organic chemistry (carroll) Flashcards
What happens during an Sn1 reaction
The leaving group departs then the nucleophile attacks. Stepwise via an intermediate
intermediate carbon only has 6 electrons around it
what happens during an Sn2 reaction
the nucleophile attacks at the same time as the leaving group departs. Sn2 is a concerted process
the carbon in the middle always has electrons around it
what does Sn1 stand for
substitution
nucleophilic
unimolecular
This means that in the mechanism only one molecule is involved in the rate determining step. The observed kinetics data is consistent with this molecularity for the reaction
what does Sn2 stand for
substitution
nucleophilic
bimolecular
which step is the slowest in the Sn1 reaction
SN1 reactions proceed by a two step mechanism in which the first step is slower than the second step.
what happens when you change the nucleophile and electrophile concentrations in sn1 reactions
If you change the concentration of the electrophile the rate goes up because the reaction is in equilibrium
If the nucleophile is kept constant then the rate of reaction is directly proportional to the electrophile. But if the electrophile is kept constant then the rate is not dependent on the nucleophile because the X- leaving group will just reattach instead
describe an sn1 reaction diagram
The big hill is always the slow step as it requires the most energy, if the second hill was bigger the reaction would not happen as nature always takes the easy way out and the reaction would go back to the starting product
what two factors affect the rate of an sn1 reaction
1) Stability of the carbenium ion intermediate - we need the energy of the starting materials and the energy of the intermediate to be as close together as possible.
2) leaving group ability of x-
The stability of the first transition state and NOT the intermediate determines the rate of reaction.
describe the stability of carbenium ions
methyl, primary, secondary, tertiary
CH3+ is sp3 hybridised and is trigonal planar, the missing two electrons is an empty p orbital. No electron density to donate to produce resonance forms. For a primary carbenium ion it is more stable as you can spread out and delocalise the positive charge producing resonance forms. To do this we need to add electron density to the empty p orbital so the R group must have some electron density to donate. Curly arrows are orbital overlap, so the R group must have an orbital with electrons in it that can overlap with the empty orbital.
The more r groups we have the more electron density we have the more stable as we have more places to donate electron density from so the more stable it is as we have spread out the positive charge more
what is hyperconjugation/ sigma conjugation
alkyl groups stabilize the carbenium ion by donating electrons from a sigma bonds
what is pie conjugation
alkyl groups stabilize the carbenium ion by donating electrons from a pie bonds
compare and contrast pie and sigma conjugation
Pie conjugation is better because in hyperconjugation we are overlapping a p orbital with an sp3 orbital. But in pie conjugation you are overlapping a p orbital with another p orbital so in this case the oribitals match in size shape and energy, so the easier the overlap the more efficient it will be to draw the curly arrow. The degree of donation also goes up in terms of pie conjugation. If we take too much electron density from the SP3 orbital the bond will break but it doesn’t matter how much electrondensity we take from the pie system as the molecule is not going to break it is only going to weaken, the molecule will stay perfectly intact. We also get more delocalisation because now it is spread over 2 atoms for each donation not just two like in hypercojugation
what possible reactions will a strong base/nuc give
either sn2 or E2
what possible reactions will a weak base/ nucleophile give
either Sn1 or E1
give an example of a strong base/ nucleophile
anything with a negative charge.
I-, Br-, N3-
-OCH3, -OH, -OCH2CH3
t-butoxide ion
give an example of a weak base/nucleophile
CH3OH and water
which sort of reactions do I-, Br- and N3- do
only SN2
what sort of reaction does t-butoxide do
only E2 as it is too bulky
what sort of reaction does -OCH3, -OH, -OCH2CH3
either Sn2 or E2
how can you distinguish between the sn2 e2 reactions
to do this we need to look at the alkyl halides
primary alkyl halides do SN2 reactions
secondary alkyl halide can give either SN2 or E2
tertiary alkyl halides only give E2
how do we differentiate between SN2 and E2 reactions for secondary alkyl halides
polar aprotic solvents gives us SN2
examples of protic solvents are HMPA, CH2CN, DMSO, acetone
how do you differentiate between E1/sn1
neither Sn1 or E1 react with primary alkyl halides and can both react with secondary ant tertiary so if the reaction has heat is is an E1 mechanism and if there is no heat it is an SN1 reatcion
