Chem 2750 Test 2 Study Guide
Chem 2750 Test 2 Study Guide Chem 2750
Popular in Organic Chemistry I
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This 5 page Study Guide was uploaded by Tyler Ebeling on Saturday February 20, 2016. The Study Guide belongs to Chem 2750 at East Carolina University taught by Shouquan Huo in Fall 2016. Since its upload, it has received 184 views. For similar materials see Organic Chemistry I in Chemistry at East Carolina University.
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Date Created: 02/20/16
Instructor: Shouquan Huo Organic Chemistry Test 2 Study Guide - There are two types of conformations (Also known as Newman Projections): Staggered and eclipsed. Staggered is more stable than eclipsed. This is because they are on completely opposite sides of the molecule, so they counteract each other’s charge and stabilize the molecule. Eclipsed is less stable, and is when the bonds line up parallel to each other. Conformations fo cyclohexane -Cyclohexane has an ideal energy of 0, so is the most common molecule that we will be working with this semester. -The “chair” conformation is free of strain, and is the lowest energy conformation of hexane. Instructor: Shouquan Huo “Cis” and “Trans” isomers -The “trans” isomer is the result when two substituents point in opposite directions. -The “cis” isomer is the result when two substituents point in the same direction. Axial position: up/down Equatorial position: Left/right A larger substituent molecule (Substituent = a molecule coming off of the “parent” molecule) almost always goes in equatorial position, because that is the most energy efﬁcient (lowest) conformation. (If it were to go in axial position, it’s molecules would be too close to the molecules of the parent molecule, and the charges would repel each other). Chapter 4: Isomers Isomers: non-identical compounds having the same molecular formula. Chirality -Achiral objects have superimposable mirror images (One half of the molecule is a perfect mirror image of the other half). -Chiral objects have non superimposable mirror images (e.g. your hands) Asymmetric center: This is the cause of chirality in a molecule. An sp3 carbon bonded to 4 different groups is an asymmetric center. Stereocenter: an atom at which the interchange of two groups produces a stereoisomer (more general than an asymmetric center). Changing bonds at a stereo center changes “cis” to “trans” (only if the two groups are different e.g. ethyl and methyl. The reason the blue Carbon isn’t a stereocenter is because it’s attached to two of the same CH3 groups) Instructor: Shouquan Huo ***** It’s really important to understand what a stereo center is, and what a stereoisomer is in order to do well on this test***** I’m going to deﬁne a few simple (yet very important) terms that are pretty much 100% going to be on the test. A mesocompound is a compound that has chirality centers but is achiral. In other words, it is two mirror images that can be manipulated to match one another (aka they are sumperimposable). To put it very simply: the molecules appear different, but if they are moved around a bit, you can see that they are exactly the same molecule. An isomer is a molecule that has the same chemical formula, but different arrangement of the atoms in space. Constitutional isomer: and isomer that typically has different connectivities, for instance, CH3OCH3 as compared to CH3CH2OH. They’re both C2H6O, but the oxygen is bonded in a different spot. Stereoisomer: generally, the atoms are attached in the same places, but arranged in different ways. Diastereomer (diastereoisomer): are also stereoisomers, but they aren’t mirror images of each other. Below are a few molecules that are stereoisomers and diastereomers. A VERY important concept that was introduced in class but is difﬁcult to understand was R/S Conﬁguration: R/S conﬁguration is basically labeling molecules from highest to lowest priority, and then assigning to them either the R or S conﬁguration. The reason this is important is because it provides a way to identify individual stereoisomers that would otherwise be indistinguishable. To assign the R or S conﬁguration, you have to follow a set of rules, much like nomenclature. 1) Examine the atoms directly attached to the stereocenter. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number. Hydrogen is the lowest possible priority substituent, because it has the lowest atomic number. When visualizing the molecule, the lowest priority substituent should always Instructor: Shouquan Huo point away from the viewer (a dashed line indicates this). If it doesn’t point away, then you have the choice to either mentally rotate the entire molecule 180º, OR simply reverse whatever you got as your answer. For instance, if something seems to be R oriented (clockwise), but hydrogen points to the front/the highest priority group points to the back, then instead of rotating the entire molecule, simply reverse your answer. So the answer would be S because, even though it’s clockwise (R), the lowest priority group isn’t pointing to the back, so it’s really S. 2) If there are two substituents with equal rank, proceed along the two substituent chains until a difference is found. Whichever difference has the atom with the highest atomic number is the highest priority. (E.g. CHOH > CH3) 3) If a chain is connected to the same kind of atom twice or three times, check to see if the atom it is connected to has a greater atomic number than any of the atoms that the Instructor: Shouquan Huo competing chain is connected to. (E.g. COOH > CHOH, because COOH has O,O,H substituents, whereas CHOH only has O,H,H) YOU NEED TO BE ABLE TO WRITE THIS INTO THE NAME OF A MOLECULE The ﬁnal important thing that you need to be able to do for this last test is knowing how to name alkenes. If you didn’t have any trouble with naming alkane compounds on the ﬁrst test, then you’ll do just ﬁne. If you DID have trouble, the only real way to get better at it is to practice. The best places to practice are on the homework, or in the book if you have the solutions manual. If you’re having trouble naming simple compounds, I would suggest watching videos on Khan Academy, as I’ve found them to be very informative and easy to understand, considering they’re taught by American people (no offense to Dr. Huo, he’s just difﬁcult to understand at times). GOOD LUCK AND I HOPE THIS HELPS!
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