Carbon atoms bonding characteristics

The really important aspect to all of this is that nuclei in similar chemical environments exhibit similar chemical shifts. Thus protons ( H nuclei) attached to a carbon atom bonded to oxygen, H-C-O, show a characteristic chemical shift (3.5-A.5 ppm), while protons attached to a carbon atom bonded to nitrogen, H-C-N, have a different chemical shift range (2.5-3.5 ppm) and, since the carbon is attached to the less electronegative N atom, resonate at lower frequency. We can therefore use chemical shifts to our great advantage when interpreting NMR spectra. 

Fig. 89. I3C CP/MAS NMR spectrum of glycine at 216 MHz (122). The carbon atom bonded to nitrogen displays a characteristic doublet arising from, 4N quadrupole interactions.

The hybridization of atomic orbitals discussed in Section 1.3 to explain the bonding characteristics of carbon atoms bonded to four other atoms can also be used to describe alkenes, compounds in which some carbon atoms are bonded to only three atoms This hybridization involves mixing a 2s orbital and two 2p orbitals of a carbon atom to form three hybrid sp orbitals (see h Figure 2.1). 

In addition to geometry, alkenes also differ from open-chain alkanes in that the double bonds prevent the relatively free rotation that is characteristic of carbon atoms bonded by single bonds. As a result, alkenes can exhibit geometric isomerism, the same type of stereoisomerism seen earlier for the cycloalkanes (Section 1.9). There are two geometric isomers of 2-butene ... 

Sol 3. This reaction is regioselective because the diazo terminal carbon atom bonds exclusively to the (3-carbon of the ester. The retention of configuration in the product with respect to both the 1,3-dipole and the dipolarophiles is a characteristic feature of 1,3-dipolar cycloadditions. Thus, stereochemistry of the substituents on the resulting five-membered cyclic ring entirely depends upon stereochemistry of the substituents on the dipolarophiles. Such a stereospecificity provides strong support for a concerted mechanism. 

The characteristics and properties of the single carbon atom were described in the preceding sections. This section is a review of the ways carbon atoms bond together to form solids, such as diamond, graphite, and other carbon polymorphs. 

The reasonable stable products are characterized by an ir-absorption near 1615 cm". The 4-protons resonate near 6.2 ppm in the H NMR spectrum (23). NMR spectra exhibit a carbonyl atom signal near 173 ppm, whereas C-4 resonates near 8 108 these positions are characteristic of other mesoionic ring carbon atoms (24). In the mass spectra, decomposition with loss of CO, rupture of the 1,5 and 2.3 bonds with elimination of R NC2R 0 and cleavage of the 1,2 and 3,4 bonds with elimination of C2R 0S is observed (11)... 

The interaction between a substituent and the ring carbon to which it is bonded could be related to some electronic characteristics of the unsubstituted ring and especially to the net charge of its various sites. In that respect the rr-net charges diagram discussed in Section 1.5 indicates that the electron-withdrawing power of the ring-carbon atoms will decrease in the order, 2>4>5. 

The enamines in which the protonation at the -carbon atom is not allowed due to the lack of coplanarity, or, in other words, the lack of electronic overlap, do not exhibit this characteristic absorption shift. For instance in the case of neostrychnine (134) where the overlap is not permitted since this would involve the formation of a double bond at the bridgehead, there is no appreciable difference in the C—C stretching region of the free amine and its perchlorate salt they absorb at 1666 cm and 1665 cm , respectively (70). 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

Bohlmann et al. (118-121) observed that an infrared absorption band between 2700-2800 cm is characteristic of a piperidine derivative possessing at least two axial carbon-hydrogen bonds in antiperiplanar position to the free-electron pair on the nitrogen atom. The possibility of forming an enamine by dehydrogenation can be determined by this test. Compounds which do not fulfill this condition cannot usually be dehydrogenated (50, 122,123). Thus, for example, yohimbine can be dehydrogenated by mercuric acetate,whereas reserpine or pseudoyohimbine do not react (124). The quinolizidine (125) enamines (Scheme 4), l-azabicyclo(4,3,0)-nonane, l-azabicyclo(5,3,0)decane, l-azabicyclo(5,4,0)undecane, and l-azabicyclo(5,5,0)dodecane have been prepared in this manner (112,126). 

A series of atom types, defining the characteristics of an element within a specific chemical context. Atom types prescribe different characteristics and behavior for an element depending upon its environment. For example, a carbon atom in a carbonyl is treated differently than one bonded to three hydrogens. The atom type depends on hybridization, charge and the types of the other atoms to which it is bonded. 

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