chapter 26 Flashcards
oxidation of aldehydes
oxidised to carboxylic acids, refluxed w/ acidified dichromate ion Cr2O72- & dilute H2SO4\
-ketones do not oxidise
C==O double bond
- react differently to alkenes
- c==o is polar, o more electronegative than c- aldehydes & ketones- react with some nucleophiles- attacks positive c atom
reducing an aldehyde
reduced to primary alcohols by NaBH4
butanal + 2[H] –NaBH4/H2O—-> butanol
reducing a ketone
reduced to secondary alcohols by NaBH4
propanone + 2[H] –NaBH4/H2O–> propan-2-ol
reaction of carbonyl compounds with HCN
hydrogen cyanide adds across C==O bond of aldehydes and ketones
colourless, v poisonous boils slightly above room temp so can’t be used in open lab– sodium cyanide and sulphuric acid used
propanal + HCN —H2SO4/NaCN—> hydroxynitrile
mechanism for reaction of aldehyde/ketone with NaBH4
- nucleophilic addition
- NaBH4 contains hydride ion :H-, acts as nucleophile
1) lone pair of electrons from hydride ion attracted and donated to delta +C in c=o bond
2) dative covalent bond formed between hydride ion and C in c=o bond
3) pi bond in c=o breaks by heterolytic fission forming a -ve intermediate
4) o atom of intermediate donates a lone pair to H atom in water- intermediate protonated to form an alcohol
mechanism for reaction of aldehyde/ketone with NaCN/H+
cyanide ion :CN- attacks electron deficient c atom in aldehyde/ketone
1) lone pair of electrons from cyanide ion attracted & donated to delta + carbon in c=o bond. Dative covalent bond forms
2) pi bond in c=o breaks by heterolytic fission, forming a negatively charged intermediate
3) intermediate is protonated by donating a lone pair of electrons to h ion form product
4) product - hydroxynitrile
detecting carbonyl compounds
2,4-dinitrophenylhydrazine (2,4-DNP)- Brady’s reagent- dissolved in methanol and sulfuric acid
1) add 5cm depth of 2,4-DNP to a clean test tube (excess)
2) add 3 drops of unknown compound w/ a pipette and leave to stand
3) if no crystals form add few drops of sulfuric acid
4) yellow/orange precipitate indicates aldehyde/ketone
distinguishing between aldehydes and ketones
tollen’s reagent- solution silver nitrate in aq ammonia
aldehyde- silver mirror
has to be made immediately before test:
1) add 3cm depth of aq AgNO3
2) add aq sodium hydroxide until a brown precipitate of silver oxide Ag2O is formed
3) add dilute ammonia solution until precipitate dissolves to colourless solution– tollens reagent
method:
1) pour 2cm depth of unknown sol to clean test tube
2) add equal vol of tollens
3) leave to stand in beaker of warm water- 50degrees, 10-15 mins
silver ions act as an oxidising agent in presence of ammonia- aldehyde oxidised and ions reduced to silver
solubility of carboxylic acids
c=o and o=h bonds are polar- form H bonds with water
- carboxylic acids with up to 4 C atoms are soluble in water
- as the number of c atoms increases, solubility decreases, as non-polar chain has a greater effect
acid reactions of carboxylic acids
-redox with metals
-neutralisation with bases
form carboxylate salts
carboxylate ion, -ate
redox reactions of carboxylic acids with metals
- form hydrogen gas & a carboxylate salt, metal disappears, effervescence eg 2CH3CH2COOH(aq) + Mg(s) --> (CH3CH2COO-)2Mg2+(aq) + H2
neutralisation of carboxylic acids with bases- metal oxides
form salt & water eg 2CH3COOH(aq) + CaO(s) --> (CH3COO-)2Ca2+(aq) +H2O(l)
neutralisation of carboxylic acids with alkalis
salt & water
CH3COOH(aq) +NaOH(aq) + NaOH(aq)–> CH3COO-Na+(aq) + H2O
ionic- H+ + OH- –> H2O
neutralisation of carboxylic acids with carbonates
CO2 gas. if carboxylic acid is in excess solid carbonate would disappear
2CH3COOH(aq) + Na2CO3(aq) –> 2CH3COO-Na+(aq) + H2O + CO2
test for carboxyl group
neutralisation w/ carbonates eg sodium carbonate- distinguish- only ones that react
what is a derivate of a carboxylic acid
a compound that can be hydrolysed to form the parent carboxylic acid
have an acyl group RCO
name acyl chlorides
-oyl chloride
acid anhydrides
formed by removal of water from 2 carboxylic acid molecules
esterification
reaction of an alcohol w/ a carboxylic acid, warmed with conc sulfuric acid
acid hydrolysis
- reverse of esterification
- ester heated under reflux w/ dilute aq acid, broken down by water - acid acting as catalyst
alkaline hydrolysis
saponification- irreversible
ester heated under reflux w/ aq hydroxide ions. carboxylate ion and methanol
eg CH3COOCH3 + NaOH –> CH3COO-Na+ + CH3OH
preparation of acyl chlorides
from parent carboxylic acid with thionyl chloride SOCl2
other products are gases- fume cupboard
propanoic acid + SOCl2 –> propanoyl chloride + SO2(g) + HCl(g)
reaction of acyl chlorides with alcohols to form esters
ethanoyl chloride + propan-1-ol –> propyl ethanoate + HCl
reaction of acyl chlorides w/ phenols to form esters
carboxylic acids not reactive enough to form esters w/ phenols- acyl chlorides and acid anhydrides much more reactive.
ethanoyl chloride + phenol –> phenyl ethanoate + hydrogen chloride
reaction of acyl chlorides w/ water to form carboxylic acids
water added- violent reaction
ethanoyl chloride + H2O –> ethanoic acid + HCl
reaction of acyl chlorides with ammonia and amines to form amides
ammonia & amines can act as nucleophiles by donating a lone pair on the nitrogen. reaction with acyl chlorides forms amides.
ammonia reacts w/ acyl chlorides forming a primary amide (nitrogen attached to 1 carbon)
ethanoyl chloride + 2NH3 –> ethanamide -primary amide + NH4Cl
primary amine reacts w/ acyl chloride to form a secondary amide (nitrogen attached to 2 carbons)
ethanoyl chloride + 2CH3NH2 (methylamine- primary) –>
N-methylethanamide (secondary) + methylammonium chloride
reactions of acid anhydrides
react in a similar way to acyl chlorides with alcohols, phenols, water, ammonia & amines. less reactive than acyl chlorides
ethanoic anhydride + phenol –> phenyl ethanoate + ethanoic acid
2(CH3CO)2O + C6H5OH –> CH3COOC6H5 + CH3COOH
