Thursday, January 24, 2013

Preparing for organic chemistry

Congratulations, you have made it through two semesters of general chemistry, next stop organic chemistry You are worried, you have heard that organic chemistry is really hard and you are wondering what you really need to know from general chemistry to make things easier. The good news for some is most of the math is not required in o-chem. There are many concepts that carry forward from general chemistry to organic chemistry, unfortunately most of these concepts were introduced in the first semester of general chemistry and many students have forgotten them by the time they take O-chem. Since very little time will be spent on these topics I feel it is important that all students refresh themselves on them before entering the course. This will help you understand what your Professor is talking about when he/she is introducing more difficult topics.

These are the areas I recommend you review:

1. Lewis structures. These are the diagrams that show the bonding between the atoms in a molecule. They also show all lone pairs on the molecules. In addition make sure you understand what a positive and negative charge means on a molecule. This is the backbone of learning how to arrow push and draw organic reaction mechanisms. Here are some examples to try. Carbon dioxide CO2, diazomethane H3CN2, methyl isocyante H3CNCO, acetamide H3CCONH2, oxalic acid (CO2H)2 .

2. Resonance forms. Be able to draw all resonance forms of a Lewis structure. Try drawing resonances on the above examples.

3. Hybridization. Understand how atomic orbitals are mixed to form new orbitals suitable for bonding. Be able to label each atom as being SP3, SP2 or SP1 hybridized. Try the above examples.

4. Understand the different intermolecular forces. (Dipole-dipole, London dispersion forces, hydrogen bonds, ion dipole, induced dipole forces). Be able to explain why they have effects on physical properties of molecules.

5. Stoichiometry. You will not need to know a lot of math for O-Chem but you will need to work out things like percentage yields and quantities of materials needed in your lab course.

6. Acid-base chemistry. Understand what makes something acidic/basic. Organic acids are all weak acids, recall that means they are not fully dissociated and are in equilibrium with its conjugate base. The lower the pKa the more acidic something is. Understand what factors affect the stability of a conjugate base and hence increases the acidity of the acid. You will not be expected to repeat the calculations from general chemistry. The reaction between a Lewis acid (accepts electrons) and a Lewis base (donates electrons) is probably the most important concept in organic chemistry. This is the back bone of reaction mechanisms.

If you understand everything above this will be a great starting point for the new materials in organic chemistry. Good luck.