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474 Chapter 25 HETEROCYCLES: HETEROATOMS IN CYCLIC ORGANIC COMPOUNDS Then, the acylium ion converts the ether oxygen into a good leaving group and allows displacement by bromide to occur. Br CH₃ H + Br CH₃ Configuration is H CH₃ H inverted here (SN2) H C C O H₃C 35. The order of basicity is the reverse of the order of the acidity of the conjugate acids (pKₐ values shown below the structures). Bases > + > > H₂O + N H N H H = -4.4 0.0 5.3 9.2 15.7 Strongest acid Weakest acid N sp₂ N 36. N sp₂ N N N S sp₂ p P sp₂ H H All have two double bonds plus one lone pair in a p orbital = 6 electrons, so all are aromatic. All have sp²-hybridized lone pairs on not tied up in the aromatic and therefore available to act in a Lewis-base manner. Pyrrole lacks an sp²-hybridized lone pair; therefore all the compounds above are stronger bases than pyrrole. CH₃ N 37. (a) (b)