3.3.8 Aldehydes and Ketones (A2) Flashcards
what is the functional group of an aldehyde?
C=O(H)
on the end of a molecule
what is the functional group of ketones?
C=O in the central region of a molecule
how are aldehydes made?
primary alcohol - one carbon bound to the carbon with -OH
oxidise - H+/K2Cr2O7 (sulfuric acid)
orange—>green
distillation
what is the aldehyde called which includes a benzene ring?
benzanal
how are ketones made?
oxidation of secondary alcohol
H+/K2Cr2O7 (potassium dichromate) orange —> green
under reflux
what are the properties of aldehydes and ketones?
C=O bond is polar
dipole-dipole interactions between molecules
higher boiling points than their counterpart alkanes - more energy needed to overcome intermolecular forces
what is the solubility like of aldehydes and ketones?
aldehydes and ketone with less than 4 carbons can dissolve in water
because carbonyl group can hydrogen bond to water.
why can’t aldehydes and ketones with more than 4 carbons dissolve in water?
the larger the amount of carbons, the more non polar the molecule, this prevents molecule from dissolving
what state is methanal at room temperature?
gas
what state are short chain aldehydes and ketones at room temperature?
liquids (apart from methanal) with a characteristic smell
what is an example of a ketone used in a household item?
propanone found in nail varnish remover
what is benzaldehyde used for? why?
scenting soaps and flavouring foods
smells of almonds
why do ketones have no fewer than 3 carbons?
the double bond oxygen has to be on neither end of the carbon chain
what is the lowest number of carbons in a ketone?
3
why is no numbering system needed for aldehydes?
R=O(H) has to be on either end of the chain
why are there no H bonds between propanone molecules?
no oxygen bonded to hydrogen
what reactions tend to happen with aldehydes/ketones?
REDOX
nucleophilic addition - the delta positive carbon is susceptible to nucleophilic attack, carbonyl group unsaturated
why can’t we use the cyanide ion in the lab?
cyanide is toxic
what group are the products of reactions with cyanide ions in?
nitrile
why do we get a racemic mixture?
C=O bond is planar
nucleophile can attack from either above or below the plane
50:50 chance of either happening
so we get a 50:50 mixture of both enantiomers
why do racemic mixtures show no optical activity?
the rotation on plane-polarised light cancels out
what is oxidation?
loss of e-
gain of oxygen
loss of H
what is reduction?
gain of e-
loss of oxygen
gain of H
what does the oxidation of an aldehyde result in?
carboxylic acid
can ketones be oxidised further? why?
no
C-C bond is too strong to be overcome
what is Fehling’s A?
CuSO4 (copper sulphate)
what is Fehling’s B?
alkali, complexing agent
what does the Fehling’s test give for an aldehyde?
colour change from blue to orange/red precipitate
what does the Fehling’s test give for a ketone?
no observable changes - remains blue
what is going on during the Fehling’s test for an aldehyde?
Aldehyde being oxidised to a carboxylic acid
this reduces the Cu2+ ions (blue) to Cu+ ions (orange/red)
why does the Fehling’s test show no results in a ketone?
ketone cannot be oxidised further
what is the Fehling’s test for?
distinguish between aldehydes and ketones
what chemical does the silver mirror test involve?
Tollens’ reagent
what happens when we put Tollens’ reagent into an aldehyde?
silver precipitate formed, “mirror” layer on inside of test tube
what is the result when we add Tollens’ reagent to a ketone?
no observable changes
what two tests do we use to distinguish between aldehydes and ketones?
Fehlings’ test
Silver mirror test (tollens’ reagent)
why do we get no observable changes when we add Tollens’ reagent to ketones?
ketones cannot be further oxidised
what happens when Tollen’s reagent is added to an aldehyde?
Aldehyde is oxidised to a carboxylic acid
silver ion reduced from Ag+ to Ag2+
what reducing agent is used to reduce an aldehyde / ketone?
sodium tetraborohydride
what is released from reducing an aldehyde/ketone?
hydride ion (H-) with a lone pair
what does reducing an aldehyde form?
primary alcohol
what is the general equation for reduction of an aldehyde?
RCHO + 2H —> RCHOH
what type of mechanism is the reduction of aldehydes?
nucleophilic addition
what is formed after the reduction of a ketone?
secondary alcohol
what is the general equation of reduction of a ketone?
RCOR’ + 2H —> RCH(OH)R’
what type of mechanism is the reduction of ketones?
nucleophilic addition
which isomer of an aldehyde does not contain a chiral carbon and therefore cannot show stereoisomerism
symmetrical ketone
what is the usual reason for molecules not showing stereoisomerism? (optical)
no chiral carbon
