Alkenes: Structure, Nomenclature, and Introduction to Reactions, Thermodynamics, and Kinetics

Molecular Formula and Degree of Unsaturation

Organic compounds can be classified by their molecular formula and degree of unsaturation, which helps identify the presence of double bonds or rings in a molecule.

• General Alkane Formula:

• General Alkene Formula:

• Degree of Unsaturation: The total number of π bonds and rings in a molecule.

• Calculation: Each degree of unsaturation corresponds to either one double bond or one ring. For every degree, subtract two hydrogens from the alkane formula.

Example: A compound with formula C8H14 has 2 degrees of unsaturation (4 fewer hydrogens than C8H18).

Saturated and Unsaturated Hydrocarbons

Hydrocarbons are classified based on the presence or absence of double bonds.

• Saturated Hydrocarbons (Alkanes): Contain only single bonds; no double bonds present.

• Unsaturated Hydrocarbons (Alkenes): Contain one or more double bonds.

Naming Alkenes

Alkenes are named by modifying the parent alkane name and following IUPAC rules.

• Replace the suffix "ane" of the alkane with "ene" to indicate the presence of a double bond.

• Common Names: Ethylene (ethene), Propylene (propene).

• Cyclic Alkenes: Named similarly, e.g., cyclopentene, cyclohexene.

• Numbering: The double bond (functional group) gets the lowest possible number in the chain.

• If multiple chains are possible, choose the longest chain containing the double bond as the parent.

Example: CH2=CHCH2CH3 is named as 1-butene or but-1-ene.

Stereoisomers: cis/trans (E/Z) Nomenclature

Alkenes can exhibit stereoisomerism due to restricted rotation around the double bond.

• cis/trans System: Used for simple alkenes; "cis" indicates substituents on the same side, "trans" on opposite sides.

• E/Z System: Used for more complex alkenes; "Z" (zusammen) means higher priority groups are on the same side, "E" (entgegen) means on opposite sides.

Example: cis-2-pentene or (Z)-2-pentene; trans-2-pentene or (E)-2-pentene.

Naming Compounds with Multiple Double Bonds

Alkenes with more than one double bond are named using the suffixes "diene" (two double bonds) or "triene" (three double bonds).

• Indicate the position and configuration (E/Z) of each double bond.

• Examples:

  • (2E,4E)-2,4-heptadiene

  • (3Z)-1,3-pentadiene

  • (3E)-1,3,5-hexatriene

Additional info: These notes cover the foundational aspects of alkene structure, nomenclature, and classification, which are essential for understanding their chemical behavior and reactivity in organic chemistry.