Structure and Synthesis of Alkenes: Chapter 7 Study Notes

Structure and Synthesis of Alkenes

Definition and Basic Properties of Alkenes

Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond. They are also known as olefins and are classified as unsaturated compounds because they contain fewer hydrogen atoms than alkanes with the same number of carbons.

• Double bond (C=C): The defining feature of alkenes, contributing to their chemical reactivity.

• Degree of Unsaturation: Each double bond introduces one degree of unsaturation, indicating the potential for additional hydrogen atoms.

• Bond Dissociation Energies (BDE):

  • $\text{C=C BDE} = 63\ \text{kcal/mol}$

  • $\text{C–C BDE} = 83\ \text{kcal/mol}$

• Functional Group: The double bond acts as a functional group, imparting unique chemical properties.

Structure of the Double Bond

The carbon atoms in a double bond are sp2 hybridized, resulting in a trigonal planar geometry. The double bond consists of one sigma ($\sigma$) bond and one pi ($\pi$) bond.

• Sigma ($\sigma$) bond: Formed by the overlap of sp2 hybrid orbitals.

• Pi ($\pi$) bond: Formed by the sideways overlap of unhybridized p orbitals.

• No Free Rotation: The presence of the $\pi$ bond restricts rotation around the double bond, leading to geometric (cis-trans) isomerism.

Nomenclature of Alkenes

Alkenes are named according to IUPAC rules, with the suffix "-ene" indicating the presence of a double bond. The longest chain containing the double bond is chosen as the parent, and the position of the double bond is indicated by the lowest possible number.

• Suffix: "-ene" for one double bond, "-diene", "-triene", "-tetraene" for multiple double bonds.

• Numbering: The chain is numbered to give the double bond the lowest possible locant.

• Examples:

  • CH2=CH–CH2–CH3: 1-butene

  • CH2=CH–CH=CH–CH2–CH3: 2,4-hexadiene

  • CH2=CH–CH2–CH3: 1-butene

  • CH2=CH–CH=CH–CH2–CH3: 1,3-hexadiene

Alkenes as Substituents

When alkenes act as substituents, they are named as alkenyl groups. Common examples include:

• Methylene group: =CH2 (methylidene group)

• Vinyl group: –CH=CH2 (ethenyl group)

• Allyl group: –CH2–CH=CH2

• Phenyl group: –C6H5

Cis-Trans and E/Z Nomenclature

Alkenes can exhibit geometric isomerism due to restricted rotation around the double bond. The cis isomer has similar groups on the same side, while the trans isomer has them on opposite sides. For more complex cases, the E/Z system is used, based on the Cahn-Ingold-Prelog priority rules.

• Cis: Two similar groups on the same side of the double bond.

• Trans: Two similar groups on opposite sides.

• E (Entgegen): Highest priority groups on opposite sides.

• Z (Zusammen): Highest priority groups on the same side.

• Example: 1-bromo-1-chloropropene can exist as E or Z isomers depending on the arrangement of Br and Cl.

Stability of Alkenes

The stability of alkenes can be assessed by measuring the heat of hydrogenation. More substituted alkenes are generally more stable, releasing less heat upon hydrogenation.

• Heat of Hydrogenation ($\Delta H^\circ$): The enthalpy change when an alkene is converted to an alkane by addition of hydrogen.

• Stability Trend: The more substituted the double bond, the lower the heat of hydrogenation and the greater the stability.

• Example:

  • 1-butene: $\Delta H^\circ = -30.3\ \text{kcal/mol}$

  • trans-2-butene: $\Delta H^\circ = -27.6\ \text{kcal/mol}$

Substitution Effects and Saytzeff Rule

Alkyl groups attached to the double bond stabilize the alkene through electron donation. The Saytzeff rule states that elimination reactions favor the formation of the more substituted (and thus more stable) alkene.

• Substitution: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted (in terms of stability)

• Saytzeff Rule: In elimination reactions, the major product is the more substituted alkene.

• Example:

  • 3-methyl-1-butene (monosubstituted): $\Delta H^\circ = -30.3\ \text{kcal/mol}$

  • 2-methyl-2-butene (trisubstituted): $\Delta H^\circ = -26.9\ \text{kcal/mol}$

Table: Alkene Substitution and Stability

Alkene	Type of Substitution	Heat of Hydrogenation ($\Delta H^\circ$)	Relative Stability
1-butene	Monosubstituted	$-30.3\ \text{kcal/mol}$	Least stable
trans-2-butene	Disubstituted	$-27.6\ \text{kcal/mol}$	More stable
2-methyl-2-butene	Trisubstituted	$-26.9\ \text{kcal/mol}$	Most stable

Additional info:

• Alkenes are important intermediates in organic synthesis and are found in many natural and synthetic compounds.

• Geometric isomerism affects physical and chemical properties, such as boiling points and reactivity.