocarbons. Studying hydrocarbons can help to understand the characteristics and preparation of other functional groups, and propane and butane are used commercially as Liquefied Petroleum Gas (LPG). Additionally, benzene, a simple aromatic hydrocarbon, serves as a raw material for the production of many synthetic drugs.
Download the JEE Previous Year Solved Questions PDF for Hydrocarbons Here
Structure of Hydrocarbon:
Table of Contents
Classification of Hydrocarbons
Uses of Hydrocarbons
The molecular formula CxHy represents hydrocarbons, organic compounds found in plants and trees. For example, carotenes, an organic pigment found in green leaves and carrots, are hydrocarbons. These compounds make up 98% of natural crude rubber and have large internal energy, giving them their importance.
⇒ Check: Basic Concepts of Organic Chemistry
Classification and Types of Hydrocarbons
Today, hydrocarbons are classified based on their structure rather than their source and properties, unlike in the past when they were classified as either aliphatic or aromatic. Aliphatic hydrocarbons were derived from the chemical degradation of fats or oils, while aromatic hydrocarbons contained substances that were a result of the chemical degradation of certain plant extracts.
Classification of Hydrocarbons
Types of Hydrocarbons
Saturated Hydrocarbons: These are the simplest hydrocarbons which are composed of carbon-carbon and carbon-hydrogen atoms held together by single bonds. These hydrocarbons don’t have double or triple bonds. The carbon atoms in these compounds are Sp3 hybridised and hence, are collectively called alkanes. The general formula for alkanes is CnH2n+2, for example, CH4 and C3H6.
Unsaturated Hydrocarbons: These compounds consist of a single, double, or triple bond between carbon-carbon atoms. Alkenes, which have double bonds, have the general formula CnH2n, while alkynes, which have triple bonds, have the general formula CnH2n-2.
Cycloalkanes: These hydrocarbons have one or more carbon rings, with hydrogen atoms attached to the carbon rings.
Aromatic Hydrocarbons: These compounds, also known as arenes, consist of at least one aromatic ring.
Aliphatic Hydrocarbons: They are organic compounds that consist of straight-chain structures with no rings.
Alicyclic Hydrocarbons: They are hydrocarbons that contain one or more rings of carbon atoms, which can be either sp, sp2 or sp3 hybridized.
Properties of Hydrocarbons
The empirical formula of hydrocarbons varies due to their distinct molecular structures. For example, alkanes, alkynes, and alkenes have decreasing amounts of bonded hydrogen, which is mainly attributed to the catenation of carbon. This is the ability of hydrocarbons to bond to themselves, allowing them to form more complex molecules such as cyclohexane and even aromatic hydrocarbons like benzene in some cases.
Meanwhile, Cracking of Hydrocarbons is a process in which heavy organic molecules are broken down into lighter molecules. This is accomplished by supplying an adequate amount of heat and pressure, and sometimes catalysts are used to speed up the reaction. This process plays a very important role in the commercial production of diesel fuel and gasoline.
⇒ Check out this link for more information on Combustion of Hydrocarbons.
Physical Properties
Alkanes with 10 or fewer C-atoms are usually gases at room temperature. For molecules with more than 10 C-atoms, they can be either gases or liquids. Alkanes typically have low boiling and melting points due to their weak Vanderwal’s interactions.
Factors Affecting Boiling Point:
Molecular Mass
Branching
Alkanes with higher molecular mass generally have higher boiling points. For example, C2H6 has a higher boiling point than CH4.
Alkanes with the same molecular mass, but a different number of branches, will have different boiling points. This is because of Vanderwal’s force, which is weaker as the area increases.
Alkanes, such as CH3-CH2-CH2-CH3, have a higher boiling point than other compounds. Additionally, they are only feebly soluble in water, but are soluble in non-polar solvents, such as Benzene and CCl4.
Preparation of Hydrocarbons: Alkanes
To Alkenes and Alkynes
The alkanes can be produced from alkenes or alkynes through hydrogenation. Hydrogen gas (H2) is passed over a metal surface such as nickel (Ni) or platinum (Pt) along with the alkenes to produce an alkane.
CH2-CH2 → (+H_2/Ni) CH_3-CH_3
The above reaction is called Sabatier-Senderens’ reaction. Other catalysts which can be used are Pt, Pd-BaSO4, Adams catalyst (Pt2O) or Wilkinson catalyst (R3PRhCl), etc.
To Alkyl Halides
Alkyl halides can be converted to alkanes through the following methods:
1. Utilizing Zn/Protic Solvents
2. Analyzing Court Reactions
Note: Alkanes with an even number of carbon atoms can be produced.
3. Utilizing Reducing Agents:
R → [H]R–H
The reducing agents that can be used are LiAlH4, NaBH4, and NaNH2, among others.
Note: Please be sure to read all instructions carefully.
LiAlH4 cannot reduce 3° halides.
NaBH4 cannot reduce a 1° halide.
2. From Aldehydes and Ketones:
3. Decarboxylation of Carboxylic Acids:
Using Soda-Lime
⇒ Also Read: Pyrolysis of Hydrocarbons
Preparation of Hydrocarbons: Alkenes
General formula: C_nH_2n
Preparation Methods
Most of the reactions involving the preparation of alkenes involve β-elimination. There are 3 mechanisms suggested for the β-elimination reactions.
E2 Mechanism
Second Order Kinetics
Single step process.
1° > 2° > 3°: Order of Reactivity
Due to steric hindrance
More favored in non-polar, aprotic solvents.
Less substituted alkenes formed as the major product.
E1 Mechanism
Two Step Process
1st Order Kinetics
Order of Reactivity: 1° < 2° < 3°
Due to the stability of carbonation.
More favored by polar, protic solvents.
Rearrangement Possible
Produces more substituted alkene as major products.
Hydration
(i) Acid-Catalyzed
Markovnikov Product
Rearrangement is possible.
(ii) Hydroboration-Oxidation:
Anti–Markonikov
No rearrangement
(iii) Oxymercuration-Demercuration:
Oxymercuration-Demercuration is a method of alkene hydration in which an alkene is converted to an alcohol by the addition of water in the presence of a mercury(II) salt.
Markonikov
* No rearrangement
Oxidation Reactions
Using Baeyer’s Reagent
Using hot KMnO4
Using O5O4
Addition of Peroxy Acid
Preparation of Hydrocarbons - Alkynes
Alkynes can be prepared from alkyl halides and alcohols.
Addition Reaction:
All addition reactions in alkenes are possible.
Preparation of Benzene
To Ethyne
To Phenol
To Aniline
Chemical Properties:
Benzene generally undergoes electrophilic substitution reactions.
Friedel-Crafts Alkylation, Halogenation, and Acylation
Uses of Hydrocarbons
Hydrocarbons are used for a variety of purposes, including fuel, lubricants, solvents, and plastics. They are also used in the production of chemicals such as methanol and ethylene.
Hydrocarbons are widely used as fuels, such as LPG (Liquefied Petroleum Gas) and CNG (Compressed Natural Gas).
They are used in the manufacturing of polymers such as polyethene, polystyrene, etc.
These organic compounds are utilized as a beginning material in the production of drugs and dyes.
They act as a lubricant for oil and grease.
Hydrocarbons: Important Topics
Important Questions about Hydrocarbons
Top 12 Most Important and Expected Questions on Hydrocarbons
Hydrocarbons - JEE Advanced Questions
“Hydrocarbons Chemistry One-Shot for JEE”
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Frequently Asked Questions on Hydrocarbons
The 4 Types of Hydrocarbons are:
- Alkanes
- Alkenes
- Alkynes
- Aromatic Hydrocarbons
Hydrocarbons are made up of hydrogen and carbon atoms.
Hydrocarbons are characterized by their molecular structure, which is composed of only carbon and hydrogen atoms. They are generally insoluble in water and have low boiling points. They are highly flammable and can be used as a source of energy.
The characteristics of hydrocarbons are:
- Made up of hydrogen and carbon
- Can be either saturated or unsaturated
- Have a high energy content
- Are flammable
Hydrocarbons are colorless and odorless.
They undergo a combustion reaction with oxygen giving carbon dioxide and water.
The greater the number of carbons in a hydrocarbon, the higher its melting point. This is due to the increased strength of the intermolecular forces between the molecules. Alkanes are the least reactive hydrocarbons because they have the fewest functional groups, which makes them less reactive to other molecules.
The product of ozonolysis of ethene is two molecules of formaldehyde.
Ethene on ozonolysis forms ozonide, which on further reduction with zinc dust forms 2 molecules of formaldehyde.
