Rules of the Day
2-20-2025
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. Esters require acid catalysis (or base promotion, see below) to hydrolyze. The acid catalyzed ester hydrolysis mechanism is the functional reverse of the Fischer esterification mechanism, illustrating the concept of microscopic reversibility.
3. Microscopic reversibility predicts that you encounter the same intermediates in either direction of a reversible reaction. You will also see the complement mechanistic elements (make a bond vs. break a bond and add a proton vs. take a proton away) at each step in either direction. Is it really true? Can mechanisms really make that much sense? Mind officially blown.
4. 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
5. 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.
6. Nitriles are hydrolyzed in strong acid to give carboxylic acids, a mechanism that involves formation of amides.
7. Base promoted ester hydrolysis is just Mechanism B followed by an acid base reaction.
8. 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).
Homework:
Read: Sections 18.8-18.11 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.
Continue working on the Homework Problem Set 5, due at 10 PM on Wednesday, February 26.
When you are ready to upload your Gradescope answers click here (If you are using OneNote you will have an extra front page that MUST be removed before uploading!)There is also an Aktiv learning homework so remember to do that as well. Click here to access the Aktiv Learning Homework 5.
