carbonyls - fundamental reactions Flashcards
(35 cards)
describe functional group conversion of carbonyls via oxidative and reductive processes
alcohol < ketone/aldehyde < carboxylic acid
increasing oxidation level at carbon
these functional groups are interconverted between via oxidation and reduction processes
how does the oxidation of an alcohol to a carbonyl product work + what kind of reaction is it?
base attacks α acidic proton, electrons move to form double bond between C and O, and alcohol H (or whatever LG its bonded to) is lost to form the carbonyl product
this always takes place via E2
give 2 classic methods of the oxidation of alcohols
jones oxidation - uses Cr2O7^-, H2SO4, H2O
- current day issues: Cr waste is very environmentally harmful so the process is not possible for industrial scale, acid is also incompatible with OH functional group
potassium permanganate (H+/OH-)
- current day issues: only works in acid/base and cannot use ant cosolvents, all substances must be water soluble, chemical is restricted in the UK as its used to make cocaine
why can secondary and tertiary alcohols not be fully oxidised?
the oxidation process works by O attacking the dichromate ion to form a dichromate ether and the chromate ion later attacking a neighbouring hydrogen - this needs to happen twice to form the carboxylic acid
secondary alcohols cannot undergo the second oxidation as there is no second proton to lose
tertiary alcohols cannot undergo either oxidation as it has no protons to lose
what are the 3 types of selectivity?
chemoselectivity, regioselectivity, stereoselectivity
chemoselectivity definition
which functional groups will react
regioselectivity definition
where the functional group will react
stereoselectivity definition
how the functional group will react
which oxidant is best for oxidising a primary alcohol to an aldehyde + why?
either jones or KMnO4 will oxidise the alcohol all the way to a carboxylic acid - the issue is water, the CrO3 derivative is needed and this doesn’t require aqueous conditions, also the aqueous conditions form a 1,1-diol which is easily oxidised
alternative method to jones reagent is PCC (pyridinium chlorochromate, a salt) - this prevents formation of the hydrate so product remains an aldehyde - this still isn’t very environmentally friendly
TPAP is a more green alternative, used for the mild oxidation of primary and secondary alcohols, utilises ruthenium in its +7 O.S and is catalysed via co-oxidant N-methylmorphine
works exactly like CrO3 mechanistically but with RuO4
what does TPAP stand for + give one disadvantage of using it ?
TPAP = tetrapropylammonium perruthenate
ruthenium is a bit endangered
what is DMP + what is it used for?
DMP = dess martin periodinade
a hypervalent iodine species which utilises iodine in its +5 O.S and is a mild stoichiometric oxidant - high hypervalency of iodine means it easily donates many lone pairs to form bonds, which isn’t preferrable, so in this state is is very electrophilic as it wants its lone pairs back
give one method used to convert an aldehyde to a carboxylic acid
pinnick oxidation using NaClO2 as the oxidant
alongside 2-methyl-2-butene as a radical scavenger - this reaction produces many O/Cl radicals so this is used to remove radical product to protect yield of reaction
what are the 2 most common types of reducing agents?
metal hydrides
lewis acid hydrides
how do metal hydrides work - give examples + explain reactivity?
e.g. LiAlH4, LiBH4, NaBH4 (decreasing reactivity)
the more reactive the metal halide the less selective - reactivity is determined by electronegativity, Al is more metal like than B so there is a larger dipole across Al-H bond making it more unstable, Li wants to lose H
counter ion reactivity depends on acidity, Li = more lewis acidic so coordinates better with O on C=O increasing carbonyl reactivity
how do lewis acid hydrides work?
e.g. B2H6
charge neutral complexes made of Al or B which only become a source of H+ when they form lewis acid-base complexes, allowing the hydride to readily coordinate with O using lone pair, best for reduction of electron rich carbonyl derivatives, can reduce amides
- these form bridged dimers when un complexes
via what process are aldehydes/ketones reduced + what product is formed?
reaction with 1 equiv. reducing agent via nucleophilic addition
forms alcohols
via what process is an ester reduced + what product is formed?
reaction with 2 equiv. reducing agent, via nucleophilic substitution
forms alcohol + carboxylic acid
via what process is an amide reduced + what product is formed?
reaction with 2 equiv. reducing agent, via nucleophilic substitution
forms amine + carboxylic acid
via what process is a carboxylic acid reduced + what product is formed?
reaction with 2 equiv. reducing agent, via nucleophilic substitution
forms alcohol via aldehyde
how does cyanide react with a carbonyl?
a chain lengthening reaction via nucleophilic addition, to form a hydroxynitrile
this is a reversible reaction
give 1 disadvantage of using cyanide
cyanide, -CN is very toxic
why is a nitrile functional group useful?
nitriles are easily converted into other functional groups e.g. carboxylic acid/amine
how are nitriles converted into a carboxylic acid?
via acid hydrolysis
- reduction of nitrile into protonated +vely charged nitrile by reaction with proton/acid
- addition of water to electrophilic carbonyl to form trigonal planar intermediate (from linear)
- PT to amine from water to form ammonium ion, then to oxygen via C=O double bond formation
- water adds to electrophilic carbonyl again forming tetrahedral intermediate, breaking double bond
- PT from water to amine group, protonation allows it to then leave
- carbonyl bond formed + proton is lost
give 2 other kind of reagents that can be used for carbon chain lengthening purposes/to form new C-C bonds
organometallic reagents - Li-R or BrMg-R
these are able to act as very strong nucleophiles/bases, and can deprotonate acidic C-H bonds to form new organometallic reagents
