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Featured Golden Rules of Chemistry: 6. Delocalization of unpaired electron density over a larger area is stabilizing.

1. Alkanes react with halogens, usually Cl2 and Br2, in the presence of radical initiators like peroxides, heat or light to generate haloalkanes via the radical chain reaction mechanism. Once radicals initiate the chain reaction, they keep reacting to make more product until a termination step.

2. Carbon radicals should be thought of as being similar to carbocations; sp2 hybridized, wanting electron density, stabilized by alkyl groups. In the case of radicals, the unpaired electron density is stabilized by being spread around to increase stability, and in the case of carbocations, it is the charge that is being spread around to increase stability (See Golden Rules).

3. The predominant site of halogen reaction corresponds to the site of the most stable radical, therefore, the order of reactivity during the halogenation reaction is: tertiary H > secondary H > primary H.

4. Allyl radicals and cations are stabilized by resonance with adjacent double bonds (pi-way). The unpaired electron density (radical) or positive charge (cation) is spread over a larger area, and that is stabilizing.

5. Allylic halogenation uses NBS and light to place a Br atom adjacent to a C=C bond. The mechanism is a free radical chain reaction, involving an allyl radical intermediate. This is VERY tricky because multiple sites can be considered for adding the Br atom, the predominant one will be the site that gives the most stable (most highly substituted) C=C in the product.

6. When deciding which possible product(s) will be the major one(s) in an allylic halogenation reaction:

A) Consider all possible allylic radical intermediates that could be formed

B) Analyze each allylic radical for both possible products

C) Choose the product(s) that is/are the most stable alkene, in other words, the alkene that is most substituted (least H atoms on sp2 C atoms) because alkyl groups stabilize alkenes more than H atoms

HOMEWORK:

Read: Sections 8.5 - 8.6.

Take the Daily Quiz 16 before 10 PM Friday night (because of Halloween tomorrow, no trick this is a treat for you). Click here to access the quiz. These quizzes are designed to review the important material from today's lecture. Together, they will count as 3% of your final grade.

Start working on the Gradescope Homework Problem Set 8, due at 10 PM on Wednesday, November 6. Click here to access the Gradescope Homework Problem Set 8.

There is no Aktive learning homework this week.The Gradescope homework is challenging and I would like you to focus on it.