Recording-2025-03-04T04:21:45.644Z

Unsaturated Hydrocarbons Overview

• Unsaturated hydrocarbons contain only carbon and hydrogen atoms, characterized by carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds without any heteroatoms.

Types of Unsaturated Hydrocarbons

• Alkenes:

  • Contain a C-C double bond.

  • Examples: Ethylene (C2H4), where each carbon atom forms four bonds, including one double bond (4 electrons total).

• Alkynes:

  • Feature a C-C triple bond.

  • More electrons involved than in alkenes, allowing for increased reactivity.

  • Example: Butyne.

• Aromatic Hydrocarbons:

  • Contain benzene rings with alternating double bonds, yielding specific spatial characteristics due to resonance.

  • Example: Benzene ring, which has six carbon atoms and three double bonds (6 electrons involved).

Reactivity of Unsaturated Hydrocarbons

• Unsaturated hydrocarbons are generally more reactive than saturated hydrocarbons (alkanes) due to rich electron density around C-C multiple bonds.

• Alkynes and alkenes are important in biological systems and are often used to synthesize drugs and other compounds.

Structural Characteristics

• Alkenes and Alkynes Shapes:

  • Alkenes have a trigonal planar shape (angle of 120 degrees) due to sp2 hybridization.

  • Alkynes have a linear shape (angle of 180 degrees) due to sp hybridization.

  • General formula for alkenes: CnH2n, which is the same as cyclic alkanes.

Naming Alkenes and Alkynes

• Identify the longest carbon chain and number the carbon atoms to give the double or triple bond the lowest number possible.

• Use the suffix "-ene" for alkenes and "-yne" for alkynes.

• Example::

  • 1-Butene (double bond at carbon 1)

  • 2-Hexyne (triple bond at carbon 2)

Substituents in Hydrocarbons

• In cases of substituents (e.g., methyl or ethyl groups), apply the same naming principle and assign numbers based on proximity to the multiple bond.

• Example:

  • 3-Methyl-1-butene (double bond at carbon 1 with a methyl group at carbon 3).

Complex Naming Examples

• Dienes:

  • When there are two double bonds, use the prefix "di-" while naming.

  • Example: 1,3-butadiene has double bonds at carbon 1 and 3.

• Cyclic Compounds:

  • Cycloalkenes can also have double bonds; the double bond is always located between carbons 1 and 2.

  • Example: 1-Methylcyclopentene has a substituent group.

Isomerism in Alkenes

• Cis-Trans Isomerism:

  • Important characteristic due to restricted rotation around C=C double bonds.

  • Cis Isomer: substituents on the same side of the double bond.

  • Trans Isomer: substituents on opposite sides.

• These isomers exhibit differences in physical properties and reactivity.

Stereoisomers vs. Constitutional Isomers

• Stereoisomers: Same connectivity but differ in three-dimensional orientation.

  • Example: Cis-2-butene and trans-2-butene.

• Constitutional Isomers: Differ in the connectivity of atoms.

  • Example: Different arrangements giving rise to different compounds even with the same molecular formula.

Reactions of Unsaturated Hydrocarbons

• Understanding the isomerism is crucial in predicting and analyzing reactions involving alkenes and alkynes.

• Future topics will cover the reactions of these hydrocarbons and their applications in organic synthesis.