Sigma bond carbon—oxygen

Pi bonds are more reactive than sigma bonds. Carbon, nitrogen, oxygen and sulfur are the only atoms that commonly form re-bonds. Phosphorous forms tc-bonds with oxygen in nucleotide phosphates such asATR... 

Mechanistically this reaction is described as a concerted pericyclic [3,3] sigma-tropic rearrangement. A carbon-oxygen bond is cleaved and a carbon-carbon bond is formed. In a subsequent step the initial product 4 tautomerizes to the stable aromatic allylphenol 3 ... 

Figure 27. Mechanism of radiation induced chain scission in PMMA. Homolysis of the mainchain-carbonyl carbon bond is indicated as the initial step. Acylcarbon-oxygen, sigma bond homolysis also occurs but rapid decarbonylation ultimately leads to the same indicated products.

However, the chemical properties of an alkene are dramatically affected by the presence of the double bond. Recall that a carbon-carbon pi bond is considerably weaker than a carbon-carbon or carbon-hydrogen sigma bond. It is possible to selectively cause a reaction to occur at a pi bond under conditions that do not affect the sigma bonds. The pi bond is the weak spot of an alkene, and it is there that most chemical reactions occur. This is why unsaturated fats spoil more readily than saturated fats. Their pi bonds provide a place for reaction with oxygen to occur, which leads to spoilage. 

In addition to rearrangements such as the previous two, it is possible for both of the components to contain pi bonds. In the following reaction the carbon-oxygen bond is broken and the new sigma bond is formed between the carbons at the other end of each of the components. This is an example of a [3,3] sigmatropic rearrangement. 

The catalyst must not cleave the carbon-oxygen sigma bond ( 360 kJ/mol). 

The oxygen atom and both carbon atoms have octets. The CH3 carbon atom is sigma bonded to four atoms, so it is sp2 hybridized (and tetrahedral). The C=0 carbon is bonded to three atoms (no lone pairs), so it is sp2 hybridized and its bond angles are about 120°. 

The double bond between carbon and oxygen looks just like the double bond in ethylene. There is a sigma bond formed by overlap of sp2 hybrid orbitals and a pi bond formed by overlap of the unhybridizedp orbitals on carbon and oxygen (Figure 2-19). 

The carbonyl carbon atom is sp2 hybridized and bonded to three other atoms through coplanar sigma bonds oriented about 120° apart. The unhybridized p orbital overlaps with a p orbital of oxygen to form a pi bond. The double bond between carbon and oxygen is similar to an alkene C=C double bond, except that the carbonyl double bond is shorter, stronger, and polarized. 

Single bonds (sigma bonds) will also be seen in the carbon-oxygen bonds discussed later in the alcohol series, where there is a C-O-H bond. Double bonds (sigma and pi bonds) also occur in aldehydes and ketones where there is a C==0 bond. These homologous series will be looked at in Module 7. 

For clarity, electrons in sigma bonds are not shown, (a) carbon and oxygen are both sp hybridized... 

In this group, the carbon atom is sp2- hydridized. As a result of the hybridization, three sigma (a) bonds and one pi (it) bond are formed. Sigma bonds, which lie in a plane with bond angles of 120°, are present between the carbon-oxygen and carbon-alkyl groups. 

FIGURE 10.29 Bonding in the carbonate ion. The carbon atom forms three sigma bonds with the three oxygen atoms. In addition, the 2p orbitals of the carbon and oxygen atoms overlap to form delocalized molecular orbitals, so that there is also a partial pi bond between the carbon atom and each of the three oxygen atoms. 

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