Ch. 11 - Alkyne Chemistry (Week 4, Quiz 3)

Question 1

How many pi bonds do alkynes have?

Answer 1

2

Question 2

Is Csp-H more or less acidic than Csp2 or Csp3 -H?

Answer 2

more acidic

Question 3

What will deprotonate Csp-H of alkyne?

Answer 3

strong bases

Question 4

Why do alkynes undergo addition reactions?

Answer 4

pi bonds are easily broken

Question 5

Why are alkynes more polarizable than alkenes?

Answer 5

electrons in the pi bonds are more loosely held

Question 6

How are alkynes named?

Answer 6

same IUPAC rules but lowest number is given to first area of unsaturation (doesnt matter if a double or triple bond)

Question 7

Do alkynes have high or low MP/BP?

Answer 7

low

Question 8

How are alkynes synthesized?

Answer 8

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)

Question 9

What do alkynes react with (nucleophile or electrophile)?

Answer 9

electrophile

Question 10

What is sodium amide?

Answer 10

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)

Question 11

Why are terminal alkynes easily deprotonated?

Answer 11

acidic Csp-H (with a strong base via BL acid base reaction)

Question 12

What type of base must be used in order to deprotonate a terminal alkyne to make an acetylide anion?

Answer 12

strong bases that have a pKa above 25 (-NH2 or -H)

Question 13

What bases cannot be used to deprotonate terminal alkynes?

Answer 13

-OR or -OH

Question 14

What reactions will acetylides undergo the fastest?

Answer 14

Sn2 with unhindered primary alkyl halides

Question 15

What will an alkyne and HX give?

Answer 15

geminal halide

Question 16

How many products form for symmetrical internal, unsym internal and terminal alkynes when HX is added?

Answer 16

sym internal = one gem halide
unsym internal = two gem halides (1:1 mixture)
terminal = one gem halide (regioselective)

Question 17

Does alkyne reacting with HX to give gem halide form carbocations?

Answer 17

yes

Question 18

Does alkyne reacting with HX to give gem halide follow mark’s rule?

Answer 18

yes, carbocations form (esp for terminal alkynes)

Question 19

Is an sp or sp2 carbocation more stable? Which will form slower?

Answer 19

sp2 is more stable so formation of sp-C+ is slower

Question 20

T/F: Carbocation is resonantly stabilized if it is bonded to a halogen?

Answer 20

true

Question 21

What will adding X2 (2 eq) to an alkyne give you (where X = Cl or Br)?

Answer 21

tetrahalide

Question 22

What type of addition occurs when adding X2 (2 eq) to an alkyne?

Answer 22

anti-addition only

Question 23

What type of mechanism occurs when adding X2 (2 eq) to an alkyne?

Answer 23

Sn2

Question 24

Does adding X2 (2 eq) to an alkyne form carbocations or follow Mark’s rule?

Answer 24

no and no

Question 25

How many bridged halonium ions does adding X2 (2 eq) to an alkyne form?

Answer 25

two

Question 26

What is made when water and sulfuric acid is added to an alkyne?

Answer 26

and enol and then through tautomerization a ketone

Question 27

What type of ketone is made when water and sulfuric acid is added to a terminal alkyne?

Answer 27

a methyl ketone

Question 28

What is an enol?

Answer 28

an unstable intermediate formed from addition of H2O or hydroboration-oxidation of alkynes where a double bond has OH connected to it

Question 29

Does addition of water to an alkyne form carbocations or follow Mark’s rule?

Answer 29

yes and yes

Question 30

What is tautomerization?

Answer 30

the equilibirum conversion between enols and ketones

Question 31

What are tautomers?

Answer 31

consitutional isomers (enol or keto form) that differ in location of the double bond and H atom

Question 32

What is the keto form of tautomers?

Answer 32

double bond is between carbon and oxygen

Question 33

Why does tautomerization equilibrium favor the keto form?

Answer 33

the pi bonds of C=O are more stable and stronger than those of C=C

Question 34

Are Csp or Csp3 carbocations more stable?

Answer 34

Csp3 is more stable

Csp carbocations are VERY unstable because of repulsion from the nucleus

Question 35

Does terminal alkyne follow marks rule for addition of water?

Answer 35

yes, it protonates regioselectively (H goes on the end carbon) BUT there is not carbocation rearrangement because it is not energetically favorable

Question 36

How many ketones does a symmetrical internal, unsym internal and terminal alkyne form via water addition?

Answer 36

sym internal = one ketone
unsym internal = two ketones (1:1 mixture)
terminal = one ketone

Question 37

What forms when an internal alkyne reacts with BH3 and H2O2 (NaOH)?

Answer 37

a mix of regioisomer ketones

Question 38

What forms when a terminal alkyne reacts with BH3 and H2O2 (NaOH)?

Answer 38

an aldehyde (anti-mark addition only)

Question 39

Does hydroboration-oxidation of alkynes form carbocations?

Answer 39

no

Question 40

Does hydroboration-oxidation of alkynes form enols?

Answer 40

yes, and they undergo tautomerization

Question 41

What is special about the tautomerization of alkynes after hydroboration-oxidation?

Answer 41

an enolate forms which resonates to eventually form ketone

Question 42

What is an enolate?

Answer 42

an enol with a negative charge