4.4 aldehydes and ketones Flashcards
primary alcohols undergo partial oxidation to form what?
aldehydes
secondary alcohols undergo oxidation to produce what?
a ketone
further oxidation of an aldehyde results in what?
a carboxylic acid being formed
what is the oxidising agent for the oxidation of primary and secondary alcohols?
- acidified potassium dichromate
- H+/Cr2O7 2-
- [O]
what is the colour change for the oxidation of primary and secondary alcohols?
orange to green
(the orange dichromate ion, Cr2O7 2- is reduced to the green chromium (III) ion, Cr3+)
can tertiary alcohols be oxidised?
no
what is a carbonyl?
any compound that contains C=O
e.g
- aldehyde
- ketone
- carboxylic acid
- ester
what is the ending when naming aldehydes?
- al
what is the ending when naming ketones?
-one
the aldehyde group should always be on carbon 1 - it has high priority (like c/a)
what are the 2 reducing agents you can use for reducing aldehydes, ketones and carboxylic acids?
- sodium borohydride, NaBH4
- lithium aluminium hybride, LiAlH4
what do you use to represent the oxidation agent of APD in equations?
what do you use to represent the reducing agent of NaBH4 and LiAlH4 in equations?
- [O]
- [H]
oxidising alcohols to carbonyls:
generally :
- if oxidise 1° alcohol = need 2[O]
- if oxidise 2° alcohol = need 1[O]
in reducing carbonyls to alcohols:
generally:
- if reducing aldehyde/ketone = 2[H]
- if reducing carboxylic acid = 4[H] —> + H2O
what is a hydride ion?
H-
what are the 4 tests for aldehydes and ketones?
- Fehling’s reagent
- Tollen’s reagent
- 2,4-dinitrophenylhydrazine (2,4-DNPH)/brady’s reagent
- iodoform test (triiodomethane)
what is fehling’s reagent? (made from)
a mixture of alkaline solution of copper (II) sulfate
does fehling’s reagent produce a positive result for aldehydes?
does fehling’s reagent produce a positive result for ketones?
- aldehydes = +ve result (will be oxidised to a c/a)
- ketones = -ve result (cannot be further oxidised)
what does Fehling’s reagent do?
- it will oxidise a carbonyl group IF IT CAN
- so will distinguish between an aldehyde and a ketone
- because aldehydes will be oxidised to a c/a
- but the test shows no reaction with ketones since they cannot be further oxidised
what is the observation of a positive result to Fehling’s reagent?
- from blue solution to a red-brown ppt
- due to the reduction of cooper (II) ions to copper (I) ions in Cu2O
what is Tollen’s reagent? (made from)
- an ammonical silver nitrate (silver nitrate, NaOH and ammonia) solution
what does Tollen’s reagent test for?
aldehydes
- the solution contains Ag+ ions
- these will be reduced if an aldehyde is present - to silver
- if no aldehyde group - no visible reaction
what is a positive result for tollen’s reagent?
- from colourless to a silver mirror
what does 2,4-DNP test for?
- aldehydes and ketones
- doesnt react with carboxylic acids, amides or esters even tho they contain carbonyl groups
what is a positive result for the 2,4-DNPH test?
- produce a red-orange ppt
what type of reaction is the 2,4-DNPH test?
addition-elimination / condensation mechanism
- crystals of different hydrazones as the products of this 2,4-DNP test have different melting points
- so can measure its melting point
- which will be specific to what it was originally
- so can identify what it was originally
what is the iodoform test? (made from)
- a mixture of iodine and sodium hydroxide (I2/NaOH)
what does the iodoform test test for?
- methyl ketone groups, CH3CO
- methyl alcohol groups, CH3COH
what is a positive result for the iodoform test?
- yellow crystals/yellow ppt
- of CHI3 (iodoform)
- antiseptic smell
AS tests:
what is the test for alkenes?
positive result?
- bromine water
- orange —> colourless
AS tests:
what is the test for carboxylic acids?
positive result?
- sodium carbonate
- fizzing
AS tests:
what is the second test for alkenes?
positive result?
- acidified potassium manganate
- purple —> colourless
AS tests:
what is the test for 1° or 2° alcohols?
positive result?
- acidified potassium dichromate
- orange —> green
AS tests:
what is the third test for alkenes?
positive result?
- alkaline potassium manganate
- purple —> green
what does aliphatic mean?
without a benzene ring
what type of reaction is the reduction of aldehydes and ketones to alcohols?
nucleophilic addition
hydrogen cyanide formula
HCN
what type of reaction is the one between HCN and an aldehyde/ketone?
nucleophilic addition
is hydrogen cyanide, HCN a weak or strong acid?
weak acid
why cant pure HCN be used as HCN in the nucleophilic addition of HCN to an aldehyde/ketone?
- because a neutral molecule is not a nucleophile
- (also its not added directly to an aldehyde/ketone because its a very poisonous gas and is also hard to store)
what is the equation for producing the nucleophile that is then used in the nucleophilic addition of HCN to an aldehyde/ketone?
HCN + OH- ⇌ H2O + CN-
(OR HCN ⇌ CN- + H+)
- CN- is the cyanide ion
- it is a nucleophile with a lone pair on the C atom
what is used as the nucleophile in the nucleophilic addition of HCN to an aldehyde/ketone?
CN- (cyanide ion)
HCN:
H - C ≡ N
- the nucleophilic addition (of HCN to an aldehyde/ketone) extends the C chain
- and it generates chiral compounds even when its not symmetrical to start
what can the product of the nucleophilic addition of HCN to an aldehyde/ketone sometimes have ?
a chiral centre
why does a racemic mixture form in the nucleophilic addition of HCN to an aldehyde/ketone?
- all double bonds (C=O, C=C) are planar
- it forms two enantiomers
- 50% of one, 50% of the other
what should you do if you want to favour one enantiomer over the other in the nucleophilic addition of HCN to an aldehyde/ketone?
- change/block one of the portion of molecule to encourage 1 side to attack more easily
whats is formed in the nucleophilic addition of HCN to an aldehyde/ketone?
- hydroxynitrile
- OH on the same carbon as CN
how do you name hydroxynitriles?
- nitrile group = high priority = 1st carbon
- e.g 2- hydroxybutanitrile
what group is the CN in organic chemistry?
nitrile group
name the type of reaction alcohols undergo to form aldehydes and ketones
oxidation
why is immediate distillation required for the oxidation of an alcohol to an aldehyde?
- because otherwise it will undergo further oxidation to form a carboxylic acid
what is a common oxidising agent used in the oxidation of alcohols?
- acidified potassium dichromate
what would be observed when acidified potassium dichromate is added to a tertiary alcohol?
- the solution will remain orange
- because potassium dichromate doesn’t react with tertiary alcohols as they cannot be oxidised
give the chemical equation for the oxidation of propane-2-ol to propanone
CH3CH(OH)CH3 + [O] —> CH3COCH3 + H2O
name the two reagents which can be used to distinguish between aldehydes and ketones
- tollen’s reagent
- fehling’s reagent
how can tollen’s reagent be used to determine whether you have an aldehyde or a ketone?
- tollen’s reagent forms a silver mirror on the test tube only when warmed with an aldehyde and not a ketone
how can Fehling’s reagent be used to determine whether you have an aldehyde or a ketone?
- fehling’s reagent is blue
- if fehling’s reagent is added to an aldehyde then the blue solution will form a brick red ppt/red brown ppt
- if fehling’s reagent is added to a ketone, nothing happens
name the type of reaction aldehydes and ketones undergo to form alcohols
reduction
- aldehydes will be reduced to a ___ alcohol
- ketones will be reduced to a ___ alcohol
- primary
- secondary
give the chemical equation for the reduction of butanone to butane-2-ol
CH3COCH2CH3 + 2[H] —> CH3C(OH)HCH2CH3
name the mechanism for the reduction of propanal to propan-1-ol
nucleophilic addition
why are hydroxynitriles important?
- they can be hydrolysed to form hydroxy acids which are very useful and are commonly used in the cosmetic industry
how does the addition of HCN to ethanal affect the C-C chain length?
- the carbon chain length will increase by one carbon atom
- due to the carbon atom in the cyanide ion, CN-
name the type of reaction taking place when aldehydes and ketones react with 2,4-DNP
nucleophilic addition-elimination reaction
(condensation reaction)
how can 2,4-dinitrophenylhydrazine be used to test for aldehydes and ketones?
- an orange-yellow ppt is formed when 2,4-DNPH reacts with aldehydes or ketones
how can 2,4-DNPH be used to identify specific aldehydes and ketones?
- add 2,4-DNP so that a ppt forms
- purify the solid by recrystallisation
- compare the melting point of the pure crystals formed with the melting points of 2,4-dinitrophenylhydrazones of all the common aldehydes and ketones
what reagents can be used for the triiodomethane (iodoform) test?
- iodine and sodium hydroxide solution
(or potassium iodide and sodium chlorate (I) solutions)
describe the method and results of the iodoform test using iodine and sodium hydroxide
- iodine is added to a small amount of aldehyde/ketone
- followed by some sodium hydroxide to remove the iodine colour
- a very pale yellow precipitate will be formed if an aldehyde or ketone is present
what is the iodoform test used to identify?
- CH3CO- groups
