ORGANIC CHEMISTRY Reactivity Predictions: For the five groups (A–E) on the follo
ID: 481799 • Letter: O
Question
ORGANIC CHEMISTRY Reactivity Predictions: For the five groups (A–E) on the following page, predict the order of reactivity from slowest to fastest for the indicated molecules:
For groups A, B, and C: Give separate predictions for both SN1 and SN2 reactivity.
For groups D and E: Give predictions for only SN2 reactivity.
Groups for Reactivity Predictions 2-chlorobutane 2-bromobutane 2-iodobutane 1-bromobutane 2-bromobutane 2-bromo-2-methylpropane Br Br bromobenzene 1-bromo-1-phenylethane 2-bromo-1-phenylpropane bromocyclopentane bromocyclohexane Br 1 -bromo-2,2- 1-bromo-2-methylpropane 1-bromopropane dimethylpropaneExplanation / Answer
Group A :
In these three compounds everthing is same except the leaving group. Better the leaving group faster will be the rate of the reaction for both SN1 and SN2 reaction. The leaving group ability among the halogens : I->Br->Cl-.
Thus, highest reaction rate will be observed from I- compound followed by Bromo compound and least in case of choloro compound.
Group B:
In this group leaving group is same in all three cases. So, you have to consider the geometry of the substrate : primary, secondary or tertiary.
In SN1 reaction , a carbocation is formed in the rate determining step.Greater the stability of the carbocation, greater will be the rate. The stability order among carbocation is tertiary>secondary>primary.
The reactivity order will be : 2-bromo 2-methyl propane > 2 bromo butane > 1bromo butane
In SN2 reaction, the incoming nucleophile attacks from the opposite side where the leaving group leaves. Greater the crowding at the reaction center, less will be the rate.
Reactivity order : 1-bromo butane > 2-bromo butane > 2-bromo-2methyl butane
Group C
Bromobenzene will not respond to SN1 reaction as phenyl carbocation are extremely unstable. Two other compounds will have similar reactivity in SN2 reaction.
In SN2 reaction Bromobenzene will not respond as backside attack is hindered by the phenyl group. 2-bromo-1-phenyl propane is more reactive than 1-bromo1-phenyl ethane due to less steric hindrance.
Group D
In Sn2 reaction a transition state is formed which has a bond angle of 120 degree (sp^2 hybridised). Small rings like cyclobutane is already very strain when the hybridisation is 109 degree. It will become even more strained if the bond angle is 120 degree. Thus reactivity is very less.
Compared to the above case, in cyclohexane can exist in chair form where bond strains are eliminated . thus sn2 reaction can occur and reactivity will be much greater than the reactivity of cyclobutane.
Group E
Greater the crowding at the C bearing leaving group less will be the reaction rate.
reactivity : 1-bromopropane > 1-bromo2-methyl propane > 1bromo 2,2 dimethyl propane
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