Chapter 7 Flashcards
Naming
Same naming as alkenes but with yne
(Substituants left and right)Acetylene
Triple bond has same priority as double bonds
-adiyne
If double and triple bond tied to one another, give it to alphabetical order so double bond (ene)
Hex-X-en-X-yne
Terminal alkynes
Triple bond at the end of the molecule, hydrogen at the end is a tiny bit acidic
Allene
2 double bonds
Structure and properties alkynes
Triple bond 180 degrees
1 sigma bond and two pi bonds
Physical properties similar to alkanes and alkenes
Boiling point higher for alkyne than an alkane for the same number of C (more polarized, so more LDF)
Electrophoresis addition reactions alkynes
Similar to alkenes
Produces vinylic cation
Step 1 and 2 are regioselective
Formation of pi-complex
Something like bromonium ion, with cycle with HCl
Primary vinylic is very unstable
Causes H and Cl to be anti to each other
However, not all addition for triple bonds are stereoselective
Alkynes energy diagram
Alkynes are less stable, but less reactive
It’s free energy of reactant is - higher than alkanes, but it’s transition state is also high (more activation energy),
Alkene made from alkyne is more reactive than the alkyne, giving an alkane, rarely only an alkene (unless halo-substituted alkene, very unreactive, electron inductive withdrawal )
Geminal dihalide
2 halides are on the same carbon
Must start with an alkyne and the addition of an electrophile
Regiosele
Addition of dihalide
Same as alkenes
Produces trans alkene after step 1
Produces tetrahalogenated alkane in second step
Alkene less reactive than alkyne,
Addition of hydrogen to alkyne
Alkene more reactive than alkyne, so produces an alkane
Stopping at alkene:
Using lindlar catalyst and H2
Hydrogens add syn
Makes cis alkenes
Trans alkene: Na or Li
Solvent is NH3 (liq)
Carbon atoms electronegativity
Sp more electronegative than sp2 than sp3
Makes hydrogen bonded to a triple bond more positive, so more acidic
Hydrogen of a Terminal alkynes are more acidic than ones of an alkanes
Terminal alkyne hydrogen will react first (with NH2- amide ion for example)(strongest base)
Amide ion strong enough to react with alkynes, hydroxide isn’t because water is a better acid than the alkyne
Alkanes are very very not acidic
Alkylation reactionk
After alkynes react and lose their H, they are electrophile, so will link with a primary (!!) alkyl halide
Will attach to most positive carbon, aka one with halide
Halide will become a leaving group
NaNH2 and NH3 will remove the H (first step)
