Chapter 6: The Reactions of Alkenes and Alkynes – The Stereochemistry of Addition Reactions

Reactions of Alkenes and Alkynes

Introduction

This chapter explores the mechanisms and stereochemistry of electrophilic addition reactions involving alkenes and alkynes. These reactions are fundamental in organic chemistry, as they transform unsaturated hydrocarbons into more complex molecules by breaking π bonds and forming new σ bonds.

Electrophilic Addition to Alkenes

General Mechanism

• Electrophilic addition involves the breaking of a π bond in an alkene and the formation of two new σ bonds.

• The alkene (with sp2 carbons) reacts with an electrophile (Y+) and a nucleophile (Z-).

• General equation: $\ce{R_2C=CR_2 + Y^+ + Z^- \rightarrow R_2C(Y)-CR_2(Z)}$

Addition of Hydrogen Halides

• Alkenes react with hydrogen halides (HBr, HI, HCl) to form alkyl halides.

• Example: $\ce{2,3-dimethyl-2-butene + HBr \rightarrow 2-bromo-2,3-dimethylbutane}$ $\ce{cyclohexene + HI \rightarrow iodocyclohexane}$

Regioselectivity: Which sp2 Carbon Gets the H+?

• In unsymmetrical alkenes, the hydrogen adds to the sp2 carbon bonded to the most hydrogens (Markovnikov's rule).

• Example: $\ce{CH_2=C(CH_3)_2 + HCl \rightarrow (CH_3)_3CCl}$ (tert-butyl chloride, major product)

Mechanism of Electrophilic Addition

• The first step is the formation of a carbocation intermediate (rate-limiting step).

• Stability of the carbocation determines the major product.

• Example mechanism: $\ce{CH_2=C(CH_3)_2 + HCl \rightarrow (CH_3)_3C^+ + Cl^- \rightarrow (CH_3)_3CCl}$

Relative Stabilities of Carbocations

• Carbocation stability increases with the number of alkyl groups attached:

Carbocation Type	Stability
Tertiary	Most stable
Secondary	Moderately stable
Primary	Less stable
Methyl	Least stable

• Alkyl groups stabilize carbocations by hyperconjugation and inductive effects.

Hyperconjugation

• Delocalization of electrons from adjacent C–H bonds into the empty p orbital of the carbocation stabilizes the positive charge.

• Occurs in alkyl-substituted carbocations, not in methyl cations.

Rate of Reaction and Carbocation Stability

• More stable carbocations form more rapidly, leading to faster reactions and major product formation.

• Reaction coordinate diagrams illustrate lower activation energy for formation of more stable carbocations.

Regioselectivity in Electrophilic Addition

• Reactions are regioselective when one constitutional isomer is formed preferentially.

• Degree of regioselectivity: completely, highly, or moderately regioselective.

• Example: $\ce{CH_3CH=CH_2 + HCl \rightarrow CH_3CHClCH_3}$ (2-chloropropane, major product)

Carbocation Rearrangement

• Carbocations can rearrange via 1,2-hydride shifts or 1,2-methyl shifts to form more stable intermediates.

• Major product results from addition to the rearranged, more stable carbocation.

Acid-Catalyzed Addition of Water

Mechanism

• Alkenes react with water in the presence of acid (H2SO4) to form alcohols.

• Mechanism involves electrophilic addition, carbocation formation, nucleophilic attack by water, and deprotonation.

• Example: $\ce{CH_3CH=CH_2 + H_2O \xrightarrow{H_2SO_4} CH_3CH(OH)CH_3}$ (2-propanol)

Regioselective vs. Stereoselective Reactions

• Regioselective: More of one constitutional isomer is formed than another.

• Stereoselective: More of one stereoisomer is formed than another.

Stereochemistry of Addition Reactions

Formation of Stereoisomers

• Some addition reactions produce products with no stereoisomers (e.g., addition to propene).

• Others produce racemic mixtures when a new asymmetric center is formed.

• Example: $\ce{CH_2=CHCH_2CH_3 + HBr \rightarrow CH_3CHBrCH_2CH_3}$ (2-bromobutane, racemic mixture)

Formation of Racemic Mixtures

• When a planar carbocation intermediate is attacked from either side, equal amounts of enantiomers are formed.

• Transition states leading to enantiomers have equal energy.

Syn and Anti Addition

• Syn addition: Both substituents add to the same side of the double or triple bond (e.g., hydrogenation with Pd/C).

• Anti addition: Substituents add to opposite sides (e.g., bromination with Br2).

• Syn addition to a cis isomer forms only cis stereoisomers; anti addition forms only trans stereoisomers.

Meso Compounds

• If the substituents added are the same, a meso compound (achiral despite stereocenters) can result.

• Example: $\ce{1,2-dimethylcyclopentene + H_2 \xrightarrow{Pd/C} meso ext{-}1,2-dimethylcyclopentane}$

Enzyme-Catalyzed Stereoselectivity

• Enzymes can catalyze addition reactions in a completely stereoselective manner, forming only one stereoisomer.

• Enzymes can block one side of the reactant, leading to selective product formation.

• Example: $\ce{fumarate + H_2O \xrightarrow{fumarase} (S) ext{-}malate}$

Reactions of Alkynes

Introduction to Alkynes

• An alkyne is a hydrocarbon containing a carbon–carbon triple bond.

• General formula for acyclic alkynes: $\ce{C_nH_{2n-2}}$

Nomenclature of Alkynes

• The suffix "-yne" is used, and the triple bond is assigned the lowest possible number.

• Substituents are numbered to give the lowest set of locants.

• Examples: $\ce{HC#CH}$ (ethyne, acetylene) $\ce{CH_3C#CCH_3}$ (2-butyne)

Structure of Alkynes

• The triple bond consists of one σ bond and two π bonds formed by side-to-side overlap of p orbitals.

• Alkyne carbons are sp-hybridized, resulting in linear geometry (180° bond angle).

Electrophilic Addition to Alkynes

• Alkynes undergo electrophilic addition reactions similar to alkenes.

• First step: addition of an electrophile to form a vinyl cation intermediate.

• Second step: nucleophilic attack forms the final product.

• Example: $\ce{CH_3C#CCH_3 + HCl \rightarrow CH_3C(Cl)=CHCH_3}$

Regioselectivity in Alkynes

• Addition to terminal alkynes is regioselective; the electrophile adds to the carbon with more hydrogens.

• Internal alkynes can form one or two products depending on symmetry.

• Example: $\ce{CH_3CH_2C#CH + HCl \rightarrow CH_3CH_2C(Cl)=CH_2}$ (2-bromo-1-butene)

Acid-Catalyzed Addition of Water to Alkynes

• Alkynes react with water (in acid) to form enols, which tautomerize to ketones.

• General reaction: $\ce{R-C#C-H + H_2O \xrightarrow{H_2SO_4} R-CO-CH_3}$

Keto–Enol Tautomerization

• Enols (compounds with a hydroxyl group attached to a double-bonded carbon) rapidly convert to ketones via tautomerization.

• Mechanism involves proton transfer and resonance stabilization.

• Equation: $\ce{RCH=CH(OH) \rightleftharpoons RCH_2CO}$

Hydrogenation of Alkynes

• Alkynes can be hydrogenated to alkanes using catalysts.

• Partial hydrogenation to cis-alkenes is possible using Lindlar's catalyst (Pd/CaCO3 with lead acetate and quinoline).

• Syn addition produces cis-alkenes; anti addition produces trans-alkenes.

Lindlar Catalyst

• Lindlar catalyst selectively hydrogenates alkynes to cis-alkenes without further reduction to alkanes.

• Composition: palladium on calcium carbonate, treated with lead(II) acetate and quinoline.

Summary Table: Regioselectivity and Stereoselectivity in Addition Reactions

Reaction Type	Regioselectivity	Stereoselectivity	Major Product
Alkene + HX	Markovnikov	Racemic or single	Alkyl halide
Alkyne + HX	Markovnikov	Racemic or single	Vinyl/geminal dihalide
Alkene + H2 (Pd/C)	Not applicable	Syn addition	Cis-alkane/alkene
Alkene + Br2	Not applicable	Anti addition	Trans-dibromo compound
Alkyne + H2 (Lindlar)	Not applicable	Syn addition	Cis-alkene

Additional info:

• Enzyme-catalyzed reactions can be used to separate enantiomers due to their stereoselectivity.

• Carbocation rearrangements (hydride and methyl shifts) are important for predicting major products in addition reactions.