Alkynes Alkynes Nomenclature Synthesis Reactions

Alkynes Alkynes Hydrocarbons that contain carbon-carbon triple bond sp hybridized carbons linear geometry Molecular formula CnH2n-2 Less common than alkenes in nature or in pharmaceutical applications acetylene

Alkynes Physical Properties: Similar to alkanes and alkenes Nonpolar Insoluble in water Densities ~0.6 – 0.8 g/mL Acetylene Most important commercial alkyne Fuel for oxyacetylene welding torches Thermodynamically unstable Stored in cylinders filled with crushed firebrick wet with acetone

IR Terminal alkynes exhibit the following peaks in an IR spectrum: sp C – H at ~ 3300 cm-1 C Ξ C at ~ 2150 cm-1 (weak) Internal alkynes do not have an sp C-H so the peak at 3300 cm-1 is absent. Internal alkynes may or may not exhibit a peak at ~2150 cm-1 for the triple bond.

Alkyne Nomenclature To name alkynes: Find the longest continuous chain that includes the triple bond Change the ending of the parent alkane to “yne” Number the chain from the end closest to the triple bond Designate the position of the triple bond using its lower-numbered carbon atom Number and name substituents

Alkyne Nomenclature Example: Give the IUPAC name for the following compounds.

Alkyne Nomenclature Example: Draw the following compounds. (R)-2-chloro-3,3-dimethylnon-4-yne 4,5-dimethyl-1-hexyne

Synthesis of Alkynes Terminal alkynes are much more acidic than other hydrocarbons. Acidity increases as the amount of s character increases sp > sp2 > sp3

Synthesis of Alkynes Acetylide ion Terminal acetylenes can be deprotonated using a very strong base such as sodium amide (NaNH2) to form the corresponding acetylide ion. Hydroxide ion and alkoxide ions are not strong enough to produce the acetylide ion. Acetylide ion

Synthesis of Alkynes Acetylide ions are strong bases and strong nucleophiles. Can serve as nucleophile in SN2 reactions Addition of an acetylide ion to alkyl halides Addition of an acetylide ion to a carbonyl compound

Synthesis of Alkynes Alkylation of Acetylide Ions SN2 addition of acetylide ion to a methyl or 1o alkyl halide forming a new alkyne with a longer carbon chain

Synthesis of Alkynes Addition of Acetylide Ion to Carbonyl Group acetylide ion adds to the electrophilic carbon in a carbonyl producing an alkynol (an alcohol with a triple bond in the backbone) H3O+

Synthesis of Alkynes Examples:

Synthesis of Alkynes Example: Draw the product for the following reaction.

Synthesis of Alkynes Synthesis of Alkynes by Double Dehydrohalogenation loss of two molecules of HX from a vicinal or geminal dihalide

Synthesis of Alkynes Loss of the second molecule of HX (from the vinyl halide) requires extremely basic conditions: KOH (fused)/200oC most stable internal alkyne formed by base catalyzed rearrangement NaNH2/150oC terminal alkyne formed Limited use due to extreme conditions and rearrangements

Reactions of Alkynes The pi bonds of alkynes are electron rich undergo addition reactions readily similar to alkenes Alkynes can add up to two moles of a reagent to the triple bond Forms the corresponding alkane

Reactions of Alkynes Catalytic Hydrogenation formation of an alkane by addition of hydrogen to the triple bond Reagents: H2 Pt, Pd, or Ni

Reactions of Alkynes Catalytic hydrogenation involves an alkene as an intermediate cannot stop at the alkene under these conditions

Reactions of Alkynes OR Catalytic Hydrogenation to cis-Alkene formation of cis-alkene using a poisoned (partially deactivated) catalyst Reagents H2 Lindlar’s catalyst (Pd/BaSO4/quinoline) Ni2B (nickel boride) OR

Reactions of Alkynes Examples:

Reactions of Alkynes Metal-NH3 Reduction to trans-Alkene formation of trans-alkene using Na/NH3 involves radical anion and radical intermediates trans isomer forms because the trans radical intermediate is more stable

Reactions of Alkynes Example:

Reactions of Alkynes Addition of Halogens (X2) addition of Cl2, Br2, or I2 across the triple bond dihaloalkene intermediate difficult to stop at the alkene most commonly used to form the tetrahaloalkane by addition of 2 moles of halogen

Reactions of Alkynes Example:

Reactions of Alkynes Addition of HX addition of HX to the triple bond monohalogenated alkene intermediate difficult to stop at the alkene Markovnikov product formed

Reactions of Alkynes Internal alkynes give a mixture of products: first mole of HX can form 2 structural isomers (with each structural isomer present as the E and Z isomer also)

Reactions of Alkynes Hydration of Alkynes to Ketones Reagents: HgSO4/H2SO4/H2O Markovnikov product produces ketone

Reactions of Alkynes Hydration of Alkynes to Aldehydes Reagents: (1) Sia2BH (di-sec-isoamylborane) (2) H2O2/NaOH Anti-Markovnikov product converts terminal alkyne to aldehyde

Reactions of Alkynes

Reactions of Alkynes Examples: H

Reactions of Alkynes Example: Predict the products of the following reactions:

Reactions of Alkynes Example: Predict the product of the following reactions. H