Chap 20 Textbook slides

Chap 20 Textbook slides - CHAPTER 20 Carboxylic Acid...

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CHAPTER 20 arboxylic Acid Derivatives and Carboxylic Acid Derivatives and Mass Spectrometry
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Relative Reactivities, Structures and Spectra of Carboxylic Acid Derivatives 20-1 Carboxylic acid derivatives undergo substitution reactions via the (often acid- or base-catalyzed) addition-elimination sequence:
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The relative reactivities of the substrates follow a consistent order:
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The order of reactivity depends upon the ability of L to act as a leaving group and what effect it has on the adjacent carbonyl nction function. Lone pairs on L can be delocalized onto the carbonyl oxygen: The resonance form on the right is most important gp in amides and somewhat less important in esters. Amides and esters are strongly stabilized by resonance. Anhydrides are more reactive than esters because the lone pairs on the central oxygen are shared over two carbonyl groups. lkanoyl halides are least stable because of their Alkanoyl halides are least stable because of their electronegatives and the poor overlap between their p-orbitals and those of carbon.
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Relative Reactivities, Structures and Spectra of Carboxylic Acid Derivatives 20-1 The greater the resonance, the shorter the C-L bond. The structures of carboxylic acid derivatives are directly related to the extent of resonance. In progressing from alkanoyl halides to exters and amides, the C-L bond becomes progressively shorter (increased double-bond character).
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The NMR spectra of N,N-dimethylformamide at room temperature xhibits two singles for the two methyl groups exhibits two singles for the two methyl groups. •Bond rotation about the C-N bond in this molecule is very slow on the NMR time scale. •The measured barrier to this rotation is about 21 kcal mol -1 . The amide nitrogen possesses sp 2 hybridization. The resultant planarity of the amide group is the most important determinator of structure (thus, function) in peptides and proteins.
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IR spectra of amides and esters also indicate the presence of sonance in the structures resonance in the structures. The C=O bond is weakened, which causes a corresponding decrease in the carbonyl stretching frequency. The IR spectra of monomeric acetic acid displays a carbonyl pp y y stretching frequency of 1780 cm -1 , similar to that of anhydrides.
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The 13 C NMR signals of the carbonyl carbons in carboxylic acid erivatives are less sensitive and fall into a narrow range near derivatives are less sensitive and fall into a narrow range near 170 ppm.
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The mass spectra of carboxylic acid derivatives typically contain eaks resulting from both leavage and McLafferty peaks resulting from both -cleavage and McLafferty rearrangement.
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Carboxylic acid derivatives are basic and acidic. Resonance in carboxylic acid derivatives affects their basicity (protonation at the carbonyl oxygen) and their acidity (enolate formation).
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This note was uploaded on 02/18/2010 for the course CHEM 140C 674224 taught by Professor Albizati during the Spring '10 term at UCSD.

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Chap 20 Textbook slides - CHAPTER 20 Carboxylic Acid...

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