- more selective - only reduces aldehydes and ketones

- less selective, more reactive - reduces aldehydes, ketones, esters, carboxylic acids, nitriles, amides

Unit 3 (Lesson 14-20) Flashcards by Delaney Johnson

lithium reagents are typically used as

strong bases

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Grignard reagents are perferred when acting as

nucleophile with epoxides and carbonyls

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when the rxn is of a grignard + epoxide, you attack

the least hindered carbon and maintain stereochemistry

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acidic proton with grignard

rxn will end up with starting material

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reactivity of aldehydes and ketones in nucleophilic addition reactions

formaldehyde>aldehyde>ketone

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alkyl groups are electron donating through

hyperconjugation and make C=O carbon less positive

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as steric hindrance of the carbonyl increases

the reactivity toward nucleophilic addition decreases

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order of nucleophilic reactivity

acid chlordide> acid anhydride> aldehyde > ketone > carboxylic acid > amide

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grignard rxn with formaldehyde produces

primary alcohol

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grignard rxn with aldehyde produces

secondary alcohol + (enantiomer sometimes)

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grignard rxn with ketone produces

tertiary alcohol + (enantiomer sometimes)

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what can be used to synthesize a carboxylic acid

addition of CO2 to a grignard

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when a carbon is associated with a metal (Mg, Li, Cu), the carbon is the more/less electronegative atom and takes on a partial () character

more, negative

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ether/solvent that Mg is added to

Et2O or THF

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what conditions promote the conversion of acetone to acetone cyanohydrin

HCL, excess NaCN

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cyanohydrin can be converted back into an aldehyde or ketone in the addition of a

base

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strong acid/weak base =

good LG

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strong base/weak acid =

poor LG

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NaBH4

more selective
only reduces aldehydes and ketones

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LiAlH4

less selective, more reactive
reduces aldehydes, ketones, esters, carboxylic acids, nitriles, amides

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why do NaBH4 and LiAl4 require H3o+

they require an aqueous workup to protonate the alkoxide

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things that add irreversibly

NaBH4, LiAlH4, RMgX, or RLi

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ketone /aldehyde + water (+acid/base catalyst)

hydrate

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degree of hydration at equilibrium (K)

= [hydrate]/starting aldehyde/ketone

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degree of hydration trends

as K increases, more hydrate is formed

why is an aldehyde with F3C instead of CH3 so reactive

so reactive because it's electron withdrawing

aldehydes and ketones are in equilibrium with their acetals in the presence of ()

alcohol

hemi-acetal

1OR + 1 OH

acetal

OR + OR

to favor acetal formation

add more (CH3OH], remove water

to favor aldehyde formation

increase [H2O] and remove methanol

acetal hydrolysis

- follows same mechanism as an acetal formation, but in reverse - acetals are good protecting groups -->

reducing agents we've used

H2/Pd H2NNH2, KOH, heat NaHB4 DIBALH LiAlH4

reduction refers to

the addition of H2 or removal of oxygen

Oxidation refers to

the addition of oxygen or removal of H2

oxidizing agents for this course

PCC H2CrO4

hydrogenation

H2 pd/c - only benzylic - reduces benzylic carbonyls to methylenes

wolff-kischner -- H2NNH2, KOH, heat

reduces all carbonyls to methylenes

primary alcohol oxidations - anhydrous conditions with ()

PCC

primary alcohol oxidations - aqueous with ()

Chromate (H2CrO4)

PCC + a secondary alcohol makes a ()

ketone

to form an imine you use a () amine

primary

to form an enamine you use a () amine

secondary

enamines have

2 R groups and a double bond in the ring

imines have

1 R group and a double bond between N and C on ring

imine reduction is a useful method for

synthesizing secondary amines

true/false: imines are more reactive than ketones and aldehydes

true -- they can selectively reduce with H2, Pb/C, or NaBH3CN

what does a wittig reaction do

replace carbonyl with alkene

first step in wittig reaction

generate phosphonium ylide

weak compounds add via ()

conjugate addition - more stable, weak bases 1,4 - Addition

strong compounds ad via ()

direct addition - strong base, nuc 1,2 - Addition

addition to a weak base is () and driven by ()

reversible, thermodynamic control

secondary alcohols oxidize to form ()

ketones, regardless of oxidizing agent

primary alcohols can be oxidized to form () or ()

aldehydes (mild oxidation) carboxylic acids (strong oxidation)

can tertiary alcohol be oxidized

true/fasle: when aldehydes and ketones are attacked by a carbon nucleophile or a hydride donor, there is no leaving group in the tetrahedral intermediate

true

when aldehydes and ketones react with water, alcohols, or amines (1 & 2), they undergo what

a nucleophilic-addition-elimintation mechanism

what are the optimal pH conditions for imines and enamines to be formed

~ 4.5

imine and enamine formation at a low pH

pH must be acidic enough to promote the dehydration step (elimination of H2O), however if solution is too acidic, the amine present will deprotonate, decreasing the availability of nucleophilic amine in the mixture

inamine and enamine formation at a high pH

if the solution isn't acidic enough, protonation of the hemiaminal intermediate is less effective and the rate decreases

imine formation last step

deprotonation

enamine formation last step

elimination of an alpha hydrogen to for a C=C, which breaks the C-N Pi bond

steps to go from hemiacetal to acetal

protonate, eliminate water, nucelophilic addition, deprotonate

reversion to carbonyl occurs when acetals are reacted with ()

water and acid