week 3 - 5 - Chemistry S-20ab Lecture 1 Chemistry E-2b Week...

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Chemistry E-2b: Lecture 1 January 28, 2010 2 More than One Alkene In a Molecule Molecules with 2 C=C bonds are dienes . Draw the structures of the following dienes: 1,4-cyclohexadiene 1,3-cyclohexadiene 1,3-butadiene 1,2-butadiene Molecules with 3 C=C bonds are trienes . Draw the structure of 1,3,5-cyclooctatriene. Reading : Chapter 15 (introduction)
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Chemistry E-2b: Lecture 1 January 28, 2010 3 Cumulated ! -Bonds: Allenes How close together can two C=C double bonds be in a single molecule? When two C=C bonds share a single carbon atom , they are called cumulated double bonds. The simplest compound that contains cumulated double bonds is allene , C 3 H 4 . Draw the structure of allene and determine the geometry and hybridization of each atom. Show the " - and ! -orbitals of allene. Is the entire molecule planar? Draw the structure of 1,2-cyclohexadiene. This molecule can not be prepared as a stable compound. Can you explain why? Reading : Section 15.1
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Chemistry E-2b: Lecture 1 January 28, 2010 4 Chirality Without Stereogenic Carbons Draw the structure of 1,3-dimethylallene. Build a model with your model kit. Draw the structure of this molecule in the space below. Are there any stereogenic carbon atoms? Is your model exactly the same as that of your neighbor? If not, what is the relationship between the two models? Here is another example of a chiral molecule that does not contain a stereogenic center. In what way is it similar to an allene? Me HO 3 S Me SO 3 H Reading : Section 15.1
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Chemistry E-2b: Lecture 1 January 28, 2010 5 Conjugated ! -Bonds: 1,3-Dienes When two double bonds are on adjacent carbons they are said to be conjugated . Draw some examples of conjugated dienes. Why are these compounds often called 1,3-dienes? Conformational analysis of rotation around the single bond in 1,3-butadiene shows that it has two stable conformations, both of which are (nearly) planar . Why are these conformations particularly stable? Which one is more stable, and why? s -cis conformation -trans conformation Draw the ! -molecular orbitals (MOs) for 1,3-butadiene. Can you explain why the planar conformations of butadiene are preferred? Reading : Section 15.1
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Chemistry E-2b: Lecture 1 January 28, 2010 6 Multiple Conjugated p -Orbitals When a lone pair, vacant p -orbital, or radical (unpaired electron) is adjacent to a ! -bond, the orbital in question can become conjugated with the ! -bond. For each of the following structures, count how many p-orbitals are conjugated together, and count the total number of !
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week 3 - 5 - Chemistry S-20ab Lecture 1 Chemistry E-2b Week...

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