20-1 Relative Reactivities,Structures and Spectra of Carboxylic Acid Derivatives CHAPTER 20 Additin-Flminatin in Carboylic Acid Derivatives Carboxylic Acid Derivatives and 个L一R Mass Spectrometry a ocalzedontoneatoaMomyoe 0: 20-1 Relave ea vative ater the orter the C-L be add derivatives are directly related t this rotation is about 21 kcal mo N-Dimcor 5=5又 The de nitrooen possesses so2 1 CHAPTER 20 Carboxylic Acid Derivatives and Mass Spectrometry 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: The relative reactivities of the substrates follow a consistent order: 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 function. Lone pairs on L can be delocalized onto the carbonyl oxygen: The resonance form on the right is most important 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. Alkanoyl halides are least stable because of their electronegatives and the poor overlap between their p-orbitals and those of carbon. 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 esters and amides, the C￾L bond becomes progressively shorter (increased double-bond character). The NMR spectra of N,N-dimethylformamide at room temperature 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 sp2 hybridization. The resultant planarity of the amide group is the most important determinator of structure (thus, function) in peptides and proteins