Ketones and Aldehydes Flashcards
3 different type of reactions for carbonyl group reactions:
- If product stable, reaction stops here: Nucleophilic Addition to C=O NaBH4/LiALH4, Grignard reaction
- If R1 good leaving group: Nucleophilic Substitution to C=O
- If Nuc has lone pair- loses OH: Nucleophilic Substitution + Replacement of O
Ketone:
two alkyl (or aryl) groups bonded to the carbonyl carbon atom
Aldehyde:
one alkyl (or aryl) group and one H bonded to the carbonyl carbon atom
Positively polarised carbon acts as an ______ (Lewis acid)
electrophile
Negatively polarised oxygen acts as an _______ (Lewis base)
nucleophile
Synthesis of Ketones and Aldehydes
step 1-4 don’t need to remember
what are steps 5-10?
- Synthesis of ketones using organolithium reagents with carboxylic acids
- Synthesis of ketones using organocuprates with acid chlorides
- Synthesis of ketones from nitriles
- Aldehyde synthesis by reduction of nitriles
- Aldehyde synthesis by reduction of acid chlorides
- Aldehyde synthesis by reduction of esters
Synthesis of Ketones and Aldehydes
step 1-4 don’t need to remember
what are steps 5-10?
- Synthesis of ketones using organolithium reagents with carboxylic acids
- Synthesis of ketones using organocuprates with acid chlorides
- Synthesis of ketones from nitriles
- Aldehyde synthesis by reduction of nitriles
- Aldehyde synthesis by reduction of acid chlorides
- Aldehyde synthesis by reduction of esters
Most common reaction is nucleophilic addition;
Addition of a nucleophile and a proton (H) across the C=O
Reactions of Ketones and Aldehydes - nucleophilic addition
Step 1: A strong nucleophile adds to the carbonyl group to form an alkoxide
Step 2: A weak acid protonates the alkoxide to give the addition product
Reverse reaction
Deprotonation, followed by loss of the nucleophile
Acid Conditions with weak nucleophile and activated carbonyl
Step 1: Protonation activates the carbonyl group toward nucleophilic attack
Step 2: A weak nucleophile adds to the activated (protonated) carbonyl group
Reverse reaction: Loss of the weak nucleophile, followed by deprotonation
Aqueous solution
ketone/aldehyde is in equilibrium with its hydrate (geminal diol)
hydration of k and a
IN ACID
weak nucleophile
hydration of k and a
IN BASE
strong nucleophile
Ketones favours the ____ ______ keto form
non hydrated
Aldehydes more likely to form _______ hydrates (Formaldehyde is the most reactive)
stable
reactions of k and e
- Addition of organometallic reagents (Grignard, organolithium- see alcohols)
- Reduction (see alcohols) - Reduces ketones (2˚alcohols) and aldehydes (1˚ alcoholes)
3/4 not remember
- Condensations with hydroxylamine and hydrazine
6.Formation of acetals (addition of alcohol)
in the formation of imines;
- acid-catalysed addition of the amine to the carbonyl group
- acid-catalysed dehydration
Substituted imine is also called a
Schiff base
Condensation Reaction
Enols and Enolates
Acidity of Hydrogens bonded to Carbon
A hydrogen alpha to a C=O is acidic because :
1) Inductive effect by electron-withdrawal results in C-H bonds being weakened
2) Delocalisation of the resulting negative charge stabilises the enolate anion and favours its formation
Tautomerism
isomerisation occurring by the migration of a proton and the movement of a double bond. The isomers that interconvert are called tautomers
Reactions under base-conditions occur via the
Enolate
Reactions under acidic-conditions occur via the
Enol
Very small amounts of the enol form is present
Carbon-Carbon Bond-Forming reaction of
Enolates ( Grignard and Wittig)
Aldol Reaction (condensation)
Carbonyl compound used as electrophile and the enolate derived as nucleophile
POOR or GOOD Aldol Reaction (condensation)?
very poor:
* Ketones are less reactive than aldehydes as electrophiles* The product from ketone self-condensation is more sterically
compressed\equilibrium favours the starting material
