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by: Alice Hsu

CHEM 51 WEEK 2 NOTES Chemistry 51

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this week in organic chemistry, we covered nomenclature, conformational analysis, isomers, and alkanes and cycloalkanes.
Organic Chemistry
Peter Jacobi
Class Notes
Chemistry, Organic Chemistry, alkane nomenclature, cycloalkane conformations, Conformations, isomers, discussion of constitutional and stereochemical isomers, Alkanes, Cycloalkanes
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This 7 page Class Notes was uploaded by Alice Hsu on Saturday September 24, 2016. The Class Notes belongs to Chemistry 51 at Dartmouth College taught by Peter Jacobi in Fall 2016. Since its upload, it has received 3 views. For similar materials see Organic Chemistry in Chemistry at Dartmouth College.


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Date Created: 09/24/16
CHEM 51 WEEK 2 NOTES ALKANES AND CONFIGURATION Functional groups: a group of atoms that show characteristic chemical behavior and are relatively independent of their molecular environment. Three classes of functional groups 1) Carbon-carbon single, double, and triple bonds, and arenes 2) Polar sigma-bond: carbon bonded to more electropositive or electronegative element 3) Multiple bonds between carbon and a more electronegative element These groups are everywhere: take a biologically active compound, like a prostaglandin, for example. Alkanes are the least reactive of all functional groups, mainly because they contain a nonpolar sigma bond. They undergo mainly two different kinds of reactions: 1) Combustion (oxidation reaction) ∆ ???????? 3???? ????2 + 53 → 3????2 + 4???? ???? 2 ℎ???????????? 2 2) Halogenation (free radical halogenation) – a substitution reaction ℎ???? ???????? ∆ ???????? ????3 + 3???? → 2 ???????? 3???? ????????2+ ???????????? General formula for alkanes: ???? ???? ????????+???? Many different structures are possible for alkanes, which have the same molecular formula. For instance: C6H14. These are isomers: same molecular formula, different structural properties Stereochemistry: the study of molecules in three dimensions  Configurational isomers: isomers having different bond connectivity  Stereoisomers: isomers having the same bond connectivity, but different projection in space o Configurational/Constitutional stereoisomers: Interconversion involves breaking sigma- bonds (see above) o Geometric stereoisomers: Breaking pi-bonds o Conformational stereoisomers: can be rotated around sigma-bonds Conformational analysis of ethane, a stereoisomer: If we look down the C-C bond, the Newman diagram looks like this: In the staggered configuration, there are 60 degrees between the front H and the back H. There are 0 degrees in the eclipsed position. In the eclipsed position, the proximity of the hydrogens cause a torsional strain of about 4 kJ each. Each space-bond in ethylene is full of electrons, and in the eclipsed position, the sigma-bonds feel too much repulsion from the electron pairs, unlike in the staggered position. Torsional strain: through space-bond repulsion. This is the energy diagram with respect to dihedral angle. Conformational analysis of propane: CH3CH2CH3 Looking down the 1-2 carbon bond, the Newman diagram: Repulsion between sp3- sp3 sigma bonds > repulsion between sp3-s sigma bonds, so the torsional strain between CH3 and H is 6 kJ. Thus, the total torsional strain on pentane in the eclipsed form is 4+4+6 = 14 kJ. Conformational analysis of butane: CH3CH2CH2CH3 Looking down the 2-3 carbon bond, the Newman diagram and energy diagram: 1) Anti-conformation: staggered when the CH3s are as far away as possible. 0 kJ. 2) Eclipsed, 6+6+4= 16 kJ. 3) Gauche formation: when it is staggered, but the CH3s are not 180 degrees away from each other. This causes steric strain between the methyl groups = 3.8 kJ. 4) When the methyl groups conflict: this is 11 kJ + 4 + 4 = 19 kJ, where the 3.8 is factored into the 11. Values to memorize: Strain kJ H-H torsional strain 4 H-methyl torsional strain 6 Methyl-methyl torsional + steric strain 11 Methyl-methyl steric strain 3.8 Kinds of strain and where they come from: Strain How Torsional (kJ) electron density repulsion Steric (kJ) repulsion due to size of group Angle (degrees, excess energy as a result in kJ) angle bent out of ideal, Total Calculated from combustion reaction CYCLOALKANES AND CONFIGURATION Conformational analysis of cyclopropane: a special case  Is perfectly flat (angles = 60)  Lots of angle strain  Overall strain energy = 115 kJ/mol (As energy increases, the stability decreases.) 1) Torsional strain: We have perfectly eclipsing C-H bond groups in a cyclopropane, 3 on top and three on the bottom, for a total of 24 kJ of energy for torsional strain. 2) Steric strain: We don’t have steric strain unless one of the C-H bonds is substituted for a group that is much larger such as a methyl group. This methyl group may or may not add steric strain depending on whether or not the n-methyl- cyclopropane is cis or trans. In cis formation, we do have steric strain, making the top face 23 kJ and the bottom face 12 kJ for 35 kJ in total. In trans formation, we do not have steric strain because there is no methyl-methyl interaction and so each face is 16 for a total of 32 kJ. 3) Angle strain: The angle for cyclopropane is 60 degrees, so angle strain itself is 109.5-60 = 49.5 degrees. Calculating the amount of energy the bonds contain based on combustion data and the total torsional + steric strain, we get 115-24 = 91 kJ/mol of energy from angle strain. This is due to the fact that there is not enough overlapping electron density, and thus it takes more energy to form the bond. This minimal electron density results in a total bond strength of 61 kcal/mol as opposed to normal propane, which is 88 kcal/mol. Conformational analysis of cyclobutane:  Total strain energy based on combustion analysis = 110 kJ  IF planar, torsional strain = 32 kJ  Angle strain: based on a perfectly flat cyclobutane, 109.5-90 = 19.5 degrees.  BUT cyclobutane is not perfectly planar. In fact, it puckers so that there is less torsional strain, despite creating more angular strain. So, we cannot predict angular strain energy because it is no longer based purely on perfectly a perfectly planar cycloalkane. Conformational analysis of cyclopentane:  Total strain energy based on combustion analysis = 26 kJ  Angle strain, if flat: 109.5-108 = 1.5 degrees  Torsional strain for a perfectly planar ring: 20 + 20 = 40 kJ/mol Because torsional > total, we know that it is impossible for cyclopentane to actually be flat. Thus, there must be relief by puckering: Puckering lowers the overall torsional strain while relatively insignificantly increasing the energy from reducing the electron density and increasing angular strain to 14 kJ/mol. Conformational analysis of cyclohexane:  Angular strain, if flat: 120-109.5 = 12.5 degrees Basically being in the perfectly flat conformation is absolutely terrible, so the cyclohexane puckers into different conformations: Chair conformation is perfectly staggered, and so there is no torsional or angle strain (total = 0kJ). Boat conformation is significantly less stable at a total of 29 kJ due to steric strain from the flagpost H-groups (blue) and a total eclipsing of the bonds. Red Hs are equatorial, where this is more space, and black Hs are axial.


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