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Organic Chemistry II Carboxylic Acids and Nitriles Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University Spring, 2011 1 Chapter Objectives • Nomenclature • Properties • Preparation • Reactions • Spectroscopy Spring, 2011 2 The Importance of Carboxylic Acids (RCO2H) • Abundant in nature from oxidation of aldehydes and alcohols in metabolism – Acetic acid (CH3CO2H) - vinegar – Butanoic acid (CH3CH2CH2CO2H) – smell of rancid butter – Caproic acid (CH3CH2CH2CH2CH2CO2H) - smell of sweaty gym socks – Long-chain aliphatic acids from the breakdown of fats • Starting materials for acyl derivatives (esters, amides, and acid chlorides) Spring, 2011 3 Nomenclature • Carboxylic Acids, RCO2H • If derived from open-chain alkanes, replace the • terminal -e of the alkane name with -oic acid The carboxyl carbon atom is C1 Spring, 2011 4 Nomenclature • Compounds with CO2H bonded to a ring are named using the suffix -carboxylic acid • The CO2H carbon is not itself numbered in this system Spring, 2011 5 Common Names • Many common names in use O H O O OH Formic H3C OH Acetic Butyric OH Propionic O O CH3CH2CH2 CH3CH2 OH CH3CH2CH2CH2 O OH CH CH CH CH CH 3 2 2 2 2 OH Caproic Valeric O CO2H CO2H OH Acrylic Crotonic Pivalic Spring, 2011 6 Common Names CO2H CO2H Benzoic CO2H CH3 Toluic HO2C CO2H CO2H Oxalic CO2H OH Salicylic CO2H CO2H Malonic Phthalic CO2H CO2H Succinic Spring, 2011 7 Nitriles • • • • • Closely related to carboxylic acids Named by adding -nitrile as a suffix to the alkane name, Nitrile carbon is numbered 1 Complex nitriles are named as derivatives of carboxylic acids. Replace -ic acid or -oic acid ending with -onitrile Spring, 2011 8 Common Names CN CH3CN Acetonitrile Spring, 2011 Benzonitrile 9 Structural and Physical Properties • Carboxyl carbon sp2 hybridized • Carboxylic acid groups are planar with C–C=O and O=C–O bond angles of approximately 120° • Carboxylic acids form hydrogen bonds, existing as cyclic dimers held together by two hydrogen bonds • Strong hydrogen bonding causes much higher boiling points than the corresponding alcohols • Acetic acid (bp = 117.9oC) •Spring, Ethanol (bp = 78.3oC) 2011 10 Hydrogen Bonding O H O R R O H O hydrogen bond Spring, 2011 11 Dissociation of Carboxylic Acids • Carboxylic acids are proton donors toward weak • • and strong bases Producing metal carboxylate salts, RCO2 +M Carboxylic acids with more than six carbons are only slightly soluble in water, but their conjugate base salts are water-soluble Spring, 2011 12 Acidity Constant and pKa • Carboxylic acids transfer a proton to water to • • • give H3O+ and the carboxylate anion, RCO2 H3O+ is a much stronger acid The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid pKa ~ 5 Spring, 2011 13 Acidity of Carboxylic Acids • Carboxylic acids – stronger acids than ROH • The pKa of ethanol is ~16, compared to ~5 for acetic acid • In an alkoxide ion, the negative charge is localized on oxygen while in a carboxylate ion the negative charge is delocalized over two equivalent oxygen atoms, giving resonance stabilization Spring, 2011 14 Resonance Spring, 2011 15 Henderson-Hasselbalch Equation • Covered in General Chemistry • Be certain you know Section 15.3 • You must know how to use the equation Spring, 2011 16 Substituent Effects on Acidity • Electron withdrawing groups increase the acidity • Promote formation of the carboxylate ion • Electron donating groups decrease the acidity Spring, 2011 17 Preparation of Carboxylic Acids • Oxidation of a substituted alkylbenzene with • KMnO4 or Na2Cr2O7 gives a substituted benzoic acid 1° and 2° alkyl groups can be oxidized, but tertiary groups do not react Spring, 2011 18 From Alkenes • Oxidative cleavage of an alkene with KMnO4 gives a carboxylic acid if the alkene has at least one vinylic hydrogen Spring, 2011 19 From Alcohols • Oxidation of a primary alcohol or an aldehyde with CrO3 in aqueous acid Spring, 2011 20 Hydrolysis of Nitriles • Hot acid or base yields carboxylic acids • Conversion of an alkyl halide to a nitrile (with • • cyanide ion) followed by hydrolysis produces a carboxylic acid with one more carbon RBr RCN RCO2H Best with primary halides because elimination reactions occur with secondary or tertiary alkyl halides Spring, 2011 21 Carboxylation of Grignard Reagents • Grignard reagents react with dry CO2 to yield a • carboxylic acid after acid hydrolysis Limited to alkyl halides that can form Grignard reagents Spring, 2011 22 Reactions of Carboxylic Acids Spring, 2011 23 Reactions of Carboxylic Acids • Carboxylic acids transfer a proton to a base to give carboxylic anions, which are good nucleophiles in SN2 reactions • Like ketones, carboxylic acids undergo addition of nucleophiles to the carbonyl group • In addition, carboxylic acids undergo other reactions characteristic of neither alcohols nor ketones Spring, 2011 24 Reduction of Carboxylic Acids • Reduced by LiAlH4 to yield primary alcohols • The reaction is difficult • Generally requires heating in THF Spring, 2011 25 Reduction with Borane • Borane in tetrahydrofuran (BH3/THF) converts • carboxylic acids to primary alcohols selectively Preferable to LiAlH4 because of its relative ease, safety, and specificity Spring, 2011 26 Chemistry of Nitriles • Nitriles and carboxylic acids both have a carbon • atom with three bonds to an electronegative atom, and both contain a bond Both are electrophilic centers Spring, 2011 27 Preparation of Nitriles • Primary alkyl halides + cyanide • SN2 reaction – same constraints KCN R X Spring, 2011 RCN 28 Preparation of Nitriles • Primary amides RCONH2 react with SOCl2 or • POCl3 (or other dehydrating agents) Not limited by steric hindrance or side reactions (as is the reaction of alkyl halides with NaCN) Spring, 2011 29 Aromatic Nitriles • Conversion of anilines into diazonium salt • Followed by CuCN/KCN NH2 Spring, 2011 HONO N2 + CuCN/KCN CN 30 Reactions of Nitriles • RCN is strongly polarized • Electrophilic carbon atom • Attacked by nucleophiles to yield sp2-hybridized imine anions Spring, 2011 31 Hydrolysis of Nitriles • Hydrolyzed by acid or base to the carboxylic acid • Ammonia or an amine is other product Spring, 2011 32 Reduction of Nitriles • Reduction of a nitrile with LiAlH4 gives a primary amine Spring, 2011 33 Nitriles with Organometallic Reagents • Grignard reagents add to give an intermediate imine anion that is hydrolyzed by addition of water to yield a ketone Spring, 2011 34 Infrared Spectroscopy • Carboxylic Acids – OH and C=O absorptions – OH is very broad over 2500 – 3300 cm-1 – C=O absorbs over 1710 – 1760 cm-1 • See next spectrum Spring, 2011 35 Spring, 2011 36 Infrared Spectroscopy • Nitriles intense C-N triple bond absorption • 2250cm-1 for saturated compounds • 2230cm-1 for aromatic and conjugated • • compounds Highly diagnostic for nitriles See next spectrum Spring, 2011 37 Spring, 2011 38 NMR Spectroscopy • Carboxyl carbons resonate between 165 – 185d • Nitrile carbons resonate between 115 – 130d • CO2H resonates near 12d • Proton is exchangeable with Deuterium Spring, 2011 39