Reference: McMurry Ch 4, George et al Ch 1.2

• "Unsaturated" hydrocarbons
• Contain at least one Cº C group

• Each carbon atom of the Cº C group is sp hybridized
• The sp hybridized carbon atoms are linear with bond angles of 180°
• The triple bond is composed of overlap of two sp hybridized orbitals to form a sigma (s ) bond and overlap of four p orbitals to form two pi (p ) bonds.

• Stem derived from the number of carbon atoms in longest chain which includes the triple bond
• Ending is yne
• Give triple bond lowest possible number
• Name the substituents as usual
• The simplest alkyne is called acetylene, H-Cº C-H

Example:

Stereoisomers are not possible as the functional group is linear.

• A triple bond at the end of a chain is called a terminal alkyne

General type of reaction is an electrophilic addition reaction. Like alkenes, the electrons of the p bonds are available for reaction. As there are two p bonds the addition can occur twice.

• It is normally difficult to stop after the addition of one equivalent of reagent
• In the case of hydrogenation it is possible to isolate the alkene when a poisoned or deactivated catalyst is used.
• Terminal alkynes may also react like an acid in the presence of a very strong base.

• Require a catalyst (Pt/C or Pd/C) to help break strong H-H bond
• Form a saturated hydrocarbon from an unsaturated one

• Partial hydrogenation is possible if a poisoned or deactivated catalyst is used
• In the case of partial hydrogenation, (Z) alkenes are obtained from non terminal alkynes

2. Halogenation

• No catalyst required for addition of Cl₂ or Br₂
• Normally an excess of halogen is used to ensure complete reaction

Example:

3. Hydrohalogenation

• Excess HCl, HBr, HI
• Markovnikov’s rule applies with addition to a terminal alkyne

4. Acid-base reaction

• Terminal alkynes only
• Require a very strong base, usually Na⁺NH₂^– in liquid NH₃ as solvent

Questions on Alkynes

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