Ch. 12 – Oxidation and Reduction (Week 5, Quiz 4)

Question 1

What is oxidation?

Answer 1

increase in the number of C-Z bonds (where Z is more EN than carbon)
or decrease in number of C-H bonds
(loss of electrons)

Question 2

What is reduction?

Answer 2

increase in number of C-H bonds or decrease in number of C-Z bonds
(gain of electrons)

Question 3

Is alkyne&raquo_space; alkene ox or red?

Answer 3

reduction

Question 4

Is alkene&raquo_space; alkane ox or red?

Answer 4

reduction

Question 5

What makes a mechanism regioselective?

Answer 5

could form two products but only forms one

indicator: Mark’s rule

Question 6

What makes a mechanism stereospecific?

Answer 6

there is only one product possible (not even a chance of others being formed)
indicator: Sn2

Question 7

What makes a mechanism stereoselective?

Answer 7

gives stereoisomers but one is favored (check)

indicator: syn or anti addition

Question 8

What are reducing agents?

Answer 8

providers of H2

Question 9

What are the three main types of reducing agents?

Answer 9

1) H2 (with metal catalyst)
2) 2H+ (from two NH3) + 2e- (from two alkali metal / Na)
3) hydride H- (from NaBH3 or LiAlH4) + H+ (from alcohol or water)

Question 10

What is general formula for an alkene reduction?

Answer 10

alkene + H2 (with metal catalyst) = alkane

Question 11

What does the addition of H2 require for catalytic hydrogenation?

Answer 11

metal catalyst: Pd, Pt or Ni

Question 12

What type of addition is H2 addition?

Answer 12

syn addition

Question 13

Is hydrogenation exo/endothermic?

Answer 13

exothermic (product bonds are stronger)

Question 14

What is the driving force of hydrogenation?

Answer 14

conversion of pi bond to two sigma bonds (sigma bonds are lower in energy)

Question 15

What does a larger heat of hydrogenation ∆Hº mean?

Answer 15

less stable starting point

Question 16

Do cis/trans alkenes produce the same alkane? What about their ∆Hº’s?

Answer 16

yes, but trans releases less energy because it is more stable to begin with

Question 17

Do more/less stable alkenes have higher ∆Hº’s?

Answer 17

less stable = higher ∆Hº

Question 18

Do more/less stable alkenes react faster in hydrogenation?

Answer 18

least stable alkenes react faster

Question 19

What happens in catalytic hydrogenation of an alkene to form an alkane?

Answer 19

H2 binds to the surface of a metal catalyst
alkene also binds
H2 is transferred to pi bond and alkane is released

Question 20

Why does syn addition occur in catalytic hydrogenation (alkene to alkane)?

Answer 20

the addition occurs on the side of the alkene bonded to the metal catalyst

Question 21

What does bottom syn addition of tri-sub E alkene give?

Answer 21

S-enantiomer

Question 22

What does top syn addition of tri-sub E alkene give?

Answer 22

R-enantiomer

Question 23

What does bottom syn addition of tri-sub Z alkene give?

Answer 23

R-enantiomer

Question 24

What does top syn addition of tri-sub Z alkene give?

Answer 24

S-enantiomer

Question 25

What does bottom syn addition of tetra-sub E alkene give?

Answer 25

(3S,4S) stereoisomer

Question 26

What does top syn addition of tetra-sub E alkene give?

Answer 26

(3R,4R) stereoisomer

Question 27

What does bottom syn addition of tetra-sub Z alkene give?

Answer 27

(3R,4S) stereoisomer

Question 28

What does bottom syn addition of tetra-sub Z alkene give?

Answer 28

(3S,4R) stereoisomer

Question 29

What are three ways an alkyne can be reduced?

Answer 29

1) add H2 (2 eq) to form alkane 2) add H2 (1 eq) in syn form to make cis alkene 3) add H2 (1 eq) in anti form to make trans alkene

Question 30

What happens in catalystic hydrogenation (reduction) of an alkyne to alkane?

Answer 30

alkyne (internal or terminal) reacts with H2 (2 eq) and metal catalyst to form an alkane

Question 31

What type of addition is addition of H2 in reduction of alkyne to alkane?

Answer 31

syn addition

Question 32

Do any st. centers form in the reduction of alkyne to alkane?

Answer 32

no

Question 33

What is the intermediate of reduction of alkyne to alkane? Can it be isolated?

Answer 33

cis alkene, cannot be isolated

Question 34

What is stereoselective alkyne reduction?

Answer 34

alkyne undergoes reduction and produces either cis or trans stereoisomer depending on conditions

Question 35

Do conditions for a terminal alkyne matter for reduction to an alkene?

Answer 35

no, there is no stereochemistry so it does not matter

Question 36

Which is more stable, pi bonds of alkyne or pi bonds of alkene?

Answer 36

pi bonds of alkene

Question 37

Why can't Pd/C be used to make an alkene from an alkyne?

Answer 37

it is too active

Question 38

What catalyst is used to make a cis alkene from an alkyne?

Answer 38

Lindlar's catalyst (less active, poisoned)

Question 39

How do you make a cis alkene from an alkyne?

Answer 39

H2 (1 eq) and Lindlar's cat

Question 40

What type of addition of H2 is the formation of a cis alkene from an alkyne?

Answer 40

syn addition only

Question 41

Is cis alkene formation from alkyne reg/stereoselective/specifc?

Answer 41

stereoselective because could form to stereoisomers but selects for one

Question 42

What type of cis alkene does internal and terminal alkyne form?

Answer 42

internal alkene and terminal alkene respectively

Question 43

Why doesn't catalytic hydrogenation work to make trans alkenes from alkynes?

Answer 43

because catalytic hydrogenation is syn addition only, trans requires anti addition

Question 44

What is dissolving metal reduction?

Answer 44

formation of trans alkene from alkyne by adding H2 anti using Na metal and NH3

Question 45

Why does the trans alkene form even though vinyl carbanion could be cis or trans?

Answer 45

trans vinyl carbanion is more stable due to less steric interactions

Question 46

What are the roles of Na metal and NH3 in dissolving metal reduction to make a trans alkene?

Answer 46

``` Na = electron source (reductant) NH3 = H source ```

Question 47

What type of addition of H2 is dissolving metal reduction to make a trans alkene?

Answer 47

anti addition only

Question 48

Is trans alkene formation from alkyne reg/stereoselective/specifc?

Answer 48

stereoselective

Question 49

What is chemoselectivity?

Answer 49

when a reagent selects for one FG in a polyfunctional molecule in which more than one FG "could" react

Question 50

How are carbonyls reduced via hydrohalogenation?

Answer 50

H2 (high pressure) and Pd/C

Question 51

What are ketones and aldehydes reduced to in hydrohalogenation with H2 (high p) and Pd/C?

Answer 51

``` ketones = 2º alcohol aldehide = 1º alcohol ```

Question 52

In a case where there is alkene and carbonyl group, which is reduced without high p H2? Why?

Answer 52

alkene pi bond reduced first because it is already high in energy; C=O is more stable

Question 53

How are molecules with polar C-X sigma bonds reduced?

Answer 53

with metal hydrides (LAH)

Question 54

What compounds can be reduced with LAH and water?

Answer 54

``` alkyl halides (lose e-) epoxides (ring opens) ```

Question 55

What type of rxn is reduction of C-X sigma bonds if H- is a strong nuc? What does this impy for reactivity and nuc attack?

Answer 55

Sn2 MeX and 1º most reactive nuc attack of H- occurs at most sub'd carbon of epoxide

Question 56

Why can't NaH be used for reduction of C-X sigma bonds Sn2 reaction?

Answer 56

because NaH is a base and will do E2

Question 57

What are the two main types of oxidizing agents?

Answer 57

O-O and metal-oxygen

Question 58

What type of oxidizing agents require H2SO4?

Answer 58

metal-oxygen with chromium: - Na2Cr2O7 - CrO3 - K2Cr2O7

Question 59

What reagent is needed to convert an alkene to an epoxide via epoxidation?

Answer 59

mCPBA (oxidizing agent)

Question 60

T/F: Epoxidation is a concerted rxn. What is its transition state?

Answer 60

True, all bonds are broken and formed at same time | Forms "butterfly" transition state

Question 61

What type of selective is epoxidation?

Answer 61

stereoselective because syn addition

Question 62

What causes the oxygen* in the central bonding mechanism to be electronically confused?

Answer 62

Hydrogen bonding between carbonyl oxygen and *oxygen's hydrogen causes oxygen to be partial negative and nucleophilic BUT... the carbonyl oxygen pulls e- density from the carbonyl carbon and thus the oxygen bonded to carbonyl carbon and then again the oxygen* bonded to that oxygen which makes oxygen* partial positive and electrophilic.