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Three Factors That Affect Carbocation Stability.pdf - Three...

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Three Factors That Affect Carbocation StabilityThree main factors increase the stability of carbocations:Increasing the number ofadjacentcarbon atoms: methyl (least stable carbocation) <primary < secondary < tertiary (most stable carbocation)Adjacent pi bonds that allow the carbocation p-orbital to be part of a conjugated pi-system system (“delocalization through resonance”)Adjacent atoms with lone pairs1. Three Factors That Stabilize CarbocationsIf electrons were money, carbocations would be the beggars of organic chemistry. Packing a meresix valence electrons, these electron-deficient intermediates figure prominently in many reactionswe meet in organic chemistry, such asnucleophilic substitution(SN1) andelimination (E1)reactionsadditions of electrophiles todoubleandtriplebondselectrophilic aromatic substitutionadditions to carbonyl compounds and enolate chemistry (albeit in masked form)That’s a huge chunk of sophomore O-chem, right there. Being electron-deficient (and thereforeunstable), formation of a carbocation is usually the rate-limiting step in these reactions. Knowingthat, then think about this:what happens to the rate of the reaction when the carbocationintermediate is made more stable?Well, the energy of the transition state leading to the reaction willbe lower. Therefore, theactivation energy will be lowered.What’s that going to do to the rate of the reaction?Since the activation energy is lowered,thereaction is going to speed up.So what are some of the factors that stabilize carbocations?If you look through all of your organic chemistry textbook, you’ll find 3 main structural factors thathelp to stabilize carbocations.1.Neighboring carbon atoms.2.Neighboring carbon-carbon multiple bonds.3.Neighboring atoms with lone pairs.Why is this? It all goes back to the core governing force in chemistry: electrostatics. Since “oppositecharges attract, like charges repel”, you would be right in thinking thatcarbocations arestabilized by nearby electron-donating groups.
2. Carbocations Are Stabilized By NeighboringCarbon Atoms.The stability of carbocations increases as we go from primary to secondary to tertiary carbons.There’s two answers as to why this is. The age-old answer that is still passed around in manyintroductory textbooks points to carbons (alkyl groups in particular) as being “electron-releasing”groups through inductive effects. That is, a carbon (electronegativity 2.5) connected to hydrogen(electronegativity 2.2) will be electron rich, and can donate some of those electrons to theneighboring carbocation. In other words, the neighboring carbon pays the carbocation withelectrons it steals from the hydrogens. The second, (and theoretically more satisfactoryexplanation) ishyperconjugation, which invokes stabilization through donation of the electrons in

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Term
Fall
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Atom, Electron, Electric charge, Chemical bond

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