carbonyls Flashcards
(22 cards)
What are the 7 functional groups that contain carbonyls? draw them
Aldehydes, ketones, carbxylic acids, acyl halides, anhydrides, esters, amides
Describe the C=O bond.
Each C atom has three sp2 orbitals and one p orbital. The lone pairs on the O atom occupy two sp2 orbitals. The sp2-hybridised orbitals on adjacent C and O atoms interact to form a sigma-type molecular orbital. 2p orbitals on adjacent C and O atoms interact to form a pi-type molecular orbital.
Are aldehydes and ketones soluble in water?
Are ketones good solvents? what can they dissolve?
Aldehydes and ketones are hydrogen bond acceptors; this makes them considerably soluble in water. Ketones such as acetone are good solvents because they dissolve both aqueous and organic compounds. Aldehydes and ketones are good electrophiles.
What makes the C=O bond important?
The C is an electrophile and the O is basic. Therefore, the C can react with nucleophiles while the O usually interacts with a Lewis acid.
How can ketones be formed?
what agents
Ketones are easily obtained from secondary alcohols using KMnO4, Na2Cr2O7 or CrO3+ H2O under acidic conditions.
For primary alcohols, it is often difficult to stop at the aldehyde and further oxidation to give the caboxylic acid can occur and oxidising agents milder than KMnO4 and CrO3/H+ are needed.
secondary alc + ox agent + h2o (acidic) = ketone
What is the angle of nucleophilic attack on a carbonyl?
- A Nu always approaches the C=O group of a ketone or aldehyde from a particular angle, not from a direction perpendicular to the plane of the C=O group but at about 107° to the C=O bond, i.e. close to the angle at which the new bond will form. This route is known as the Bürgi–Dunitz trajectory. Any other portions of the molecule that cause steric hindrance to the Burgi–Dunitz trajectory will greatly reduce the rate of addition. This is another reason why aldehydes are more reactive than ketones.
Describe the addition of H2O to aldehydes and ketones.
EDGs & EWGs
Addition of H2O to an aldehyde or ketone gives a product called a hydrate or a gem-diol. The equilibrium conversion to the hydrate varies widely and depends on the electronic properties (EWG, electron withrawing, or EDG, electron donating) and steric hindrance of the various substituents. EWGs on the carbonyl C make addition more favourable (larger K) > the formation of the gem-diol derivative is favoured. Whereas EDGs on the carbonyl C make addition less favourable (smaller K) so the formation of the gem-diol derivative is disfavoured. Larger size of groups have the same effect.
What are the factors influencing equilibria in carbonyl addition reactions?
reactant stability (alkyl group) and product stability (sterics)
Reactant Stability:
* Recall that alkyl groups stabilize C=C > This works for C=O too; meaning that ketones are more stable than aldehydes. Therefore, the addition to ketones is less favoured than addition to aldehydes.
Product Stability:
* The four groups in the product are closer together than the three groups attached to the carbonyl carbon in the reactant;
* Alkyl groups cause more steric destabilisation in the tetrahedral addition product than does hydrogen, thus the ketone addition product is less favoured than that from aldehyde addition.
Aldehyde - reactant less stable, product more stable
Ketone - reactant more stable, product less stable
Describe the three step reaction of hydration under acidic conditions.
Protonation of the C=O. Addition to the C=OH+. Deprotonation.
Describe the two step reaction of hydration under basic conditions.
Addition of OH- to the C=O. Protonation of the C-O-.
Describe the key points of the synthesis of acetals and ketals. And describe the hydration of acetals and ketals.
Acetals and ketals are synthesised by the addition of alcohol and the removal of H2O. Hemiacetals are formed as the intermediate.
mechanism PADPEAD
Protonation of carbonyl -> Addition by nuc attack -> Deprotonation giving hemiacetal -> Protonation of the alcohol -> Elimination of water -> Addition by nuc attack of alc -> Deprotonation
- Back reaction is obtained in the presence of an excess of H2O and acid catalysts.
Describe the synthesis of imines.
what catalyst
Imine formation is a chemical reaction where an aldehyde or ketone reacts with a primary amine to create an imine. H+ (cat): TsOH or H2SO4
Describe the synthesis of carboxylic acids via addition reaction to CO2.
metal
CO2 + RLi / RMgX = carboxylate salt which is protonated with acid gives a carboxylic acid (with one more carbon atom than the starting organometallic). The reaction is usually done by adding solid CO2 to a solution of the given RM in THF or Et2O.
Describe the addition of metal hydrides to aldehydes and ketones.
Nuc attack by H- on carbonyl C forms an intermediate which is then protonated to form an alcohol.
H- is present in NaH, has a high charge density that it reacts as a base, but when attached to B or Al such as NaBH4 or LiAlH4, nucleophilic H are present. Since all of the electrons are in B–H σ orbitals, draw curly arrows from this bond.
> key points
Aldehydes are more reactive. NaBH4 is a mild hydride donor, a more powerful one is LiAlH4. Its mechanism is the same. LiAlH4 is less selective and it cannot be used with protic solvents.
Describe the synthesis of amines via reductive amination.
Use of Na(CN)BH3. Na(CN)BH3 is a mild hydride donor. Reaction is carried out at pH 6. Primary amines (when R’ = R” = H), secondary amines (any R’ and R” but H) and tertiary amines can be prepared.
primary = NH3 + Na(CN)BH3
secondary = NH2R’ +Na(CN)BH3
tertiary = NHR’’
What are the products from metal hydrides reacting with aldehydes, esters, ketones, imines/iminiums
Metal hydrides give:
Primary alcohols from aldehydes
Secondary alcohols from ketones;
Primary alcohols or aldehydes from esters.
Primary, secondary or tertiary amines from imines/iminiums.
Nature uses the cofactor NAD(P)H to perform redox chemistry equivalent to
metal hydrides in metabolism
What does the degree of delocalisation depend on?
The degree of delocalization depends on the electron-donating power of the substituent and increases along the series of compounds below from almost no
delocalization from the Cl atom to complete delocalization in the carboxylate anion, where the negative charge is equally shared between the two oxygen atoms.
Describe the key points of the formation of esters (Fischer’s reaction).
what catalyst
Acid Cat.: H2SO4 or HCl. No tertiary alcohols. Removal of H2O.
Mechanism
protonation -> addition -> deprotonation -> protonationn -> elimination -> deprotonation
Describe the mechanism and reactants of the formation of esters from acyl chlorides.
Acyl chloride + alcohol
Anhydrides give the same product following a similar mechanism.
mechanism
Addition -> deprotonation -> elimination
Describe the key points of the formation of amides from acyl chlorides.
ACID/BASE reaction: carboxylic acids react with amines to an ammonium salt because the amines themselves are basic. Anhydrides give the same product following a similar mechanism.
Mechanism
Addition -> deprotonation -> elimination
How are enols formed?
The conversion of a carbonyl compound into its enol is an isomerisation reaction called enolisation. This involves the intramolecular transfer of a proton, and nothing else. This belong to is called a tautomerism
What is the mechanism of the acid-catalysed aldol additions?
what conditions
protonation of the aldehyde -> deprotonation to form the enol -> addition of the enol to a second protonated aldehyde molecule. Under acidic conditions, the aldol addition product is not stable; it undergoes acid-catalysed dehydration to form the aldol condensation product.
