Grade 11 → Hydrocarbons → Hydrocarbons ↓
Preparation and Properties of Alkenes
Alkanes are a fundamental class of hydrocarbons, which are organic compounds composed exclusively of carbon and hydrogen atoms. Their general formula is C n H 2n+2, where n represents the number of carbon atoms. Alkanes are also known as paraffins and are saturated hydrocarbons because they contain only single bonds. This document will explore various methods of preparing alkanes and discuss their properties with examples to enhance understanding.
Preparation of alkenes
There are many ways to prepare alkanes. These methods include natural, chemical processes and laboratory methods. The well-known methods are given below:
1. Hydrogenation of alkenes
Hydrogenation involves adding hydrogen to an alkane to form an alkane. For example, consider the conversion of ethene to ethane:
C 2 H 4 + H 2 → C 2 H 6
This reaction usually requires a catalyst such as platinum, palladium, or nickel.
Visual Diagram:
2. Decarboxylation of carboxylic acids
Alkenes can also be prepared by decarboxylating carboxylic acids. In this process, alkene is formed by removing carbon dioxide molecule from carboxylic acid.
RCOOH + NaOH → RH + NaCO 3
Decarboxylation is carried out in the presence of soda lime.
Example response:
CH 3 COONa + NaOH + CaO → CH 4 + Na 2 CO 3
3. Wurtz reaction
The Wurtz reaction is a coupling reaction in which two alkyl halides react in the presence of sodium to form higher alkenes. This mechanism, usually carried out in dry ether, involves radical species.
RX + 2Na + XR' → RR' + 2NaX
Example:
2CH 3 Cl + 2Na → C 2 H 6 + 2NaCl
4. Kolbe electrolysis
Alkenes can be formed by electrolyzing an aqueous solution of the sodium or potassium salt of a carboxylic acid. During electrolysis, carboxylate ions are oxidized to form radicals, which bivalently react to form alkenes.
Example:
2 CH 3 COOK → C 2 H 6 + 2 CO 2 + KOH
Properties of Alkenes
Alkanes are characterized by their saturated structure, composed entirely of single bonds between carbon atoms. Their properties can be discussed in different contexts, such as physical, chemical, and structural properties.
Physical properties
- State: Lower alkanes (e.g., methane, ethane) are gases at room temperature, whereas intermediate alkanes are liquids, and higher alkanes (e.g., octadecane) are solids.
- Melting point and boiling point: Alkanes have relatively low melting and boiling points, which increase with molecular weight due to increase in van der Waals forces.
- Solubility: Alkanes are generally nonpolar and insoluble in water, but are soluble in nonpolar solvents such as hexane or benzene.
Chemical properties
Alkenes are less reactive due to their strong carbon-carbon and carbon-hydrogen single bonds. However, they undergo specific types of reactions:
Combustion
Alkanes burn readily in the presence of oxygen, producing carbon dioxide, water, and energy.
C n H 2n+2 + (3n+1)/2 O 2 → nCO 2 + (n+1)H 2 O
Substitution reactions
Although alkenes are less reactive, they can undergo reactions such as halogenation under specific conditions, especially under UV light.
Example:
CH 4 + Cl 2 → CH 3 Cl + HCl
Structural properties
The structure of alkanes may be linear (straight-chain) or branched. Their isomerism is mainly constitutional isomerism, as branching increases with the number of carbons.
Example of structural difference:
Butane ( C4H10 ):
CH 3 -CH 2 -CH 2 -CH 3 (n-butane)
Isobutane ( C4H10 ) :
CH 3 -CH(CH 3 )-CH 3 (2-methylpropane)
In summary, alkanes, although simple, display a remarkable variety of preparation methods and properties. Understanding these hydrocarbons involves balancing a consideration of their chemical structure with their chemical and physical behavior in various environments. This knowledge serves as a foundational basis for further exploration of more complex organic compounds in chemistry.
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