I teach general and organic chemistry to high school and University students. I have tutored many different courses from various colleges.
I currently have organic students from Dr Gould's (ASU), Dr. Pruis(ASU) and Dr Grubb's (GCU) courses. In previous years I have tutored courses from other community colleges and even online courses.
I have several general chemistry students from community colleges, ASU, and some highschools. My high school students are studying for honors, AP and IB chemistry.
I also help people prepare for the MCAT, DAT and OAT.
Please enjoy my blog and visit my website
Arizona chemistry tutor website
I also tutor for a great agency. If you are looking for tutoring in any other subject I recommend Wyzant.
Sunday, October 14, 2012
Students often complain there is a lot to remember in organic chemistry especially the hundreds of different mechanisms. I reply in disagreement, I think its the lack of understanding the curly arrows and perhaps the lack of pattern recognition that makes people think there is a lot to memorize. I think if you really understand your arrows you should be able to predict mechanisms. In this post I aim to make you understand curly arrows and the definitions that often accompany them.
Curly arrows are used by organic chemists to show the movement of electrons, in basic conditions this is referred to as electron pushing. Some people like to use the term electron pulling when in acid conditions since its the pull of the positive charge of the acid which drives the reaction forward. Remember a covalent bond is basically two electrons being shared between two atoms. Curly arrows are used to indicate bonds being formed and bonds being broken. I like to make the analogy of 2 people. One person is holding a ball with a tail attached to it, this person throws the ball to the other person but keeps hold of the tail. The path of the ball from one person to the other is the curly area, the tail between the two people is the bond.
Electrons always have to flow from an area where there is more electrons (HOMO, highest occupied molecular orbital) to an area which may be lacking in electrons (LUMO, lowest unoccupied molecular orbital). When we use a curly arrow we are indicating the movement of two electrons from the tail to the head of the arrow. If we only want to show the movement of one electron we use a fish hook as in free radical reactions.
Bonds being made
A new bond may be formed when none bonded electrons move towards a positive center. You may see the area the electrons are moving from referred to as a nucleophile. A simple definition of a nucleophile is an electron donor (Lewis Base) and is described a nucleus loving. While the area where the electrons are accepted is called the electrophile (Lewis acid) and is described as electron loving.
The tail is on the end where the none bonded electrons are and the head points to where the new bond attaches to.
Bonds being broken.
The arrows tail begins in the middle of the bond being broken to indicate that’s the bond being broken. The head of the arrow goes to the element which accepts the electrons from the broken bond.
It should be noted that over all in a reaction charge is conserved.
General rules for arrow pushing.
- Identify which is the nucleophile and while atom is the electrophilic centre.
- Decide what is the driving force of the mechanism in other words where are the electrons being pushed from.
- Draw the molecules in such a way that you can show the bonds being formed and broken.
- Curly arrows should always flow in one direction and never meet head on.
- Mark charges on all reactants and intermediates.
Thursday, October 11, 2012
If you reading this you have probably completed all the general requirements to take the MCAT. Thinking back over your semesters of chemistry you would of used your calculator a lot to complete the math problems ranging from the basics of stoichiometry to the complex problems of solving the pH of a weak acid. You may be wondering how on Earth are you expected to solve logs and square roots without a calculator. Bear in mind its a multiple choice exam so the math is actually done for you, so all you have to do is approximate and pick the best answer.
As a chemistry tutor I would like to offer some of my tips for getting through the mathematical problems of the exam. If you have not studied general chemistry for several years I suggest you get really familiar with the different types of calculation problems. The easiest way of doing this is to pick up a text book and work through the different types of problems. Initially build up your confidence using your calculator. Once you understand the problems, find a set of multiple choice questions and practice without a calculator.
Although I have not got room to tell you all tips for the MCAT in this blog post, I would like to share with you some simple tips.
- Know exactly what the question is asking you. The only way to get good at this is practice.
- When ever possible approximate and eliminate any wrong answers quickly.
- Do not take too much time solving complex problems, remember all questions are worth the same amount of points. If you spend along time solving one problem you may run out of time and miss out on many more simple points.
- Round your figures to make approximations. For example 94 rounds up to 100, so 94 could be thought of as 100-6. 273 rounds up to 300, so 273 could be thought of as 300-27. Now we can add up the terms 100+300=400, 27+6=33, so 273+94=400-33=367
- Square roots of scientific notation seems really scary for example what is the square root of 5.2X10-9? Lets make it simple. Do you know your square table? 1X1=1, 2X2=4, 3X3=9 etc. Lets look at this scary root again. This time lets change the 5.2 to 52. What is the nearest square number of 52? Its 7X7=49. Since we have changed the number to 52 we need to change the scientific notation to X-10. To calculate the square root of this side we simple divide by 2. So the answer becomes approximately 7X10-5. On the calculator the answer is 7.2X10-5. Not so scary after all!
- Logs as in pH calculations. Change the number that you are trying to take the log of to scientific notation. For example 0.0001 becomes 1X10-4, so the -log of that becomes 4. If the number was 2X10-4, the pH would be less than 4 but a lot more than 3. It may also be useful to remember log 1=0, log 2=0.301 and log 3=0.477 as you should be able to solve any log question if you know these values. For example if you need to calculate log 6 its simply log (3X2)=log3+log2=0.301+0.477=0.778. Going back to the above problem -log(2X10-4)=-(0.301-4)=3.7
- Another useful approximation is knowing the equation pH=0.5pKA-0.5log[HA] using this equation you should be able to solve the pH for a weak acid very easily.
- For buffer solutions we can use the Henderson Hasselback equation to approximate. pH=pKa+log[moles of conjugate base/moles of conjugate acid]. This means if the ratio of acid to base is approximately 1 then the pH=pKa. Looking at the equation how do you think the pH will be affected if there is more acid to base and vice versa? Remember buffers are only affected by the ratio of base/acid not the concentrations.
I hope this post has given you some insight into preparing for the MCAT. If you have enjoyed reading this post, please email me and arrange a tutoring session.
Please note I have also published this post under my blog at http://www.wyzant.com/Tutors/chemistryheidi
Tuesday, October 9, 2012
Drawing Cyclohexane rings.
Students often complain they are terrible at drawing organic chemistry structures especially chair conformations of the cyclohexane rings. Since getting the questions right often depends on the person marking your papers being able to interpret your ring correctly I feel its worth spending extra time getting your ring right.
I will walk you through two methods for getting a perfect ring. Firstly it is unlikely you will be able to draw a perfect ring without picking your pen up off the paper. Please refer to my diagrams when reading the methods
Step 1. Start with a pair of lines drawn in pencil as guides. Draw a triangle as shown in the picture.
Step 2. Draw a pair of parallel lines coming off this triangle. Note the lines should not be horizontal but pointing upwards towards the pencil line. The two top points should both be on the upper pencil guide line.
Step 3. Close the ring by drawing two more lines to make another triangle.
The second method makes use of the fact it is made up of three sets of parallel lines. In my example I have chosen the middle two lines to draw first, you may prefer the end two. As you can see in both my examples of have used the same color for the lines that are parallel.
Step a. I have again started with a pair of lines drawn in pencil as guides. Although not essential to draw the lines it is important that the two top points should be on the upper pencil line.
Step. 2 more parallel lines.
Step c. Close both sides of the ring with another pair of parallel lines.
Now that you have the perfect ring we need to place the axial and equatorial bonds. Once the perfect ring has been drawn its easy to place the axial bonds. They should all be drawn vertical. I find it easiest to start on a bottom point and draw a vertical line down, any up points should have a vertical line up. This is an alternating pattern of up and down. When ever I answer a problem involving energy conformations I will always start with a quick diagram of a cyclohexane ring with the axial positions drawn in.
One common mistake is to start with two lines horizontal. This makes it difficult to put the axial positions on correctly as they would no longer be vertical. Another common mistake is to have the axial position going in the wrong direction i.e going up when it should be going down. This makes it impossible for the carbons to be tetrahedral. Do not draw your rings like the examples in the next two pictures.
Next drawing the equatorial positions. Notice if the axial position goes down the equatorial position goes up. This is why I recommend putting the axial positions in first. The equatorial positions should be drawn parallel to the lines in the ring. Again in my example I have color coded which line is parallel with each side. Its worth noticing that there should be 3 sets of 4 lines now all parallel.