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