Carbonyl compounds Flashcards
What is the ketone functional group?
Where a carbonyl group (C=O) is found on a carbon chain, not at the end
What is the aldehyde functional group?
Where a carbonyl group (C=O) is found at the end of a carbon chain
Whst are carbonyl compounds?
A compound where C=O appears on its own.
Ketones and aldehydes are examples, but not carboxylic acids because its C=O is not on its own.
What intermolecular bonds can aldehydes and ketones form?
London and pd-pd
Oxygen has lone pairs present, but there is no delta positive H.
No H atoms bonded to an electronegative O, N or F atom. So cannot form H-bonds
What bonds do aldehydes and ketones form with water?
hydrogen bonds
- oxygen atom with a lone pair on the carbonyl
- delta positive hydrogen atom on the water molecule (due to the electronegativity difference between the H and O)
Why does carbonyl solubility (in water) decrease as chain length increases?
Energy required to overcome 2 H-bonds in water is compensated by the forming of the single H-bond between the carbonyl and water
Energy required to break the stronger london forces between carbonyl molecules (more e- in longer carbon chain carbonyls) is not compensated when a single H-bond forms between a carbonyl and water
The reactions of carbonyl compounds with Fehlings/Benedicts solution
Distinguishing aldehydes from ketones (tests)
Fehling’s solution is a blue which includes copper(II) ions dissolved in sodium hydroxide
Aldehydes – warmed with Fehling’s solution, the blue sol forms a brick-red precipitate of copper(I) oxide
The Cu²⁺ ions oxidise the aldehyde to a carboxylic acid. The aldehyde reduces the Cu²⁺ ions to Cu⁺ ions
*RCHO + [O] –> RCHOO
Cu²⁺ + e⁻ –> Cu⁺
RCHO + 2Cu²⁺ + 5OH⁻ –> Cu₂O (red ppt) + 3H₂O + RCOO⁻ (propanoate)*
Ketones - No reaction
The reactions of carbonyl compounds with Tollens’ reagent (‘silver mirror’)
Distinguishing aldehydes from ketones (tests)
Tollen’s reagent contains the complex ion silver diammine [Ag(NH₃)₂]⁺
Aldehydes – warmed with Tollen’s reagent, metallic silver is formed around the inside of the test tube (in reality a plack ppt will probably form)
Aldehyde oxidised to carboxylic acid and Ag⁺ ions are reduced to Ag(s)
Ag⁺(aq) + e⁻ –> Ag(s)
RCHO + 2Ag⁺(aq) + 3OH⁻ –> RCOO⁻ + 2Ag(s) + 2H₂O
Ketones – no reaction
The reactions of carbonyl compounds with Acidified dichromate(VI) ions (Potassium dichromate(VI) and sulfuric(VI) acid
Distinguishing aldehydes from ketones (tests)
K₂Cr₂O₇ is a strong oxidising agent, oxidising an aldehyde to a carboxylic acid
The orange Cr₂O₇²⁻ ions turn green as they are reduced to Cr³⁺ ions
Aldehydes - warmed with potassium dichromate(VI) orange —> green
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ —> 2Cr³⁺ + 7H₂O
RCHO + [O] + H₂O —> RCOOH + 2H⁺ + 2e⁻
Ketones - no reaction (sol stays orange)
The reaction of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in ether
Reduction of carbonyls and carboxylic acids
LiAlH₄ is a strong reducing agent, and it acts as a source of hydride (H⁻) ions, which are very reactive
The reagent must be used in a solution of dry ether (ethoxyethane), as it will hydrolyse in water
reduces:
* carboxylic acids and aldehydes to primary alcohols (cannot be stopped at aldehydes, very reactive)
* ketones to secondary alcohols
ex. ethanoic acid to ethanol
CH₃COOH + 4[H] —> CH₃CH₂H + H₂O
The reaction of carbonyl compounds with lithium aluminium hydride [MECHANISM]
Reduction of carbonyls and carboxylic acids
- AlH₄⁻ acts as a source of H⁻ ions (which act as a nucleophile)
- H⁻ ions attack the delta⁺C=O forming an alkoxide
- followed by warming with dilute HCl (hydrolysis) to protonate the O⁻ and release the alcohol
The H- ions from the LiAlH4 are strong reducing agents, however they cannot reduce a C=C to a C-C. Why not?
AlH₄⁻ acts as a source of H⁻ ions which act as nucleophiles, therefore they attract delta- carbon atoms. C=C is electron rich, so it would repel the H⁻, and no reaction would occur
The reaction of carbonyl compounds with HCN (hydrogen cyanide) in the presence of KCN as a nucleophilic addition
Hydrogen cyanide is a toxic gas (made by reacting KCN and dil. sulfuric acid). KCN must be in excess, so there are free cyanide ions (which are important for the mechanism)
conditions: room temp and pressure, small amount of alkali
general reaction:
aldehyde/ketone + HCN —> hydroxynitrile
In HCN the H and CN split up, the H joins the now single bonded C-O to form an alcohol (hydroxyl group) and the CN attaches to the carbon to form a nitrile group
The CN carbon atom is always numbered 1 in the chain
The reaction of carbonyl compounds with 2,4-dinitrophenylhydrazine (2,4-DNPH) as a test for the carbonyl group
Also known as Brady’s reagent
When 2,4-DNPH and a carbonyl compound are added together, a yellow/orange ppt should form
If it doesn’t form straight away, add dil. sulfuric acid and warm in a water bath
This reaction can be classed as a condensation reaction
Identifiying individual aldehydes and ketones from their DNPH derivatives (test for carbonyl)
- When carbonyl and 2,4-DNPH are added together an orange ppt forms
- filter off the ppt
- dissolve in the minimum quanitity of hot ethanol, filter, cool and allow to recrystalise
- filter off crystals, dry and measure melting point
- compare melting temperature with know 2,4-DNPH derivatives in a data book
The reaction of carbonyl compounds with iodine in the presence of alkali
identifies a CH₃C=O group
tests for a methyl group adjacent to an oxygen containing group
reagents: 3I₂ + 4NaOH (iodine and sodium hydroxide)
positive result: yellow ppt (triiodomethane - CHI₃)
- makes a carboxylic acid (or technically, it makes a carboxylic salt)
- way of reducing the carbon chain by one
- all 4 H atoms in the methyl group are substituted by iodine atoms
- CI₃–C bond is broken forming triiodomethane (iodoform) and the salt of an acid
Increasing the length of a carbon chain
1. Preparation of grignards reagents:
* Unstable so are made in situ or just before use
* Mg is added to a halogenoalkane dissolved in dry ether
* they have to be completely dry during preparation otherwise the grignard reagent reacts explosively with water:
CH₃CH₂MgI + H₂O —> CH₃CH₃ + Mg(OH)Br
2. Reaction of carbonyl compounds with grignard reagents:
* carbonyl compound reacts with grignards to form alcohols
Mechanism
step 1: addition reaction (anhydrous conditions)
step 2:hydrolysis in the presence of dilute acid
