Single bonds carbon-nitrogen

Amide resonance is a powerful stabilizing force and gives rise to a number of structural effects. Unlike the pyramidal arrangement of bonds in ammonia and amines, the bonds to nitrogen in amides lie in the same plane. The carbon-nitrogen bond has considerable double-bond character and, at 135 pm, is substantially shorter than the normal 147-pm carbon-nitrogen single-bond distance observed in amines. 

Catalyt c hydrogenation (palladium or Raney nickel catalyst) surprisingly results in reduction of the carbon-nitrogen single bond rather than the double bond.4,12,40 The imines, or possibly enamines, are usually not isolated and their existence has only been inferred in most instances. Harvey and Ratts have shown that this reaction with azirine (165) does not proceed first to the aziridine which is then reduced to 166, since aziridine (167) is inert to hydrogen and palladium on carbon.40... 

There is little difference between the double and single bond systems. However, if a triple carbon/nitrogen bond is being broken, this will release about 250 kJ mol-1, but a further 295 kJ mol-1 would be released if the reaction proceeded to the carbon/nitrogen single bond stage. Using the rule of thumb... 

Carbon-nitrogen single bonds are freely rotating however, amide carbon-nitrogen bonds are much more rigid. Why is this true What effect does this property of amide bonds have on the shapes that proteins can assume ... 

Ring Closure to Form Carbon-Nitrogen Single Bonds. 548... 

D. RING CLOSURE TO FORM CARBON-NITROGEN SINGLE BONDS... 

The Eigen model requires two different hydrogen-bonded intermediates eorresponding to the two alkene produets that are formed. Eaeh intermediate leads to a different transition state, structures 2 and 3 drawn in Scheme 2. The former, 2, leads ultimately to 1-methylcyclopentene as the final product. This transition state has no overall molecular symmetry, but retains the methyl rotor as well as free rotation about the carbon-nitrogen single bond for a net symmetry number a = 9. Because transition state 2 is chiral, the reaction path degeneracy is equal to (18/ ) = 4. 

Peptide bond structure. The peptide bond structure favors coplanar N, C, and O atoms. Although a peptide bond is formally a carbon-nitrogen single bond, the unpaired electrons on the carboxyl oxygen and on the nitrogen can overlap through their pi orbitals to make the three-atom system partially double-bonded in character. The partially double-bonded system makes it harder to rotate the peptide bond in solution. As a result, peptide bonds can exist in one of two conformational isomers, with the two carbons either cis or trans to each other. 

One of the chief values of imines is that the carbon-nitrogen double bond can be reduced to a carbon-nitrogen single bond by hydrogen in the presence of a nickel or other transition metal catalyst. By this two-step reaction, called reductive amination, a primary amine is converted to a secondary amine by way of an imine, as illustrated by the conversion of cyclohexylamine to dicyclohexylamine ... 

The carbon-nitrogen double bond of an imine can be reduced by hydrogen in the presence of a transition metal catalyst to a carbon-nitrogen single bond ... 

The dipolar contributor is so important that the carbon-nitrogen bond behaves much like a double bond. Consequently, the nitrogen and the carbonyl carbon, and the two atoms attached to each of them, lie in the same plane, and rotation at the C—N bond is restricted. Indeed, the C—N bond in amides is only 1.32 A long—much shorter than the usual carbon-nitrogen single bond length (which is about 1.47 A). 

Enamines have a carbon-nitrogen single bond with an adjacent carbon-carbon double bond. 

FIGURE 18.18 The structures of some simple acyl compounds. Notice how the carbon-oxygen and carbon-nitrogen single bonds are shortened by the double-bond character that results from the resonance shown in Figure 18.17. 

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