Study Guide: Alkenes
Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). They
are unsaturated compounds with the general formula CnH2n. Alkenes are important in
organic chemistry due to their reactivity, especially in addition reactions, and are commonly
used in industrial applications such as polymer production.
1. Structure and Properties
- General formula: CnH2n
- Bonding: One sigma bond and one pi bond form the C=C double bond.
- Geometry: Each carbon in the double bond is sp2 hybridized; bond angle ~120°.
- Isomerism: Structural isomers and cis-trans (geometric) isomerism are possible.
- Physical properties: Nonpolar, insoluble in water, soluble in organic solvents.
2. Nomenclature
- Identify the longest continuous carbon chain containing the double bond.
- Number the chain to give the double bond the lowest possible number.
- Replace the -ane suffix of the corresponding alkane with -ene.
- Indicate the position of the double bond and substituents.
Example: CH3-CH=CH-CH3 → But-2-ene.
3. Preparation of Alkenes
- Dehydration of alcohols: Using concentrated H2SO4.
- Dehydrohalogenation: Removal of HX from haloalkanes with strong base.
- Cracking of alkanes: High temperature breaking of larger hydrocarbons.
- Wittig reaction: Conversion of carbonyl compounds into alkenes.
4. Reactions of Alkenes
- Hydrogenation: Addition of H2 in presence of catalyst (Ni, Pt).
- Halogenation: Addition of X2 (Cl2, Br2) across the double bond.
- Hydrohalogenation: Addition of HX following Markovnikov’s rule.
- Hydration: Addition of H2O in presence of acid catalyst to give alcohols.
- Oxidation: * Mild oxidation with KMnO4 → glycols.
* Strong oxidation (ozonolysis or hot KMnO4) → aldehydes, ketones, or carboxylic acids.
- Polymerization: Formation of polyalkenes like polyethylene and polypropylene.
5. Applications and Importance
- Production of plastics (polyethylene, polypropylene).
- Synthesis of alcohols, aldehydes, and other industrial chemicals.
- Role as intermediates in organic synthesis.
- Basis for many addition polymerization reactions.
6. Summary
Alkenes are unsaturated hydrocarbons with diverse industrial and laboratory applications.
Their characteristic reactivity is due to the presence of the carbon-carbon double bond,
which undergoes electrophilic addition reactions. Understanding their structure,
preparation, and reactions is fundamental in organic chemistry.