Rules of the Day
Nucleophilic Aromatic Substitution and Carbohydrate Recorded Modules 4-2023
Click here for a copy of the lectures notes I wrote during these modules
Click here for a copy of the handouts I used in these modules.
Click here to watch a video I made that explains the nucleophilic aromatic substitution reaction. THIS MATERIAL WILL BE ON THE FINAL
Click here for the first Carbohydrate Video (Module) you are also required to see. THIS MATERIAL WILL BE ON THE FINAL
Click here for the second Carbohydrate Module Video (Module) you are also required to see. THIS MATERIAL WILL BE ON THE FINAL
Click here to watch a video I made that explains the nucleophilic aromatic substitution reaction. THIS MATERIAL WILL BE ON THE FINAL
1. If an aromatic ring is loaded with electron withdrawing groups, it is no longer nucleophlic, but can react as an electrophile. In this case, strong nucleophiles can displace a halogen on an aromatic ring. We will only see one example of this very rare and special reaction, in which 2,4-dinitrofluorobenzene reacts with strong nucleophiles such as amines. This is called nucleophilic aromatic substitution, and is rare compared to electrophilic aromatic substitution reactions.We will not cover this reaction in class, but you are responsible for it. Click here to watch a video I made that explains the nucleophilic aromatic substitution reaction.
Click here for the first Carbohydrate Video (Module) you are also required to see. THIS MATERIAL WILL BE ON THE FINAL
2. Carbohydrates differ by the stereochemistry at almost all of their chiral centers, which is easily seen using Fischer projections. In Fischer projections, swapping the H and OH groups at a chiral carbon changes the stereochemistry. Recall that all of the common carbohydrates are D-carbohydrates because the carbon farthest from the carbonyl has the same stereochemistry as D-Glyceraldehyde.
3. Carbohydrates exist in the cyclic, hemiacetal form in solution. Click here for a second molecule of the day that discusses much of this. These are generally shown as Haworth projections. The mechanism involves the same steps at the hemiacetal formation mechanism you learned in Chapter 16.
5. A new chiral center is created (at the anomeric carbon) as the carbohydrate cyclizes and the OH group can be axial (alpha equals axial for glucose) or equatorial (beta equals equatorial for glucose).
6. Five-membered ring carbohydrates like D-Ribose are called furanoses named after the organic molecule furan, and six-membered ring carbohydrates like D-Glucose are called pyranoses named after the organic molecule pyran.
7. Certain OH groups on carbohydrates can be substituted for N atoms in nature, a common case is N-Acetyl-D-Gluclosamine, or GlcNAC.
Click here for the second Carbohydrate Module Video (Module) you are also required to see. THIS MATERIAL WILL BE ON THE FINAL
For monosaccharides, the alpha and beta forms are in equilibrium, and their interconversion is catalyzed by acid. I FORGOT TO SAY IT IN THE VIDEO, BUT THIS PROCESS OF INTERCONVERTING ALPHA TO BETA IS CALLED "MUTAROTATION" OF A CARBOHYDRATE
8. Carbohydrate monomers can be linked together via acetal bonds, and this linkage can be alpha or beta. (For glucose, alpha is axial) This type of acetal bond is called a glycosidic bond and is named as alpha or beta referring to the stereochemistry of the anomeric carbon and also by the numbers of the ring carbons attached via the -C-O-C- linkage (i.e. alpha 1,4-). Click here for a third molecule of the day that discusses much of this.
9. You can now understand the true complexity of carbohydrates: You can have any of the different carbohydrates linked as either alpha or beta and to carbons 2,3,4 and 6. Because there are always glycosidic bonds, recall that the linkages always involve carbon 1. There are many, many combinations of different carbohydrates and different linkages and they exist in complex distributions of different molecules on cell surfaces and attached to certain proteins. We are just beginning to understand these incredibly complex structures and their functions in biology and medicine.
10. Both starch and cellulose are polymers of glucose, but starch has the alpha (axial) glucose linkages so it is bent and therefore not rigid (potatoes). Cellulose has the glucose monomers linked via beta(equatorial) glucose linkages so it is flat and the chains can pack together nicely to create rigid cellulose (wood). Click here for a molecule of the day that discusses most of this.
11. Nucleic acid have the so-called bases conected to carbohydrates via glycosidic bonds! In nucelic acids, the carbohydrates are numbered with prime numbers, 1', 2', etc.), examples include D-Ribose (in RNA) or D-2'-Deoxyribose (DNA).