Rules of the Day 320N

. A reaction mechanism describes the sequence of steps occurring during a reaction.

1. The general features of carbonyl reaction mechanisms can be related to the plot for the first Star Wars movie starring Nuc "the nucleophile" Skywalker, Old "Pi Bond" Kinobe, Han "the proton" Solo, and their faithful robots sp3O and SN2E2.

2. It is important that you are able to write a balanced equation for each mechanism that we cover. To do this, keep track of all the atoms in all of the products produced during a reaction mechanism and balance that on the starting materials side of the equation. This will help you keep track of "acid or base catalyzed" vs. "acid or base promoted" as well as EQUIVALENTS. Doing this establishes a critical link between reaction mechanisms and how to use them properly in synthesis reactions!There are four new concepts associated with chapter 19. Click on these one at a time to get more information.

A) Reactions favor the formation of weaker bases at equilibrium (provides a motive), so you need to have a Table of pKa values

B) Enolates as nucleophiles

C) Beta-Dicarbonyl species are especially acidic

D) Conjugate addition to alpha,beta unsaturated carbonyl species.

3. The C atom next to a carbonyl is called the apha carbon, and a carbon atom two carbon atoms away is called the beta carbon. H atoms attached to the alpha carbon are called alpha hydrogens, and H atoms attached to the beta carbon are called beta hydrogens.

4. Important acid-base concepts to remember are that 1) acid-base reactions favor formation of the weaker acid/weaker base (quantitatively, the equilibrium constant can be derived from the difference of the pKa values of the acids), 2) Weaker conjugate bases are derived from stronger acids and vice versa, and 3) stronger acids have lower pKa values. Putting 2) and 3) together means that conjugate bases of acids with lower pKa values are favored at equilibrium.

5. Enolates are stabilized by a combination of delocalization of charge and formation of a pi-way. By far the most important reactions of enolates are with carbonyl groups to make carbon-carbon bonds.

6. When a catalytic amount of base is added to an aldehyde, the enolate that is formed reacts with an aldehyde carbonyl via mechanism A to create a new carbon-carbon bond. The reaction is called the aldol reaction. See how the concept of enolates was ripped of in an old popular movie staring Mike Myers. Often, two new chiral centers are created in an aldol reaction, meaning racemic mixtures of two pairs of enantiomers are created. Aldol reactions run using catalytic base are favored for aldehydes, but not ketones.

7. Aldol products can dehydrate to give a mixture of E and Z alpha,beta unsaturated aldehydes when heated in dilute acid. The more stable alkene (usually E) generally predominates.

Here are somepractice problems. You will not turn these in, but I have posted answers for you to check.

If you are having even a little trouble with mechanisms: Print out multiple copies of the new Mechanism Sheet 18 Microscopic Reversibility. Practice writing arrows and reagents in both directions (Acid catalyzed ester hydrolysis, starting at the top then working down and Fischer esterification, starting at the bottom then working up), taking advantage of the common structures drawn in red. Try to UNDERSTAND why each step occurs and why acid catalyzed ester hydrolysis uses water or H3O+ in several steps while Fischer Esterification used an alcohol or protonated alcohol. Do this several times until it makes sense. Then print out several copies of the Fischer esterification and acid catalyzed ester hydrolysis sheets and work them until it all makes sense. Keep asking yourself why each step occurs as it does (add a proton, make a bond etc.) until you understand why every step is the best choice. Compare your answers to the sheets you filled out in class.

If you are having even a little trouble with synthesis questions: You must know all the reactions, full stop. After you know your reactions very well, you need to learn the KRE's. The best way to learn these is to turn it into a game. Find some friends and get a list of all the reactions we have learned this semester. Each of you needs to write down an example of each reaction we have learned, with the product on a separate sheet of paper. For example, write an n-butyl Grignard reacting with 2-pentanone and put the product on a separate sheet. Make up examples of each reaction. Then give the product sheet to your friends and have them write down possible reactions that gave the products. There are often several ways to make the same product by the way. Do this for an hour and you will know your KRE's. When working synthesis problems, work backwards. Count carbons in products vs. reactants to identify where new C-C bonds are formed. When you know where the new C-C bonds are formed you can identify KRE's. Be systematic and you will get these.

Homework:

Read: Sections 19.3, 19.5 in the ebook textbook. This text is part of the Longhorn Textbook access program.

Take the Daily Quiz 11 before 10 PM tomorrow. Click here to access the quiz.

There is no homework because of the exam this week.