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Rules of the Day

2-27-2024

Click here for a copy of the lectures notes I wrote in class

Click here for a copy of the handouts I used in class today

Featured Golden Rule of Chemistry: 10. A reaction mechanism describes the sequence of steps occurring during a reaction.10. A reaction mechanism describes the sequence of steps occurring during a reaction.

When learning mechanisms, the key to "catching the wave" is to always ask yourself WHY each step happens the way it does. When you start understanding the answers to that question at each step, the whole thing will make sense.

You should keep a few things in mind when writing these complex mechanisms to make sure you are not making any mistakes: 1) Identify the bonds to be made and broken 2) Avoid "mixed media errores". In acid all intermediates are positively-charged/neutral, in base all intermediates are negatively-charged/neutral, in neutral solution the intermediates can be positvely-charged or negatively-charged 3) Proton transfers are fast 4) Evaluate properties of intermediates to predict next step.

1. The characteristic reaction of carboxylic acid derivatives is Mechanism B. If the attacking nucleophile is an alkyl group or hydride, a ketone or aldehyde is produced as an intermediate that then continues to react via Mechanism A to give the alcohol product.

2. Reactions with rings are harder to keep track of, so I suggest that you number the ring atoms and use the following for practice.

Microscopic Reversibility Practice Sheet 3: Lactone Hydrolysis

Microscopic Reversibility Practice Sheet 4: Lactone Formation

3. Esters require acid catalysis or base promotion to hydrolyze. These are the reactions used to make soap or biodiesel from triglycerides (the glycerol triester of fatty acids).

4. Heating in very strong aqueous acid hydrolyzes amides in a mechanism analogous to the acid catalyzed ester hydrolysis reaction except for amides the reaction is only promoted by acid, not catalyzed by it, because the last step involves transfer of a proton to the amine group that departed.

5. Nitriles are hydrolyzed in strong acid to give carboxylic acids, a mechanism that involves formation of amides.

6. It is best to hydrolyze esters in base and amides in acid, although either will hydrolyze in acid or base.

7. Acid chlorides, anhydrides, and esters (need heat) react with amines involving an initial attack to create a zwitterionic tetrahedral intermediate, followed by a proton transfer, followed by loss of the leaving group to give the amide.

 

 

Homework:

Read: Sections 19.1-19.3 in the ebook textbook. This text is part of the Longhorn Textbook access program.

Take the Daily Quiz 10 before 10 PM tomorrow. Click here to access the quiz.

Start working on the Homework Problem Set 6, due at 10 PM on Monday, March 4. Click here to access the Homework Problem Set 6. Remember to do both the Aktiv learning and Gradescope portions of the homework! We will grade one problem for accuracy, and the rest for completeness.