Positive charge lone pair next

Both examples exhibit a lone pair next to a positive charge. In each case, we can bring down the lone pair to form a pi bond ... 

The curved arrows show how one resonance structure relates to another. Notice that the formal negative charge is located on the ortho and para positions, exactly where reaction takes place most quickly. Other ortho- and para-directing groups include —NH2, —Cl, and —Br. All have an atom with a lone pair of electrons next to the ring, and all accelerate reaction. 

There are three resonance structures for the intermediate formed in each form of electrophilic substitution, but there are two crucial ones to consider (Fig. N), arising from ortho and para substitution. These resonance structures have the positive charge next to the OH substituent. If oxygen only had an inductive effect, these resonance structures would be highly unstable. However, oxygen can act as a nucleophile and so can use one of its lone pairs of electrons to form a new rbond to the neighbouring electrophilic centre (Fig. O). This results in a fourth resonance structure where... 

The same is true for the following substituents alkoxy (-OR), esters (-OCOR), amines (-NH2, -NHR, -NIT,) and amides (-NHCOR). In all these cases, there is either a nitrogen or an oxygen next to the ring. Both these atoms are nucleophilic and have lone pairs of electrons which can be used to form an extra bond to the ring. The ease with which the group can do this depends on the nucleophilicity of the attached atom and how well it can cope with a positive charge. 

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. 

Protonation of lone pairs adjacent to the partial positive charge of a carbonyl is less favored than protonation of the earbonyl itself. Protonation of the ester carbonyl produces a delocalized cation, whereas protonation of the ester s ether oxygen is destabilized by having a positive charge next to the partial positive carbonyl carbon. 

Greater resonance stabilization in the products. Delocalization of a lone pair on the oxygen joining the two phosphoms atoms is not very effective because it puts a positive charge on an oxygen that is next to a partially positively charged phosphoms atom. When the phosphoanhydride bond breaks, one additional lone pair can be effectively delocalized. 

The main effect of the oxygen atom next to a reaction centre in chemical terms is to stabilise the buildup of positive charge on the central atoms the lone pairs of electrons on the oxygen atom release electrons into the vacant p orbital of an adjacent carbonium ion centre. This means that, compared with nucleophilic substitutions at purely carbon centres, the C—X bond can break to a much greater extent before there is much C—Y bond formation. Indeed, in the limit, for most processes in water the C—X bond completely breaks first and the oxocarbonium ion is a discrete intermediate. 

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