Organic chemistry is built on the study of hydrocarbons—compounds made of carbon and hydrogen. Among them, alkanes, alkenes, and conjugated dienes hold special importance because they not only form the foundation of organic structures but also demonstrate fascinating reactivity patterns. Their study helps us understand reaction mechanisms, stability, and applications in pharmaceuticals, petrochemicals, and polymer industries.
This unit explores their hybridization, properties, important reactions such as substitution, elimination, addition, and rearrangements, as well as the special behavior of conjugated dienes in reactions like the Diels–Alder reaction.
Introduction to Alkanes
Alkanes are a class of saturated hydrocarbons consisting of only carbon and hydrogen atoms joined by single covalent bonds. The term “saturated” means they contain the maximum possible number of hydrogen atoms for their number of carbon atoms, with no double or triple bonds. Alkanes are a fundamental topic in organic chemistry and serve as the basis for understanding other classes of organic compounds.
General Formula & Nomenclature
The general chemical formula for an alkane is CnH2n+2, where n is the number of carbon atoms. For example, if n=1, the formula is CH4 (methane), and if n=2, it is C2H6 (ethane).
The naming of alkanes follows the IUPAC system. The name consists of a prefix that indicates the number of carbon atoms in the longest continuous chain, followed by the suffix “-ane” to signify that it’s an alkane.
| Number of Carbons (n) | Prefix | Alkane Name |
| 1 | Meth- | Methane |
| 2 | Eth- | Ethane |
| 3 | Prop- | Propane |
| 4 | But- | Butane |
| 5 | Pent- | Pentane |
Hybridization in Alkanes (sp³)
Alkanes, also known as paraffins, are saturated hydrocarbons containing only single bonds. Each carbon atom in an alkane is sp³ hybridized, forming four sigma (σ) bonds arranged tetrahedrally with bond angles of ~109.5°. This makes them chemically stable and relatively less reactive compared to unsaturated hydrocarbons.
Halogenation of Alkanes
One of the most important reactions of alkanes is halogenation, where halogen atoms (Cl, Br, etc.) replace hydrogen atoms under UV light or heat. This reaction proceeds via a free radical chain mechanism:
- Initiation: Homolytic cleavage of halogen molecules.
- Propagation: Radical substitution of hydrogen atoms.
- Termination: Combination of free radicals.
This reaction is the basis for producing important industrial compounds like chloroform and carbon tetrachloride.
Uses of Paraffins
- Fuels (e.g., methane, propane, butane)
- Waxes, lubricants, and greases
- Raw materials for petrochemical industries
Introduction to Alkenes
Alkenes are a class of unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). The presence of this double bond makes them more reactive than alkanes, as the double bond can be broken to form new single bonds.
General Formula & Nomenclature
The general chemical formula for an alkene with one double bond is CnH2n, where n is the number of carbon atoms. For example, ethene (n=2) has the formula C2H4, and propene (n=3) is C3H6.
The naming of alkenes follows the IUPAC system. The name consists of a prefix that indicates the number of carbon atoms in the longest continuous chain containing the double bond, followed by the suffix “-ene”. The position of the double bond is indicated by a number before the name. The chain is numbered from the end that gives the double bond the lowest possible number.
| Number of Carbons (n) | Alkene Name |
| 2 | Ethene |
| 3 | Propene |
| 4 | Butene |
| 5 | Pentene |
Hybridization in Alkenes (sp²)
Each carbon in the double bond is sp² hybridized, forming three σ bonds and one π bond. The bond angle is ~120°, making the double bond planar and more reactive than a single bond.
Stability of Alkenes
The stability of alkenes depends on the number of alkyl groups attached to the double bond. More substituted alkenes (like tertiary > secondary > primary) are more stable due to hyperconjugation and inductive effects.
Reactions of Alkenes
Ozonolysis
Alkenes react with ozone to form ozonides, which upon cleavage give carbonyl compounds (aldehydes, ketones). This is useful for locating double bonds in unknown compounds.
Electrophilic Addition Reactions
The double bond acts as a nucleophile and undergoes addition with electrophiles.
- Markovnikov’s Rule: In the addition of HX, the hydrogen atom attaches to the carbon with more hydrogens, and halogen attaches to the more substituted carbon.
- Anti-Markovnikov Addition: In the presence of peroxides, the opposite orientation occurs due to a free radical mechanism.
Free Radical Addition
Hydrogen bromide (HBr) can add to alkenes through a radical mechanism (Anti-Markovnikov orientation).
Stability of Conjugated Dienes
Conjugated dienes have alternating double and single bonds, leading to delocalization of π-electrons, which increases stability. For example, 1,3-butadiene is more stable than isolated dienes.
Reactions of Conjugated Dienes
- Diels–Alder Reaction:
- A conjugated diene reacts with a dienophile to form a six-membered cyclic compound.
- Widely used in the synthesis of complex natural products and pharmaceuticals.
- Electrophilic Addition:
- Addition can occur at either the 1,2-position or 1,4-position.
- At low temperature, kinetic (1,2-addition) product forms, while at higher temperature, thermodynamic (1,4-addition) product dominates.
- Free Radical Addition:
- Similar to alkenes but involves resonance-stabilized allylic radicals.
- Allylic Rearrangement:
- Involves shifting of the double bond when allylic carbocations or radicals are formed.
- Provides diverse reaction products useful in synthesis.