Alkynes Flashcards
Naming of alkynes
Find the longest chain. If it has the triple bond in it, it’s an alkyne. If the triple bond is a substituent, it’s an alkynyl substituent. If a double bond and a triple bond are equidistant from either terminus, the ene is numbered first. Otherwise, whichever is closest. The “ene” comes first in the nomenclature, because it’s alphabetically first. An alcohols takes precedence over a triple bond, as does a ring.
Why are terminal alkynes acidic?
Carbons in the triple bond are sp hybridized. This creates an orbital with 50% s character, which is far more electronegative than other carbons. This makes the hydrogen attached to them quite acidic.
Alkynes from alkynes
Alkynyl anions can act as nucleophiles, attacking primary haloalkanes through SN2 to create longer chains. With sec or tert, gets super complex w/ E2 as well. Unusual reaction, works for oxacylcopropanes and carbonyl functions
Catalytic hydrogenation with platinum or palladium on charcoal as catalysts
Under these conditions, the triple bond becomes completely saturated –> it goes to an alkane.
Catalytic hydrogenation with Lindlar’s catalyst
Fosters the syn addition of two hydrogens. Thus, it results in a cis alkene from the alkyne.
Alkynes from alkanes
Double E2 elimination of dihaloalkanes. Remember, the halogens must be trans to each other because E2 requires the leaving group and the H to be anti periplanar. Can do this from an alkene, then bromination.
To do the reaction, use NaNH2, NH3 liquid. Aqueous workup is H+ and H20, which is usually implied. However, without it the NH2 base might deprotonate a terminal alkyne, leaving us with a carbanion.
Trans alkene from an alkyne
Uses Na and liquid NH3, plus H2O for protonation. Uses stepwise mechanism – adds one hydrogen at a time.
HX reaction with excess HX
two X atoms on the same carbon – “geminal”. Goes to the more substituted carbon. Works with internal or terminal alkynes
X2 reaction
needs two equivalents of X2. Creates double anti addition, just like for alkenes. Four halides are added to the two carbons which were in the triple bond.
Alkyne hydration
HgSO4 is the catalyst, with H2O as the solvent. No NaBh4 is necessary. Adds OH to the more substituted carbon. BEWARE: tautomerization. The hydrated version isn’t stable, so it shifts into a version of that in which the H is gone and the oxygen is a carbonyl. No double bond between the carbons. Tautomerization can be catalyzed by either H+ or OH-
Radical Hydrobromination
Attaches a bromine and a hydrogen either syn or anti to the less substituted carbon (anti-markovnikov!). Thus can have either cis or trans products. Use HBr and ROOR.
Hydroboration-Oxidation
Uses two bulky R groups in a R2BH to ensure that only one pi bond is hydrated. Results in anti-markovnikov hydration. BEWARE tautomerization. The double bond goes to the oxygen, catalyzed in either H+ or OH-
Do alkeynyl halides undergo substitution reactions?
NO
How do we react w/ alkenyl halides?
Organometallics!
HX reaction with one part HX
The X goes for the more substituted carbon, anti addition of H and X just like for alkenes. Results in a trans alkene
