Chapter 9 Notes - Alkynes
Structure
- carbon-carbon triple bond
- sp hybridization (linear)
- no cis-trans possibilities
- the two pi bonds are perpendicular
- high electron density
(usually more reactive than alkenes)
Nomenclature
- -yne infix (with number)
- rules similar to alkenes
- with both -enes and -ynes, suffix is -enyne and numbering
is from the end closer to a multiple bond
(E)-4-hexen-1-yne
- common names: alkyl acetylene or dialkyl acetylene
Properties
- acidity
sp C-H bonds are relatively acidic
alkanes (sp3 C-H), pKa ~ 62
alkenes (sp2 C-H), pKa ~ 45
alkynes (sp C-H), pKa ~ 26
- sp C-H bonds can be deprotonated by bases with pKa > 26
( e.g., NaNH2 but not NaOH )
- alkyne anions are called acetylides and are good nucleophiles
- alkylation
substitution of an alkyl group, using acetylide anion
SN2, so alkyl group must be CH3
or 1°
Preparations - eliminations
- NaNH2 in liquid NH3
removes 2 HX from dihalides
Reactions - Hydrogenation
- catalytic hydrogenation to an alkane
- Lindlar catalyst + H2 - forms cis-alkene
- reduction by sodium in liquid NH3
- forms trans-alkene
Addition reactions
- hydration
addition of water with acid catalysis or Hg+2
as catalyst
follows Markovnikov addition (unlike hydroboration/oxidation)
- goes through an enol intermediate
- keto-enol tautomers
carbonyl compound is generally more stable than the enol isomer
tautomers - isomers differing by the position of a hydrogen
& double bond
- halogenation
addition of two equivalents of Cl2 or
Br2
- hydrohalogenation
addition of two equivalents of HCl or HBr
forms geminal dihalide
- ozonolysis
oxidation to form two equivalents of carboxylic acids - one from
each half
Organic synthesis
- retrosynthetic strategy
- think backwards -
What is a reaction that could be used to make this product?
- visualize the construction of the carbon framework
What reactions make new C-C bonds?
- carry out functional group interconversions as necessary
