Ch. 11 - Alkyne Chemistry (Week 4, Quiz 3) Flashcards
How many pi bonds do alkynes have?
2
Is Csp-H more or less acidic than Csp2 or Csp3 -H?
more acidic
What will deprotonate Csp-H of alkyne?
strong bases
Why do alkynes undergo addition reactions?
pi bonds are easily broken
Why are alkynes more polarizable than alkenes?
electrons in the pi bonds are more loosely held
How are alkynes named?
same IUPAC rules but lowest number is given to first area of unsaturation (doesnt matter if a double or triple bond)
Do alkynes have high or low MP/BP?
low
How are alkynes synthesized?
strong base (NaNH2 or KOtBu) removes two eq of HX from a vicinal or geminal dihalide (E2)
can also make from alkyl halides and lithium acetylide (Sn2)
What do alkynes react with (nucleophile or electrophile)?
electrophile
What is sodium amide?
A VERY strong base that is not a good nucleophile, used to synthesize alkynes (strong because N, which isnt very electronegative, has two lone pairs it wants to give up)
Why are terminal alkynes easily deprotonated?
acidic Csp-H (with a strong base via BL acid base reaction)
What type of base must be used in order to deprotonate a terminal alkyne to make an acetylide anion?
strong bases that have a pKa above 25 (-NH2 or -H)
What bases cannot be used to deprotonate terminal alkynes?
-OR or -OH
What reactions will acetylides undergo the fastest?
Sn2 with unhindered primary alkyl halides
What will an alkyne and HX give?
geminal halide
How many products form for symmetrical internal, unsym internal and terminal alkynes when HX is added?
sym internal = one gem halide
unsym internal = two gem halides (1:1 mixture)
terminal = one gem halide (regioselective)
Does alkyne reacting with HX to give gem halide form carbocations?
yes
Does alkyne reacting with HX to give gem halide follow mark’s rule?
yes, carbocations form (esp for terminal alkynes)
Is an sp or sp2 carbocation more stable? Which will form slower?
sp2 is more stable so formation of sp-C+ is slower
T/F: Carbocation is resonantly stabilized if it is bonded to a halogen?
true
What will adding X2 (2 eq) to an alkyne give you (where X = Cl or Br)?
tetrahalide
What type of addition occurs when adding X2 (2 eq) to an alkyne?
anti-addition only
What type of mechanism occurs when adding X2 (2 eq) to an alkyne?
Sn2
Does adding X2 (2 eq) to an alkyne form carbocations or follow Mark’s rule?
no and no
How many bridged halonium ions does adding X2 (2 eq) to an alkyne form?
two
What is made when water and sulfuric acid is added to an alkyne?
and enol and then through tautomerization a ketone
What type of ketone is made when water and sulfuric acid is added to a terminal alkyne?
a methyl ketone
What is an enol?
an unstable intermediate formed from addition of H2O or hydroboration-oxidation of alkynes where a double bond has OH connected to it
Does addition of water to an alkyne form carbocations or follow Mark’s rule?
yes and yes
What is tautomerization?
the equilibirum conversion between enols and ketones
What are tautomers?
consitutional isomers (enol or keto form) that differ in location of the double bond and H atom
What is the keto form of tautomers?
double bond is between carbon and oxygen
Why does tautomerization equilibrium favor the keto form?
the pi bonds of C=O are more stable and stronger than those of C=C
Are Csp or Csp3 carbocations more stable?
Csp3 is more stable
Csp carbocations are VERY unstable because of repulsion from the nucleus
Does terminal alkyne follow marks rule for addition of water?
yes, it protonates regioselectively (H goes on the end carbon) BUT there is not carbocation rearrangement because it is not energetically favorable
How many ketones does a symmetrical internal, unsym internal and terminal alkyne form via water addition?
sym internal = one ketone
unsym internal = two ketones (1:1 mixture)
terminal = one ketone
What forms when an internal alkyne reacts with BH3 and H2O2 (NaOH)?
a mix of regioisomer ketones
What forms when a terminal alkyne reacts with BH3 and H2O2 (NaOH)?
an aldehyde (anti-mark addition only)
Does hydroboration-oxidation of alkynes form carbocations?
no
Does hydroboration-oxidation of alkynes form enols?
yes, and they undergo tautomerization
What is special about the tautomerization of alkynes after hydroboration-oxidation?
an enolate forms which resonates to eventually form ketone
What is an enolate?
an enol with a negative charge
