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Effect of conformation on reactivity

Conformational effect on reactivty

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LOVELY PROFESSIONAL
UNIVERSITY
TOPIC:- EFFECT OF CONFORMATION ON REACTIVITY
COURSE CODE:-CHE 541
SUBMITTED TO:-DR.NITIN TONDON
DEPTT.:- SCHOOL OF PHYSICAL SCIENCES AND
CHEMICAL ENGINERRING
SUBMITTED BY:-Madan kumar
Course:-M.sc(HONS.)chem.
Reg. No:-11614536
Roll No:-A14 1
INTRODUCTION
Relation between conformation and
reactivity and stability of acyclic compound
was first introduced by BARTON in
1950.According to which there is profound
effect of conformation on chemical reactivity
because the stereoelectronic and steric factor
are among the essential condition for
reaction. In the upcoming slides we discuss
the effect of conformation on reactivity with
examples.
2
ACYCLIC COMPOUND
3
E2 REACTIONS:-E2 reactions are stereospecific and most of
these are anti elimination. In anti elimination the five atoms
involved must be in same plane and the eliminating groups
must be trans to each other, the conformation is called
antipreplanar this is stereoelectronic requirement of anti
elemination.
EXAMPLE:-2-Bromo butane can give either cis- or trans-2-
butene
H3C
C
H
C
CH3
H
trans-2-butene
C
C
H
Br
CH3
H
H
H3C
2-Bromobutane
4
C
C
H
Br
CH3
H
H
H3C
2-Bromobutane
H3C
C
H
C
H
CH3
cis-2-butene
(Major)
5
EXAMPLE:-Debromination of 2,3-dibromobutane with a metal such as Zn.
C
C
Br
Br
CH3
H
H
H3C
meso-2,3-dibromobutane
H3C
C
H
C
H
CH3
cis-2-butene
C
C
Br
Br
CH3
H
H
H3C
meso-2,3-dibromobutane
H3C
C
H
C
CH3
H
trans-2-butene
racemic mixture
CYCLIC COMPOUND
The effect of conformation on reactivity has been
studied mostly in the case of cyclohexanes because the
ring system is mostly occurs in natural products such as
steroids, terpenoids. Environments of axial and
equatorial group are different, hence the reactivity of
a given group depend on the conformation i.e.,
weather the group is axial or equatorial. Generally, but
not always, the equatorial substituents are more
reactive than the corresponding axial substituent.
6

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Effect of conformation on reactivity

  • 1. LOVELY PROFESSIONAL UNIVERSITY TOPIC:- EFFECT OF CONFORMATION ON REACTIVITY COURSE CODE:-CHE 541 SUBMITTED TO:-DR.NITIN TONDON DEPTT.:- SCHOOL OF PHYSICAL SCIENCES AND CHEMICAL ENGINERRING SUBMITTED BY:-Madan kumar Course:-M.sc(HONS.)chem. Reg. No:-11614536 Roll No:-A14 1
  • 2. INTRODUCTION Relation between conformation and reactivity and stability of acyclic compound was first introduced by BARTON in 1950.According to which there is profound effect of conformation on chemical reactivity because the stereoelectronic and steric factor are among the essential condition for reaction. In the upcoming slides we discuss the effect of conformation on reactivity with examples. 2
  • 3. ACYCLIC COMPOUND 3 E2 REACTIONS:-E2 reactions are stereospecific and most of these are anti elimination. In anti elimination the five atoms involved must be in same plane and the eliminating groups must be trans to each other, the conformation is called antipreplanar this is stereoelectronic requirement of anti elemination. EXAMPLE:-2-Bromo butane can give either cis- or trans-2- butene
  • 5. 5 EXAMPLE:-Debromination of 2,3-dibromobutane with a metal such as Zn. C C Br Br CH3 H H H3C meso-2,3-dibromobutane H3C C H C H CH3 cis-2-butene C C Br Br CH3 H H H3C meso-2,3-dibromobutane H3C C H C CH3 H trans-2-butene racemic mixture
  • 6. CYCLIC COMPOUND The effect of conformation on reactivity has been studied mostly in the case of cyclohexanes because the ring system is mostly occurs in natural products such as steroids, terpenoids. Environments of axial and equatorial group are different, hence the reactivity of a given group depend on the conformation i.e., weather the group is axial or equatorial. Generally, but not always, the equatorial substituents are more reactive than the corresponding axial substituent. 6
  • 7. a. Substitution reaction 7 • SN2 reactions take place more readily with axial substituent than with equatorial .For example the SN2 reaction of thiophenoxide ion with 4-t- butylcyclohexylbromide having an axial bromine takes place about 60 times faster than as compared to the equatorial isomer because the attack of the thiophenoxide ion on the equatorial isomer is hindered by Beta axial hydrogens. - H H H Me3C H Br H SPh H H Br H Me3C SPh
  • 8. SN1 OTs H H Me3C H H Me3C H H H H OTs H Me3C Slower + H Me3C H H + Faster •Proceeds through formation of carbocation which relives the steric strain of the axial isomer due to 1,3 diaxial interaction. •SN1 reaction is sterically assisted for an axial substituent, such type of steric strain not produce in equatorial substituent. •For example the acetolysis of cis-4-t- butylcyclohexyl tosylate with an axial tosylate group, is about 3.4 times faster than that of trans isomer. 8
  • 9. ESTERFICAION AND HYDROLYSIS Me Me H H OCOMe MeOC O H H Cholestanyl acetate (equqtorial ester) Epicholestanyl aceate (axial ester) Equatorial and axial conformational isomer usually react at different rate due to 1,3 -diaxial interaction. In the axial isomer esterfication and hydrolysis occur less readily as compare to equatorial conformation. For example , cholestanyl acetate having acetate group placed on the equatorial reacts more readily than the epicholastenyl having acetyl group placed on axial position. 9
  • 10. ELIMNATION REACTION • As we have already noted that the steroelectronic requirement of an E2 reaction is that the eliminating group should be antiperiplanar. In cyclohexane this relationship exists when the substituent on the two adjacent carbon are in axial position. • In neomenthyl both the axial and equatorial eliminating groups are at axial position thus it undergoes readily to elimination and give the products. • In menthyl chloride eliminating group is at equatorial. consequently methyl chloride reacts slowly as compared to neomenthyl chloride. • Similarly, the cis isomer of 4-ter-butylcyclohexyl tosylate undergo elimination in the presence of sodium ethoxide in ethanol 100 times faster as compared to its trans isomer this is because of the position of tosyl group. 10
  • 11. 11 + H H H Cl Cl H H Cl EtONa 25% 75% 100% EtONa Neomenthyl chloride Menthyl chloride three substiuents are all euatorial is the more stable chair conformation Conformation of high energy OTs H H H CMe3 H TsO H H CMe3 H trans-isomer undergoes slower elemination cis-isomer undergoes faster elemination
  • 12. MOLECULAR REARRANGMENT • Example of pinacol-pinacolone rearrangement of 1,2-dimethyl-1,2 cyclohexanediol. • The cis isomer of this diol on treatment with acid undergoes a methyl shift to give 2,2- dimethylcyclohexanone. • The trans isomer undergoes ring contraction to give cyclopentane derivative. 12
  • 14. NEIBGHOURING GROUP PARTICIPATION • In neighbouring group participation, the group participating as a neighbouring group must be in a position to attack from the just opposite side to leaving group. • Acetolysis of trans-2-iodo cyclohexyl brosylate which undergoes 2.7*10^6 times faster acetolysis than the cis-isomer in which iodo group cannot attack from the backside. 14
  • 15. 15 .. + I .. I OBs I I BsO .. -OBs AcOH OAc cis isomer undergoes very slow acetolysis trans isomer undergoes very fast acetolysis
  • 16. OXIDATION • Reaction having no steric requirement also display rate differences between reaction of axial and equatorial isomers. • Oxidation of both cis and trans isomer of 4-tert- butylcylohexyl with chromic acid gives the same ketone. • In the preferred conformation the cis isomer has an axial which is removed in the oxidation to allow relief from van der waals strain(1,3-diaxial interaction with beta hydrogen), thus the reaction is three times faster than that of trans isomer. 16