Alkenes and Alkynes I: Properties and Synthesis – Elimination Reactions of Alkyl Halides

Alkenes and Alkynes I: Properties and Synthesis

Elimination Reactions of Alkyl Halides

This chapter introduces the fundamental properties and synthetic methods for alkenes and alkynes, focusing on elimination reactions of alkyl halides. Key concepts include stereochemistry, stability, and mechanisms of elimination reactions.

(E)-(Z) System for Designating Alkene Diastereomers

Priority Assignment and Stereochemistry

The (E)-(Z) system is used to describe the stereochemistry of double bonds in alkenes based on the Cahn-Ingold-Prelog priority rules.

• Z (zusammen): If the groups of highest priority on each carbon of the double bond are on the same side, the alkene is designated as Z.

• E (entgegen): If the highest priority groups are on opposite sides, the alkene is designated as E.

• Priority is determined by atomic number: higher atomic number = higher priority (e.g., Cl > F, Br > H).

Example: (Z)-2-Bromo-1-chloro-1-fluoroethene vs. (E)-2-Bromo-1-chloro-1-fluoroethene; (Z)-2-Butene (cis-2-butene) vs. (E)-2-Butene (trans-2-butene).

Relative Stabilities of Alkenes

Cis vs. Trans Alkenes

Alkene stability is influenced by steric hindrance and substitution patterns.

• Cis alkenes are generally less stable than trans alkenes due to increased steric crowding between substituents on the same side of the double bond.

• Trans alkenes have substituents on opposite sides, reducing steric interactions and increasing stability.

Example: Crowding in cis-2-butene vs. trans-2-butene.

Heat of Hydrogenation

The relative stabilities of alkenes can be measured by their heats of hydrogenation, which are the enthalpy changes ($\Delta H$) when alkenes are converted to alkanes by addition of hydrogen.

• Lower (more negative) $\Delta H$ indicates less stable alkene.

• More substituted alkenes have less negative $\Delta H$ and are more stable.

Example:

Alkene	Heat of Hydrogenation ($\Delta H$)
1-Butene	$-127$ kJ mol$^{-1}$
cis-2-Butene	$-120$ kJ mol$^{-1}$
trans-2-Butene	$-115$ kJ mol$^{-1}$

Overall Relative Stability of Alkenes

Alkene stability increases with the number of alkyl substituents attached to the double bond.

• Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted > Unsubstituted

Example: Tetrasubstituted alkenes are the most stable due to hyperconjugation and electron-donating effects of alkyl groups.

Synthesis of Alkenes via Elimination Reaction

E2 and E1 Mechanisms

Alkenes are commonly synthesized from alkyl halides via elimination reactions. The two main mechanisms are E2 and E1.

• E2 (bimolecular elimination) reactions are favored for secondary and tertiary alkyl halides and with strong bases such as sodium ethoxide or potassium tert-butoxide.

• E1 (unimolecular elimination) reactions can be problematic due to competing side reactions.

• Bulky bases (e.g., potassium tert-butoxide) are used for E2 reactions of primary alkyl halides to favor elimination over substitution.

General E2 Reaction Equation:

$\text{R-CH}_2\text{-CH}_2\text{-X} + \text{Base} \rightarrow \text{R-CH}=\text{CH}_2 + \text{Base-H} + \text{X}^-$

Zaitsev’s Rule: Formation of the Most Substituted Alkene

Regioselectivity in Elimination

Zaitsev’s Rule states that when elimination can yield more than one alkene, the most highly substituted alkene is the major product.

• This rule applies when a small base (e.g., ethoxide) is used.

• Major product: more substituted alkene (greater stability).

Example:

Alkene Product	Relative Yield
2-Methyl-2-butene	69% (more stable)
2-Methyl-1-butene	31% (less stable)

Transition State and Double Bond Character

The transition state of the elimination reaction resembles the more substituted alkene, which has a lower activation energy ($\Delta G^{\ddagger}$) and forms faster.

• Kinetic control: the product formed fastest is the major product.

Example: Reaction coordinate diagrams show lower activation energy for formation of the more substituted alkene.

Formation of the Least Substituted Alkene Using a Bulky Base

Hofmann Product

Bulky bases favor elimination of the most accessible (least hindered) hydrogen, leading to the formation of the less substituted alkene (Hofmann product).

• Bulky bases cannot easily approach hindered hydrogens.

• Major product: less substituted alkene.

Example:

Alkene Product	Relative Yield
2-Methyl-2-butene	27.5% (more substituted)
2-Methyl-1-butene	72.5% (less substituted)

Additional info: These notes cover the first part of Chapter 7, focusing on alkene stereochemistry, stability, and elimination mechanisms. Further topics such as alkynes, carbocation rearrangements, and retrosynthetic analysis are typically included in a full chapter.