Student%20Notes%20-%20Section%2015

Student%20Notes%20-%20Section%2015 - Section 15: Aldehydes...

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Section 15: Aldehydes and Ketones 217 Section 15: Aldehydes and Ketones 15.1 Nomenclature and Preparation 15.1.1 Nomenclature Aldehydes and ketones are part of a larger group of organic molecules known as carbonyl compounds – they contain a carbonyl (C=O) group. RH O aldehyde, RCHO RR ' O ketone, RCOR' Simple aldehydes are named by replacing the – ane suffix with – al . The parent chain must contain the aldehyde functional group, and the carbonyl group is numbered 1. There are a number of aldehydes with trivial names. O H H formaldehyde (methanal) O H H 3 C acetaldehyde (ethanal) O H propionaldehyde (propanal) O H butyraldehyde (butanal) O H valeraldehyde (pentanal) O H acrolein (propenal) H O benzaldehyde (benzenecarbaldehyde) Simple ketones are named by replacing the –ane suffix of the parent hydrocarbon with – one. The parent chain is the longest one containing the carbonyl group, and is numbered from the end closest to the carbonyl group.
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Section 15: Aldehydes and Ketones 218 In cases where the group (R-CO) is a substituent, the side chain is named as an acyl group. Acyl groups are named as an alkyl chain, with the – yl suffix replaced by – oyl . Once again, a number of ketones and acyl groups are identified by trivial names. H 3 CC H 3 O acetone O CH 3 acetophenone O benzophenone H O formyl group H 3 C O acetyl group O benzoyl group Where other groups are present, and the carbonyl group is part of the parent chain, the oxo - prefix is used, with a number to indicate its position on the parent chain. 15.1.2 Preparation of Aldehydes Aldehydes may be prepared by oxidation of primary alcohols. Alkenes with at least one vinylic hydrogen may be oxidatively cleaved to yield aldehydes by ozonolysis. Similarly substituted cyclic alkenes may yield dialdehydes or oxo-aldehydes.
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Section 15: Aldehydes and Ketones 219 Hydroboration of terminal alkynes will generate aldehydes. 1. BH 3 /THF 2. H 2 O 2 /HO - /H 2 O O Carboxylic esters may be reduced to aldehydes. Again, the choice of reducing agent is critical: sodium borohydride will not react with esters, and lithium aluminum hydride will react to form alcohols. We require a reducing agent with intermediate reactivity: one such agent is diisobutylaluminum hydride (DIBAH). Reaction is generally also carried out at low temperatures. 15.1.3 Preparation of Ketones Ketones may be generated by oxidation of secondary alcohols. Ozonolysis of alkenes where one end of the alkene is disubstituted will generate ketones. Similarly substituted cycloalkenes may generate diketones.
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Section 15: Aldehydes and Ketones 220 Aryl ketones are generated by Friedel-Crafts acylation with an acid chloride and AlCl 3 catalyst. Recall that there are a number of limitations to this reaction, depending on the presence of other substituents on the aromatic ring.
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Student%20Notes%20-%20Section%2015 - Section 15: Aldehydes...

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