CH17_Lecture - Chapter 17 Aldehydes and Ketones...

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Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O Summary Aldehydes (RCHO) and ketones (RCOR') are very important functional groups, characterized by the presence of an acyl (carbonyl) group (C=O) bonded either to H or another C. There are two contrasting, but equally important reactivity patterns for these compounds: 1. Aldehydes and ketones are attacked by nucleophiles due to the electrophilic character of the carbonyl C. 2. Treatment of aldehydes and ketones with a suitable base can lead to the formation of a nucleophilic species called an enolate that reacts with electrophiles. In this chapter, we will be looking at the reactions of aldehydes and ketones with nucleophiles: 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). The implications of these effects are: 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 Structure: The carbonyl group consists of an O atom bonded to a C atom via a double bond, C=O, via an sp 2 hybridization model similar to that of ethene, with bond angles close to 120 ° . O is connected to the carbonyl C via a s and a p 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.
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Reactivity: The O atom in the C=O is a region of high electron density due to the lone pairs. The carbonyl oxygen atoms are Lewis bases (site of protonation) There is low electron density on the C atom of the C=O group. Therefore the general mode of reaction of aldehydes and ketones is attack of a nucleophile at the electrophilic carbonyl C: IMPORTANT: The reactivity of aldehydes and ketones can be easily rationalized by considering the important resonance contributor which has a + δ C and - δ O. In general the reactivity order towards nucleophiles is as shown to the right: aldehydes > ketones The substituents have two contributing factors Size of the substituents attached to the C=O. Larger groups sterically hinder the approach of Nu. The electronic effect of the substituent; alkyl groups are weakly electron donating so they make the C in the carbonyl less electrophilic. In general the reactivity order towards nucleophiles is: aldehydes > ketones The substituents have two contributing factors on the reactivity at the carbonyl C: 1) Size of the substituents attached to the C=O. Larger groups will tend to sterically hinder the approach of the Nu.
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