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Topic 10a

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Western University
Chemistry 2223B
Felix Lee

Chemistry 2213a  Fall 2012  Western University Topic 10a. Carboxylic Acids A. Structure and Nomenclature O  The carboxyl group results from the RCOOH or RCO H 2 connection of a hydroxyl group to aR OH carbonyl group.  The functional group retains some properties of a carbonyl group, some of a hydroxyl group, plus some properties unique to the carboxyl group.  In IUPAC nomenclature, the O O carboxyl group is always given Br priority, with its carbon assigned OH OH as atom #1. The e in the word alkane is changed to oic acid.propanoic acid 4-bromo-2-butenoic acid  For complicated compounds, we can use the ending carboxylic acid. COOH If we use this system, the COOH carboxylic acid name does not count as part of the parent hydrocarbon. trans-1,2-cyclopentanedicarboxylic acid Carboxylic Acids  2  The simplest acids are known by their common names. HCOOH formic acid CH COOH acetic acid COOH 3 HOOC–COOH oxalic acid benzoic acid O O  If a molecule contains a carbonyl in addition to the carboxyl group, the carbonyl oxygen is denoted by oxo. HO 3-oxobutanoic acid  Like carbonyl compounds, Greek letters are often used to indicate substituent positions. O O R an -amino acid HO HO NH 2 Carboxylic Acids  3 B. Properties  As seen in Chapter 2, carboxylic acids are acidic. Acids generally taste sour. Vinegar is 5% acetic acid by volume.  They are very polar compounds that can have extensive H-bonding and have bp higher than similarly sized alcohols. CH 3H C2 CH2OH2 bp 118° CH 3H C2OH bp 141°  In solution, the carboxyl groups of O - O H O R - + adjacent molecules hydrogen bond   with each other to form dimers. R + O H R O H O  Carboxylic acids have polar bonds located at one end of the molecule, and it is this polar end, termed the hydrophillic end, that interacts with water. The other end of the molecule, represented by the R group, is non-polar (termed the hydrophobic end) and repels water.  When the R group is small (< 6 carbons), the carboxylic acid is water-soluble. As the length of the C chain increases, solubility in water goes down. Carboxylic Acids  4 C. Spectroscopy  The IR spectrum of an acid has two characteristic, strong peaks: the OH at −1 −1 2400–3400 cm and the C=O at 1700–1725 cm .  In H NMR, the acidic H of the COOH appears far downfield, between 10–13 ppm. Often this is off of the scale for the plot, so it is “offset” by an amount. Carboxylic Acids  5 D. Reactions 1. Reduction to 1° alcohols  The COOH group is quite resistant to 1. LiAlH / ether reduction. However, it can be reduced 4 OH CH to a 1° OH using Li4lH followed bR OH 2. Water R 2 treatment with2H O.  Note that NaB4 is not strong enough of a reducing agent, and that catalytic hydrogenation does not reduce carboxyl groups. Therefore, selective reduction can be done by carefully choosing the reducing agent. O H2/ Pt O OH OH O OH 1. LiAlH / ether NaBH 4 4 2. 2 O O OH OH OH OH Carboxylic Acids  6 2. Decarboxylation of -keto acids  Carboxylic acids with a -keto group decarboxylate when heated, to eliminate CO2. (i.e. break C–C bond) O O O  CO2 R OH R CH3  The reaction proceeds through a cyclic conformation, which is ideally arranged for the facile elimination of CO . The -keto group accepts the electrons from 2 the broken bond.
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