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

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

Molecular Formula and Degree of Unsaturation

Understanding the molecular formula and degree of unsaturation is essential for classifying hydrocarbons and predicting their reactivity.

• General Formula for Alkanes: $C_nH_{2n+2}$

• General Formula for Alkenes: $C_nH_{2n}$

• Degree of Unsaturation: The total number of π bonds and rings in a molecule. Each degree of unsaturation corresponds to either one double bond or one ring.

• Formula for Degree of Unsaturation: $\text{Degree of Unsaturation} = \frac{2n + 2 - \text{number of hydrogens}}{2}$

• Example: A compound with formula $C_8H_{14}$ has 2 degrees of unsaturation (4 fewer hydrogens than $C_8H_{18}$).

Type	Formula	Degree of Unsaturation
Alkane	$C_5H_{12}$	0
Alkene	$C_5H_{10}$	1
Cyclic Alkane	$C_5H_{10}$	1
Cyclic Alkene	$C_5H_8$	2

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, Alkynes) Contain one or more double or triple bonds.

• Example: Ethane ($C_2H_6$) is saturated; ethene ($C_2H_4$) is unsaturated.

Naming Alkenes

Alkenes are named by replacing the “-ane” ending of the corresponding alkane with “-ene.” The position of the double bond and substituents must be specified according to IUPAC rules.

• Step 1: Identify the longest carbon chain containing the double bond.

• Step 2: Number the chain so that the double bond gets the lowest possible number.

• Step 3: Replace “-ane” with “-ene.”

• Step 4: Name and number substituents, placing them before the parent name.

• Example: $CH_2=CHCH_3$ is named propene (common name: propylene).

• Functional Group Priority: The double bond (alkene functional group) gets the lowest possible number.

Examples of Alkene Names

• Ethene (common name: ethylene)

• Propene (common name: propylene)

• Cyclopentene

• Cyclohexene

Stereoisomers: cis/trans and E/Z Nomenclature

Alkenes can exhibit stereoisomerism due to restricted rotation around the double bond. The relative positions of substituents are described using cis/trans or E/Z notation.

• cis-2-pentene or (Z)-2-pentene: Substituents on the same side of the double bond.

• trans-2-pentene or (E)-2-pentene: Substituents on opposite sides of the double bond.

• E/Z System: Used when there are different substituents; based on Cahn-Ingold-Prelog priority rules.

Naming Compounds with Multiple Double Bonds

When a molecule contains more than one double bond, the suffixes “diene” (two double bonds) or “triene” (three double bonds) are used.

• Example: (2E,4E)-2,4-heptadiene or (3Z)-1,3-pentadiene

• Double bonds: Indicated by “diene” or “triene” in the name.

Naming Alkenes with Substituents

Substituents are named and numbered according to their position on the carbon chain. The chain is numbered to give the double bond the lowest possible number, and substituents are listed in alphabetical order.

• Example: (E)-4-methyl-2-pentene, (Z)-3-methyl-3-heptene

• Complex Substituents: 4-pentoxy-1-butene, (3Z)-4-methyl-1,3-pentadiene

Compound Name	Structure/Description
(E)-4-methyl-2-pentene	Double bond at position 2, methyl group at position 4
(Z)-3-methyl-3-heptene	Double bond at position 3, methyl group at position 3
4-pentoxy-1-butene	Pentoxy group at position 4, double bond at position 1
(3Z)-4-methyl-1,3-pentadiene	Two double bonds at positions 1 and 3, methyl group at position 4

Additional info: These notes cover the first part of alkene nomenclature, structure, and classification. Further sections would include reaction mechanisms, thermodynamics, and kinetics as indicated by the chapter outline.