ROTD - 10-26-11 Featured Golden Rule of Chemistry: 3....

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
10-26-11 Featured Golden Rule of Chemistry: 3. Nature hates unpaired electrons. If a molecule must have an unpaired electron ( a.k.a. radical), it is better to have the unpaired electron distributed over as many atoms as possible through resonance, inductive effects, and hyperconjugation. 1. Carbon radicals should be thought of as being similar to carbocation s ; sp2 hybridized, wanting electron density, stabilized by alkyl groups. In the case of radicals, the unpaired electron density is stabilized by being spread around to increase stability, and in the case of carbocations, it is the charge that is being spread around to increase stability (See Golden Rules) . 2. The predominant site of halogen reaction corresponds to the site of the most stable radical, therefore, the order of reactivity during the halogenation reaction is: tertiary H > secondary H > primary H. 2. Hammond's postulate, i.e. transition states resemble more closely the structures of the species they are closest to in energy, explains why bromination is more selective than chlorination. Fluorine is too reactive, iodine is not selective. Bottom line: Always use Br2 and light when reacting alkanes to give alkyl halides. 10-28-11 2 . Allyl radicals and cations are stabilized by resonance with adjacent double bonds (pi-way). The unpaired electron density (radical) or positive charge (cation) is spread over a larger area, and that is stabilizing. 3.Allylic halogenation uses NBS and light to place a Br atom adjacent to a C=C bond. The mechanism is a free radical chain reaction, involving an allyl radical intermediate. This is VERY tricky because multiple sites can be considered for adding the Br atom, the predominant one will be the site that gives the most stable (most highly substituted) C=C in the product. 4. The non-Markovnikov addition of HBr to an alkene is a very useful reaction, involving a radical chain mechanism. 10-31-11 1. Using a peroxide (ROOR) and light in the presence of HBr and an alkene leads to a free radical chain reaction that leads to non-Markovnikov addition of the HBr. This is a very useful reaction that can make primary alkyl halides (haloalkanes) from terminal alkenes. 2. Remember, pi bonds don't just show up, they react with strong electrophiles . 3. SN2 (Substitution, Nucleophilic, Bimolecular) - reaction involves a single step in which nucleophile attacks backside of a carbon-leaving group bond (the electrophile) , making a new bond as the leaving group departs. Click here to see a movie of an SN2 reaction . 4. Bulky groups that increase non-bonded interaction strain with the incoming nucleophile on the back of an alkyl halide inhibit an SN2 reaction. 5. Flashback Rule of the Day : Trans alkenes are more stable than cis alkenes because cis alkenes have some non-bonded interaction strain. In addition, more highly substituted alkenes are more stable than less substituted alkenes (we don't really tell you why, just learn it.) 6. E2 (Elimination, Bimolecular) -
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 01/25/2012 for the course CH 310 M taught by Professor Iverson during the Spring '09 term at University of Texas at Austin.

Page1 / 6

ROTD - 10-26-11 Featured Golden Rule of Chemistry: 3....

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online