Lone pairs, on oxygen atoms

Alumina behaves as an amphoteric support due to acidity connected to OH surface groups (resulting from hydration of AI2O3) and basicity related to the available lone pairs on oxygen atoms. 

Lone-pairs on oxygen atoms are often included either as point charges or dipoles. This is as much an approximation as is the atomic-charge model and less effort has been directed to parameterizing this approach. 

The lone pair electrons on an amine nitrogen is much more available for protonation than lone pairs on oxygen atoms. 

SkillBuilder 2.4 Identifying Lone Pairs on Oxygen Atoms... 

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. 

In this example, we can see that one of the lone pairs on oxygen is coming down to form a bond, and the C=C double bond is being pushed to form a lone pair on a carbon atom. When both arrows are pushed at the same time, we are not violating either of the two commandments. So, let s focus on how to draw the resonance structure. Since we know what arrows mean, it is easy to follow the arrows. We just get rid of one lone pair on oxygen, place a double bond between carbon and oxygen, get rid of the carbon-carbon double bond, and place a lone pair on carbon ... 

Answer We need to read the arrows like a road map the first arrow is going from a lone pair on HO to form a bond with the carbon atom of the C=0. The second arrow goes from the C=0 bond to form a lone pair on oxygen. We use this info to draw the resulting intermediate ... 

We predict the oxygen atom is sp2 hybridized because it is bonded to one atom (carbon) and has two lone pairs, requiring a total of three hybrid orbitals. We cannot experimentally measure the angles of the lone pairs on oxygen, however, so it is impossible to confirm whether the oxygen atom is really sp2 hybridized. 

But we have not told you the whole story about BF3. Boron is in group 3 and thus has only six electrons around it in its trivalent compounds. A molecule of BF3 is planar with an empty p orbital. This is the reverse of a lone pair. An empty orbital on an atom does not repel electron-rich areas of other molecules and so the oxygen atom of acetone is attracted electrostatically to the partial positive charge and one of the lone pairs on oxygen can form a bonding interaction with the empty orbital. We shall develop these ideas in the next section. 

Silyl enol ethers hydrolyse by a slightly different mechanism, though the first step is the same— protonation at carbon using the lone pair on oxygen. We have already seen how easy it is to attack silicon with nucleophiles, especially those with oxygen or a halogen as the nucleophilic atom. This tips the balance towards attack by water at silicon for the next step. 

The compounds are stable at ordinary temperatures but decompose on heating. They can act both as weak Lewis bases using lone pairs on oxygen, and as weak Lewis acids using vacant d orbitals. For example, SeOQ2(pyrazole)2 has a square pyramidal structure with apical O and trans Cl atoms in the base. 

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