MCAT Organic Chemistry > Aldehydes and Ketones
An aldehyde has one alkyl (or aryl) group and one hydrogen bonded to a carbonyl group.
A ketone has two alkyl (or aryl) groups bonded to a carbonyl group.
Aldehyde and Ketone Nomenclature
Aldehydes are named with the suffix –al or -aldehyde.
You should know the common aldehydes formaldehyde and acetaldehyde by structural formula and common name
When aldehydes are attached to a ring, the suffix –carbaldehyde is used in the IUPAC name.
If the aldehyde does not hold priority in the molecule, it is named as a substituent with the prefix formyl-.
Ketones are named with the suffix –one or ketone.
When it is necessary to name the ketone as a substituent, the prefix oxo- or keto- is used.
A common ketone is acetone which is a solvent used in many lab procedures as a final drying or purifying agent.
Aldehydes and ketones are characterized by the carbonyl group (C=O) which is a carbon double bonded to an oxygen. This bond is polar which gives the carbon a partial positive charge and the oxygen a partial negative charge. Understanding this electrostatic principle is key to mastering this concept.
The dipole moment associated with the carbonyl group causes an elevation of boiling points associated with their alkane counterparts in solution. The boiling points of aldehydes and ketones are less than carboxylic acids however. The elevation of boiling point is not as high as alcohols because there is no hydrogen binding.
Aldehydes are more reactive towards nucleophiles than ketones.
The infrared spectroscopy of the C=O bond of aldehydes and ketones usually falls around 1700 cm-1
Effect of Substituents on C=O Double Bond
When substituents are added to either side of a carbonyl group, reactivity will be reduced because reagents will have a more difficult time getting to the carbonyl group. This process is called steric hinderance. More and larger substituents near the carbonyl group will decrease reactivity further.
Carbanions and Acidity of Alpha Hydrogens
The alpha hydrogens are acidic because the resulting carbanion is stabilized by resonance.
Alpha beta unsaturated carbonyl compounds, their resonance structures.
α,β-unsaturated carbonyls reacted with a nucleophile will result in the addition of the nucleophile at the β position. A nucleophile attacks the beta hydrogen, pushing the α,β-unsaturated carbonyl into the enol form, which tautomerizes to the original carbonyl.