carbonyl compounds Flashcards
what is a carbonyl group?
a carbonyl group is composed of a carbon atom which is double bonded to an oxygen atom
what are the 2 types of carbonyl compounds?
- aldehydes (RCOH)
- ketones (RCOR’)
where R is an alkyl or aryl group
aldehydes end with -al
ketones end with -one
why do aldehydes & ketones have higher boiling points that corresponding hydrocarbons with similar number of electrons in the molecule?
the carbonyl group is polar -> pd-pd interactions exist between molecules in aldehydes & ketones in addition to dispersion forces -> requires more energy to overcome compared to only dispersion forces present in non-polar alkenes -> carbonyl compounds have higher boiling point
why do aldehydes & ketones have lower boiling points than their corresponding alcohols & carboxylic acids?
alcohols & carboxylic acids can form hydrogen bonds between molecules, which are stronger and require more energy to overcome than pd-pd interactions in the corresponding aldehydes & ketones -> carbonyl compounds thus have lower boiling points than the corresponding alcohols & carboxylic acids
why are only small alipathic aldehydes & ketones soluble in water?
- smaller alipathic aldehydes & ketones are soluble in water due to their ability to form hydrogen bonds with water
- carbonyl compounds with more than 5 carbon atoms/aromatic rings are virtually insoluble in water.
- no. of carbon atoms in alkyl chain increases -> strength of dispersion forces between molecules of the carbonyl compound increases -> energy released from hydrogen bonding formed between the carbonyl group & water is less able to overcome increasingly stronger dispersion forces & existing hydrogen bonding in water.
- hydrogen bonding is also disrupted by the larger non-polar alkyl chain
since aldehydes & ketones have both polar & non-polar regions, they can act as solvents to both polar & non-polar solutes
how can aldehydes & ketones be prepared?
aldehydes: oxidation of primary alcohols
ketones: oxidation of secondary alcohols or oxidative cleavage of subsituted alkenes
why do carbonyl compounds undergo nucleophilic addition but alkenes cannot?
- the C=O bond is polar so the carbonyl compound has a δ+ charge. the δ+ carbon attracts nucleophiles, so aldehydes & ketones react with nucleophilic reagents with HCN
- the C=C bond is non-polar and has no δ+ carbon to attract nucleophiles. the electron-rich C=C bond repels nucleophiles, so alkenes cannot react with nucleophilic reagents
what are aldehydes generally more reactive to nucleophilic addition than ketones?
- STERIC REASON: the presence of 2 relatively large substituents in ketones versus just one large substituent in aldehydes means the attacking nucleophiles are able to approach the carbonyl carbon in aldehydes with less steric hindrance than in ketones
- ELECTRONIC REASON: aldehydes only have 1 electron-donating alkyl group while ketones have 2, so an aldehyde-carbon has a higher partial positive charge compared to a ketone-carbon, so the aldehyde-carbon is more susceptible to nucleophilic attack.
(less e- donating alkyl groups -> more e- deficient -> higher partial positive charge -> more susceptible to nucleophilic attack)
what are the reagents & conditions for the nucleophilic addition of hydrogen cyanide to aldehydes/ketones?
reagents & conditions: HCN with trace KCN, cold
KCN is a catalyst and is used to raise rate of reaction
mixture must be kept cold as HCN is a toxic gas at rt, so it must be kept a liquid
how can the rate of reaction for nucleophilic addition of HCN be increased?
1 - add a trace amount of KCN as catalyst
- KCN dissociates completely to provide free CN- ions while HCN only dissociates partially to give CN-
- if only HCN is used, the reaction takes place very slowly as the concentration of CN- is low.
2 - add a small amount of base (eg NaOH)
- HCN + H2O ⇌ H3O+ + CN-
- when a small amount of base is added, H3O+ ions are neutralised.
- by LCP, POE of the above reaction shifts right to increase [H3O+], and [CN-] increases as well -> rate of reaction increases
why can a racemic mixture be formed in nucleophilic addition to aldehydes/ketones?
*assuming the resulting sp3 carbon is chiral
- the geometry about the carbonyl carbon atom in aldehydes & ketones is planar
- the nucleophile can hence attack the δ+ carbonyl carbon atom in equal probability from the top or bottom of the plane, producing equal amounts of both enantiomers to give a racemic mixture, if the resulting sp3 carbon is chiral
what are the reagents, conditions & observations for condensation of aldehydes/ketones with 2,4-DNPH?
reagents & conditions: 2,4-DNPH, room temperature
observations: orange ppt formed
test for presence of aldehydes/ketones
what are the reagents & conditions for reduction of aldehydes/ketones to form primary/secondary alcohols respectively?
reagents & conditinos: LiAlH4 in dry ether, room temperature
other reducing agents like NaBH4 in methanol or H2 (g) & Ni in high P can be used
when the aldehyde/ketone reacts with LiAlH4, the alkoxide ion is formed. H2O must be added to protonate the alkoxide to form the alcohol
why is LiAlH4 a stronger reducing agents than NaBH4?
aluminium is less electronegative than boron -> more of the negative charge in the AlH4- ion is borne by the hydrogen atoms (larger diff in electronegativity between Al & H compared to between B & H) -> LiAlH4 is a stronger reducing agent & less selective than NaBH4
bc LiAlH4 & ether is hazardous, NaBH4 is preferred
both LiAlH4 & NaBH4 do not reduce the non-polar electron-rich alkene grou
what are the reagents, conditions & observations for the oxidation of aldehyde to carboxylic acid with acidified K2Cr2O7/KMnO4?
reagents & conditions: K2Cr2O7/KMnO4 with dilute H2SO4, heat under reflux
observation: orange solution turns green (for K2Cr2O7)/purple solution turns colourless (for KMnO4)
ketones do not have a hydrogen atom attached directly to the carbonyl carbon, so they are not easily oxidised
what are the reagents, conditions & observations for reaction of aldehydes with tollens’ reagent?
reagents & conditions: tollens’ reagents, heat
observation: silver mirror formed
tollens’ reagents [Ag(NH3)2]+ is unstable and is freshly prepared by:
1. one drop of dilute NaOH (aq) is added to 3cm3 of AgNO3 (aq) to produce a dark brown ppt of Ag2O
2. dilute NH3 (aq) is added dropwise until the brown ppt first formed just redissolves
what are the reagents, conditions & observations for reaction of alipathic aldehydes with fehling’s reagent?
ONLY alipathic (straight chain) aldehydes - NO benzaldehydes
reagents & conditions: fehling’s reagent, heat
observations: reddish-brown (brick-red) ppt formed
ketones & benzaldehydes do NOT reduce fehling’s reagent
what are the reagents, conditions & observations for iodoform test?
reagents & conditions: aqueous I2 with NaOH (aq), warm
observation: yellow crystals of CHI3 are formed
only for aldehydes & ketones with the COCH3 group!!!
