Smith ch08_Lecture_edit


General Features of Elimination Reactions

  • Involves losing elements from starting materials.

  • Forms new bonds in the product.

Elimination of HX

  • Involves a base removing elements of an acid, HX, from organic starting material.

Dehydrohalogenation

  • Defined as the removal of elements HX.

  • Example of β elimination.

  • Illustrated using curved arrow formalism (breaking/forming bonds).

Common Bases for Dehydrohalogenation

  • Predominantly negatively charged oxygen compounds such as:

    • HO− (hydroxide)

    • RO− (alkoxide derivatives).

Drawing Products of Dehydrohalogenation

  • Identify the α carbon.

  • Locate β carbons with H atoms.

  • Remove elements H and X from α and β carbons to form a π bond.

Alkenes

  • Hydrocarbons containing a carbon-carbon double bond.

  • Carbons in the double bond are sp2 hybridized.

  • Geometry is trigonal planar.

  • Bond angles: 120 degrees.

Alkene Structure

  • Comprised of a σ bond and a π bond formed by the double bond.

Classifying Alkenes

  • Classification based on the number of carbon atoms bonded to the carbons of the double bond.

Restricted Rotation About Double Bonds

  • Free rotation occurs around single bonds but is restricted around double bonds.

Stereoisomers of Alkenes

  • Restricted rotation leads to the possibility of stereoisomers, such as cis-2-butene and trans-2-butene.

  • Classified as diastereomers (non-mirror image stereoisomers).

Alkene Diastereomers

  • Two different groups on each end of a double bond lead to potential cis-trans isomerism.

Stability of Trans Alkenes

  • Trans alkenes are generally more stable due to reduced steric interactions arising from distance between bonded groups.

Stability in Alkenes

  • Stability increases with the number of R groups on double bond carbons.

  • Alkene stability linked to electron density acceptance based on hybridization.

  • Sp2 carbons (greater s-character) accept electron density better than sp3 carbons.

Relative Stability of Butenes

  • Disubstituted 2-butenes more stable than monosubstituted 1-butene.

  • Trans-2-butene more stable than cis-2-butene due to lesser crowding.

Elimination Mechanisms

  • Two mechanisms exist: E2 (bimolecular elimination) and E1 (unimolecular elimination).

  • Differentiated based on bond cleavage and formation timing, relating to SN1 and SN2 substitutions.

E2 Mechanism

  • Major mechanism for dehydrohalogenation.

  • Exhibits second-order kinetics; both alkyl halide and base affect the rate.

  • Concerted reaction: all bonds broken and formed in a single step.

Energy Diagram for E2 Reaction

  • Illustrates energy changes throughout the reaction process.

Bases in E2 Mechanisms

  • Strong, negatively charged bases such as −OH and −OR used.

  • Rate increases with stronger bases.

Effects of Leaving Group and Solvent on E2 Reactions

  • Better leaving groups enhance reaction speeds.

  • Polar aprotic solvents increase E2 reaction rates.

Effect of Alkyl Halide Structure on E2 Reactions

  • In E2, as the R group number increases, the reaction rate also increases.

Transition States in E2 Mechanisms

  • Transition state stability: double bond formation increases with higher alkyl substitution, lowering activation energy (Ea).

Product Stability and Rate of E2 Reactions

  • More R groups lead to the formation of stable alkenes.

  • Disubstituted products more favorable than monosubstituted in reaction speed.

E2 Mechanism Summary

  • A closed overview of E2 mechanisms and their importance in organic reactions.

E2 Reaction in Organic Synthesis

  • Example: E2 reactions in synthesizing complex natural products such as quinine.

The Zaitsev (Saytzeff) Rule

  • Predicts that the major product of β elimination is the more substituted double bond, which is more stable.

Regioselectivity of E2 Reactions

  • Dominant formation of one constitutional isomer when multiple are possible.

Stereoselectivity of E2 Reactions

  • More stable stereoisomers preferentially formed due to stereoselectivity.

E1 Mechanism

  • Illustrated through the example of dehydrohalogenation with H2O to form alkenes.

  • Exhibits first-order kinetics; follows a two-step mechanism.

E1 Mechanism Differences

  • E1 involves the leaving group breaking before β proton removal, contrasting with E2.

Energy Diagram for an E1 Reaction

  • Provides visualization of energy changes throughout the E1 process.

Effect of Alkyl Halide Structure on E1 Reactions

  • Rate increases with the number of R groups on the leaving group carbon.

Effect of Base on the E1 Reaction

  • Strong bases favor E2, while weaker bases favor E1 mechanisms.

Regioselectivity of E1 Reactions

  • Zaitsev’s rule applies; E1 reactions favor more substituted, stable alkenes.

E1 Mechanism Summary

  • E1 reactions less useful than E2 due to competing SN1 reactions.

Comparison of E1 and E2 Mechanisms

  • Both begin with carbocation formation but differ in subsequent steps—nucleophile vs base role.

Stereochemistry of E2 Reactions

  • Transition state consists of aligned atoms necessary for elimination; reveals two coplanar ways for C—H and C—X bonds.

Two Possible Geometries for E2 Reactions

  • Preference for anti-periplanar geometry, resulting in lower energy reacting conformations.

Anti Periplanar Geometry

  • Impact on compounds like chlorocyclohexane existing in chair conformations.

Trans Diaxial Geometry for E2 Reactions

  • C—Cl bond must be anti-periplanar to β C—H bond, limiting the reactive conformer.

Trans Diaxial Geometry Examples

  • Visual of trans-diaxial geometry in chlorocyclohexane.

E2 Reactions of Cis and Trans Isomers

  • Reaction pathways vary based on axial arrangements of substituents resulting in different alkene products.

Regiochemistry of E2 Reactions on Cyclohexanes

  • Different outcomes based on axial β hydrogen orientations.

Axial Leaving Groups for E2 Reactions

  • Required conformational arrangements for successful E2 reactions in trans isomers.

Anti Zaitsev Products for E2 Reactions

  • Limited reaction pathway due to single axial β hydrogen, contrasting typical Zaitsev outcomes.

Comparison of E1 and E2 Mechanisms

  • Primary focus on base strength for elimination mechanism determination.

E2 Reactions and Alkyne Synthesis

  • Sequential elimination reactions yield alkynes from alkenes.

E2 Reactions and Alkyne Synthesis Needs

  • Stronger bases required for alkyne synthesis to break sp2 hybridized C—H bonds.

Dehydrohalogenation of Dihalides

  • Reactivity and product formation during dihalide elimination.

Reaction Mechanism Prediction

  • Nucleophile vs base activity influences preference for reaction pathways.

Bulky Bases Favoring Elimination

  • Bulky bases, due to steric hindrance, promote elimination over substitution processes.