Synthesis and Reactions of Alkenes and Alkynes

Structure and Synthesis of Alkenes

Methods for Synthesis of Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. Their synthesis is fundamental in organic chemistry, providing access to a wide range of functionalized molecules.

• Dehydrohalogenation of Alkyl Halides: Removal of a hydrogen halide (HX) from an alkyl halide using a strong base, typically yielding an alkene via an E2 mechanism.

• Dehalogenation of Vicinal Dihalides: Elimination of two halogen atoms from adjacent carbons, often using zinc in acetic acid.

• Dehydration of Alcohols: Acid-catalyzed removal of water from alcohols to form alkenes, usually via an E1 mechanism for secondary and tertiary alcohols.

• Dehydrogenation of Alkanes: Removal of hydrogen from alkanes at high temperatures, often using a catalyst.

• Alkyne Reduction: Partial reduction of alkynes to alkenes using Lindlar's catalyst (cis-alkene) or sodium in liquid ammonia (trans-alkene).

• Wittig Reaction: Reaction of aldehydes or ketones with phosphonium ylides to form alkenes.

Example Equations

• Dehydrohalogenation: $\ce{R-CH_2-CH_2Br} + \ce{KOH} \rightarrow \ce{R-CH=CH_2} + \ce{KBr} + \ce{H_2O}$

• Dehydration: $\ce{R-CH_2-CH_2OH} \xrightarrow{H_2SO_4, \Delta} \ce{R-CH=CH_2} + \ce{H_2O}$

Key Terms and Concepts

• Alkene: Hydrocarbon with at least one carbon-carbon double bond.

• Dehydrohalogenation: Elimination of HX from an alkyl halide to form an alkene.

• Dehydration: Elimination of water from an alcohol to form an alkene.

• Wittig Reaction: Formation of alkenes from aldehydes/ketones and phosphonium ylides.

• Beta Elimination (E2): Simultaneous removal of a proton and a leaving group from adjacent carbons.

• Zaitsev's Rule: The most substituted alkene is favored in elimination reactions.

Reactions of Alkenes

Electrophilic Addition Reactions

Alkenes undergo a variety of addition reactions due to the electron-rich nature of the double bond.

• Hydrohalogenation: Addition of HX (X = Cl, Br, I) to form alkyl halides. Follows Markovnikov's rule.

• Hydration: Addition of water (acid-catalyzed) to form alcohols.

• Halogenation: Addition of X2 (Cl2, Br2) to form vicinal dihalides.

• Halohydrin Formation: Addition of X2 and H2O to form halohydrins.

• Hydroboration-Oxidation: Anti-Markovnikov addition of water using BH3 and H2O2.

• Ozonolysis: Cleavage of double bonds with ozone to form carbonyl compounds.

• Polymerization: Alkenes can undergo addition polymerization to form polymers.

Example Equations

• Hydrohalogenation: $\ce{CH_2=CH_2} + \ce{HBr} \rightarrow \ce{CH_3-CH_2Br}$

• Hydration: $\ce{CH_2=CH_2} + \ce{H_2O} \xrightarrow{H_2SO_4} \ce{CH_3CH_2OH}$

• Halogenation: $\ce{CH_2=CH_2} + \ce{Br_2} \rightarrow \ce{BrCH_2CH_2Br}$

• Ozonolysis: $\ce{RCH=CHR'} + \ce{O_3} \rightarrow \ce{RCHO} + \ce{R'CHO}$

Regioselectivity and Stereochemistry

• Markovnikov's Rule: In the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens, and the halide to the more substituted carbon.

• Anti-Markovnikov Addition: Observed in hydroboration-oxidation, where the less substituted carbon receives the OH group.

• Syn vs. Anti Addition: Syn addition adds both groups to the same face of the double bond (e.g., hydroboration), while anti addition adds to opposite faces (e.g., halogenation).

Oxidative Reactions

• Epoxidation: Formation of epoxides using peracids (e.g., mCPBA).

• Dihydroxylation: Addition of two hydroxyl groups (syn or anti) using OsO4 or KMnO4.

• Oxidative Cleavage: Ozonolysis or hot KMnO4 to cleave double bonds into carbonyl compounds.

Reactions of Alkynes

Acidity and Synthesis

Alkynes are more acidic than alkenes and alkanes due to the sp-hybridized carbon. Terminal alkynes can be deprotonated to form acetylide ions, which are useful nucleophiles in synthesis.

• Alkylation of Acetylide Ions: Reaction with primary alkyl halides to form new carbon-carbon bonds.

• Preparation from Dihalides: Double dehydrohalogenation of vicinal or geminal dihalides yields alkynes.

Example Equation

• $\ce{RC#CH} + \ce{NaNH_2} \rightarrow \ce{RC#C^- Na^+}$

Addition Reactions of Alkynes

• Hydrogenation: Complete reduction to alkanes (Pd/C) or partial reduction to cis-alkenes (Lindlar's catalyst) or trans-alkenes (Na/NH3).

• Halogenation: Addition of X2 (Cl2, Br2) to form di- or tetrahalides.

• Hydrohalogenation: Addition of HX, following Markovnikov's rule; excess gives geminal dihalides.

• Hydration: Acid-catalyzed addition of water (with HgSO4) yields ketones via enol intermediates.

• Hydroboration-Oxidation: Anti-Markovnikov addition of water, yielding aldehydes from terminal alkynes.

• Ozonolysis/Oxidative Cleavage: Cleavage of triple bonds to carboxylic acids or ketones.

Example Equations

• Hydrogenation: $\ce{RC#CH} + 2\ce{H_2} \xrightarrow{Pd/C} \ce{RCH_2CH_3}$

• Partial Hydrogenation: $\ce{RC#CH} + \ce{H_2} \xrightarrow{Lindlar's} \ce{RCH=CH_2}$ (cis-alkene)

• Hydration: $\ce{RC#CH} + \ce{H_2O} \xrightarrow{HgSO_4, H_2SO_4} \ce{RCOCH_3}$

Key Terms and Concepts for Alkynes

• Acetylide Ion: The conjugate base of a terminal alkyne, a strong nucleophile.

• Lindlar's Catalyst: A poisoned palladium catalyst used for syn (cis) hydrogenation of alkynes to alkenes.

• Markovnikov and Anti-Markovnikov Addition: Regioselectivity in addition reactions, similar to alkenes.

• Ozonolysis: Cleavage of triple bonds to form carboxylic acids or ketones.

Summary Table: Key Reactions of Alkenes and Alkynes

Reaction	Reagents	Product	Regioselectivity/Stereochemistry
Hydrohalogenation (Alkene)	HX	Alkyl halide	Markovnikov
Hydration (Alkene)	H2O, H2SO4	Alcohol	Markovnikov
Hydroboration-Oxidation (Alkene)	1. BH3; 2. H2O2, OH-	Alcohol	Anti-Markovnikov, syn
Halogenation (Alkene)	X2	Vicinal dihalide	Anti addition
Hydrogenation (Alkyne)	H2, Pd/C	Alkane	Syn addition
Partial Hydrogenation (Alkyne)	Lindlar's catalyst	Cis-alkene	Syn addition
Hydration (Alkyne)	HgSO4, H2SO4	Ketone (from terminal alkyne)	Markovnikov
Hydroboration-Oxidation (Alkyne)	1. BH3; 2. H2O2, OH-	Aldehyde (from terminal alkyne)	Anti-Markovnikov

Additional info:

• These notes are based on textbook summary pages and essential terms from chapters on alkenes and alkynes, including synthesis, reactions, and key terminology.

• Study problems and worked examples are included in the original material to reinforce concepts and provide practice with reaction mechanisms and synthetic planning.