Featured Golden Rule of Chemistry:
3. Nature hates unpaired electrons.
If a molecule must have an
unpaired electron (
radical), it is better to have the unpaired electron distributed over as many
atoms as possible through resonance, inductive effects, and hyperconjugation.
should be thought of as being
similar to carbocation
; 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 >
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
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
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
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.
pi bonds don't just show up, they react with strong electrophiles
SN2 (Substitution, Nucleophilic, Bimolecular) -
reaction involves a single step in which
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
Bulky groups that increase non-bonded interaction strain with the incoming nucleophile
the back of an alkyl halide inhibit an SN2 reaction.
Flashback Rule of the Day
alkenes are more stable
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.)
E2 (Elimination, Bimolecular) -