Rxns with Alkynes

Question 1

what are the 2 types of triple bonds in alkyne rxns?

Answer 1

internal and terminal

Question 2

what type of rxn is needed in order to create an alkyne: addition, substitution, elimination, or radical?

Answer 2

elimination

Question 3

Do alkynes with terminal triple bonds act like bases or acids?

Answer 3

acids

Question 4

how do you know if a given base is good enough to deprotonate the terminal triple bond on an alkyne?

Answer 4

check the pka table on p 471

the base must be stronger than the alkyne’s conjugate base.

Question 5

what kind of rxn would you use if you needed to MAKE an alkyne out of an alkane?

Answer 5

elimination (base takes H away, creating double bond that kicks off leaving group)

You’d need 2 successive eliminations to go from single bond to triple bond

Question 6

how many equivalents of strong base do you need in order to make an alkyne from an internal dihalide?

Answer 6

2 (one for each H that needs to be attacked in order to kick off the 2 leaving groups/form the 2 pi bonds)

Question 7

what is a dihalide?

Answer 7

molecule whose carbon has 2 halogens

Question 8

what does it mean to call a dihalide ‘geminal’?

Answer 8

two leaving groups are attached to the same carbon in the a molecule

Question 9

what does it mean to call a dihalide ‘vicinal’?

Answer 9

two leaving groups are attached to two DIFFERENT carbons in the same molecule

Question 10

how many equivalents of strong base do you need in order to make an alkyne from a terminal dihalide?

Answer 10

3 total:

(one for each H that needs to be attacked in order to kick off the 2 leaving groups/form the 2 pi bonds

AND

one more to remove the terminal H. This H gets added back if you add H3O+ or H2O in a later step

Question 11

when using 3 equivalents of strong base in order to make an alkyne from a terminal dihalide, why do we bother removing the terminal H with the 3rd equivalent, only to add the H back in a later step?

Answer 11

this is the most thermodynamically favorable

Question 12

what is it called when you add H3O+ or H2O to an alkynide ion in order to protonate it/make it into an alkyne?

Answer 12

a water work up step

Question 13

does the water work up step of an alkyne making rxn have to occur last/at the end of the process?

Answer 13

YES!

Question 14

what do you call an terminal alkyne that is missing its terminal H?

Answer 14

alkynide ion

Question 15

in the context of rxns involving alkynes, why are alkynide ions useful?

Answer 15

they can act like strong nucleophiles for SN2 rxns

Question 16

what do you call it when an alkynide ion acts like a nucleophile and does an SN2 rxn?

Answer 16

alkylation

Question 17

what strong base is commonly used to remove an alkyne’s terminal H in an alkylation rxn?

Answer 17

NaNH2

Question 18

what makes a good leaving group for an electrophile in an alkylation rxn?

Answer 18

has the thing you need to alkyne to attack, plus a halogen (or some other excellent leaving group)

Question 19

what does it mean to reduce an alkyne?

Answer 19

reduce its triple bond to a double bond (and then reduces its double bond to a single bond)

Question 20

which kind of rxn do you need in order to reduce an alkyne: substitution, elimination, addition, or radical?

Answer 20

addition i.e. halogenation

Question 21

when/why do you need to use a poisoned catalyst (like Lindler’s catalyst) when reducing an alkyne through addition?

Answer 21

use poison when you want to go from triple bond to double bond rather than speeding right from triple bond to single bond