alkyne
an unsaturated hydrocarbon that contains one or more carbon-carbon triple bonds
IUPAC
- parent chain: longest carbon chain containing the alkyne
- number the carbon chain giving the lowest number to the first multiple bond (DB beats TP)
- -infix- = “yn”
physical properties of alkynes
- only attractive forces between alkynes are dispersion forces
- colorless
- nonpolar
- insoluble in water
- less dense than water
acidity
sp hybridization leads to increased acidity of terminal alkynes (pka~25) relative to alkenes (sp2; pka ~44) and alkanes (sp3; pKa~51)
-alkanes > alkenes > alkynes
strong bases capable of deprotonating terminal alkynes
- Hydride: “Hard” base (100% s character, very EN), decreases chance of it “attacking” atoms, and deprotonates instead
- Lithium disopropyl amide (LDA): “Bulky” base, decreases chance of it “attacking” atoms, and deprotonates instead
Preparation of Alkynes
A. alkylation of acetylide anions with primary haloalkanes
B. Alkynes from Alkenes (dehydrohalogenation reaction)
alkylation reaction
any reaction in which a new carbon-carbon bond to an alkyl group is formed
dehydrohalogenation reaction
removal of -H and -X from adjacent carbons
addition of bromine and chlorine to Alkyne
stereoselective: during the first addition -Br adds from the opposite face of the triple bond (anti stereoselective), yielding a trans isomer. The second addition removes stereo centers
addition of hydrogen halides
regioselective: maokovnikov addition (halogen goes onto the more substituted carbon for both additions)
hydroboration-oxidation
bulky boron is used to prevent multiple hydroborations, which occurs for terminal alkynes with BH3, but not internal alkynes
- keto-enol tautomerism
- regioselective: non-Markovnikov addition (-H goes onto the carbon with fewer hydrogens)
- stereoselective: in the first step hydrogen and boron add from the same face of the triple bond, but the final products do not contain new stereocenters
enol
a compound containing a hydroxyl group bonded to a C=C double bond
tautomers
constitutional isomers in equilibrium with each other that differ in the location of a hydrogen atom and a double bond relative to a heteroatom
keto-enol tautomerism
a type of isomerism involving keto (from ketone) and Enol tautomers
acid-catalyzed hydration
regioselective: Markovnikov addition (-H goes onto the carbon with more hydrogens)
stereoselective: In the first step water and Hg add from the opposite face of the triple bond but the final products do not contain new stereocenters
catalytic reduction
- lindlar catalyst
- stereoselective: both hydrogens add from the same face of the triple bond
lindlar catalyst
Pd-based catalyst that reduces alkynes to cis alkenes
hydrobration-protonolysis
-stereoselective: in the first step hydrogen and boron add from the same face of the triple bond and maintain that configuration upon protonolysis and a carboxylic acid
protonolysis
cleavage of a chemical bond by acid
dissolving-metal reaction
stereoselective: hydrogens add from the opposite face of the triple bond and yield a trans double bond
organic synthesis
a series of reactions by which a set of organic starting materials are converted to a more complicated structure
*functional groups often react in the same fashion, whether in a simple or complex molecule
retrosynthetic analysis
working backward from the desired product in order to plan a synthesis
target molecule => starting materials
analyze the target molecule in the following ways:
- carbon skeleton: how can we put it together? only way to make a C-C bond is alkylation with alkyne anions
- functional groups: what are they, how can be transformed, and how can the choice of reagents provide the exact target?
