Topic 5: alkenes

Question 1

Define saturated

Answer 1

• The most amount of H it can have
• An open-chain alkane

Question 2

How many hydrogens removed is 1 degree of unsaturation?

Answer 2

2 H

Question 3

Give examples of 1 degree of unsaturation

Answer 3

• Double bonds
• Ring/cyclo

Question 4

Give examples of 2 degrees of unsaturation

Answer 4

• Triple bond

Question 5

Give examples of 4 degrees of unsaturation

Answer 5

Benzene

Question 6

Give examples of 6 degrees of unsaturation

Answer 6

Caffeine

Question 7

Give examples of 10 degrees of unsaturation

Answer 7

Anthracene

Question 8

Explain how to find the degree of unsaturation with only chemical formula

Answer 8

• Compare to fully saturated alkane chain = CnH2n+2 with the carbons available
• Subtract from saturated value then divide by 2 = degree
  1) Pure hydrocarbon = compare to saturated directly
  2) Halogen = add number of halogens to number of H > then compare with saturated
  3) Oxygen = doesn’t affect number of H > ignore + compare
  4) Nitrogen = remove 1 H for every N > compare
  5)

Question 9

Describe alkene stability

Answer 9

• More unstable = more reactive
• Cis = bigger molecules near each other = more steric strain/repulsion between the e-
• Trans = bigger groups are opposite = less repulsion as no interaction

Question 10

Describe the electronic structure of alkene

Answer 10

• The C=C is made of a sigma bond in the middle surrounded by 2 π bonds
• π bonds = make alkenes e- rich
• e- rich = lewis base = nucleophile = attracted to +ve charge
• React with lewis acids = electrophiles

Question 11

What is the reaction that happens to alkene?

Answer 11

Electrophilic addition

Question 12

Define electrophilic addition

Answer 12

• Adding X-Y across double bond
• Break double bond and C-X + C-Y created

Question 13

Describe electrophilic addition of HX

Answer 13

• Alkene + hydrogen halide = alkyl halide
• Electrophile = HF/HCl/HBr/HI
• Solvent = CCl4 = non-polar

Question 14

Describe the electrophiles

Answer 14

• HF = too reactive
• HCl = gaseous
• HBr = gaseous
• HI = too unreactive > use KI in presence of H3PO4

Question 15

Describe the carbocation

Answer 15

• C+ = reaction intermediate
• sp2 hybridization
• Trigonal planar geometry = 120
• Empty 2p orbital

Question 16

What is Markovnikov’s rule?

Answer 16

• In electrophilic addition reaction HX + alkene
• H > alkene carbon with most H attached directly
• X > carbocation

Question 17

Describe carbocation stability

Answer 17

• Most stable > least
• Tertiary > secondary > primary > methyl
• Carbocation attached to how many carbons
• Alkyl groups = e- donating = reduces total +ve charge = stabilize carbocation

Question 18

Describe Markovnikov’s rule in producing products of electrophilic addition

Answer 18

• Major product = more stable
• Minor product = less stable
• More stable = lower energy = lower AE = speedier reaction = major product

Question 19

Describe electrophilic reaction with X-X

Answer 19

• When X-X comes close to e- rich double bond = top X will have repulsion
• e- move to bottom X = make it δ- and the top X δ+ = polarized = induced dipole

Question 20

Describe anti addition

Answer 20

• Stereospecific reaction
• X-X added to cycloalkene
• The atoms attach across the C=C but are trans to each other = reduce steric strain

Question 21

Describe addition of H2O to alkenes

Answer 21

• Hydration reaction
• Produces alcohols
  3 METHODS:
  1) Brute force = Markovnikov addition
  2) Oxymercuration = Markovnikov addition
  3) Hydroboration = anti- Markovnikov addition

Question 22

Explain brute-force hydration

Answer 22

• Acid catalyst needed e.g. H2SO4
• Problems = high acidic conditions + high temp
• However cheaper = ideal for industrial setting

Question 23

Explain oxymercuration

Answer 23

Step 1) Mercuric acetate = Hg(OAc)2
- Tetrahydrofuran = THF = solvent
Step 2) Sodium borohydride = NaBH4

Question 24

Explain hydroboration

Answer 24

Step 1) BH3 + ET2O
Step 2) NaOH + H2O2
- Syn addition = H + OH added to same side of ring

Question 25

Describe alkene hydrogenation

Answer 25

- React with H2 in presence of metal catalyst - Reduction = gain of e- density - Doesn't reduce C=O/C≡N/benzene - C=C > C-H = C is more electronegative compared to H = e- flow to C - 1 of each H2 for each C

Question 26

What are the metal catalysts used in alkene hydrogenation

Answer 26

- Pt/C = platinum powder + activated charcoal - Pd/C = palladium powder + activated charcoal - PtO2 = Adam's reagent

Question 27

Describe alkene hydration in cycloalkene

Answer 27

- Even though higher steric strain = syn addition = cis product

Question 28

How can we tell by hydrogenation how many double bonds + rings?

Answer 28

- If you start with 2 degrees and go to 1 degrees = something didn't absorb hydrogen - Rings don't absorb = only C=C absorbs

Question 29

Describe oxidation of alkenes

Answer 29

- Addition of oxygen + loss of e- density - O is more electronegative than C = e- flow to O in C-O 2 TYPES: 1) Ozonolysis 2) Treatment with KMnO4

Question 30

Explain ozonolysis

Answer 30

- Using O3 - Cut the double bond down the middle + attach an O to each - Product = aldehyde or ketone Step 1) O3 Step 2) Zn/H+ or (CH3)2S

Question 31

Explain alkene oxidative cleavage

Answer 31

- KMno4 = splits alkene into 2 pieces - Solution is purple = if reacts will decolorize - Will keep oxidizing until fully oxidized = aldehydes will oxidize into carboxylic acids BUT ketone no further oxidation - If CO2 produced = alkene terminal