organic chemistry (carroll)

Question 1

What happens during an Sn1 reaction

Answer 1

The leaving group departs then the nucleophile attacks. Stepwise via an intermediate
intermediate carbon only has 6 electrons around it

Question 2

what happens during an Sn2 reaction

Answer 2

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

Question 3

what does Sn1 stand for

Answer 3

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

Question 4

what does Sn2 stand for

Answer 4

substitution
nucleophilic
bimolecular

Question 5

which step is the slowest in the Sn1 reaction

Answer 5

SN1 reactions proceed by a two step mechanism in which the first step is slower than the second step.

Question 6

what happens when you change the nucleophile and electrophile concentrations in sn1 reactions

Answer 6

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

Question 7

describe an sn1 reaction diagram

Answer 7

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

Question 8

what two factors affect the rate of an sn1 reaction

Answer 8

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.

Question 9

describe the stability of carbenium ions
methyl, primary, secondary, tertiary

Answer 9

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

Question 10

what is hyperconjugation/ sigma conjugation

Answer 10

alkyl groups stabilize the carbenium ion by donating electrons from a sigma bonds

Question 11

what is pie conjugation

Answer 11

alkyl groups stabilize the carbenium ion by donating electrons from a pie bonds

Question 12

compare and contrast pie and sigma conjugation

Answer 12

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

Question 13

what possible reactions will a strong base/nuc give

Answer 13

either sn2 or E2

Question 14

what possible reactions will a weak base/ nucleophile give

Answer 14

either Sn1 or E1

Question 15

give an example of a strong base/ nucleophile

Answer 15

anything with a negative charge.
I-, Br-, N3-
-OCH3, -OH, -OCH2CH3
t-butoxide ion

Question 16

give an example of a weak base/nucleophile

Answer 16

CH3OH and water

Question 17

which sort of reactions do I-, Br- and N3- do

Answer 17

only SN2

Question 18

what sort of reaction does t-butoxide do

Answer 18

only E2 as it is too bulky

Question 19

what sort of reaction does -OCH3, -OH, -OCH2CH3

Answer 19

either Sn2 or E2

Question 20

how can you distinguish between the sn2 e2 reactions

Answer 20

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

Question 21

how do we differentiate between SN2 and E2 reactions for secondary alkyl halides

Answer 21

polar aprotic solvents gives us SN2
examples of protic solvents are HMPA, CH2CN, DMSO, acetone

Question 22

how do you differentiate between E1/sn1

Answer 22

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