Module 5 Flashcards
What are the 2 factors that determine the barrier height of activation energy?
- energy needed for a collision
2. orientation of that collision
Arrhenius plot
ln(k) vs. (1/T)
What is the slope of an Arrhenius plot?
m=-Ea/R
Kinetics
the rate of a reaction is determined by its path (energy heights, etc)
rates are path dependent
Thermodynamics
the free energy difference between reactants and products
path independent
What are the five factors that can influence the rate of a reaction?
- nature of reactants
- concentration
- temperature
- surface area
- prescence of a catalyst
Rate of reaction vs. rate constant
Rate of reaction is the change in concentration of reactant/product per time
Rate constant does not depend on the concentrations of reactants/products present and simply gives a constant of proportionality at a specific temperature
Units of rate of reactions
M/sec
Units of rate constant
change depending on the order of reaction
What are the two plausible mechanisms for an overall alkyl halide substitution?
SN1 or SN2
What is the barrier in an SN1 mechanism?
carbocation stability
What is the barrier in an SN2 mechanism?
steric hindrance
Alkyl halide reactivity in decreasing order (SN1)
3º>2º»1º
or conjugated allylic or benzylic
Alkyl halide reactivity in decreasing order (SN2)
1º>2º»3º
SN1 nucleophile
weak (typically neutral)
SN2 nucleophile
strong (typically charged)
SN1 solvent
polar protic
- can H+ donate
- ex: alchohols
SN2 solvent
polar aprotic
- can’t H+ donate
- ex: DMSO, acetone
Acetone chemical formula
C3H6O
SN1 stereochemistry
mix of retention and inversion
racemic
SN2 stereochemistry
inversion
If you see an alkyl halide what argument are you making?
kinetics
looking at intermediate stability primarily
If you see a carbonyl group what argument are you making?
thermodynamics
looking at most stable products
Why in a SN1 reaction can the incoming nucleophile attack from top or bottom?
the carbocation intermediate is sp2 hybridized, and therefore it has an empty p-orbital
this p-orbital can be attacked from either the top or bottom
this also leads to the racemic mixture of products
Why does a SN2 reaction have an inversion?
there is no open p-orbital, so the nucleophile has to attack the sigma-star orbital of the leaving group
this forces the nucleophile to attack the backside of the leaving group and causes an inversion
Substitution reaction
chemical reaction where one functional group in a chemical compound is replaced for another functional group
SN1 meaning
nucleophilic substitution reaction where the RDS is unimolecular
Sn2 meaning
nucleophilic substitution reaction where the RDS is bimolecular
What is the LUMO in an SN1 reaction?
pi
carbocation is trigonal planar, sp2 hybridized
What is the LUMO in an SN2 reaction
sigma-star
tetrahedral, sp3 hybridized with no empty p-orbital
What are relative rates reported to?
relative the slowest rate in a series
Why is I- a better leaving group than F- in SN2?
F- has a higher pKa and is a more reactive base, therefore it is a worse leaving group
I- is bigger and can spread out the electron density better, which makes it a better leaving group
Why would SN1 occur for (CH3)3Br, but not CH3Br or Ch3CH2Br?
1-butyl and 2-butyl have carbocations that are not stabilized by hyperconjugation as much and therefore are too high in energy for an SN1 reaction to occur
Hyperconjugation
partial overlap between the empty p-orbital on the C+ and the sigma bond on the neighboring carbon
stabilizes the carbocation intermediate
How can 1º alykyl halides proceed with an SN1 mechanism? (this is an execption)
if the carbocation can be stabilized by resonance
if not, the 1º carbocation is too high in energy to proceed with SN1
Effect of nucleophilic strength for SN2 reaction
stronger nucleophile=greater rate of reaction
higher pKa = stronger nucleophile
When in the SAME row of the periodic table
Effect of nucleophilic strength for SN2 reaction when in different rows of periodic table
larger molecular has more electron density to overlap during the transition state
this lowers the energy of the transition state and increases the rate of the reaction
DMSO
dimethyl sulfoxide
(CH3)2SO
*polar aprotic solvent
What molecule does hydrogen bonding from the polar protic solvent stabilize?
the nucleophile in a SN2 reaction
Why does a polar protic molecule slow the speed of a SN2 reaction?
the nucleophile is stabilized by hydrogen bonding, which lowers it in energy
therefore, by lowering the energy of the starting product, it takes more energy to reach the peak of activation energy and takes more time
Why do SN1 reactions prefer a polar protic solvent?
the H-bonding stabilizes the carbocation intermediate
Cyclohexane rate of SN2 reaction
When leaving group is in the axial position, it is higher in energy than when it is in the equatorial position
therefore, the axial position has starting products higher in energy, which makes less of a energy barrier to overcome to reach the transition state
Methyl tosylate
good leaving group because incredibly stabilized by resonance
it is also a very strong acid, so a very weak nucleophile leaving group
abbreviated Ts
What is something to watch for in Sn2 reaction with CN- as nucleophile?
Since CN- is attached to a carbon, the priority of the assymetric carbon attachments could change
ex: CN- is less priority than CSO2
Inversion still happens, but could remain in the same configuration (i.e., S=>S)
What do constitutional isomers in the products indicate?
a carbocation rearrangment
*SN1 reaction
Allylic system
double bond next to the carbocation
this stabilizes the carbocation
*SN1
Where do curved arrows have to start from for a hydride shift?
the sigma bond
not the hydrogen itself!
we are moving electrons!
Why is it called a hydride shift instead of a proton shift?
hydrogen is bringing its electrons with it from the sigma bond
What are the two ways that a carbocation can shift?
1) Hydride shift
2) through resonance
Why do two successive hydride shifts never occur?
first shift must lead to a more stable carbocation
if not, it just wastes too much energy to shift to a new carbocation that is not lower in energy
Why do rearrangements occur?
makes hyperconjugation of carbocation greater
carbocation is stabilized by having more carbons to conjugate with
