Rules of the Day CH320M/328M

Featured Golden Rules of Chemistry: Nothing today

1. Many reaction mechanism intermediates and products are chiral and formed as racemic mixtures. To avoid having to write too many structures, for mechanism questions, you will only need to draw one of the stereoisomers formed as an intermediate or product but you will have to LABEL ALL CHIRAL CENTERS WITH AN ASTERISK (*) AND WRITE "RACEMIC" EVERY PLACE IT IS TRUE.

2. Alkenes react with X2 in the presence of water to give halohydrins. Holonium ion intermediates again so anti adition leading to trans products. Regiochemistry is Markovinikov (OH ends up on more substituted carbon atom) because of extra charge on the more substituted carbon atom of the halonium ion intermediate.

3. Of the intermediates we have seen or will see, only carbocations rearrange, all other intermediates such as the halonium ions do not.

4. When two new chiral centers are produced in an alkene reaction, you need to work through each example on a case-by-case basis just like the Dude says. Make sure you follow the regiochemistry rules to understand which consititutional isomer(s) will predominate (Markovinikov, non-Markavnikov, etc). Then apply the anti, syn, or mixed geometry of addition based on attack from either face of the flat alkene to predict which stereoisomer products are produced. Review the two stereochemistry Lecture sheets (stereochemistry of addition 1 and stereochemistry of addition 2) to help understand this.

5. When trying to predict reaction products, first recognize the chemistry based on the reagents (HBr, H2O/H2SO4, Br2/H2O, etc.), then work out the regiochemistry (which carbon gets what new bond, Markovnikov, etc.) then see if any chiral centers are created. If they chiral centers are formed, focus on the stereochemistry of the reaction (anti, mixed, etc.) to determine which of the possible stereoisomers are made ON A CASE BY CASE BASIS. DO NOT LOOK FOR ONE-SIZE-FITS-ALL SHORT CUTS OR HACKS THAT DO NOT EXIST. CASE BY CASE IS THE ONLY WAY TO GO.

6. For each reaction of an alkene we have covered you need to remember to call a NIRRS:

Nature of the reaction; what is the starting material/product? (i.e. alkene converted to an alcohol)

Intermediate (or "Important transition state" if applicable) of the reaction, the key to the mechanism (carbocation, halonium ion, etc.)

Reagents Learn the exact way to designate the reagents for each reaction

Regiochemistry What is the regiochemistry of addition? (Markovnikov, non-Markovinikov, etc.)

Stereochemistry of addition (anti, syn or mixed)

7. When studying for the exam, remember this material is very different then the first midterm material. You CAN do this. I suggest you follow these steps before looking through old exams: 1) Read each Rule of the Day so you can identify those things you are not familiar with. 2) Binge-watch the lectures recordings, skipping the things you undersrtand and spending time watching the things you do not yet understand (last year's class used this study method and they did great!) 3) Consult the Missed the Wave topics to see if you can get help from those recordings 4) Ask questions at office hours and the Review Session on Tuesday night 5) Start working through the old exams.

8. Alkenes react with 1) BH3, 2) H2O2/HO- to give an alcohol. The reaction involves a four-membered ring transition state in which the H ends up on the more highly substituted carbon, largely for steric reasons, so the reaction shows non-Markovnikov regiochemistry. Note the BH3 reacts three times, so one BH3 molecule reacts with three alkenes. The last step does not mix the stereochemistry, so overall it is syn stereochemistry.

HOMEWORK:

Read: Sections 6.5 and 6.6 in the eBook.

Take the Daily Quiz 13 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture.

Finish Working on Homework Problem Set 6, due at 10 PM on Wednesday, October 15.

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Start Working on Homework Problem Set 7, due at 10 PM on Wednesday, October 22.

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