Unit 4

Question 1

Halohydrin synthesis from alkene reagents, conditions, mechanism

Answer 1

• X-X in presence of H2O/ROH solvent
• anti addition
• regiospecifc reaction (Nu attacks the more subbed side
• circling around mechanism which forms a 3 member end ring.
• leans towards the less subbed side

Question 2

Oxymercuration-reduction reaction

Answer 2

• Markovnikov hydration, avoids carbocations
• step 1: Hg(OAc)2, water/Alcohol as solvent
• step 2: NaBH4, NaOH
• Regiospecific - Markovikov

Question 3

When referring to steriospecifictiy, what 2 words are associated? What do they mean?

Answer 3

• Anti (Hs will add to different sides to each other)
• Syn (Hs will add to the same side as each other)

Question 4

What does it mean when we say regiospecific? What 2 words are associated with it?

Answer 4

• Markovnikov - when Hs add to the less subbed side, Xs add to the more subbed side, and usually goes through a carbocations intermediate (except the oxymercuration-reduction)
• anti-Markovnikov - opposite of Markovnikov

Question 5

Hydroboration-Oxidation reaction

Answer 5

• Anti-Markovnikov addition, so prevents carbocations
• step 1: BH3/B2H6
• step 2: H2O2, NaOH
• stereospecificity: Syn (concerted)

Question 6

Epoxidation

Answer 6

• uses peroxycarboyllic acids - has a C=O, O-O-H (extra O-O bond which is very weak and easy to break)
• doesn’t switch stereospecificty
• Syn addition, concerted.

Question 8

Sun-dihydroxyltion with OsO4

Answer 8

• 2 methods: Stoichiometric and catalytic
  1: OsO4 with H2S or Zn metal
  2: Catalytic OsO4 w NMO (an amine oxide)
• stereo specific, concerted syn addition

Question 9

Ozonolysis

Answer 9

• reductive work up:
• O3, S(CH3)2
  Forms ketones and aldehydes

Oxidative work up:
- O3, H2O2 and water
Forms ketones and carboxylic acids

Question 10

Hydrogenation of alkenes

Answer 10

• H2 + Pt or Pd/C catalyst
• syn addition

Question 11

Hydrohalogenation

Answer 11

• H-X
• Markovnikov addition, involves carbocation intermediates (regiospecific)

Question 12

Hydration

Answer 12

• H2O + catalytic H2SO4 (or any non nu strong acid)
• Markovnikov addition, involves carbocation intermediates

Question 13

Alkene isomerization with catalytic acid

Answer 13

• catalytic H2SO4
• Markovnikov addition
• follows E1, Zaitzev’s rule

Question 14

Halogenation

Answer 14

• X2
• anti-addition
• involves c+ formation intermediate

Question 16

Structure and reactivity of alkyne func group

Answer 16

• 2 perpendicular pi bonds
• NOT in a conjugated system with each other, not overlapping.
• very electron rich area

Question 17

Making Alkynes using 2 E2
Normal adjacent (vicinal halogens)
Terminal (vicinal halogens)
Geminal halogens

Answer 17

• Dibromoalkane with 2 eq NaNH2 (or very strong base)
• triple bond will form on 2 carbons with LGs.
• if on the edge of the molecule, then use NaNH2 because terminal alkyne is very acidic, so will be deprotonated, do acidic work up.
• if on the same Cs, (geminal), will give you 2 diff products

Question 18

SN2 and electrophillic addition with alkynes

Answer 18

• HOMO: lone par in C sp orbital in alkyne with deprotonated terminal alkyne (Nu Sub)
• HOMO: electron pair in CC pi bond in alkyne (Nu addition)

Question 19

Acetylide

Answer 19

• terminal alkyne that is deprotonated - can be organometallic!
• is a strong Nu, strong base

Question 20

SN2 with alkyl halides and alkynes

Answer 20

• react alkyne with very strong base - eg NaNH2 or NaH to from acetylide.
• Then react acetylide with unhindered alkyl halides (to avoid E2), so only methyl and primary unhindered (remember inversion!)

Question 21

Alkyne hydrogenation

Answer 21

• H2 and Pt or Pd/C catalyst
• each alkyne carbon gets 2 Hs.
• syn addition (twice)

Question 22

Partial reduction: hydrogenation w Lindlar’s catalyst

Answer 22

• H2 and Pd/CaCO3 (poisoned catalyst), less reactive catalyst
• poisin by adding Pb(OAc)2, Quinoline (relative of pyridine), with extra benzene ring - can only chew through one pi bond
• Syn addition
• forms a cis alkene as the final product

Question 23

Partial reduction of alkynes: hydrogenation w sodium metal in ammonia

Answer 23

• react alkyne with Na metal, liquid ammonia
• forms trans alkene
• reaction involves radicals
• radical anion intermediate - spreading out e- and minimizing steroids (trans geometry) lowers energy of this intermediate
• ends with acid/base with radical anion and ammonia.

Question 24

Hydrohalogenation alkynes

Answer 24

• Markovnikov addition
• Can stop at alkene product with halogen attached
• carbocation intermediate
• 2nd addition slower due to inductive effect of X (need excess of H-X to get to alkane product with geminal halogen)

Question 25

Alkyne halogenation

Answer 25

- Forms Bromium ion - anti addition (SN2) - second addition is slower due to inductive effect of X - Thisnal Geminal halogens (like 4 halogens)

Question 27

Tautomers

Answer 27

- constitutional isomers that differ in he location of a double bond and a H atom

Question 28

Hydration of alkynes with catalytic Hg2+

Answer 28

- needs HgSO4, H2SO4 and water as solvent - Markovnikov addition - will involve tautomerization - enol will appear - will end up with carbonyl as final product (ketone)

Question 29

Hydration of alkynes with hydroboration-oxidation

Answer 29

1. R2BH (big bulky group like dicylohexylborane) 2. H2O2+NaOH in water - anti-Markovnikov addition

Question 30

Why do we need 2 bulky groups when doing hydration of alkynes with hydroboartion oxidation?

Answer 30

- ensures 1:1 stoichiometry with alkyne, and protects and preserves 2nd pi bond, which is required for C=O product.