What is the general structure of an aldehyde ?
What is the general structure of a ketone ?
What is the functional group of aldehydes and ketones ?
The carbonyl group C=O
Describe the Carbonyl functional group, including bond angles
Planar, with bond angles of 120º
The Carbonyl Carbon atom has 3 σ bonds.
The C=O bond is shorter and stronger than the C-O.
What is an Aryl group ?
Benzene minus 1 hydrogen e.g. C6H5 Phenyl
Name five important features of the Carbonyl group
- Acidity of the alpha hydrogen
- Keto-enol tautomerisation
- Hydrogen bonding
Sketch the two resonance structures of a ketone
Why is polarity important in the Carbonyl group ?
It may participate in nucelophilic or electrophilic reactions. Nucleophiles attack the partially positive carbon, and electrophiles attack the partially negative oxygen.
In both of these reactions, the character of the double bond is altered.
Where is the alpha carbon located in a Carbonyl compound ?
The alpha carbon is always adjacent to the functional group.
What is the relationship between resonance structures and stability ?
If you can draw resonance structures for a compound, then it displays increased stability.
Explain why acidity of the alpha hydrgen occurs in a carbonyl compound ?
The acidity is a result of the resonance stabilisation of the alpha carbanion formed.
Are the beta and gamma hydrogens also acidic in a carbonyl compound ?
No, only the alpha hydrogen is acidic
What happens to acidity if the alpha hydrogen is situated between 2 Carbonyl groups ?
It will display increased acidity
What are tautomers ?
Constitutional isomers that readily interconvert (= tautomerization).
This happens so quickly that they are usually considered to be the same compound.
What is an enol ?
Alkene & Alcohol
What are conjugated double bonds ?
Double bonds separated by a single bond
Sketch the equilibrium present in tautomerization
Describe tautomerization ?
Fast interconnection between constitutional isomers
e.g. Carbonyl and Enol
How do Carbonyl groups participate in hydrogen bonding ?
Why don't aldehydes and ketones form hydrogen bonds with themselves ?
There is no hydrogen on the carbonly oxygen
What influences reactivity of the Carbonyl compounds ?
The carbonyl group is the functional group, but adjacent groups can influence reactivity, e.g. an e- withdrawing group adjacent to the carbonyl carbon will increase the partially positive charge on the carbon making the carbonyl group more attractive to a nucleophile.
An e- donating ligand would decrease the reactivity of the carbonyl group.
How does steric hindrance influence the reactivity of the carbonyl compounds ?
Generally, aldehydes oxidise and more easily undergo nucleophilic addition than ketones.
Explain oxidation of Carbonyl compounds ?
Aldehydes oxidise to Carboxylic acids (using agents such as KMnO4, CrO3 (Jones reagent), HNO3, Ag2O (Tollens)
Describe a diagnostic tool for the presence of an aldehyde group ?
Using Tollen's reagent (Ag2O) metallic silver Ag is produced if an aldehyde is present.
Ketones cannot be oxidised to Carboxylic acids.
Describe 7 reactions that result in the formation of aldehydes or ketones ?
- 1º alcohols → Aldehydes (Oxidation)
- 2º alcohols → Ketones (Oxidation)
- Friedel-Crafts Acylation
- Hydration of terminal alkynes
Explain the difference in boiling points between alcohols and ketones ?
Alcohols exhibit hydrogen bonding, ketones exhibit dipole-dipole bonding (weaker intermolecular forces) therefore ketones have a lower boiling point.
Why do alkanes have a lower boiling point than ketones or alcohols ?
- Alkanes - Intermolecular (london) forces
- Ketones - Dipole dipole forces
- Alcohols - hydrogen bonding
London forces < dipole dipole < hydrogen bonding
(In terms of relative bond strength)
What is the impact of increasing alkyl groups on reactivity of the carbonyl group ?
The more alkyl groups present (and therefore, the longer the carbon chain), the more stable the carbonyl group.
There is more polarisation in an aldehyde than a ketone, therefore a ketone is more stable.
What are the two classes of reactions that carbonyl compounds undergo ?
- Nucleophilic addition reactions at the C=O bond
- Reactions at adjacent positions
Describe nucleophilic addition reactions in Carbonyl compounds ?