Introduction to Hydrocarbons
What are Hydrocarbons?
Hydrocarbons are found abundantly in petroleum and natural gas. They are frequently used as domestic and industrial fuels due to their high energy content.
Generally, hydrocarbons have many synthetic applications. Unsaturated hydrocarbons, especially alkenes are used as the starting materials for a number of industrial products. Hydrocarbons are the source of the majority of goods including polymers, plastics, and fragrances.
Aliphatic and Aromatic Hydrocarbons
Aliphatic Hydrocarbons
The compounds that are not aromatic are called aliphatic hydrocarbons. Aliphatic compounds can be both saturated or unsaturated, open chain or cyclic.
Types of aliphatic hydrocarbons include alkanes, alkenes, alkynes, and cycloalkanes. Cycloalkanes are cyclic aliphatic hydrocarbons and alkanes are acyclic hydrocarbons.
Aromatic Hydrocarbons
Aromatic hydrocarbons are a special class of cyclic hydrocarbons that have high carbon to hydrogen ratio and are based on benzene (C₆H₆) or resembling compounds.
Benzene is the basic aromatic compound and other compounds such as toluene and phenol are its derivatives. The term aromatic was derived from the Greek word ‘aroma’ meaning “fragrance” and used in organic chemistry for a special class of compounds having characteristic odour.
Alkanes or Paraffins
Introduction to Alkanes
Alkanes are the simplest organic compounds made up of carbon and hydrogen only. Methane (CH₄) is the simplest member of the family. Their general formula is CₙH₂ₙ₊₂.
| IUPAC Name | Molecular Formula | Condensed Structure |
|---|---|---|
| Methane | CH₄ | CH₄ |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
| Pentane | C₅H₁₂ | CH₃CH₂CH₂CH₂CH₃ |
Nomenclature of Alkanes
Branched chain alkanes are named according to specific rules:
1. Selection of longest continuous chain: Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane.
2. Numbering of longest continuous chain: Number the longest chain beginning from the end nearest to the substituent.
3. Select the location of substituent group: Use the numbers obtained to designate the location of the substituent group.
4. Location of more than one substituent: When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain.
5. Identical position of substituents: When two substituents are present on the same carbon atom, use that number twice.
Alkenes
Introduction to Alkenes
Alkenes are unsaturated hydrocarbons; unsaturated hydrocarbons are compounds of hydrogen and carbon only whose molecules contain carbon to carbon double bonds. Alkenes have two hydrogen atoms less than the corresponding saturated hydrocarbons.
Ethene (C₂H₄) is the simplest alkene. Each carbon atom in ethene is sp² hybridized and forms three equivalent sp² hybrid orbitals which lie in the same plane.
Preparation of Alkenes
Alkenes are prepared by the removal of small atoms (H, OH, X) from the adjacent carbon atoms of the saturated compounds, so as to create a double bond between carbon atoms. These reactions are known as elimination reactions.
Method 1: Dehydration of alcohols – Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at 180°C. The reaction involves the removal of a water molecule from the alcohol molecule.
Method 2: Dehydrohalogenation of alkyl halides – On heating, ethyl bromide with alcoholic KOH, ethene is formed. Removal of hydrogen and halogen takes place from adjacent carbon atoms to create a double bond.
Reactions of Hydrocarbons
Reaction Mechanism
Reaction mechanism is the series of steps that take place in the course of the overall reaction. Like all chemical reactions, organic reactions involve the breaking of chemical bonds.
There are two ways in which covalent bonds can break:
Homolytic fission occurs when the covalent bond breaks evenly to produce free radicals. Free radical is a species that contains an unpaired electron and is very reactive.
Heterolytic fission occurs when the covalent bond breaks in such a way that the originally shared pair of electrons is gained by one atom only and oppositely charged ions are produced.
Reactions of Alkanes
Under high temperature or suitable conditions, alkanes undergo two types of reactions: Thermal and Catalytic Reactions (e.g., Combustion and cracking) and Free radical substitution reactions.
Halogenation: Substitution reactions of alkanes take place in the presence of light energy through the formation of highly reactive free radicals. An example is the substitution of a halogen atom with a hydrogen of an alkane.
Alkanes react with chlorine and bromine in the presence of sunlight or UV light resulting in the successive replacement of hydrogen atoms with halogens. The extent of halogenation depends upon the amount of halogen used.
Reactions of Alkenes
Alkenes are unsaturated hydrocarbons and they undergo electrophilic addition reactions mostly. Other minor types of reactions include oxidation, combustion and polymerization reactions.
Electrophilic Addition Reactions: The reactions in which an electrophile is added to the double bond of an alkene is called electrophilic addition reaction. The C = C in alkenes is a region of high electron density making susceptible to attack by electrophiles.
Types of electrophilic addition reactions include halogenation, hydrohalogenation, hydrogenation, hydration, halohydration, epoxidation, ozonolysis, and polymerization.