08022010 - Carboxylic Acid Deriva1ves A carboxylic...

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Unformatted text preview: Carboxylic Acid Deriva1ves A carboxylic acid deriva1ve is a compound that reacts with water (acidic or basic condi1ons) to yield a carboxylic acid. This reac1on is called hydrolysis. We will focus on 5 types of carboxylic acid deriva1ves, 4 of which are also carbonyl compounds: O R C Cl R O C O O C R' O R C O R' R O C N R' R C N acid chloride acid anhydride ester amide R" nitrile Since nitriles do yield carboxylic acids upon hydrolysis, and since many other reac1ons of the nitrile func1onal group are similar to carbonyl group reac1ons, nitriles are tradi1onally lumped in with the carbonyl ­containing acid deriva1ves. Nomenclature Acyclic acid chlorides are named by replacing the “–ic acid” suffix of the parent acid name with the suffix “–yl chloride.” The ending “–carbonyl chloride” is used if the acid chloride moiety is a subs1tuent on a ring: CH3 CH3CHCH2 4 3 2 O C 1 Br 4 3 2 1 O C Cl Cl 3 ­methylbutanoyl chloride 4 ­bromocyclohexanecarbonyl chloride Symmetric anhydrides (both acyl groups iden1cal) are named by replacing the word “acid” in the name of the parent acid with the word “anhydride.” Mixed anhydrides (acyl groups are different) are named by ci1ng the the 2 parent acids in alphabe1cal order. O CH3CH2 C O O C CH2CH3 H O C O O C Ph benzoic formic anhydride (a mixed anhydride) propanoic anhydride (a symmetric anhydride) Acyclic esters are named by first naming the group aSached to oxygen, and then the parent acid, with the “–ic acid” ending replaced by “–ate” (similar to the way carboxylate salts are named). O CH2CH2CH O α β γ 4 3 2 Cl O CH2CHCH3 1 2 3 C 1 2 ­chloropropyl 2 ­methylbutanoate A cyclic ester is called a lactone. Lactone names are frequently derived from the common name of the acyclic acid having the same number of carbon atoms. The point of aSachment of oxygen to the carbon chain is indicated by a Greek leSer. The “–ic acid” ending is replaced by the suffix “–olactone.” CH3 O δ (a δ lactone) δ ­valerolactone (5 ­pentanolactone) Amides are classified as primary (1°), secondary (2°), or ter1ary (3°) according to the number of carbon atoms directly aSached to nitrogen. O C NH2 CH3 O C N H CH2CH3 O H C N CH3 CH3 (3° amide) (2° amide) (1° amide) benzamide N ­ethyl ­3 ­methylcyclopentanecarboxamide N,N ­dimethylformamide Acyclic amides are named by replacing the “–ic acid” or “–oic acid” ending of the parent acid name with the suffix “–amide.” The suffix “–carboxamide” is used if the amide moiety is a subs1tuent on a ring. Subs1tuents on nitrogen in 2° and 3° amides are indicated with the designa1on N ­ in the prefix. A cyclic amide is called a lactam. Lactams are named in a manner similar to lactones, except the suffix “–olactam” is used. O α β NH β ­propionolactam (3 ­propanolactam) (a β lactam) The nitrogen analog of an anhydride is called an imide. Cyclic imides tend to be encountered much more frequently than acyclic imides. O NH O O succinimide NH O phthalimide Simple acyclic nitriles are named by adding the suffix “–nitrile” to the alkane name. The nitrile carbon atom is numbered C1. Alterna1vely, a nitrile can be named as a carboxylic acid deriva1ve by replacing the “–ic acid” ending with “–onitrile,” or the CN “–carboxylic acid” ending with “–carbonitrile.” CH2CH3 CH3CH2CHCH2CN 5 4 3 2 1 CH3 Ph C C N N CH3 CH3 acetonitrile benzonitrile 3 ­ethylpentanenitrile 2,2 ­dimethylcyclohexanecarbonitrile Spectroscopy Let’s compare the IR spectra of various deriva1ves of butanoic acid: O CH3CH2CH2 C Cl • C=O stretch, 1802 cm–1 • weaker band ~ 1712 cm–1 C=O stretch O CH3CH2CH2 C O O C CH2CH2CH3 • 2 C=O stretching bands, 1819 cm–1 & 1750 cm–1 C=O stretch (symmetric and unsymmetric) • C–O stretch ~ 1000 cm–1 C–O stretch O CH3CH2CH2 C O CH2CH3 • C=O stretch, 1739 cm–1 • C–O stretch ~ 1200 cm–1 C=O stretch C–O stretch O CH3CH2CH2 C N H H C=O stretch N–H stretch N–H bend • C=O stretch, 1662 cm–1 • N–H bend, 1634 cm–1 • N–H stretch, ~3200 cm–1 & 3400 cm–1 CH3CH2CH2 C N • C≡N stretch, 2206 cm–1 C≡N stretch What is the rela1onship between stretching frequency and bond strength? Stronger bonds have higher vibra1onal frequencies • Not surprising that C≡N stretch (~2200 cm–1) is higher frequency than C=O stretch (1650–1820 cm–1) – triple bonds are typically stronger than double bonds • Not surprising that N–H stretch is higher frequency than C–H stretch – nitrogen is smaller than carbon, so nitrogen tends to form stronger bonds • Why so much variability in the posi1on of the C=O stretching absorp1on? Shouldn’t one C=O bond be just as strong as another? In fact, IR data suggests that the C=O bond in an amide is weaker than the C=O bond in an ester, which in turn is weaker than the C=O bond in an anhydride or an acid chloride. • Why does the C–O bond of an ester appear to be stronger than the C–O bond of an anhydride? Another mystery becomes evident if we look at the 1H NMR spectrum of butanamide: 3H O CH3CH2CH2 C N aH H b 2H 2H 1H 1H a b c d e e d c The protons on nitrogen are not equivalent!!! All of these ques1ons can be cleared up if we remember one word: RESONANCE!!! The carbon–nitrogen bond of an amide is not quite a single bond, because an amide is actually a resonance hybrid... The carbon–oxygen bond of an amide is not quite a double bond, because an amide is actually a resonance hybrid... O R C N H H O R C N H H Because the structure on the right is of reasonable importance there is: • Signficant single bond character to the C=O bond • Signficant double bond character to the C–N bond If there is signficant single bond character to the C=O bond, it will be weaker than a “true” C=O bond. Lower frequency C=O stretching vibra1on If there is signficant double bond character to the C–N bond, we might expect rota1on about this bond to be somewhat restricted. O R C N H R' R O C N R' H “trans” “cis” • Barrier to rota1on is ~20 kcal/mol [approx. ⅓ strength of π bond (66 kcal/mol)] • Rota1on is restricted, and is “slow” on the NMR 1me scale. This is why the two protons on nitrogen in butanamide produce two dis1nct signals in the 1H NMR spectrum. Further evidence that the second resonance structure for an amide is significant: • The N atom of an amide has trigonal planar geometry. This suggests sp2 hybridiza1on, which is necessary if the lone pair is delocalized (the lone pair needs to be in a p orbital that is aligned parallel to the π bond of the carbonyl group). • The N atom of an amide is not nucleophilic or basic. Again, this suggests that N does not have a localized lone pair, which would be expected to be nucleophilic/basic (as we’ll see in a few weeks when we discuss the chemistry of amines). Let’s now consider the resonance structures we might write for the other kinds of carboxylic acid deriva1ves (ester, acid anhydride, acid chloride): O R C O R' O R C O R' The structure on the right is rela1vely less important as compared to an amide, because it places a posi1ve charge on a more electronega1ve oxygen atom. Since the second structure is less important, the carbonyl double bond of an ester is rela1vely stronger than the carbonyl double bond of an amide, and the carbonyl stretching absorp1on is observed at a higher frequency. O R C O O C R' R O C O O δ– C δ+ R' (another equivalent resonance structure can be wriSen which shows delocaliza1on of the central oxygen lone pair onto the other carbonyl oxygen) The second resonance structure is now even less important than it was for an ester, because not only have we placed a posi1ve charge on oxygen, this posi1ve charge is adjacent to an electron ­withdrawing carbonyl group (δ+ exists on the adjacent carbonyl carbon; adjacent like charges destabilize the molecule) • Since the second structure is of rela1vely minor importance, the carbonyl double bond is stronger than it was in an ester or an amide, and the stretching frequency is higher. • The bonds between carbon and the central oxygen are closer to being “true” single bonds in an anhydride than the are in an ester. These bonds are therefore somewhat weaker than the analogous C–O bond in an ester, and therefore the C–O stretch appears at a lower frequency. O R C Cl O R C O O C R' R O C O O δ– C δ+ R' (another equivalent resonance structure can be wriSen which shows delocaliza1on of the central oxygen lone pair onto the other carbonyl oxygen) The second resonance structure is now even less important than it was for an ester, because not only have we placed a posi1ve charge on oxygen, this posi1ve charge is adjacent to an electron ­withdrawing carbonyl group (δ+ exists on the adjacent carbonyl carbon; adjacent like charges destabilize the molecule) • Since the second structure is of rela1vely minor importance, the carbonyl double bond is stronger than it was in an ester or an amide, and the stretching frequency is higher. • The bonds between carbon and the central oxygen are closer to being “true” single bonds in an anhydride than the are in an ester. These bonds are therefore somewhat weaker than the analogous C–O bond in an ester, and therefore the C–O stretch appears at a lower frequency. O R C Cl O R C Cl The second resonance structure is rela1vely unimportant for an acid chloride because of a mismatch in orbital size – Cl, a 3rd period element, has orbitals which are significantly larger than the orbitals used by C, a 2nd period element. Orbitals that are not similar in size have less effec1ve orbital overlap, so delocaliza1on of a lone pair on Cl cannot occur. ...
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This note was uploaded on 10/25/2010 for the course CH 310n taught by Professor Iverson during the Summer '08 term at University of Texas at Austin.

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