aldehydes and ketones Flashcards
what is the carbonyl group?
C=O
where is the carbonyl carbon found in aldehydes?
at the end of a carbon chain (func. group = COH)
what suffix is used to name aldehydes?
-anal (e.g CH3CH2CH2COH = butanal)
aldehydes are…
easily oxidised to form carboxylic acids
what is functional group for carboxylic acids?
-COOH
what suffix is used for carboxylic acids?
-ioc acid (e.g. CH3CH2COOH = propanoic acid)
where is the carbonyl carbon found in ketones?
in the middle of a carbon chain
what suffix is used for ketones?
-one (e.g CH3C(O)CH3 = propanone)
out of aldehydes and ketones, which is easier to oxidise?
aldehydes
what two chemical tests are used to distinguish between aldehydes and ketones?
- Fehling’s solution
- Tollens reagent (silver mirror)
both Tollen’s and Fehling’s are…
weak oxidising agents - therefore change is observed when there is an aldehyde, NOT A KETONE
what is Fehling’s solution?
an alkaline solution of copper(II) sulfate - complex Cu2+ ions give it its blue colour
what happens to the solution in the presence of Fehling’s solution?
Cu2+ is reduced (Cu2+ + e- → Cu ) to form a brick red precipitate of
copper(I) oxide which has the formula Cu2O
what happens to the ALDEHYDE in the Fehling’s solution reaction?
it is oxidised to form a carboxylic acid
what is the process of using Fehling’s solution?
- a few drops of the organic solution added to a test tube containing Fehling’s
- gently warmed in a hot water bath
how do you prepare Tollens reagent?
- add a drop of sodium hydroxide to silver(I) nitrate solution to make a precipitate of silver(I) oxide
- dissolve this precipitate by adding dilute ammonia
what complex ion does Tollen’s reagent contain?
[Ag(NH3)2]+
what happens to the complex ion in Tollen’s reagent in the presence of an aldehyde?
[Ag(NH3)2]+ is reduced to form a precipitate of metallic silver (Ag+(aq) + e- → Ag(s)
what happens to the ALDEHYDE in the presence of Tollen’s reagent?
it is oxidised to form a carboxylic acidw
what forces exist in aldehydes and ketones?
- van der waals forces
- permanent dipole-dipole forces
what is the effect of the forces on BP of aldehydes and ketones?
it means that the b.p. of aldehydes and ketones are higher than those of alkanes with a similar Mr
bp of aldehydes/ketones: as the length of the carbon chain increases…
the b.p. of the aldehydes/ ketones increases too. This is due to a greater number of electrons,
and therefore stronger van der Waals’ forces
as branching increases…
the b.p. of the aldehydes/ketones decreases.
* This is due to less effective permanent dipole-dipole forces.
* Think of it as though the branches are getting in the way of the Intermolecular forces, while they’re trying to pack the molecules
together.
how soluble are aldehydes and ketones in water?
all short chain aldehydes and ketones are soluble in water because the
polar carbonyl groups are able to form H-bonds with water molecules.
what is the effect of increasing chain length on solubility?
- As the chain length increases, the aldehydes and ketones become less
soluble. - This is because the hydrocarbon chain that doesn’t have the carbonyl group on it is non-polar.
- The longer this chain is, the less polar the molecule is so therefore the less soluble
what kind of reaction do carbonyls undergo?
nucleophilic addition reactions
why do carbonyls undergo nucleophilic addition reactions?
oxygen is more electronegative therefore attracts the electrons from the covalent bond - leaves the carbon with a partial positive charge
what are some examples of nucleophilic addition reactions which aldehydes and ketones undergo?
- The reduction of aldehydes to primary alcohols, and ketones to secondary alcohols, using the reducing agent NaBH4 in aqueous
solution. - The reaction of the -:CN ion with aldehydes and ketones to produce
hydroxynitriles (KCN, followed by dilute acid)
what is NaBH4?
sodium borohydride
- it is a complex anion but behaves like a hydride ion (H-) So we draw it as a H- in the mechanism; that’s the species acting as the
nucleophile
what happens in the intermediate of the addition reaction with NaBH4?
gains a H+ from the acid
what are the conditions for the reduction of aldehydes by NaBH4?
Reagent: Aqueous NaBH4 followed by H2SO4
Conditions: Room temp.
Mechanism: Nucleophilic - H- = nucleophile
Reaction type: Addition/Reduction
Product: Primary alcohol
what are the conditions for the reduction of ketones by NaBH4?
Reagent: Aqueous NaBH4 followed by H2SO4
Conditions: Room temp.
Mechanism: Nucleophilic - H- = nucleophile
Reaction type: Addition/Reduction
Product: Secondary alcohol
what is the overall equation for the reaction of aldehydes with sodium borohydride?
R-CO-H + 2[H] –> R-CH2(OH)
what is the overall equation for the reaction of ketones with sodium borohydride?
R-CO-R + 2[H] –> R-CH(OH)-R
what are the conditions for the reaction of carbonyls with hydrogen cyanide?
Reagent: HCN - formed by reaction with KCN followed by dilute H2SO4
Conditions: Room temp.
Mechanism: Nucleophilic SO CN- is the nucleophile
Reaction type: Addition
Product: Hydroxynitrile
what is the overall equation for the reaction of aldehydes with hydrogen cyanide?
R-CO-H + HCN –> RCH(OH)CN
what is the overall equation for the reaction of aldehydes with hydrogen cyanide?
R-CO-R’ + HCN → RC(OH)R’CN
what are the hazards of using KCN?
HCN gas is not used directly in the lab as it is an extremely toxic + flammable gas. Instead it is formed in situ in the reaction mixture by
adding a dilute acid (H2SO4) to an aqueous solution of KCN.
* KCN must be handled carefully as it is toxic when ingested + forms HCN when in contact w/ acid.
what do aldehydes and unsymmetrical ketones form?
mixtures of enantiomers (when they react with KCN followed by dilute acid)
- this is because the products of the reactions contain chiral carbon atoms
nucleophillic addition reactions of KCN followed by dilute acid can produce…
a racemic mixture of enantiomers which is optically inactive
because:
1. The C atom in the planar carbonyl group can be attacked by a
nucleophile from above or below.
2. There is an equal chance of attack from above or below.
