Oxygen lone pairs

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

We can visualize these heterocycles as similar to the simpler aromatic systems pyrrole, furan and thiophene. For example, in imidazole, each carbon and nitrogen will be sp hybridized, with p orbitals contributing to the aromatic rt system. The carbon atoms will each donate one electron to the rt system. Then, as in pyrrole, the NH nitrogen supplies two electrons, and, as in pyridine, the =N- supplies one electron and retains a lone pair. Oxygen or sulfur would also supply two electrons, as we saw in furan and thiophene. 

The carbon-oxygen sigma (3[Pg.251]

For H2CO, low-lying excited states are obtained when (essentially) a non-bonding (or lone-pair) oxygen electron of the ground state is promoted into a 7t-electron orbital. This excitation is normally discussed in terms of a molecular orbital description for the Tc-electrons, and we shall initially use this type of treatment here. 

Figure C2.14.2. The hydrogen bond in water. The oxygen lone pairs (shaded blobs) are the donors, and the hydrogen atoms the acceptors [ 177, 178].

Other compounds containing lone pairs of electrons readily form co-ordinate links and in each case a change in spatial configuration accompanies the bond formation. The oxygen atom in dimethyl ether, CHj—O—CHj, has two lone pairs of electrons and is able to donate one pair to, for example, boron trichloride ... 

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming H30, and diethyl ether can donate to an acceptor such as boron trifluoride ... 

In some force fields the interaction sites are not all situated on the atomic nuclei. For example, in the MM2, MM3 and MM4 programs, the van der Waals centres of hydrogen atoms bonded to carbon are placed not at the nuclei but are approximately 10% along the bond towards the attached atom. The rationale for this is that the electron distribution about small atoms such as oxygen, fluorine and particularly hydrogen is distinctly non-spherical. The single electron from the hydrogen is involved in the bond to the adjacent atom and there are no other electrons that can contribute to the van der Waals interactions. Some force fields also require lone pairs to be defined on particular atoms these have their own van der Waals and electrostatic parameters. 

Typical auxochromes are hydroxyl, alkoxyl and aroxyl, amino, alkyl-amino and arylamino, all of which promote conjugation with lone pairs on oxygen or nitrogen atoms. 

Sr) and the nonbonding orbital formed by the Oxygen 2s and 2pz orbitals eombining to form the "lone pair" orbital direeted along the z-axis away from the two Hydrogen atoms). 

Cyanide is a good anion, and the cation is stabilised by a lone pair of electrons on oxygen. Draw the disconnection again using the lone pair. 

Because the acylated product has a delocahsed lone pair and is less reactive than PhNHi. You may have been surprised that LiAlHi reduction completely removes the carbonyl oxygen atom. To help explain this, please draw the likely intermediate. 

The obvious intermediate, 239A, will now react with some aluminium species to give an intermediate like 239B, which can react further if the lone pair on nitrogen halps to expel the oxygen atom. Try now to complete the mechanism. 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

The lone pair on oxygen cannot be directly involved m carbocation stabilization when attack is meta to the substituent... 

Oxygen lone pair cannot be used to stabilize positive charge in any of these stmctures all have six electrons around positively charged carbon... 

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