Alkynes - Nomenclature and Acidity

Learning Objectives

• Draw an alkyne (with proper bond angles) given the IUPAC name.
• Explain why terminal alkynes can be deprotonated by strong bases.

Key Takeaways

• Alkynes consist of carbon-carbon triple bonds.
• Following IUPAC nomenclature, alkynes must have the lowest possible numbering on the carbon chain.
• Terminal alkynes are more acidic than alkenes or alkanes, allowing them to be deprotonated by strong bases (e.g., NaNH2, NaH, LDA).

Structure and Properties

• Alkynes: Generally linear and sp hybridized, featuring one sigma and two pi bonds.
• Example: Ethyne (Acetylene)
• Similar physical properties to alkanes and alkenes with only dispersion forces.
• Melting and boiling points increase as molecular weight increases.

IUPAC Nomenclature

• Alkynes are named similarly to alkanes and alkenes.
• Suffix: -yn- indicates the presence of a triple bond.
• Number the carbon chain ensuring the alkyne receives the lowest possible number.
• Example: 3-methyl-1-butyne

Acidity of 1-Alkynes

• Hydrogen on an alkyne is far more acidic than on alkenes or alkanes.
• This acidity allows for deprotonation.
• pKa of Alkynes is lower than that of Alkenes and Alkanes, making them reactive towards strong bases:
• Strong bases such as NaNH2, NaH, and LDA can deprotonate alkynes.
• Example reaction: Ethyne + sodium amide (NaNH2)