Chemistry 3B Lecture 1

Chemistry 3B Lecture 1 - Chemistry 3B Lecture 1 Thursday...

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Chemistry 3B Lecture 1 Thursday 01-22-2009 The carbon is sp3 hybridized, and it is known as an allylic system. Molecules like this are in onions. The Carbon next to a pi bond which is sp3 hybridized is known as an allylic carbon. Hyrdogens attached to an allylic carbon are called allylic hydrogens. This molecule here has 1 allylic carbon which is circled; it must be sp3 hybridized and next to a double bond. It only has to be next to one double bond. Nomenclature : This is 3-chloro-1-propene, and this is 2-propen- 1-ol, and this is trans, or E -2-buten-1-ol, because it’s a disubstituted alkene. Common names include allyl chloride, allyl alcohol, and the last one doesn’t have a common name. But it is an allylic alcohol. Three different observations with respect to allylic systems: 1) This is a bond homolysis. This is propene (not allene) vs. propane. For propene, its 87 kcal/mol, and for propane it’s 98 kcal/mol. It is for a typical C-H bond in the molecules respectively. The C-H bond in propene is weaker than the analogous one in propane . The conclusion: allyl radicals are more stable than alkyl radicals. This is for primary radicals in both cases. Propene gives rise to an allyl radical. Even though it’s no longer sp3 hybridized, it used to be, so since it was an allylic carbon, it gave rise to an allyl radical. It was derived from an allylic system; we pulled off an allylic hydrogen. The bond energy for allyl radicals is weaker. The C-H bond is weakest in for tertiary systems, then secondary, primary, and it is strongest in methane (105 kcal/mol). This is a thermodynamic observation. 2) we are going to compare propene and propane again. Now there is 2 electron movement. Now hydrogen is being dissociated so H+ is formed, and C- is formed. It’s an acid-base
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reaction. What is the pka for the hydrogen? Both will be extremely weak acids. The pka for propene is 40, meaning it is a very weak acid, and the pka for propane is 50. It’s a large difference. The conclusion is that the allyl anion is more stable than alkyl anion. It has a lower pka and the proton comes off more easily giving rise to a stable conjugate base. By writing the equation down, it’s an acid base reaction. To estimate K equilibrium, it is 10^(pka product – pka reactant) and it’s a big number implying that K equilibrium lies way to the right. We can use alkyl anions to deprotonate allylic hydrogens to make allyl anions. The only method to make carbanions used to be using Grignards. Our new method only works for allylic systems, though it can still work with Grignards. An example being cyclohexene: we have a cyclohexene and an alkyl anion. You should recognize that the anion is a strong base and the hydrogens are a little more acidic in the conjugate acid of this base. This is
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Chemistry 3B Lecture 1 - Chemistry 3B Lecture 1 Thursday...

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