Addition Reactions to Alkenes and Alkynes

Flashcards on Addition Reactions to Alkenes and Alkynes

Addition Reaction

A reaction where a pi bond is converted to a sigma bond.

Lewis Base in Addition Reactions

A pi bond can act as a Lewis base (nucleophile or Bronsted-Lowry base) due to higher reactivity than sigma bonds.

Markovnikov's Rule

In general, hydrogen atoms tend to add to the carbon already bearing more hydrogen atoms in asymmetrical alkenes.

Regioselective Reaction

A reaction where one constitutional isomer is formed in greater quantity than another.

Anti-Markovnikov Products

Observed when reactions are performed in the presence of peroxides such as H2O2.

Carbocation Rearrangements

Hydride or methyl shifts occur if the shift makes the carbocation more stable.

Thermodynamic Expression

Equilibrium in addition/elimination reactions is assessed using ΔG (free energy) to determine which side the equilibrium favors.

Hydration Reaction

The components of water (-H and -OH) are added across a C=C double bond using an acid catalyst.

Oxymercuration-Demercuration

An alternative process that can yield Markovnikov products without the possibility of rearrangement; begins with mercuric acetate.

Mercurinium Ion

Stabilized by resonance; a good electrophile that can be attacked by a weak nucleophile like water; formed during oxymercuration.

Hydroboration-Oxidation

A process used to achieve anti-Markovnikov hydration, occurring in two steps and resulting in syn addition.

Halogenation

The addition of two halogen atoms across a C=C double bond, a key step in the production of PVC.

Halohydrins

Formed when halogens (Cl2 or Br2) are added to an alkene with water as the solvent, resulting in the addition of –X and –OH across a C=C double bond.

Catalytic Hydrogenation

The addition of H2 across a C=C double bond, resulting in syn addition if a chirality center is formed; typical catalysts include Pt, Pd, and Ni.

Asymmetric Hydrogenation

A chiral catalyst can produce one desired enantiomer over another.

Anti Dihydroxylation

Achieved through a multi-step process starting with epoxide formation.

Epoxide

Generally stable if solution isn’t acidic; opening is acid catalyzed; results in anti orientation diols.

Syn Dihydroxylation

Adds two –OH groups across a C=C double bond in one step using reagents like OsO4.

Oxidative Cleavage

A reaction where C=C double bonds are reactive, such as ozonolysis.

Alkynes

Hydrocarbons containing carbon-carbon triple bonds; named using IUPAC nomenclature similar to alkanes, with modifications.

Acetylene

The simplest alkyne, used in blow torches and synthesis of more complex alkynes.

Alkyne Acidity

Terminal alkynes have a lower pKa than other hydrocarbons but still require a strong base to react.

Preparation of Alkynes

Achieved by elimination reactions, usually via an E2 mechanism, using geminal or vicinal dihalides.

Reduction of Alkynes

Can undergo hydrogenation with H2 and a catalyst to form alkanes or can be selectively reacted using poisoned catalysts like Lindlar's catalyst to produce cis alkenes.

Dissolving Metal Reductions

Using dissolving metals (Na/NH3) results in anti addition, producing trans alkenes.

Hydrohalogenation of Alkynes

Undergo hydrohalogenation similar to alkenes, with Markovnikov regioselectivity; peroxides promote anti-Markovnikov addition.

Hydration of Alkynes

Acid-catalyzed Markovnikov hydration produces ketones; hydroboration-oxidation gives anti-Markovnikov product (aldehyde).

Alkyne Halogenation

One equivalent produces cis and trans dihalo alkene; excess produces tetrahalide.

Alkyne Ozonolysis

Produces carboxylic acids for internal alkynes and carboxylic acid and CO2 for terminal alkynes.

Alkylation of Terminal Alkynes

Terminal alkynes can be deprotonated and converted into good nucleophiles, which can attack methyl or 1° alkyl halides to develop molecular complexity.

Synthetic Strategies

Halogenation of an alkene followed by two dehydrohalogenation reactions can decrease saturation, converting an alkene to an alkyne.