Alkynes: Introduction and Nomenclature

Definition and Structure of Alkynes

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. The general formula for alkynes is CnH2n-2. The triple bond imparts unique chemical properties, including increased acidity of terminal alkynes and distinct reactivity patterns.

• Naming Alkynes: The longest carbon chain containing the triple bond is selected as the parent chain. Number the chain so that the triple bond receives the lowest possible number.

• Terminal Alkynes: Alkynes with the triple bond at the end of the carbon chain are called terminal alkynes. The hydrogen attached to the terminal carbon is unusually acidic.

Example: 1-butyne (CH≡C–CH2–CH3) is a terminal alkyne.

Acidity and Reactivity of Terminal Alkynes

• Acidity: The C–H bond of a terminal alkyne is more acidic than typical alkane or alkene hydrogens due to the high s-character of the sp-hybridized carbon.

• Deprotonation: Terminal alkynes can be deprotonated by strong bases such as sodium amide (NaNH2), forming an acetylide anion.

• Alkylation: The acetylide anion can react with haloalkanes (alkyl halides) to form new internal alkynes via nucleophilic substitution.

Equation:

Alkyne Reactions: Mechanisms and Products

Reduction of Alkynes

• Hydrogenation with Lindlar Catalyst: Partial reduction of alkynes using H2 and Lindlar catalyst (Pd, CaCO3, Pd(OAc)2) yields cis-alkenes (Z-alkenes). No detailed mechanism is typically required for this transformation.

• Reduction with Sodium in Liquid Ammonia (Na/NH3): This method produces trans-alkenes (E-alkenes) from alkynes. No detailed mechanism is typically required for this transformation.

Equations:

Addition Reactions to Alkynes

• Hydrohalogenation (HBr Addition): Addition of HBr to alkynes places the bromine on the more substituted carbon (Markovnikov addition). With excess HBr, two bromines add to the same carbon.

• Hydration (H2SO4, HgSO4, H2O): This reaction adds water across the triple bond, forming an enol intermediate that tautomerizes to a ketone. The ketone forms on the more substituted carbon.

• Hydroboration-Oxidation (B(sia)2H, H2O2, NaOH): This method adds water across the triple bond, but the ketone forms on the less substituted carbon (anti-Markovnikov addition). Tautomerization from enol to ketone occurs.

• Bromination (Br2): Addition of Br2 to alkynes yields dibromoalkenes, with bromines trans to each other. Excess Br2 leads to tetrabromoalkanes (two bromines at each former triple bond carbon).

Equations:

Hydration (Markovnikov):

Hydroboration-Oxidation (Anti-Markovnikov):

Bromination:

Predicting Products in Multi-Step Sequences

Students should be able to use the above reactions in sequence to predict the final product of a multi-step synthesis involving alkynes. This requires understanding the regioselectivity and stereochemistry of each transformation.

Haloalkanes: Nomenclature and Physical Properties

Naming Haloalkanes

Haloalkanes (alkyl halides) are organic compounds containing one or more halogen atoms (F, Cl, Br, I) attached to an alkane chain. The position and identity of the halogen(s) are indicated in the name, and the chain is numbered to give the halogen(s) the lowest possible locants.

• Haloalkanes can also contain alkene or alkyne groups, which must be considered in the numbering and naming.

Example: 3-bromo-1-butyne (BrCH2CH2C≡CH)

Physical Properties of Haloalkanes

• Boiling Point Trends: Boiling points increase with the size of the halogen due to greater polarizability and increased van der Waals forces.

• Solubility: Larger haloalkanes are less soluble in water due to increased hydrophobic character.

• Density: Haloalkanes tend to be denser than water, especially those containing heavier halogens.

Table: Boiling Point Trends of Haloalkanes

Additional info: The trends above are inferred from general organic chemistry principles and typical textbook data.