Alkenes and Alkynes- Reactions of alkynes

Question 1

Describe the structure of alkynes

Answer 1

1. Contain a carbon-carbon triple bond
2. They are linear with the substituents 180 degrees from each other
3. sp hybridised orbitals on each carbon make up 2 sigma-bonds
4. Remaining p-orbitals on each carbon overlap to form 2 pi-bonds
5. The two pi-bonds are perpendicular to each other

Question 2

Which is easiest to deprotonate- terminal alkynes, alkenes or alkanes

Answer 2

1. Terminal alkynes- have a lower pKa so good acid

Question 3

Why are alkynes the easiest to deprotonate

Answer 3

1. Alkynes are deprotonated to give anion in an sp-hybridised orbital
2. Increased s-character means the anion is closer to the nucleus and therefore has greater stability
3. Alkenes- sp2 and alkanes- sp3 so less s-character and further from the nucleus

Question 4

What can deprotonated alkynes be used for

Answer 4

1. They can be deprotonated using a strong base
2. They are good nucleophiles
3. Can react with electrophiles in SN2 substitution reactions
4. Alkyne alkylation

Question 5

What must the pKaH of the base be to complete deprotonation of a terminal alkyne and give 2 examples

Answer 5

1. > 25

2. E.g. n-Bu-Li or NaNH2

Question 6

How can you from alkynes

Answer 6

1. 1,2-dibromoalkanes can react with strong bases to form alkynes
2. Two sequential E2 elimination reactions with a bromoalkene intermediate
3. A strong base is required for the second elimination step

Question 7

Describe the mechanism for the elimination to form alkynes from 1,2-dibromoalkanes

Answer 7

1. First elimination gives (E)-vinyl bromide- Minimise steric interactions in TS
2. Second elimination is slow due to disfavoured syn-periplanar arrangement so strong base is needed
3. The product is deprotonated under reaction conditions

Question 8

What is formed when alkynes react with water and strong acid

Answer 8

1. Form ketones

2. Addition gives Markovnikov selectivity

Question 9

What is a vinyl cation

Answer 9

1. The vinyl cation is a carbocation with the positive charge on an alkene carbon.
2. It is highly reactive
3. sp hybridised

Question 10

Describe the mechanism for the formation of ketones from alkynes

Answer 10

1. Proton adds to give most stable vinyl carbocation

2. Water adds to form an enol, which undergoes tautomerisation

Question 11

What is another method to form ketones from alkynes

Answer 11

1. Oxymercuration/ demercuration sequence
2. First add Hg(OAc)2
3. Then NaBH4

Question 12

Describe the mechanism for the oxymercuration/demercuration sequence for the hydration of alkynes

Answer 12

1. Reaction proceeds via a mercurinium ion intermediate

2. Ring-opening occurs at the most substituted position, overall Markovnikov selectivity

Question 13

Describe the hydroboration of alkynes

Answer 13

1. Form vinyl boranes which can be oxidised with aqueous hydrogen peroxide and base
2. Hydroboration has syn-selectivity with boron atom adding to least hindered end
3. Oxidation gives an enol, which tautomerises to give an aldehyde (or ketone) with overall anti-Markovnikov selectivity
4. Hydroboration of terminal alkynes to give aldehydes requires a sterically demanding borane to avoid side reactions

Question 14

What is a vinyl group

Answer 14

1. CH=CH2 group

Question 15

How can alkynes form alkanes

Answer 15

1. Reduced using hydrogen and a palladium on carbon catalyst
2. The alkyne is first reduced to the (Z)-alkene by syn-addition
3. This is rapidly reduced into the alkane

Question 16

How is the reduction of alkynes to alkanes performed

Answer 16

1. H2 is obtained from a cylinder
2. Reactions can be performed under high pressure using bomb apparatus
3. Reactions can be performed at low pressure using a balloon

Question 17

How can you reduce alkynes to just a (Z)-Alkenes

Answer 17

1. Using hydrogen and a poisoned palladium catalyst

2. Poison reduces the activity of the palladium and prevents further reduction of the alkene product

Question 18

Describe the reduction of alkynes to z-alkenes

Answer 18

1. Hydrogenation is a syn-addition leading to the z-alkene

Question 19

What is a common palladium poisoned catalyst used

Answer 19

1. Lindlar’s catalyst is a common palladium catalyst poisoned with lead and quinoline

Question 20

How can you reduce alkynes to E-alkenes

Answer 20

1. Dissolving metal conditions can be used to reduce alkynes into E-alkenes
2. The Birch Reduction uses sodium in liquid ammonia

Question 21

Describe the reduction of alkynes to E-alkenes

Answer 21

1. Reaction is radical in nature and the sodium is a source of single electrons
2. Practically, need to use a cold finger to condense ammonia gas onto metallic sodium
3. Solvated electrons are deep, inky blue in colour

Question 22

Describe the mechanism for the Birch Reduction

Answer 22

1. Electron adds to alkyne to form a radical anion, which adopts a favourable E-geometry
2. The vinyl anion is basic enough to deprotonate ammonia