Alkanes, Alkenes, and Alkynes - SS2 Chemistry Lesson Note

In organic chemistry, hydrocarbons are classified into various categories based on their structure and properties. Among these classifications, alkanes, alkenes, and alkynes are fundamental groups of hydrocarbons.

Alkanes:

Alkanes, also known as saturated hydrocarbons, are hydrocarbon compounds composed solely of carbon and hydrogen atoms. They consist of single covalent bonds between carbon atoms, forming either straight chains or branched structures. Alkanes are characterised by their molecular formula CnH2n+2, where "n" represents the number of carbon atoms in the molecule. For example, methane (CH4) is the simplest alkane with one carbon atom, while ethane (C2H6) and propane (C3H8) are examples of alkanes with two and three carbon atoms, respectively.

Properties of Alkanes:

●     Alkanes are relatively unreactive due to the presence of only single bonds between carbon atoms.

●     They are nonpolar compounds with low boiling points and are generally insoluble in water but soluble in nonpolar solvents.

●     Alkanes exhibit a gradual increase in boiling points with an increase in molecular weight, following the trend of increasing London dispersion forces.

Reactivity of Alkanes:

●     Combustion: Alkanes are highly combustible and are commonly used as fuels. They undergo combustion reactions with oxygen to produce carbon dioxide and water.

●     Halogenation: Under appropriate conditions, alkanes react with halogens (e.g., chlorine or bromine) to undergo a substitution reaction, resulting in the replacement of a hydrogen atom by a halogen atom.

●     Radical Substitution: Alkanes can undergo radical substitution reactions, such as free radical halogenation, where a hydrogen atom is replaced by a halogen atom in the presence of a halogen and UV light.

Alkenes:

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C). They have the general molecular formula CnH2n. The presence of the double bond introduces unsaturation, making alkenes more reactive than alkanes.

Properties of Alkenes:

●     Alkenes are relatively more reactive than alkanes due to the presence of the carbon-carbon double bond.

●     They are nonpolar compounds with higher boiling points compared to alkanes of similar molecular weight.

●     Alkenes are generally insoluble in water but soluble in nonpolar solvents.

Reactivity of Alkenes:

●     Addition Reactions: The carbon-carbon double bond in alkenes readily undergoes additional reactions with various reagents, such as hydrogen (hydrogenation), halogens (halogenation), water (hydration), and hydrogen halides (hydrohalogenation).

●     Polymerization: Alkenes can undergo polymerization, a process where monomers join together to form long chains or branched structures called polymers. This property is exploited in the production of plastics.

Alkynes:

Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond (C≡C). They have the general molecular formula CnH2n-2. Alkynes are even more reactive than alkenes due to the presence of the triple bond.

Properties of Alkynes:

●     Alkynes are the most reactive among alkanes, alkenes, and alkynes due to the presence of the carbon-carbon triple bond.

●     They have higher boiling points compared to alkanes and alkenes of similar molecular weight.

●     Alkynes are generally insoluble in water but soluble in nonpolar solvents.

Reactivity of Alkynes:

●     Addition Reactions: The carbon-carbon triple bond in alkynes readily undergoes addition reactions, similar to alkenes. These reactions include hydrogenation, halogenation, hydration, and hydrohalogenation.

●     Acidity: The hydrogen attached to the carbon adjacent to the triple bond in alkynes is relatively acidic. It can be deprotonated using strong bases to form alkoxide ions.

Alkanes, alkenes, and alkynes represent different types of hydrocarbons with varying levels of saturation and reactivity. Alkanes are saturated hydrocarbons with only single bonds, while alkenes and alkynes are unsaturated hydrocarbons with double and triple bonds, respectively. Understanding their structures, properties, and reactivity is crucial in the study of organic chemistry and in various applications, such as fuel production, synthesis of organic compounds, and the manufacturing of polymers.