CHE 3332 – Organic Chemistry 2 – Spring 2021 Week 8 Resource
Hello everyone! Virtual group tutoring for this class will be every Tuesday from 6-7 pm, and you can reserve your spot by making an appointment at www.Baylor.edu/Tutoring. This week, we’ll cover Chapter 18: Aromatic Compounds Keywords: aromaticity, Huckels rule, benzylic position, delocalization
Naming Monosubstituted Aromatic Compounds – use the common name as the root and add the substituent name as the prefix. If the substituent chain is longer than 6 carbons, use a phenyl substituent in the name.
Chlorobenzene Ethylbenzene 1-phenylheptane
Naming Polysubstituted Aromatic Compounds Aromatic ring position names:
Use the common name as the root, add the substituent names as prefixes, and number the substituents or label with the ring position.
Meta-bromotoluene: 3,5-dibromophenol:
Aromaticity A compound must follow these two rules for it to be aromatic: 1. Flat ring with continuously overlapping p-orbitals 2. Obey Huckels rule of 4N + 2 pi electrons N can be any integer. A compound can only have one of the following numbers of pi electrons to be aromatic when following Hucks rule: 2, 6, 10, 14, 18, etc. If a compound doesn’t follow #1, it is nonaromatic. If it follows #1, but not number #2, it is antiaromatic. Antiaromatics follow a pattern of 4N pi electrons.
nonaromatic – not in a ring
nonaromatic – doesn’t have continuously overlapping p-orbitals
antiaromatic – violates Huckels rule, 4 pi electrons
aromatic – 6 pi electrons
antiaromatic – violates Huckels rule, 8 pi electrons
aromatic – 10 pi electrons
nonaromatic – doesn’t have continuously overlapping p-orbitals
aromatic – 10 pi electrons
aromatic – 6 pi electrons, localized lone pair doesn’t contribute to conjugation system Localized vs Delocalized lone pairs A localized lone pair is one that does not participate in the conjugated system and will not be counted in the pi electrons. A double bond is usually on the element with the localized lone pair. A delocalized lone pair does participate in the conjugated system and will be counted in the pi electrons. If the element (such as N or O) has a double bond directly on it, then its lone pair doesn’t count in the conjugation system; if there’s a single bond, then its lone pair is counted. If an element has two lone pairs on it, one pair is localized and the other is delocalized.
Nitrogen’s lone pair does not have a double bond directly on it, so it’s counted. Only one of oxygen’s lone pairs is counted; the other is not. Both structures have 6 pi electrons so they’re aromatic.
Reactions at the Benzylic Position On a benzene ring, the benzylic position is any allylic position to any of the double bonds in the ring:
1. Oxidation – this needs at least one proton (hydrogen) at the benzylic position. Carboxylic acid gets added at the benzylic position. If there’s no available proton, then no reaction happens.
2. Free Radical Bromination – bromine is added to the benzylic position with a free proton.
3. Substitution – this can act as an SN1 or SN2 reaction to add a new functional group.
4. Elimination – this can act as E1 or E2 to leave a double bond at the benzylic position.
5. Hydrogenation – this reagent selectively reduces a double bond at the benzylic position and leaves the aromatic ring intact. H2/Ni will reduce a whole aromatic ring.
6. Birch Reduction – an aromatic ring undergoes a dissolving-metal reduction to create a diene.
• If an electron withdrawing group is a substituent on the ring, don’t put it adjacent to the double bonds on the product:
• If an electron donating group is a substituent on the ring, do put it adjacent to a double bond on the product:
Electron withdrawing/donating groups Electron withdrawing groups are electronegative, usually have a carbonyl, and always pull electrons toward them. Electron donating groups have free electrons that can be used; they do not pull electrons toward them.
Electron withdrawing group: electron donating group:
Quiz: 1. Determine aromaticity:
2. Determine aromaticity:
3. Predict the product:
Answers: All questions sourced from the Wiley Organic Chemistry Second Semester textbook.
1. Nonaromatic – doesn’t have overlapping p-orbitals
2. Antiaromatic – only one lone pair on sulfur counts. The lone pair on nitrogen counts. The total number of pi electrons is 8 so that doesn’t follow Huckels rule.
3.