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Organic Chemistry > Ch. 12 – Oxidation and Reduction (Week 5, Quiz 4) > Flashcards

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

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1
Q

What is oxidation?

A

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)

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2
Q

What is reduction?

A

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

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3
Q

Is alkyne&raquo_space; alkene ox or red?

A

reduction

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4
Q

Is alkene&raquo_space; alkane ox or red?

A

reduction

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5
Q

What makes a mechanism regioselective?

A

could form two products but only forms one

indicator: Mark’s rule

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6
Q

What makes a mechanism stereospecific?

A

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

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7
Q

What makes a mechanism stereoselective?

A

gives stereoisomers but one is favored (check)

indicator: syn or anti addition

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8
Q

What are reducing agents?

A

providers of H2

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9
Q

What are the three main types of reducing agents?

A

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)

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10
Q

What is general formula for an alkene reduction?

A

alkene + H2 (with metal catalyst) = alkane

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11
Q

What does the addition of H2 require for catalytic hydrogenation?

A

metal catalyst: Pd, Pt or Ni

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12
Q

What type of addition is H2 addition?

A

syn addition

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13
Q

Is hydrogenation exo/endothermic?

A

exothermic (product bonds are stronger)

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14
Q

What is the driving force of hydrogenation?

A

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

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15
Q

What does a larger heat of hydrogenation ∆Hº mean?

A

less stable starting point

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16
Q

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

A

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

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17
Q

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

A

less stable = higher ∆Hº

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18
Q

Do more/less stable alkenes react faster in hydrogenation?

A

least stable alkenes react faster

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19
Q

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

A

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

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20
Q

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

A

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

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21
Q

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

A

S-enantiomer

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22
Q

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

A

R-enantiomer

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23
Q

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

A

R-enantiomer

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24
Q

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

A

S-enantiomer

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25
Q

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

A

(3S,4S) stereoisomer

26
Q

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

A

(3R,4R) stereoisomer

27
Q

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

A

(3R,4S) stereoisomer

28
Q

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

A

(3S,4R) stereoisomer

29
Q

What are three ways an alkyne can be reduced?

A

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

30
Q

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

A

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

31
Q

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

A

syn addition

32
Q

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

A

no

33
Q

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

A

cis alkene, cannot be isolated

34
Q

What is stereoselective alkyne reduction?

A

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

35
Q

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

A

no, there is no stereochemistry so it does not matter

36
Q

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

A

pi bonds of alkene

37
Q

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

A

it is too active

38
Q

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

A

Lindlar’s catalyst (less active, poisoned)

39
Q

How do you make a cis alkene from an alkyne?

A

H2 (1 eq) and Lindlar’s cat

40
Q

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

A

syn addition only

41
Q

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

A

stereoselective because could form to stereoisomers but selects for one

42
Q

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

A

internal alkene and terminal alkene respectively

43
Q

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

A

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

44
Q

What is dissolving metal reduction?

A

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

45
Q

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

A

trans vinyl carbanion is more stable due to less steric interactions

46
Q

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

A 
Na = electron source (reductant)
NH3 = H source
47
Q

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

A

anti addition only

48
Q

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

A

stereoselective

49
Q

What is chemoselectivity?

A

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

50
Q

How are carbonyls reduced via hydrohalogenation?

A

H2 (high pressure) and Pd/C

51
Q

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

A 
ketones = 2º alcohol
aldehide = 1º alcohol
52
Q

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

A

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

53
Q

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

A

with metal hydrides (LAH)

54
Q

What compounds can be reduced with LAH and water?

A 
alkyl halides (lose e-)
epoxides (ring opens)
55
Q

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?

A

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

56
Q

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

A

because NaH is a base and will do E2

57
Q

What are the two main types of oxidizing agents?

A

O-O and metal-oxygen

58
Q

What type of oxidizing agents require H2SO4?

A

metal-oxygen with chromium:

  • Na2Cr2O7
  • CrO3
  • K2Cr2O7
59
Q

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

A

mCPBA (oxidizing agent)

60
Q

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

A

True, all bonds are broken and formed at same time

Forms “butterfly” transition state

61
Q

What type of selective is epoxidation?

A

stereoselective because syn addition

62
Q

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

A

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.

Organic Chemistry (26 decks)

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