Ch. 12 – Oxidation and Reduction (Week 5, Quiz 4) Flashcards
What is oxidation?
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)
What is reduction?
increase in number of C-H bonds or decrease in number of C-Z bonds
(gain of electrons)
Is alkyne»_space; alkene ox or red?
reduction
Is alkene»_space; alkane ox or red?
reduction
What makes a mechanism regioselective?
could form two products but only forms one
indicator: Mark’s rule
What makes a mechanism stereospecific?
there is only one product possible (not even a chance of others being formed)
indicator: Sn2
What makes a mechanism stereoselective?
gives stereoisomers but one is favored (check)
indicator: syn or anti addition
What are reducing agents?
providers of H2
What are the three main types of reducing agents?
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)
What is general formula for an alkene reduction?
alkene + H2 (with metal catalyst) = alkane
What does the addition of H2 require for catalytic hydrogenation?
metal catalyst: Pd, Pt or Ni
What type of addition is H2 addition?
syn addition
Is hydrogenation exo/endothermic?
exothermic (product bonds are stronger)
What is the driving force of hydrogenation?
conversion of pi bond to two sigma bonds (sigma bonds are lower in energy)
What does a larger heat of hydrogenation ∆Hº mean?
less stable starting point
Do cis/trans alkenes produce the same alkane? What about their ∆Hº’s?
yes, but trans releases less energy because it is more stable to begin with
Do more/less stable alkenes have higher ∆Hº’s?
less stable = higher ∆Hº
Do more/less stable alkenes react faster in hydrogenation?
least stable alkenes react faster
What happens in catalytic hydrogenation of an alkene to form an alkane?
H2 binds to the surface of a metal catalyst
alkene also binds
H2 is transferred to pi bond and alkane is released
Why does syn addition occur in catalytic hydrogenation (alkene to alkane)?
the addition occurs on the side of the alkene bonded to the metal catalyst
What does bottom syn addition of tri-sub E alkene give?
S-enantiomer
What does top syn addition of tri-sub E alkene give?
R-enantiomer
What does bottom syn addition of tri-sub Z alkene give?
R-enantiomer
What does top syn addition of tri-sub Z alkene give?
S-enantiomer
What does bottom syn addition of tetra-sub E alkene give?
(3S,4S) stereoisomer
What does top syn addition of tetra-sub E alkene give?
(3R,4R) stereoisomer
What does bottom syn addition of tetra-sub Z alkene give?
(3R,4S) stereoisomer
What does bottom syn addition of tetra-sub Z alkene give?
(3S,4R) stereoisomer
What are three ways an alkyne can be reduced?
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
What happens in catalystic hydrogenation (reduction) of an alkyne to alkane?
alkyne (internal or terminal) reacts with H2 (2 eq) and metal catalyst to form an alkane
What type of addition is addition of H2 in reduction of alkyne to alkane?
syn addition
Do any st. centers form in the reduction of alkyne to alkane?
no
What is the intermediate of reduction of alkyne to alkane? Can it be isolated?
cis alkene, cannot be isolated
What is stereoselective alkyne reduction?
alkyne undergoes reduction and produces either cis or trans stereoisomer depending on conditions
Do conditions for a terminal alkyne matter for reduction to an alkene?
no, there is no stereochemistry so it does not matter
Which is more stable, pi bonds of alkyne or pi bonds of alkene?
pi bonds of alkene
Why can’t Pd/C be used to make an alkene from an alkyne?
it is too active
What catalyst is used to make a cis alkene from an alkyne?
Lindlar’s catalyst (less active, poisoned)
How do you make a cis alkene from an alkyne?
H2 (1 eq) and Lindlar’s cat
What type of addition of H2 is the formation of a cis alkene from an alkyne?
syn addition only
Is cis alkene formation from alkyne reg/stereoselective/specifc?
stereoselective because could form to stereoisomers but selects for one
What type of cis alkene does internal and terminal alkyne form?
internal alkene and terminal alkene respectively
Why doesn’t catalytic hydrogenation work to make trans alkenes from alkynes?
because catalytic hydrogenation is syn addition only, trans requires anti addition
What is dissolving metal reduction?
formation of trans alkene from alkyne by adding H2 anti using Na metal and NH3
Why does the trans alkene form even though vinyl carbanion could be cis or trans?
trans vinyl carbanion is more stable due to less steric interactions
What are the roles of Na metal and NH3 in dissolving metal reduction to make a trans alkene?
Na = electron source (reductant) NH3 = H source
What type of addition of H2 is dissolving metal reduction to make a trans alkene?
anti addition only
Is trans alkene formation from alkyne reg/stereoselective/specifc?
stereoselective
What is chemoselectivity?
when a reagent selects for one FG in a polyfunctional molecule in which more than one FG “could” react
How are carbonyls reduced via hydrohalogenation?
H2 (high pressure) and Pd/C
What are ketones and aldehydes reduced to in hydrohalogenation with H2 (high p) and Pd/C?
ketones = 2º alcohol aldehide = 1º alcohol
In a case where there is alkene and carbonyl group, which is reduced without high p H2? Why?
alkene pi bond reduced first because it is already high in energy; C=O is more stable
How are molecules with polar C-X sigma bonds reduced?
with metal hydrides (LAH)
What compounds can be reduced with LAH and water?
alkyl halides (lose e-) epoxides (ring opens)
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?
Sn2
MeX and 1º most reactive
nuc attack of H- occurs at most sub’d carbon of epoxide
Why can’t NaH be used for reduction of C-X sigma bonds Sn2 reaction?
because NaH is a base and will do E2
What are the two main types of oxidizing agents?
O-O and metal-oxygen
What type of oxidizing agents require H2SO4?
metal-oxygen with chromium:
- Na2Cr2O7
- CrO3
- K2Cr2O7
What reagent is needed to convert an alkene to an epoxide via epoxidation?
mCPBA (oxidizing agent)
T/F: Epoxidation is a concerted rxn. What is its transition state?
True, all bonds are broken and formed at same time
Forms “butterfly” transition state
What type of selective is epoxidation?
stereoselective because syn addition
What causes the oxygen* in the central bonding mechanism to be electronically confused?
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.