Alkenes and alkynes, categorised as unsaturated hydrocarbons, are fundamental to the realm of organic chemistry. Their unique structural characteristics, coupled with their diverse reactivity, make them indispensable in various industrial and biological applications.
Nomenclature of Alkenes and Alkynes
Alkenes
- General Formula: CnH2n
- Naming: Derived from the corresponding alkanes by replacing the suffix "-ane" with "-ene".
- Example: Ethene (C2H4), Propene (C3H6)
- Position of Double Bond: For larger alkenes, the location of the double bond is specified by a number.
- Example: 1-Butene has the double bond starting at the first carbon, whereas 2-Butene has it at the second.
Alkynes
- General Formula: CnH2n-2
- Naming: Derived from the corresponding alkanes by replacing the suffix "-ane" with "-yne".
- Example: Ethyne (C2H2), Propyne (C3H4)
- Position of Triple Bond: The location of the triple bond is indicated by a number for larger alkynes.
- Example: 1-Butyne versus 2-Butyne
Properties of Alkenes and Alkynes
Physical Properties
- Boiling Point: Alkenes and alkynes have boiling points that increase with molecular weight. However, increased branching leads to decreased boiling points due to reduced surface area and weaker van der Waals forces.
- Density: Both groups have densities less than water, making them float on its surface. Their non-polar nature renders them insoluble in water but soluble in organic solvents like benzene and ether.
- State: Lower members (up to C4) are typically gases at room temperature, while higher ones can be liquids or even solids.
Chemical Properties
- Alkenes:
- Electrophilic Addition: The electron-rich double bond in alkenes makes them susceptible to attack by electrophiles. Common agents include halogens (Br2, Cl2), hydrogen (H2), and water (H2O).
- Oxidation: Alkenes can be oxidised using agents like potassium permanganate (KMnO4) to produce diols. Stronger oxidising agents can break the double bond to form compounds like ketones or carboxylic acids.
- Alkynes:
- Electrophilic Addition: Alkynes, similar to alkenes, can undergo addition reactions. However, due to the presence of a triple bond, they can add two equivalents of the adding species.
- Reduction: Alkynes can be reduced to alkenes using hydrogen in the presence of Lindlar's catalyst. Further reduction can produce alkanes.
Importance in Organic Synthesis
Alkenes
- Intermediates: Alkenes are often used as intermediates in the synthesis of more complex molecules. Their double bond can be transformed into a variety of functional groups, expanding the range of possible products.
- Polymerisation: Many commercially important polymers are derived from alkenes. Ethene, for instance, can be polymerised to produce polyethene, a common plastic. Similarly, propene can be polymerised to form polypropene.
Alkynes
- Versatility in Synthesis: Alkynes can be transformed into a multitude of functional groups, including aldehydes, alcohols, and carboxylic acids. This versatility makes them valuable starting materials in the synthesis of complex molecules.
- Pharmaceutical Applications: Alkynes play a crucial role in the synthesis of many pharmaceutical compounds. Their ability to be transformed into various functional groups allows for the creation of diverse drug molecules with specific biological activities.
FAQ
Cis-trans isomerism can significantly influence the biological activity of molecules. The spatial arrangement of atoms or groups in cis-trans isomers can lead to different interactions with biological receptors or enzymes. For instance, the cis and trans isomers of a drug might have different efficacies or side effects because they fit differently into the active site of a target protein. In some cases, one isomer might be biologically active while the other is inactive or even harmful. This is why the specific geometric isomer of a compound is crucial in drug design and pharmacology.
Chiral centres are vital in drug design because the two enantiomers of a chiral drug can have vastly different biological activities. One enantiomer might be therapeutic and beneficial, while the other could be ineffective or even produce adverse effects. This is due to the specific three-dimensional shape of the enantiomers, which can interact differently with biological targets such as enzymes or receptors. Recognising and understanding chirality is essential to ensure the safety and efficacy of drugs. In modern pharmaceuticals, efforts are often made to produce and administer only the active enantiomer of a chiral drug.
The presence of double or triple bonds in alkenes and alkynes introduces regions of high electron density, making these molecules more reactive than alkanes. Physically, the double or triple bonds lead to a decrease in symmetry and can result in higher boiling points for alkenes and alkynes compared to alkanes with similar molecular weights. However, increased branching in alkenes can reduce boiling points. Alkynes, with their triple bonds, are generally less dense than corresponding alkenes and alkanes. Both alkenes and alkynes are non-polar, making them insoluble in water but soluble in non-polar solvents.
Alkynes and alkenes are both unsaturated hydrocarbons, but they differ in the type and number of bonds between their carbon atoms. Alkenes contain a double bond (C=C) between two carbon atoms, while alkynes contain a triple bond (C≡C). The presence of the triple bond in alkynes means they have two π bonds and one σ bond between the carbon atoms, whereas alkenes have one π bond and one σ bond. This difference in bond structure imparts distinct chemical properties and reactivities to these two classes of compounds.
Alkenes are termed unsaturated hydrocarbons because they contain fewer hydrogen atoms than alkanes with the same number of carbon atoms. The term "unsaturated" indicates the presence of a double bond between carbon atoms, which means that the molecule can add more atoms or groups without breaking the carbon skeleton. This contrasts with alkanes, which are saturated and contain the maximum possible number of hydrogen atoms for their carbon count, with only single bonds between carbon atoms.
Practice Questions
Geometric (cis-trans) isomerism in alkenes arises due to restricted rotation around the double bond. It results in different spatial arrangements of substituents. For instance, in 2-butene, the cis-isomer has both methyl groups on the same side of the double bond, while the trans-isomer has them on opposite sides. On the other hand, optical isomerism occurs in molecules with chiral centres, typically a carbon atom bonded to four different groups. These molecules are non-superimposable mirror images called enantiomers. An example is 2-chlorobutane, which has two enantiomers that rotate plane-polarised light in opposite directions.
Alkenes play a pivotal role in the petrochemical industry, primarily as starting materials for a plethora of chemical products. They are obtained from the cracking of larger hydrocarbons present in crude oil. One of the most significant applications of alkenes in the industry is in the production of polymers. Polymers are large molecules formed by the repetitive addition of small alkene units. An example is polyethene, derived from the polymerisation of ethene. This polymer is extensively used in packaging, containers, and many other everyday products due to its versatility and durability.
