Hybridized Carbon Atoms

Chains of sp-hybridized carbon atoms one-dimensional system 

The reports of Sladkov [36] on a carbon allotrope (carbyne) which would add to the two known carbon hybridization states a third one, namely sp-hybridization in macromolecular chains, were followed by data collected by Whittaker et al. [37] who preferred the name chaoite. 

If carbon atoms inside polyacetylenic chains are substituted by nitrogen atoms, positively-charged nitrilium or isonitrilium centers result boron substitution leads to negatively-charged centers. No experimental or theoretical data are yet available on such systems, which (if charges would be correctly placed) would benefit from electrostatic interactions, in addition to Van der Waals forces. 

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This chapter discusses the chemistry of 1,3-dioxanes, -oxathianes, -dithianes, their unsaturated and benzo-fused derivatives, and their tautomers. All possible degrees of unsaturation in the six-membered ring, from zero to four 5/7 -hybridized carbon atoms, are included. Following a brief treatment of theoretical, structural, and spectroscopic studies, the reactivity of the different ring systems is examined, categorized by the formal degree of unsaturation. 

Simplest alkene is C2H4 (ethylene), which is described as containing 5/7 hybridized carbon atoms... 

When in a set of ZW groups, W is CH2, CH, C, C=0, C6H4, CH==CH, or C C, no problem arises as the hybridized carbon atom our previous results suggest that the delocalized effect is insignificant. Thus, we expect that for (T/,zch2 <1 OdZCH2>... 

The latter reaction is an example of the di-n-methane rearrangement This rearrangement is a very general reaction for 1,4-dienes and other systems that have two % systems separated by an 5/7 -hybridized carbon atom ... 

The insertion of a carbonyl group in between the 5/7 -hybridized carbon atoms of an alkene can be accomplished through the formation of cyclopropanone thioacetals from the appropriate epoxide or dibromocyclopropane and treatment of these compounds with aqueous trifiuoroacetic acid (Scheme 16), ... 

The transformation of ethylene to the carbene requires the re-pairing of three electron pairs. It is a phase-preserving reaction, so that the loop is an ip one. The sp -hybridized carbon atom formed upon H transfer is a chiral center consequently, there are two equivalent loops, and thus conical intersections, leading to two enantiomers. 

To ensure that the arrangement of four atoms in a trigonal planar environment (e.g., a sp -hybridized carbon atom) remains essentially planar, a quadratic term like V(0) = (fe/2) is used to achieve the desired geometry. By calculating the angle 9 between a bond from the central atom and the plane defined by the central... 

Begin with two sp hybridized carbon atoms and four hydrogen atoms... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

A hydrocarbon that contains two double bonds is called an alkadiene, and the rela tionship between the double bonds may be described as isolated conjugated or cumu lated Isolated diene units are those m which two carbon-carbon double bond units are separated from each other by one or more sp hybridized carbon atoms 1 4 Pentadiene and 1 5 cyclooctadiene have isolated double bonds... 

Scheme 6 Isomers of aromatic compounds with an ip -hybridized carbon atom...

Figure 1.15 An sp-hybridized carbon atom. The two sp hybrid orbitals (green) are Oriented 180° away from each other, perpendicular to the two remaining P orbitals (red/blue).

When two sp-hybridized carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp-sp a bond. In addition, the pz orbitals from each carbon form a pz-pz it bond by sideways overlap and the py orbitals overlap similarly to form a py-py tt bond. The net effect is the sharing of six electrons and formation of a carbon-carbon triple bond. The two remaining sp hybrid orbitals each form a bond with hydrogen to complete the acetylene molecule (Figure 1.16). 

Figure 1.16 The structure of acetylene. The two sp-hybridized carbon atoms are joined by one sp-sp rr bond and two p-p 7t bonds.

Much current research is centering on polyynes—linear carbon chains of sp-hybridized carbon atoms. Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

You might recall from Section 1.9 that a carbon-carbon triple bond results from the interaction of two sp-hybridized carbon atoms. The two sp hybrid orbitals of carbon lie at an angle of 180° to each other along an axis perpendicular to the axes of the two unhybridized 2py and 2pz orbitals. When two sp-hybridized carbons approach each other, one sp-sp a bond and two p-p -rr bonds are... 

Handedness is also important in organic and biological chemistry, where it arises primarily as a consequence of the tetrahedral stereochemistry of 5p3-hybridized carbon atoms. Many drugs and almost all the molecules in our bodies, for instance, are handed. Furthermore, it is molecular handedness that makes possible the specific interactions between enzymes and their substrates that are so crucial to enzyme function. We ll look at handedness and its consequences in this chapter. 

Alkyl halides contain a halogen bonded to a saturated, sp3-hybridized carbon atom. The C-X bond is polar, and alkyl halides can therefore behave as electrophiles. 

The electronic structure of benzyne, shown in Figure 16.19, is that of a highly distorted alkyne. Although a typical alkyne triple bond uses sp-hybridized carbon atoms, the benzyne triple bond uses sp2-hybridized carbons. Furthermore, a typical alkyne triple bond has two mutually perpendicular it bonds formed bv p-p overlap, but the benzyne triple bond has one tt bond formed by p-p overlap and one tt bond formed by sp2 sp2 overlap. The latter tt bond is in the plane of the ring and is very weak. 

Intramolecular Arylation of Aromatic and Alkene sp2-Hybridized Carbon Atoms 261... 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

On the other hand, the rate constant k does not depend on the changing steric influence of substitutents in the 8-position, but correlates surprisingly well with the Hammett-Brown constant cr. This result indicates that the formation of an sp3-hybridized carbon atom (at the 1-position of the o-complex) leads to a compound without significant steric interaction of the electrophile with substituents in the 8-position. The o-complex cannot be planar and is asymmetric. The preferred conformation of a o-complex of this type is illustrated in Figure 12-6. The pseudoax-ial position of the electrophile E reduces the steric interaction between this group and the peri substituent R. 

Graphite, the most important component of the lead of pencils, is a black, lustrous, electrically conducting solid that vaporizes at 1700°C. It consists of flat sheets of sp2 hybridized carbon atoms bonded covalently into hexagons like chicken wire (Fig. 5.22). There are also weak bonds between the sheets. In the commercially available forms of graphite, there are many impurity atoms trapped between the sheets these atoms weaken the already weak intersheet bonds and let... 

FIGURE 5.22 Graphite consists of layers of hexagonal rings of sp2 hybridized carbon atoms. The slipperiness of graphite results from the ease with which the layers can slide over one another when there are impurity atoms Iving between the planes. 

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

All conducting polymers have a common feature a long chain of sp2 hybridized carbon atoms, often with nitrogen or sulfur atoms included in the chains. Polyacetylene, the first conducting polymer, is also the simplest, consisting of thousands of —CH=CH -units ... 

H2—He at 8.5 5. This ion is an example of a homoaromatic compound, which may be defined as a compound that contains one or more sp -hybridized carbon atoms in an otherwise conjugated cycle. °... 

We may now proceed to an analysis of the significance of the d values. If the transition state were close to the product, then the P values of the disubstituted ethylenes should be small, because in the product, the substituent is bonded to an sp hybridized carbon atom. Thus, it is incapable of resonance interaction. As the observed values of p for the trans-disubstituted dienophiles are very large, we conclude that the transition state is closer to reactants than to products. The cis-disubstituted dienophiles show a much smaller value of P than do the trans compounds. It therefore seems likely that the transition state for the CIS compounds will be closer to product than is the transition state for the trans compounds. The values of p for the reaction of the tnws-disubstituted dienophiles with 9,10-dimethylanthracene, while large, are much smaller than... 

This example illustrates that a negative charge on an sp hybridized carbon atom is more stable than a negative charge on an sp hybridized nitrogen atom. For this reason, NH2 can be used as a base to deprotonate a triple bond. 

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