Lecture 1: Introduction to Aldehydes and Ketones in Organic Chemistry by Professor Krische, Study notes of Chemistry

Download Lecture 1: Introduction to Aldehydes and Ketones in Organic Chemistry by Professor Krische and more Study notes Chemistry in PDF only on Docsity! 318N – Krische, Lecture 1: Tu - 01/20/09, Course Introduction & Aldehydes and Ketones • Today, we go over the course syllabus, with attention to exam dates, course policies and other administrative items. Please note that 1H, 13C NMR and IR spectroscopy were covered in Professor Krische’s section of 318M last semester. If you were not in Professor Krische’s section of 318M last semester or have not been exposed to these topics you should reconsider enrollment in this course. • We now begin the chapter(s) covering aldehydes and ketones. • Structure begets reactivity: C=O double bond is polarized such that the oxygen bears a partial negative charge and the carbon bears a partial positive charge. This naturally follows from the electronegativities of carbon and oxygen (the latter being more electronegative). The polarity/dipole of the C=O double bond is reflected by a zwitterionic resonance structure. • In accordance with the polarization of the C=O double bond, electrophiles react at oxygen and nucleophiles react at carbon. From the standpoint of mechanism, it is important to recognize that in reactions with electrophiles (Brønsted acids or Lewis acids), oxygen reacts using its lone pairs (not the C=O pi-bond). • Based on our understanding of the structure of the carbonyl group, we began to explore its reactivity with oxygen nucleophiles. Specifically, acid catalyzed hydration was discussed and a comparison was made with the reactivity of alkenes under similar conditions. It was shown that protonation activates the carbonyl group as an electrophile, rendering it highly susceptible to nucleophilic addition. This activation is manifested by an increase in the C-O bond length of the carbonyl group upon protonation, i.e. 1.20 angstroms vs. 1.27 angstroms. This is substantial considering a normal C-O single bond has a length of 1.43 angstroms. This lengthening reflects the contribution of the non-octet resonance structure of the protonated carbonyl. • We observed that formaldehyde hydrates more readily than acetaldehyde, and that acetaldehyde hydrates more readily than acetone. These effects were rationalized on the basis of steric and