Key Reactions and Mechanisms from Organic Chemistry Chapter 8: Alkenes and Alkynes

Reactions and Mechanisms of Alkenes and Alkynes

Addition Reactions of Alkenes

Addition reactions are a hallmark of alkene chemistry, where reagents add across the carbon–carbon double bond. These reactions are central to the transformation of alkenes into a variety of functional groups.

• Hydrohalogenation (Addition of HX): Alkenes react with hydrogen halides (HX) to form alkyl halides. The reaction follows Markovnikov's rule, where the hydrogen atom adds to the carbon with more hydrogens.

• Hydration (Addition of Water): Water adds to alkenes in the presence of acid to form alcohols. This also follows Markovnikov's rule.

• Halogenation (Addition of X2): Alkenes react with halogens (Cl2, Br2) to form vicinal dihalides via an anti addition mechanism.

• Hydroboration-Oxidation: A two-step process where borane adds to the alkene (anti-Markovnikov), followed by oxidation to yield an alcohol.

• Oxymercuration-Demercuration: Water adds to alkenes in a Markovnikov fashion without carbocation rearrangement.

• Epoxidation: Alkenes react with peracids (e.g., mCPBA) to form epoxides (oxiranes).

• Dihydroxylation: Addition of two hydroxyl groups across the double bond, either syn (OsO4) or anti (epoxidation followed by acid hydrolysis).

• Oxidative Cleavage: Strong oxidants (e.g., KMnO4, O3) cleave alkenes to form carbonyl compounds.

Example: The addition of HBr to propene yields 2-bromopropane as the major product due to Markovnikov addition.

Alkene Synthesis and Dehydration

Alkenes can be synthesized by elimination reactions, such as the dehydration of alcohols or dehydrohalogenation of alkyl halides.

• Alcohol Dehydration: Heating alcohols with acid (e.g., H2SO4) removes water to form alkenes via an E1 or E2 mechanism.

• Dehydrohalogenation: Alkyl halides react with strong bases to eliminate HX and form alkenes.

Equation:

$\ce{R-CH2-CH2-OH} \xrightarrow{H_2SO_4, \ \Delta} \ce{R-CH=CH2} + \ce{H2O}$

Addition Reactions of Alkynes

Alkynes undergo similar addition reactions as alkenes, but can add two equivalents of reagents due to their two π bonds.

• Hydrogenation: Alkynes can be fully hydrogenated to alkanes or partially hydrogenated to cis-alkenes (Lindlar's catalyst) or trans-alkenes (Na/NH3).

• Hydrohalogenation: Addition of HX to alkynes yields vinyl halides or geminal dihalides, depending on equivalents used.

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

• Halogenation: Addition of X2 forms di- or tetrahalides.

• Ozonolysis/Oxidative Cleavage: Cleavage of alkynes yields carboxylic acids or ketones.

Example: Hydration of terminal alkyne (e.g., propyne) with HgSO4/H2SO4 yields acetone.

Organometallic Reactions

Organometallic reagents such as Grignard reagents and organolithium compounds are used to form carbon–carbon bonds.

• Grignard Reagents (RMgX): React with carbonyl compounds to form alcohols after hydrolysis.

• Organolithium Reagents (RLi): Similar reactivity to Grignard reagents, often more reactive.

• Alkyne Alkylation: Terminal alkynes can be deprotonated and alkylated with alkyl halides.

Equation:

$\ce{R-C#C-H} + \ce{NaNH2} \rightarrow \ce{R-C#C^- Na^+}$

$\ce{R-C#C^- Na^+} + \ce{R'-X} \rightarrow \ce{R-C#C-R'} + \ce{NaX}$

Oxidation and Reduction of Alkenes and Alkynes

Oxidation and reduction reactions modify the degree of saturation and functional groups in organic molecules.

• Reduction: Hydrogenation of alkenes/alkynes to alkanes using H2 and metal catalysts (Pt, Pd, Ni).

• Oxidation: Strong oxidants (KMnO4, OsO4, O3) cleave double/triple bonds to form carbonyl compounds or carboxylic acids.

Summary Table: Major Reactions of Alkenes and Alkynes

Reaction Type	Reagent(s)	Product(s)	Regioselectivity	Stereochemistry
Hydrohalogenation	HX	Alkyl halide	Markovnikov	Mixed
Hydration	H2O/H+	Alcohol	Markovnikov	Mixed
Hydroboration-Oxidation	1. BH3; 2. H2O2, OH-	Alcohol	Anti-Markovnikov	Syn
Halogenation	Cl2, Br2	Dihalide	None	Anti
Oxymercuration-Demercuration	1. Hg(OAc)2, H2O; 2. NaBH4	Alcohol	Markovnikov	Mixed
Epoxidation	mCPBA	Epoxide	None	Syn
Dihydroxylation	OsO4, KMnO4	Glycol	None	Syn
Ozonolysis	O3, (CH3)2S	Aldehyde/Ketone	None	None

Additional info:

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

• Anti-Markovnikov addition: The reagent adds in the opposite fashion, often via a radical or concerted mechanism.

• Syn addition: Both groups add to the same face of the double bond; anti addition: groups add to opposite faces.

• Oxymercuration-Demercuration avoids carbocation rearrangement, making it useful for Markovnikov hydration without side products.