Rules of the Day
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Featured Golden Rules of Chemistry: 6. Delocalization of unpaired electron density over a larger area is stabilizing.
1. 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.
2. 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.
3. 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
4. Using a peroxide (ROOR) and light in the presence of HBr and an alkene causes a free radical chain reaction that leads to non-Markovnikov addition of the HBr. This is a very useful reaction that can make primary haloalkanes from terminal alkenes. Dr. Morris Kharasch figured this all out and made Markovnikov famous, but no one remembers poor Morris!
5. A leaving group is a group capable of making a single bond to carbon that can create a stable species when it departs. The largest three halogens (I, Br, Cl) are good leaving groups, but not F.6.There are four different mechanisms possible when a haloalkane reacts with a nucleophile/base. It is important to categorize the nucleophile/base with respect to nucleophile and base strength. Click here for a table of nucleophiles grouped by nucleophile and base strength and a mechanism decision map.
A) SN2 (Substitution, Nucleophilic, Bimolecular) - reaction involves a single step in which nucleophile attacks backside of a carbon-leaving group bond (the electrophile), making a new bond as the leaving group departs. Click here to see a movie of an SN2 reaction. There is an InVERSiON of stereochemistry at the site of an SN2 reaction.
B) E2 (Elimination, Bimolecular) - anti-periplanar - involves a base reacting with the proton that MUST be anti-periplanar to the leaving group (that simultaneously leaves) in a single step to give an alkene. Click here to see the movie.When there is a choice in an E2 reaction, the more stable alkene predominates. This is referred to as Zaitsev's rule.
HOMEWORK:
Read: Sections 9.3 - 9.4.
Take the Daily Quiz 17 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture. Together, they will count as 5% of your final grade.
If you finish Homework Problem Set 7, you can start working on Homework Problem Set 8, due at 10 PM on Friday, November 11. Click here to access the Homework Problem Set 8. Note there are Aktiv Learning and Gradescope Questions, and you MUST DO BOTH by November 11.