Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

Aldehydes and Ketones

Nomenclature:
Aldehydes: functional group suffix = -al (review)
Ketones: functional group suffix = -one (review)
Functional group prefix = oxo-
(Note that an aldehyde is higher priority than a ketone)
 

Physical Properties:

• The polar nature of the C=O (due to the electronegativity difference of  the atoms) means dipole-dipole interactions will occur.
• Though C=O can not hydrogen-bond to each other, the C=O can accept hydrogen bonds from hydrogen bond donors (e.g. water, alcohols).

• higher melting and boiling points compared to analogous alkanes
• lower boiling points than analogous alcohols
• more soluble than alkanes but less soluble than alcohols in aqueous media

• The carbonyl group consists of an O atom bonded to a C atom via a double bond, C=O, via an sp² hybridisation model similar to that of ethene, H₂C=CH₂ (review ?) with bond angles close to 120^o.
• O is connected to the carbonyl C via a σ and a π bond.
• The C=O and the two other atoms attached to the C are co-planar.
• This implies that the two lone pairs on O are in sp² hybrid orbitals.

Reactivity:

The image shows the electrostatic potential for methanal (formaldehyde)  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  The O atom in the C=O is a region of high electron density (red) due to the lone pairs. The carbonyl oxygen atoms are Lewis bases (site of protonation) There is low electron density (blue) on the C atom of the C=O group.

The image shows the electrostatic potential for propanone (acetone).  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  The O atom in the C=O is a region of high electron density (red) due to the lone pairs. The carbonyl oxygen atoms are Lewis bases (site of protonation) There is low electron density (blue) on the C atom of the C=O group

IMPORTANT : The reactivity of aldehydes and ketones can be easily rationalised by considering the important resonance contributor which has a +ve C and -ve O.  Note that this matches the ideas shown in the electrostatic potential diagrams shown above.

In general the reactivity order towards nucleophiles is as shown to the right : aldehydes > ketones  The substituents have two contributing factors   Size of the substitutents attached to the C=O. Larger groups sterically hinder the approach of the Nu. The electronic effect of the substituent. Alkyl groups are weakly electron donating so they make the C in the carbonyl less electrophilic.

You can look at the accessiblity effect in the following series of aldehydes and ketones:
 

© Dr. Ian Hunt, Department of Chemistry, University of Calgary