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Order of rate of acid catalysed dehydration of alcohols
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+ Elimination
+ Chemistry
+ Carbocation
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Melvin Moraa
Order of rate of acid catalysed dehydration of alcohols
Your basic assumptions are correct.
It can be observed as such:
In Q the carbocation is stablised by resonance, inductive and hyperconjugative effects so it is quite stable and will be formed fastest.
Comparing P and R: In P, a secondary carbocation is formed and there are 4+1 or 5 hyperconjugative structures possible. Moreover, the inductive effect is also greater in P than in R. Hence, it forms carbocation faster.
Both R and S form primary carbocations. Inductive effect is more prevelent in R than in S, while S has 3 hyperconjugating structures.
But R offers capacity to rearrange and stablise the carbocation, by methyl migration to produce highly stable tertiary carbocation, also stablised by resonance over the adjoining phenyl ring.
But, S also forms a carbocation which is primary, but offers no stablising effect other than 2 +1 or 3 hyperconjugating structures.
Similarly, this rearrangement may also be attributed to the comparison of P and R, where hydride shift and methyl shift occur. The migration amplitude of hydride, being lower, leads to rearrangement of positive charge quickly and hence is kinetically also favoured.
In Q the carbocation is stablised by resonance, inductive and hyperconjugative effects so it is quite stable and will be formed fastest.
Comparing P and R: In P, a secondary carbocation is formed and there are 4+1 or 5 hyperconjugative structures possible. Moreover, the inductive effect is also greater in P than in R. Hence, it forms carbocation faster.
Both R and S form primary carbocations. Inductive effect is more prevelent in R than in S, while S has 3 hyperconjugating structures.But R offers capacity to rearrange and stablise the carbocation, by methyl migration to produce highly stable tertiary carbocation, also stablised by resonance over the adjoining phenyl ring.But, S also forms a carbocation which is primary, but offers no stablising effect other than 2 +1 or 3 hyperconjugating structures.
Similarly, this rearrangement may also be attributed to the comparison of P and R, where hydride shift and methyl shift occur. The migration amplitude of hydride, being lower, leads to rearrangement of positive charge quickly and hence is kinetically also favoured.
R has to have only one rearrangement to form tertiary resonance stablised carbocation. But, carbocation in S is quite isolated and primary. It can be argued that in S the first hydride shift will form secondary carbocation comparatively more stable than the initial. But then a second rearrangement has to occur to get resonance stablised carbocation, similar to Q. Then also inductive effect and hyperconjugation will be higher in R than S in the final carbocation formed after multiple rearrangement steps jn S.More
Then the whole process of multiple rearrangement will be somewhat tough, and although Hammond Postulate provides for the carbocation rearrangement to occur i.e. from primary, secondary to tertiary and transition states are effectively stablised, the process will be long, tedious and the whole process will make the rate determining step slow.More
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An acid catalysed dehydration reaction of alcohols will always proceed with the removal of -OH as H2O molecule and therefore, will be dependent on the formed carbocation's stability because the rate deternining step for this process is formation of carbocation. Carbocation Carbocation can be stabilized by inductive effect, field effect, mesomeric effect (or) resonance and hyperconjugation effect.
Q should be the fastest because it's corresponding carbocation is the most stable because it is favoured by all the effects.
P is the next fastest as it's carbocation gains maximum stability through a hydride shift as the secondary carbocation becomes tertiary carbocation which is stabilised by both resonance with the phenyl group and the hyperconjgation of H in the attached methyl and ethyl group.
R would be faster than S because it's carbocation becomes stable after it undergoes a methyl shift which is less favourable than hydride shift but results into similar stability.
S shall be the slowest because it has the most unstable carbocation after its formation.
An acid catalysed dehydration reaction of alcohols will always proceed with the removal of -OH as H2O molecule and therefore, will be dependent on the formed carbocation's stability because the rate deternining step for this process is formation of carbocation. Carbocation Carbocation can be stabilized by inductive effect, field effect, mesomeric effect (or) resonance and hyperconjugation effect.
Q should be the fastest because it's corresponding carbocation is the most stable because it is favoured by all the effects.P is the next fastest as it's carbocation gains maximum stability through a hydride shift as the secondary carbocation becomes tertiary carbocation which is stabilised by both resonance with the phenyl group and the hyperconjgation of H in the attached methyl and ethyl group.R would be faster than S because it's carbocation becomes stable after it undergoes a methyl shift which is less favourable than hydride shift but results into similar stability.S shall be the slowest because it has the most unstable carbocation after its formation.
Your basic assumptions are correct.
It can be observed as such:
Similarly, this rearrangement may also be attributed to the comparison of P and R, where hydride shift and methyl shift occur. The migration amplitude of hydride, being lower, leads to rearrangement of positive charge quickly and hence is kinetically also favoured.
Thus finally the order may be:
Q>P>R>S
Your basic assumptions are correct.
It can be observed as such:
Similarly, this rearrangement may also be attributed to the comparison of P and R, where hydride shift and methyl shift occur. The migration amplitude of hydride, being lower, leads to rearrangement of positive charge quickly and hence is kinetically also favoured.
Thus finally the order may be:
Q>P>R>S
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An acid catalysed dehydration reaction of alcohols will always proceed with the removal of -OH as H2O molecule and therefore, will be dependent on the formed carbocation's stability because the rate deternining step for this process is formation of carbocation. Carbocation Carbocation can be stabilized by inductive effect, field effect, mesomeric effect (or) resonance and hyperconjugation effect.
Q should be the fastest because it's corresponding carbocation is the most stable because it is favoured by all the effects. P is the next fastest as it's carbocation gains maximum stability through a hydride shift as the secondary carbocation becomes tertiary carbocation which is stabilised by both resonance with the phenyl group and the hyperconjgation of H in the attached methyl and ethyl group. R would be faster than S because it's carbocation becomes stable after it undergoes a methyl shift which is less favourable than hydride shift but results into similar stability. S shall be the slowest because it has the most unstable carbocation after its formation.
An acid catalysed dehydration reaction of alcohols will always proceed with the removal of -OH as H2O molecule and therefore, will be dependent on the formed carbocation's stability because the rate deternining step for this process is formation of carbocation. Carbocation Carbocation can be stabilized by inductive effect, field effect, mesomeric effect (or) resonance and hyperconjugation effect.
Q should be the fastest because it's corresponding carbocation is the most stable because it is favoured by all the effects.P is the next fastest as it's carbocation gains maximum stability through a hydride shift as the secondary carbocation becomes tertiary carbocation which is stabilised by both resonance with the phenyl group and the hyperconjgation of H in the attached methyl and ethyl group.R would be faster than S because it's carbocation becomes stable after it undergoes a methyl shift which is less favourable than hydride shift but results into similar stability.S shall be the slowest because it has the most unstable carbocation after its formation.
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