Ch18Su08

Ch18Su08 - Ketones and Aldehydes The carbonyl group is of...

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Ketones and Aldehydes The carbonyl group is of central importance in organic chemistry because of its ubiquity. Without studying the carbonyl group in depth we have already encountered numerous examples of this functional group (ketones, aldehydes, carboxylic acids, acid chlorides, etc). The simplest carbonyl compounds are aldehydes and ketone. A ketone has two alkyl (or aryl) groups bonded to the carbonyl carbon. An aldehyde has one alkyl (or aryl) group and one hydrogen bonded to the carbonyl carbon. Structure of the carbonyl group The carbonyl carbon is sp 2 hybridized, and has a partially filled unhybridized p orbital perpendicular to the σ framework. The oxygen is also sp 2 hybridized, with the 2 lone pairs occupying sp 2 orbitals. This leaves one electron in a p orbital. These p orbitals from the carbon oxygen π bond. The C=O double bond is like a C=C double bond except the carbonyl double bond is shorter and stronger . 975a7924d70b92811c9232c6f3d5e7502606901f.doc Page 1 R C H O R C R O aldehyde ketone
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The carbonyl group has a large dipole moment due to the polarity of the double bond. Oxygen is more electronegative than carbon, and so the bond is polarized toward the oxygen. The attraction of the weakly held π electrons toward oxygen can be represented by the two following resonance structures. The first resonance structure is the major contributor, but the other contributes in a small amount, which helps explain the dipole moment. It is this polarization that creates the reactivity of the carbonyl groups (carbon is electrophilic/LA, and the oxygen is nucleophilic/LB). Nomenclature IUPAC nomenclature requires ketones to be named by replacing the -e ending of the alkyl name with -one. Alkane alkanone E.g. 975a7924d70b92811c9232c6f3d5e7502606901f.doc Page 2 R C R O R C R O + - H 3 C C CH 2 -CH 2 -CH 3 O C CH 2 -CH 3 O O CH 3 2-butanone 1-phenyl-1-propanone 3-methylcyclopentanone
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Systematic names for aldehydes are obtained by replacing -e with -al. An aldehyde has to be at the end of a chain, and therefore it is carbon number 1. If the aldehyde is attached to a large unit, the suffix -carbaldehyde is used. A ketone or aldehyde group can also be named as a substituent on a molecule with another functional group as its root. The ketone carbonyl is given the prefix oxo-, and the aldehyde group is named as a formyl- group. (Especially common for carboxylic acids). Common Names The wide spread use of carbonyl compounds means many common names are entrenched in their everyday use. E.g. 975a7924d70b92811c9232c6f3d5e7502606901f.doc Page 3 CH 3 C H O H 3 C CH 2 CH CH CHO ethanal pent-2-enal CHO cyclohexanecarbaldehyde CH 3 CH 2 C CH 2 -CHO O 3-oxopentanal C CO 2 H O H H 3 C C O CH 2 -CO 2 H 2-formylbenzoic acid 3-oxobutanoic acid H 3 C C CH 3 O acetone C O CH 3 acetophenone C O benzophenone
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Syntheses of the Aldehydes and Ketones (Recap?) From Alcohols (Ch 11) Secondary alcohols are readily oxidized to ketones by Chromic acid (or KmnO 4 ). Complicated ketones can be made by the oxidation of alcohols, which in
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This note was uploaded on 02/23/2010 for the course CHEM 202 taught by Professor Smith during the Spring '10 term at Duke.

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Ch18Su08 - Ketones and Aldehydes The carbonyl group is of...

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