CC bonds

Next, we consider one pair of it electrons and one pair of cj elections. The cj electrons may originate from a CH or from a CC bond. Let us consider the loop enclosed by the three anchors formed when the electron pair comes from a C-H bond. There are only three possible pairing options. The hydrogen-atom originally bonded to carbon atom 1, is shifted in one product to carbon atom 2,... 

The orbitals from which electrons are removed and those into which electrons are excited can be restricted to focus attention on correlations among certain orbitals. For example, if excitations out of core electrons are excluded, one computes a total energy that contains no correlation corrections for these core orbitals. Often it is possible to so limit the nature of the orbital excitations to focus on the energetic quantities of interest (e.g., the CC bond breaking in ethane requires correlation of the acc orbital but the 1 s Carbon core orbitals and the CH bond orbitals may be treated in a non-correlated manner). 

Figure 2.13. Symmetrised two-dimensional INADEQUATE experiment with isopinocampheol (2) [ CDshCO, 250 mg in 0.3 ml, 25 °C, 50 MHz, 256 scans and exp.], (a) Stacked plot of the section between 8c = 20.9 and 48.2 (b) complete contour plot with cross signal pairs labelled a-k for the 11 CC bonds of the molecule to facilitate the assignments sketched in formula 2...

The //NMR spectrum (Fig. 2.19) displays anAB system for the protons adjacent to this bond the coupling constant = 72 Hz. From this can be deduced first that the dihedral angle 9 between the C7/bonds is about 180°, second that conformer 14b with minimised steric repulsion between the substituents predominates and third that there is restricted rotation around this CC bond. 

The relative configuration of the protons which is deduced from the coupling constant Jab confirms the conformation of this part of the structure of this molecule. On the other hand, the V//// coupling constant of the ethyl group attached to oxygen (7 Hz, Fig. 2.19) reflects equal populations of all stable conformers around the CC bond of this ethyl group. 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

The cross signals in the INADEQUATE plot show the CC bonds for two part structures A and B. Taking the C signal at 8c = 174.1 as the starting point the hydrocarbon skeleton A and additional C3 chain B result. 

The correlation signals of the INADEQUATE experiment directly build up the ring skeleton A of the compound. Elere characteristic C shifts (5c = 123.1, 137.6 148.9, 109.1) establish the existence and position of two double bonds and of one tetrahedral C-0 single bond (5c = 70.5). DEPT spectra for the analysis of the CH multiplicities become unnecessary, because the INADEQUATE plot itself gives the number of CC bonds that radiate from each C atom. 

CC INADEQUATE detects CC bonds (carbon skeleton) present in the sample... 

When this type of transform is applied mechanistically to 85, retron generation is simple, for example by the change 85 => 86, and the sequence 86 => 90 disconnects two rings and provides an interesting synthetic pathway. Radical intermediate 88, which is disconnected at p-CC bond a to produce 89, may alternatively be disconnected at the P-CC bond b which leads to a different, but no less interesting, pathway via 91 to the acyclic precursor 92. The analysis in Chart 11 is intended to illustrate the mechanistic transform method and its utility it is not meant to be exhaustive or complete. 

CC bond distances in localized allylic systems have been held at 1. 5A and 1.3A (typical of CC single and double bond lengths, respectively), and at 1.4A for delocalized systems. 

Chemists use curved arrows to show the electronic changes that occur during a chemical reaction. Fot example, the arrows describing the Sn2 reaction below show formation of a CC bond and loss of a Cl bond. 

Among the possible CC bond conformers for 2,2 -dichlorobiphenyl (DCBP) are syn and anti planar structures and two twist structures. 

What is the difference between the two secondary carboeations Compare CC bond distances in the reactant to those in the two carboeations. What changes does Br loss cause in each of the carboeations How do you explain these changes (Hint Changes in C hybridization, such as sp —> sp, may be responsible for some changes in distance.)... 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

Finally, examine transition states for cyanide addition cyanide+formaldehyde, cyanide+acetone, cyanide+ benzophenone) What relationship, if any, is there between the length of the forming CC bond and the various carbonyl properties determined above Try to rationalize what you find, and see if there are other structural variations that can be correlated with carbonyl reactivity. 

Enolate reactivity depends on the electrophile. Enolates generally form CC bonds with carbon electrophiles, and OSi bonds with silicon electrophiles. 

Enolates react with alkyl halides to form a new CC bond. A mixture of stereoisomers may result. For example, each of the reactions shown below gives two products, with the major product constituting > 90% of the mixture. 

Cyclohexatriene to benzene displays a sequence of structures from 1,3,5-cyclohexatriene (withCC single and double bonds initially set to 1.5 and 1.3 A, respectively) to benzene (witb all CC bonds set to 1.4 A) and back to cyclohexatriene. Plot energy (vertical axis) vs. CC bond length (horizontal axis). How many energy minima are there Do the minima look more like 1,3,5-cyclohexatriene or benzene What is the correct interpretation of the resonance picture ... 

Aeeording to your Lewis structure(s) and to the actual geometry of the molecule, is the bonding in planar corannulene fully delocalized (as in benzene), or are some CC bonds long and some short Do your results support the notion that planar corrannulene is resonance stabilized Explain. 

Buckminsterfullerene (Cm or Buckyball ) is structurally related to corannulene. In which molecule would you expect 7U-orbital overlap be more effective Explain. How many chemically unique carbons are there in C6o Measure CC bond distances. How many unique distances are there Is each benzene fully delocalized or is one resonance contributor more important than the other ... 

The CC bond distances in cyclopentadienyl anion, C5H5, are all equal, because the anion is aromatic (see Chapter 12, Problem 10). Electrophiles that interact electrostaticaUy with the anion, such as Na", interact equally with all five carbons, and do not disturb the anion s aromatic character. On the other hand, electrophiles that make covalent bonds, such as H", might interact more strongly with one particular carbon and destroy the aromaticity of the ring. 

Next, consider how Fe " interacts with C5H5. Examine the geometry of ferrocene, Fe(C5H5)2. Are the FeC distances all the same, or does iron bond more strongly to some carbons than to others Are the CC bond distances all the same Which of the above models, the electrostatic or covalent, gives the better description ... 

Examine the structure of bicyclohexane and note the spatial relationships of the various CH and CC bonds. Do all the bonds separating and Ha lie in the same plane ... 

Finally, examine the geometry of the lower-energy transition state. Measure all CC bond lengths. Draw a Lewis structure representing partial bonds in terms of... 

Step through the sequence of stmctures depicting Cope rearrangement of 1,5-hexadiene. Plot energy (vertical axis) vs. the length of either the carbon-carbon bond being formed or that being broken (horizontal axis). Locate the transition state. Measure all CC bond distances at the transition state, and draw a structural formula for it... 

The construction of the pair of (bond orbitals is carried out by combining a carbon hybrid with the Is orbital on hydrogen in a manner similar to the construction of the CC bond orbitals. The interaction diagram is shown below in Fig. 3. The bonding orbital is occupied by the two bond electrons. These two... 

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