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Level 194

Substitution & Elimination Reactions


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initiation, propagation, termination, and inhibition
Four steps to free radical halogenation
initiation
the homolytic cleavage of a molecular halogen by heat or a photon to generate two halogen radicals; yields a net increase in radicals
propogation
the goal of this step is to generate alkyl halides; first step entails an alkyl radical colliding with a n alkane, causing homolytic cleavage of C-H, formation of H-X and formation of alkyl radical.…
Termination
when the # of free radicals DECREASE
how does the inhibition stage work
when an alkyl radical combines with molecular oxygen, it forms a less reactive alkyl peroxy radical; this is a reversible step
how does the racemization of the alkyl radical occur
the halogen molecule can react to form a C-X bond above or below plane. racemization occurs if substitutions are unsymmetrical since bond formation can occur to an equal extent from either side of the unhybridized p orbital
selectivity
alkyl radical stability has an effect on its
selctivity = reactivity (% yield)/# sites available; ratio= selectivity of (i.e. tertiary vs primary sites)
formula for selectivity of different sites for free radical halogenation and ratio of selectivity for different sites
bromination of alkanes is much more selective than chlorination because
it is far less reactive and less exothermic than chlorination (observe enthalpies of rxt to determine)
Lewis bases
nucleophiles are also known as
major differences btween free radical halogenation and nucleophilic substitutions
1)free radical halogenation = homolytic cleavage; substitution = heterolytic cleavage
nucleophilicity gauges
the strength of a nucleophile
increases with negative charge
what three characteristics determine nucleophile strength?
nucleophile examples
chloride, alcohols, ammonia, cyanide, hydroxides, ethene, CH3-MgBr
Lewis acids (electron pair acceptors); incomplete octet
Electrophiles can also be known as__. They often have an
pericyclic reactions (i.e. claisen rearrangement)
apart from free radical halogenation, which other rxts do not have nuc-electro attraction?
Covalent
Bonding Characteristic: Electrons are shared between 2 or 3 elements or compounds
what makes a good leaving grp for nuc sub?
halides because they are very electronegative, (they take the electrons and run)
Sn2 mechanism
backside attack simultaneous with leaving grp leaving leads to Inversion of configuration (stereochemistry Change)
single concerted step
# steps in Sn2
Sn2 reactants and kinetics
bimolecular (indicated by the '2' in Sn2; relating to 2 reactants); rate = k(nuc)(electr)
CH3>primary>secondary>tertiary
reactivity of Sn2 substrate
best solvents for Sn2
polar aprotic (non-H bonding); polar aspect dissolves the Nu while the aprotic solvent increases rate by binding to cation to free Nu (avoid H2O and OH bc they hinder backside attack)
why are there no rearrangements for Sn2?
not possible d/t concerted mechanism (no carbocations are present in solution)
strong, non-bulky bases
what type of nucleophiles favor Sn2?
stereochemical characteristics of Sn2
concerted mechanism creates a pentavalent transition state for inverted prd
polar aprotic solvent examples
DMSO, acetone and DMF (dissolve Nu and leaving grps without completely solvating Nu)
what type of intermediate forms in Sn1 rxts; logic?
planar carbocation (carbonium) with 120 degree angles - energetically unfavorable because charge is lost; stability = tertiary>secondary>primary>methyl carbocation because the more substituents, the more stable and the faster the reaction
what is the stereochemical result of Sn1 rxt
racemic mixtures due to nucleophilic attack on either side of the p orbital
reactivity of substrate for Sn1
teriary>secondary>primary due to stabilization of the electrophile substituents to the carbocation
Sn1 kinetics
rate = k(electrophile)
polar protic with H bonding
Sn1 favors which type of solvents; (2) roles of the solvents?
Rearrangements in Sn1
If a carbocation can rearrange, it will
favored Sn1 nucleophiles
non-basic, weaker nucleophiles (water and H2O)
treated with strong mineral acids to make their bad (OH) leaving grps good (H2O) leaving grps
alcohols also undergo nuc sub as alkyl halides do, what is the solvent they are treated with? explain the diff between their Sn1 and Sn2
Strong Acids
Acids that completely ionize in aqueous solutions
weak basicity of oxygen
ethers are generally unreactive d/t
leaving grp (alcohol)
protonation of ether leads to a good
2 alkyl halides
prd of ether sub
sp2
hybridization for alkyl amine nitrogens
sp2 d/t aromatic ring
hybridization for aryl amine nitrogens
pyramidal
a molecule with one unshared pair and 3 atoms coming off of a central atom. Ex: NH3
alkyl amines undergo rapid
pyramidal inversion at the sp3 nitrogen; a low energy barrier of activation of only 6-7kcal/mol
chiral
due to this interconversion, alkyl amines are not
during exhaustive alkylation of methylamine with methyl iodide:
the Hs of amine are sequentially replaced with methyl grps, resulting in a quarternary ammonium prd
pi bonds
2 sigma bonds in eliminations are converted to
2
how many steps for E1
E1 mech
-leaving grp falls off, leaving behind a carbocation
to stabilize carbocations
why does E1 work best with tertiary substrates
concentration of substrate
E1 rxt kinetics: rxt rate only d/t on
Strong acid
An acid that donates its H+ ions to water in a reaction that goes completely to products
adjacent proton
E1 cannot occur if there is no
hydride or methide shifts (carbocation rearrangements)
since highly substituted alkenes are thermodynamically favored, what is advocated
relative stability of the prds
regioselectivity and stereoselevtivity of E1 rxts is determined by
strong acid and high temps ;#REF!
req for dehydration for alcohols; mech is
E1 mech for alcohols
protonation of hydroxyl to make water a leaving grp
substituted
major alkene prd is most
why are carbocations thermodynamically unstable?
deficiency of electrons (no octet); therefore it is very difficult to form and once it forms, it is very reactive
rearrangement (3>2>1>CH3)
how can carbocations become more stable
most common carbocation rearrangements?
1,2 hydride (H-) shifts and 1,2 methide (CH3-) shifts
Sn1; competition
E1 conditions are similar to__. Therefore, what occurs
E1 kinetics
rate = k [haloalkane]
weak bases and high temperatures
when is E1 favored over Sn1?
1 step (concerted)
E2, similar to Sn2, favors what mech
what type of nucleophiles are used in E2
strong bases that remove beta hydrogen while the leaving grp leaves and 2x bond forms at same time
alkyl halides (therefore, not alcohols)
what is the only type of substrate able to participate in E2
antiperiplanar (similar to backside rxts of Sn2)
E2 works best if the proton being removed is __ to the leaving grp
transition state
In each individual step of a reaction, the state of highest energy between reactants and products. The transition state is a relative maximum (high point) on the reaction-energy diagram.
E2 kinetics depend on
rate = k[haloalkane][base] = bimolecular
what has a large effect on the regiochemistry of E2 rxt
choice of base; small bases can remove H from most substituted to yield most thermodynamically favorable alkene as a major prd. Large bases have steric hindrance and have to choose a more accessible beta pro…
why do E2 and Sn2 compete?
bc bases can act as Nu
Rule to discern whether E2 or Sn2 will be used
strong bases yield E2 rxts UNLESS the substrate is primary
3>2>1
E2 reactivity
E2 kinetics
reaction rate = k[base][substrate]
intermolecular and intramolecular
what types of H bonding to phenols participate in
MP and BP
intramolecular bonding decreases opportunity for intermolecular bonding and decreases which properties
large difference in electronegativity between oxygen and hydrogen
the reason alcohols are relatively acidic is the same reason they engage in H bonding:
increased acidity of phenols in comparison to alcohols is due to
phenoxide anion is stabilized by resonance while alkoxide ion is not; e withdrawing substituents on phenols increase acidity
what do electron donating grps do to acidity
decreases acidity because the phenoxide anion becomes destabilized
PBr3, PCl3 and SOCl2 (thionyl chloride
common reagants to prd alkyl halides from (i.e. alcohols) for sub/elim rxts
SO2 gas bubbles out
byproduct of alcohols and thionyl chloride?