Aldehydes and Ketones - Organic Chemistry - Lecture Notes | CHM 231, Study notes of Organic Chemistry

Download Aldehydes and Ketones - Organic Chemistry - Lecture Notes | CHM 231 and more Study notes Organic Chemistry in PDF only on Docsity! 13-1© 2005 John Wile & Sons, IncAll rights reserved Chapter 13 Aldehydes and Ketones 13-2© 2005 John Wile & Sons, IncAll rights reserved The Carbonyl Group • Functional group: carbonyl group, C=O • Aldehyde: carbonyl group bonded to a H atom • Ketone: carbonyl group bonded to two alkyl groups sp2 hybridized sp2 hybridized one  sigma and one  bond 1 2 13-5© 2005 John Wile & Sons, IncAll rights reserved Nomenclature • How to name compounds when several functional groups are present in a molecule? • If a molecule contains a carboxyl-group (-COOH) => it is an acid • All other functional groups are treated as substituents • If a molecule contains an aldehyde-group (-CHO), and no carboxyl-group => it is an aldehyde • All other functional groups are treated as substituents • Etc. 13-6© 2005 John Wile & Sons, IncAll rights reserved Physical Properties • Oxygen is more electronegative than carbon (3.5 vs 2.5) and, therefore, a C=O group is polar • aldehydes and ketones are polar compounds and interact by dipole-dipole interactions • they have higher boiling points and are more soluble in water than nonpolar compounds of comparable molecular weight C O C O – Polarity of a carbonyl group -+ C O + More important contributing structure : :: : : 13-7© 2005 John Wile & Sons, IncAll rights reserved Physical Properties 13-10© 2005 John Wile & Sons, IncAll rights reserved Addition of Alcohols: Hemiacetals • Nucleophile: an alcohol molecule • Hemiacetal (half-acetal): a molecule containing an -OH group and an -OR group bonded to the same carbon • Formed by the reaction of one molecule of an aldehyde or ketone with one molecule of an alcohol CH3CCH3 O H OCH2CH3 H+ CH3 C-OCH2 CH3 CH3 OH A hemiacetal + H: from aldehyde R: from ketone H(R) 13-11© 2005 John Wile & Sons, IncAll rights reserved Mechanism of Hemiacetal Formation • Acid or base catalyzed • Base catalyzed • Acid catalyzed +ROH -RO- RO- OR OR hemiacetal -H+ 13-12© 2005 John Wile & Sons, IncAll rights reserved Addition of Alcohols: Acetals • Acetal: a molecule containing an two -OR groups bonded to the same carbon • Formed by the reaction of a hemiacetal with one molecule of an alcohol H: from aldehyde R: from ketone H(R) CH3C-OCH2CH3 CH3 OH CH3CH2OH H + CH3 C-OCH2CH3 CH3 OCH2CH3 H2 O A diethyl acetal ++ A hemiacetal R 13-15© 2005 John Wile & Sons, IncAll rights reserved Acetals and Hemiacetals • Typically, hemiacetals are minor components of an equilibrium mixture between alcohol, hemiacetal and acetal • Exception: Hydroxyaldehydes or Ketones where an intramolecular hemiacetal with a 5- or 6- membered ring can form 4-Hydroxypentanal A cyclic hemiacetal (major form present at equilibrium) OH H O O H O Hredraw to show the OH close to the CHO group O OH 1 2 3 4 5 1 23 45 1 23 45 13-16© 2005 John Wile & Sons, IncAll rights reserved Acetals and Hemiacetals • Important example: most saccharides are polyhydroxyaldehydes and –ketones • Intramolecular hemiacetals -D-glucose and -D-glucose are anomers 13-17© 2005 John Wile & Sons, IncAll rights reserved Imines • Aldehydes and ketones react with ammonia or primary amines to immines (need acid catalyst) • Imine: a compound containing a C=N bond; also called a Schiff base • formed by the reaction of an aldehyde or ketone with ammonia or a 1° amine An imineAmmoniaCyclohexanone ++ NH3 H2OO NH H+ CH3CH O H2N CH3 CH=N H2 O+ + Ethanal Aniline An imine H + 13-20© 2005 John Wile & Sons, IncAll rights reserved Rhodopsin • reaction of vitamin A aldehyde (retinal) with an amino group on the protein opsin gives rhodopsin C=O H H2N-Opsin+ C= H N-Opsin 11 12 11-cis -Retinal Rhodopsin (Visual purple) 13-21© 2005 John Wile & Sons, IncAll rights reserved Keto-Enol Tautomerism • Tautomer: structural isomers that readily interconvert by a chemical rection • This reaction is called tautomerization • Keto-Enol tautomerization: Equilibrium between the keto and enol tautomer • Enol: a molecule containing an -OH group (-ol) on a carbon-carbon double bond of an alkene (-en) 13-22© 2005 John Wile & Sons, IncAll rights reserved Keto-Enol Tautomerism • The degree of Tautomerization depends on the system • the keto form typically predominates for most simple aldehydes and ketones • However, significant amounts of the enol tautomer exist if the enol form is stabilized What factors stabilize the enol form in these examples? 13-25© 2005 John Wile & Sons, IncAll rights reserved Oxidation • Aldehydes are easily oxidized to carboxylic acids • Using chromic acid • Using oxygen CHO H2 CrO4 COOH Hexanal Hexanoic acid 2 CHO O2 2 COOH Benzoic acidBenzaldehyde + 13-26© 2005 John Wile & Sons, IncAll rights reserved Oxidation • Using Ag+ ions • Using Ag(NH3)2+ – Tollens reagent R-C + 2Ag(NH3)2+ + 2OH- =O -H =O -O H R-C + 2Ag + 2NH3 + H2O • the Ag precipitate forms a smooth mirror-like deposit on the wall of the test tube -> silver mirror test 13-27© 2005 John Wile & Sons, IncAll rights reserved Reduction • aldehydes can be reduced to 1° alcohols • ketones can be reduced to 2° alcohols 13-30© 2005 John Wile & Sons, IncAll rights reserved NaBH4 Reductions • reductions with NaBH4 are most commonly carried out in aqueous methanol, in pure methanol, or in ethanol • one mole of NaBH4 reduces four moles of aldehyde or ketone • the key step in metal hydride reductions is transfer of a hydride ion to the C=O group to form a tetrahedral carbonyl addition compound Na + H-B-H H R-C-R' O H R-C-R' H O BH3 Na + H2O R-C-R' H O-H from water from the hydride reducing agent + 13-31© 2005 John Wile & Sons, IncAll rights reserved Aldehydes and Ketones End Chapter 13