AROMATICS: Heteroaromatics [00:05:10] / electrophilic aromatic substitution [00:10:40] / KKK and SSS rule [00:34:00] / halogenations [00:42:20] / regioselectivity [00:43:20] nitrations [00:55:50] / sulfonations [01:12:20] / Friedel-Crafts alkylation [01:18:40] / thermodynamic and kinetic control [01:20:50] / Friedel-Crafts acylation [01:25:00] In part 14, we delve deeper into the world of aromatics. First, we repeat when free electron pairs of heteroatoms are counted as part of the pi system of an aromatic and when not (00:02:30). Then we will introduce the heteroaromatics purine (00:05:10), adenine and guanine (00:06:00), isoalloxazine (00:07:10), an anthracene derivative found in flavin adenine dinucleotide (FAD), and porphyrins (00:09:10), which we find in cytochromes. We will then look at the reactivity of aromatics: while alkenes prefer additions, aromatics prefer substitutions (00:10:40). An addition would lead to a non-aromatic product. In contrast, substitutions yield aromatics as products, and the aromatic stabilization energy is retained (00:15:50). Electrophilic aromatic substitutions (SEAr) proceed via a non-aromatic carbenium ion, which is also called a sigma complex or Wehland intermediate (00:27:30). In the following, you will learn about halogenations (00:42:20), nitrations (00:55:50), sulfonations (01:12:20) and Friedel-Crafts alkylations and acylations of arenes (00:31:50). Due to the high activation barrier, most of these substitutions require catalysts; high temperatures and irradiation with light lead to radical substitution in the benzylic position (00:34:00). The electrophilic species is formed by either a Lewis or Broensted acid catalyst. Reactions with monosubstituted aromatics can lead to the formation of three different regiosiomers: the para, meta and ortho product (00:38:10). Transformations of aromatics with electron-donating groups lead preferentially to the para and ortho constitutional isomers (00:43:20), while those with electron-withdrawing substituents preferentially give the meta product (01:02:20). This selectivity can be rationalized by stabilizing and destabilizing the positive charge in the Wehland intermediate by the substituent. The example of a Friedel-Crafts alkylation (01:18:40) illustrates the difference between kinetic and thermodynamic control (01:20:50). Finally, an exemplary Friedel-Crafts acylation with thiophene is used to show that the regioselectivity of substitutions in the case of heteroaromatics can also be attributed to the relative stability of carbenium ion intermediates (01:25:00). The playlist with all parts of the course and live streams can be found at: • Organic Chemistry A description of the contents of all course parts and the script are available at the following link: https://www.peterhuylab.de/youtube/ The password for the “teaching materials” area with the course script can be obtained from Prof. Huy (peter.huy[at]uni-rostock.de). For this course unit, pages 13 – 41 of “Chapter 7: Aromatics” were used. This video is an unedited recording of the live streaming of the lecture “Organic Chemistry for Life Sciences and Biology Teaching” by Prof. Dr. Peter Huy from June 1st, 2021 at the Institute of Chemistry at the University of Rostock.