Rules of the Day CH320M/328M

Welcome to our reaction festival!

1. 1° and 2° alcohols react with PBr3 and SOCl2 to give bromo alkanes and chloroalkanes, respectively, via a mechanism involving SN2 attack and thus InVERSiON of stereochemistry. The reagent SOCl2, reacts with alcohols to give chloroalkanes in a mechanism analogous to PBr3,

2. Alcohols react with sulfonyl chlorides (RSO2Cl) such as methanesulfonyl chloride or p-toluenesulfonyl chloride to give alkyl sulfonates that can be isolated. Alkyl sulfonates should be thought of as being similar to bromoalkanes in terms of how they react. The sulfonate anion is a good leaving groups, so this is yet another way to convert the OH group into a good leaving group. (Hint: do not confuse RSO2Cl with SOCl2, THEY ARE DIFFERENT)

3. Note how there is a profound stereochemical difference between the reaction sequence of a chiral secondary alcohol with PBr3 followed by reaction with a nucleophile (a net retention of configuration for the entire process) and the reaction sequence of reaction with a sulfonyl chloride followed by reaction with nucleophile (a net InVERSiON of configuration for the entire process).

4. The mechanism for alcohol oxidationsusing chromium reagents involves formation of chromic esters, followed by deprotonation and loss of HCrO3. Watch the recorded video for a complete description of this important reaction. Click here for the filled-in mechanism sheet

5. For primary alcohols, the initially formed aldehyde reacts with water to give the geminal diol, which reacts again to give a carboxylic acid with H2CrO4. Because the PCC reagent does not have water, the geminal diol does not form and the PCC reaction stops at the aldehyde stage.

6.Thiols can be oxidized to sulfinic acids (RSO2H) and sulfonic acids (RSO3H), and they can form disulfide bonds (2RSH goes to R-S-S-R). Disulfide bonds stabilize the three-dimensional structures of proteins by forming a covalent link.

7. Ethers are generally unreactive in all conditions except very strong acid. They are therefore often used as solvents, and they are considered to be polar aprotic solvents. They do not dissolve very well in water because they can only accept hydrogen bonds, they cannot donate them.

8. Ethers are synthesized using an SN2 reaction between a primary haloalkane and an alokoxide (called the Williamson ether synthesis).

9. Crown ethers are cyclic polyether molecules that have an electron dense inner cavity and hydrophobic outer surface. They are thus capable of making small cations like K+ soluble in organic solvents.

10. Ethers can also be synthesized using an alkene that can form a relatively stable carbocation and an alcohol, catalyzed by acid (H2SO4) "Same song different verse" when compared to acid catalyzed hydration of an alkene.

11. Epoxides are important because the ring strain within epoxides allows them to react with nucleophiles. This provides the motive for reactions of epoxides with nucleophiles.

A) When strong nucleophiles attack epoxides at neutral or basic pH, the LESS hindered carbon is attacked.

B) Epoxides react with nucleophiles under acid catalysis, the nucleophile attacks preferentially MORE substituted carbon atoms that possess greater positive charge in the positively-charged intermediate. Analogous to bromonium ion intermediates.

Click here to watch a video that explains epoxide reactions in detail

HOMEWORK:

Read: No reading during exam week

There is no quiz during exam week

Continue working on 3rd Midterm Practice Homework. You will not turn it in, it is designed to help you prepare for the midterm next week. The answers are posted for your reference.