Solvent intervention in nucleophile-electrophile combination reactions

Journalof Molecule Structure, 120 (1986) 107-116 TIIEOCHEM Ekevier Science Publishers B.V.,

107

Amsterdam- Printedin The Netherlands

SOLVENTINTERVENTION IN NUCLEOPHILE-ELECTROPHILE COMBINATIONREACTIONS. 3. BERTRAN'and J. ANGUIANO* ‘Departamento de Quimica Fisica, 2Oepartamento de Quimica Fisica,

Universi dad AutBnomade Barcelona Universidad de Sevilla

(Spain I

(Spain)

ABSTRACT The combination reactions between the ethylene and different solvated el ectrophiles are studied by means of MIND0/3 method in order to check the hypothesis of partial desolvation of the electrophile at the transition state. It has been found that the desolvation depends at the same time on the nucleophile, the electrophi 1e and the solvent. INTRODUCTION Some of

Ritchie’s

papers

(ref.l-3)

on nucleophile-electrophile

reactions have shown that the nucleophilic activity of a 1arge nucleophiles, in their reaction with electrophiles, is correlated by

combination number of

log k = loh k, + N, where k is the rate constant for a reaction of an electrophi 1e with a given nucleophi lit system, k, is dependent solely on the identity of the electrophi le and N, is a parameter characteristic prising lectivity

result

of this

principle,

relation

because,

relative reactivity system is constant

of the nucleophilic

is its since

N,

incompatibility is

system. The most surwith the reactivity-se-

independent of the electrophile,

the

of different electrophiles with a given nucieophilic and independent of the said nucleophilic system, which

imp1ies that the selectivity of these kinds of reactions does not depend on To explain this fact, Ritchie concludes that there are not their reactivity. specific interactions between electrophile and nucleophile at the transition state,

and that electrophi le desolvation has not begun. On the contrary, Pross data could be interpreted much better by (ref .4) showed that experimental supposing that the electrophi le had undergone parti al desolvation at the transition state; the less extensive this desolvation the stronger the electroIntroducing the assumption that the degree of electroPhi le desolvation phile. dependent only on the electrophile and not on the at the transition state is solvent,

he comes to

the

conclusion

that

said

degree

of

desolvation

for

a

particular electrophile is inversely proportional to the difference in solvation energy of that electrophile in two solvent. theoretical calculations on the In order to clarify this controversy, 01661280/85/$03.30

Q 1985 ElsevierScience Publishers B.V.

108 attack

of

the

with

one

the

results

tion

at

annnonia molecule, obtained

the

philes

time

of

two

on the

To get ber

of

Ritchie’s state

solvents

el ectrophi

nucleophile,

electrophi

must

have

the

insight

1es

and the

fine

the

are

directly

reaction

this

previously

studied

at

on the and for

us to

discuss

whether

dinate

at the

transition

the

the

into

state

solvation

the

present

electrothat

the

at

the

same

study

the

num-

have been dispersed. le

state,

The transition

of

states the

analysis

the

as

to de-

transition both for

vector

form part

H+,

and pyri dine In order

these

hypersurface

parameters

two

solvent.

account

al

From

1e desol va-

depends

have been considered.

potenti

or

5-7).

appears

ammonia, acetonitri

new ones.

state

(ref.

only

clearly

in

transition

complete the

Although it

and the

water,

out

molecule

model s on electrophi

dependence,

taken

one water

carried

transition

as nucleophile

coordinate

located

the

electrophile

and sol vents

ethylene

been

considered,

at

CH3+ and C2H5+ as electrophiles, sol vents,

have

with

be questioned.

been

into

either

and Pross’

1e desolvation

a deeper

solvated

on ethylene

both

transition

and

degree

an CH3+. both

proton

cases al lows

reaction

coor-

or not.

METHODOF CALCULATION Given the

the

impossibi

majority

tions the

which

help

interesting

dent

are

computation

the

method time

remaining

(ref.81

within

been

made

by

means

of

reasonable

step,

using

the the the

on the

the

the

limits.

method

points

effect

of

parameters

opposite

Dewar’s

study. indepen-

the

system

the

semiempiri -

to

MIND0/3

most

geometrical

of the

of

order

the

maintain

program

are the

(ref.91

and

surface

has

(ref.111

by

10) have been used. proposed

points by

minimizes

which

on the

McIver the

potenti

al

and Konornicki

gradient

diagonalizes

the

norm.

In

a subsequent

force

constant

matrix,

model

(ref -121

has been

solvent

molecule

is determined. soivation,

electrophile side

two

to be optimized, in

in reducing

present

one or

for

approxima-

directly

For each value

chosen

stationary

FORCE program

of those

To study employed,

using

of

(ref.

SIGMA program which

the

the nature

placed

1ocation

surface,

variables.

been

consists

locating

are used in the

potential

geometrical

has

in

hypersurface

two basic

are

The first

second,

number of parameters

GEOMOprogram of Rinaldi The direct

the

potential

there

difficulty.

and the

of

total

interest,

as independent

al 1

the

Both approximations

dimension

taken

this

surface,

Because of the

MINDO/S

the

of chemical

on it.

the

calculating

overcoming the

points

variables

cal

in of

To reduce

optimized.

of

of reactions

dimensions

parameters

1 ity

being to

the

the

supermolecule

sol vated direction

with of

only

attack.

one

109

RESULTS AND For the metric

DISCUSSION

C2H5+ and C3H7+ cations,

parameters,

plexes.

These

(ref.131

and with

has been taken the

shows that

results the

into

IT complexes

C atom of

n

complexes

good

calculations

remain

with

in

For this

imposing

le

are more stable

agreement

(ref.l4-18).

studied,

el ectrophi

the

in

ab initio

account

are

the

are

MIND0/3method, optimizing

the

the

which

the

than the

0

only

energy

the formation the

point

comdata

correlation

that

middle

the geo-

experimental

the

reason,

restriction

over

the

all

attacking

of the

of H or

C-C bond in

ethylene. Figure xes

1 illustrates

between

represents

ethylene the

ene,

and E is

cess

is

very

an electron

the

and the

distance the

profiles

three

from the

energy

related

no Paul i

of

el ectrophi

electrophile to

no energy

exothermic, cloud

energy

that

the

formation

les

considered.

to the

barrier

being

forces

2

3

d

atoms of the ethylFor the

obtained.

appear.

TT comple-

The parameter

carbon

of the reactants.

repulsion

of the

Since

For the

H+ the prothe

H* lacks

CH3+ the

process

E kcal /mol 0

-100

-150

1.3

Fig.

1.

Energy

C3H7+ 1- --I,

profiles

in

and C4Hg+ I--

the C atoms of ethylene.

the --3,

formation d is

the

4

of

the

distance

d/A

‘TT compiexes

C2H5+ (-1.

between the electrophile

and

110 Electron

HOMO

affinity

CH3+ Affinity

H+ Affinity

C2H5+ Affinity

E eV

-5

C2H5+

-

-

-10

-

C5H5N

H20

-

C2H4

-

NH3

-

CH3CN

-

C5H5N

H20

-

C2H4

-

NH3

-

CH3CN

-

exothermic,

slight

CH3+

barrier

possesses

appear.

Finally,

exothermic,

giving

a barrier

of

of

understand since

the

state

in

the the

in accordance

with

heat that

us now see the

one solvent

back

exothermic

reactants

of the

electrophi

barrier.

holding

more

The order

by the

this

molecule

The of

and the

course

the

potential is,

sooner

it

ot the

three

the founti

side

at

of the

in

A.

to

the

is

even

contri buus

transition

co-ordinate.

the ethylene

in Figure solvated

direction

to

named case,

the

in the reaction

is

is

allows

is

presented

A

The geometric

last

with

affi-

repulsive

reaction

the

les

3.7

process

(ref.191

electrophile opposite

at

strong

2.7

more alike

electrophi

affinities,

when the

on the

is

barrier

and ethyl

kcal/mol

C2H5+ the

kcal/mol

the

methyl

cloud,

the

Hammond’s postulate

electron

processes placed

10.12 in

0,57

electron for

a process

reaction of its

le

of

an

already

25%

Let

the

a

forces

change experienced tes

an?

Because

obtained. less

NH3

H+

less

exchange

-

H20

Fig. 2. Electron affinities of electrophiles, and proton, nities of nucleophi 1es together with HOMOenergies.

is

H20

,NH3

CH3+

-

-

-

-

is

2. with

of

just

attack.

111 TABLE 1 The Reaction barrier,

heats,

E,

group

the

and the

of electrophi

and distances

position

of the

desolvation

les

in

transition

the

and solvents

studied.

Exo d :

the

34.29 1.30 2.G9 32.06 1.10

32.56 1.80 1.83 62.93 0.43

objective

form

nucleophile

to

water

given

in

‘ZH5+ found

at

water

the

i

is

has

transition 0.57

part the

of

of

this

the

eletrophile,

as an example and the

and

one

d,

with i

,

distance

D is is

and the

On these

A.

1.42

A.

in

When the

found

at

of

Given

water

the i

from

obtained,

(Fig

transition

1,

from

electrophile

the

ethylene

with

lines

as a are

When the

transition

C2H5+ with

to

a molecule

desolvation.

the

the

In Figure

isoenergetic

state

molecule

and D=Z.Ol

the

this

by

3a)

solvation

distance

using

the

the

variables.

solvating

on solvating

solvating

d=Z.lO

the

independent

accompanied

that

whether

both

surfaces

are

a molecule

clarify

distance as

two surfaces

reactions

D=1.74

produced. state

the

to

coordinate,

be chosen

aannonia.

Both

solvated d=l.89

is

C2H5+ as an electrophile, of

kcal/mol.

paper

reaction

D, must necessarily

been

the.

53.38 1.31 2.22 53.34 1.20

parameters

of

for

C2H5+

23.72 2.10 2.01 68.02 0.57

: des

we present

CH3+

14.78 1.95 1.85 53.16 0.41

Endo d

3,

potential

in kcal/mo’!

23.26 1.32 2.02 25.33 1.00

: E des

nucleophile

are expressed

-0.27 1.89 i -74 51.33 0.32

Endo

solvent,

indicated

-13.16 2.08 1.61 29.44 0.22

: E des

the

Energies

D, the

are

-12.33 2.70 0.97 0.79 0.01

Endo

that

d,

state

in Angstroms.

H+

Given

state,

transition

state

is

a molecule

of

a desolvation

is

mania

the

desolvation

(Fig

of

0.32

3b)

the

now being

E.

In Tab1 e 1 the

results

obtained

for

the

group of el ectrophi

les

and solvents

2

Fig.

3.

ethylene

Potential and the

arznonia molecule. distance.

surfaces C2Hgi, d is

The scaling

for

the

solvated the

a)

C-C2H5’

of isoenergetic

L

I

3

4

forma-Lion with distance lines

cne

of

the

water

complexes

molecule

anti C is
IT the

between the

and b1 with

electrophile-solvent

in kcal/mol.

one

113 studied

is

process,

expressed

tial

energy

are

indicated.

are

exothermic

solvents 2.

when solvated are

The are

in

is

a

In

electrophi

between

from

solvents

When the

it

is

correspondingly Desolvation

is

0.22

!,

is

If

that

we analyze

tion

at

The weaker

vated

with

phile

is parallel the

the

between the

latter

also

with

relate the

process

state less

wi 11

possible the takes

place

depends

not

with

what is

with

to

has

been stated

le,

the

observed

less in

in

the

the

transition of

the

desolvation

of

the

studied. the

other of

while

an

for

the

reaction

less

advanced is

establish

a

desolvation the

in

the

the

thermodynamic

model

al so

desolvation but

latter it the

(ref.191

increase

in

agreement

position

is

of

maintained us to

The more exothermic and this

in

Table

between the

transition

wi 11 also 1,

it

reaction

a similar in

observed

allows

desolvation

In

obtai-

is

and the

heat.

electro-

a good

agreement

indicated

state.

sol-

C2H5+ sol -

the results

with

is

state,

relationship

we can

used as the

solvents,

state

its

check

desolva-

general

of the

increases

transition

factor

is

to

le

CH3+ and the

and this

results

in

On comparing

reaction

order

is found in the transi-

the

There

transition

barriers,

for-

CH3+ it

Ritchie’s

in

solvent,

transition

the

quantitative

potential

on the

the

the

ic

for

electrophi

when water

and therefore

following

However,

the

of

different

state

to

state

at the transition

solvents

Hammond’s postulate

magnitudes

H+ but

discount

desolvation

co-ordinate,

barrier.

transition electrophil

for

desolvation

solvent.

in

of the

degree

comparing

process.

solvated

the

desolvation

other

of the

respect

be more reactant-like

only

in

electrophile.

with

potential

with

other

transference

in

occurs

the

to a more endothermic

different

of

the

apparent

than

process,

These results which

electrophile

the

to

level

z.

of annnonia. The greater

along

advanced.

the

weakness

hardly

obtained

electrophi

of

is,

with

solvents

weaker

that

state,

processess

clearly

of the

the

poten-

affinities

but

desolvation the

independence

of

level

these

0.32

the

character

the

to

stronger

results

desolvation the

is

state

a molecule

nucleophilic

remainder

an exothermic

state to

on the

the

a given

the

whereas

electrophilic

water,

water,

according

the

An exception

state.

that

Pross,

opposite

tion

is

according

C2H5+ it

transition

the

is

all

is

of

the

transition

that

this

understood

ethylene

transition

the

‘s hypothesis

ned for

the

of

and the

be

molecule

extensive

the

can

greater

Pross

vent.

to

for

than

the

of water,

reason

nature

state,

endothermic.

in

less

indicate

water is

and for

and favour state

it

in

we observe

HOMO and the

water

endothermic

produced

a molecule

nucleophile

or

of the transition

table,

The

the

way

solvating

ce.

this

with

the

stronger this

le

of

exothermic

position

desolvation

endothermic.

energy

Ethylene solvents.

and the

E,

When we analyse

ethylene

other

in kcal /mol , the

barrier,

they

Figure

In each case the

1 i sted.

be

is

not

heat

and

way to what

transition

al so on a kinetic

state factor,

114 1 inked it

to

the

nature

of the

can be deduced that

lit

character

is

has a greater sions

On the

lower

by this

reduced

independent tric

the

This

as

el ectrophi

in

the

is

the

protonated

4b)

it

is

trile

to

the

nate

both

For other

to

greater

negative

dinate,

tion

our

in

water.

the

There

state

the

transition

vation

parameter state.

case

For

of the

take

vector

geome-

co-ordinate

using

these

nature

may

structures

hypersurface

has subsequently

with

a molecule observed

is

points

by the

have been obtained

4 indicates

when proton

it

of

the

separation

when the

the

and CHB’ are

of

the

the

and the

used

When the

reaction

co-ordinate

acetonitrile

electrophile in

of

components

acetonitrile.

that

co-ordinate

desolvation

stationary

value the

are

in

is

from

CH3+ (fig.

transition

CH3+ from

state the

approximation

is

acetoni-

of the elec-

components of the reaction

of

to

degree

to

For the place

sol vation reactions

of

the

the

co-ordi-

in

plays in

the no which

proton

the

level

parameter

solvated

transition part

in

state, the

a parti al

with

of

solvation

a molecule and parallel

reaction

to

the

co-orof

approxima-

solvation

does

state.

desolvation

desolvation

symmetry

a molecule

by the

parameter

of

‘with

of

the that

the reaction

solvated

of

indi-

shift

corresponding

constituted the

a

conditions

in the transition

between the proton

the

that

Frequently

represents

delineating

ethylene;

which

matrix.

components

fundamentally

co-ordinate

connection

with

vector

approximately

is

force

value

incompatible

attack

proton

have been obtained

constant

negative

4c the

shown,

the

points

the

second

co-ordinate

vector.

does not

saddle Figure

fundamental

In Figure

value

and the

other

potential

and their

transference

structures,

a close

tion

are

of the reaction

is

reaction

complete

authentic

contrary,

by the

to

sol vated

not form part

al so contains

reason,

on the

reaction

studied

study.

the

solvent

conclu-

the

state.

The reaction of

the

Both the

asymmetric in

When the

attacked

d and D have been used as

the

For that

constituted

nucleophile

corresponding

leads

4a)

one negative

imposed

of

transition

On the

that

reactions

more than

vector

is

ethylene.

distances

as solvents.

solvated

state

in the transition

cate

as a whole

whose nucl eophi -

attacked.

nucleophile

SIGMA program,

studied

(fig.

ethylene.

the

solvents

co-ordinate

points

the

constituted

to

for

obtained

FORCE program. cases

observed

fundamentally trophile

the

proton

transition

the

two

obtained.

the

atom of

1es,

electrophile

the

and pyridine

each

results

nucleophile

reduction

states

using

acetonitrile

valid

that

reaction

drastic

different

to

only

stationary

using

Among the

only

of the

The true

points,

been analysed,

is

character

surfaces

have been localised,

relating

that

From the

le.

model are inverted.

transition

as a starting

using

than

variables.

parameters.

falsify

Pross ‘model

nucleophilic

reached

electrophi

in the transi forms to

of

desol-

this,

the

in

that

co-ordinate takes

part

of water

-

place

in

the

115

0.23C H 0.23eH \ 0.26%_

H+0.29 / /H--)0.29 c+030 \ Hj0.29

--_------_--N.+C' 0.x 0.30

a

0.23

b

C

Fig.

4. Components related

tions

of ethylene

vated with acetronitrile transition

state,

to each atom ot the transition

with CH3+ solvated

with acetonitrile

vector

from the reac-

(b1 and with proton sol-

(a1 or with water (cl.

the

reaction

co-ordinate

there

is

constituted

both by tine

solvati on parameter nucleophile. Finally,

and by the distance between the el ectrophi le and the for the reactions with high desolvation in the transition

state,

co-ordinate

the reaction

in that state

is fundamentally

constituted

by

the solvation parameter. In al 1 c2s2s, the process consists in the transference of the electrophile from the solvent to the nucleoghile, and this transference can take place before, during or afLer the transition state. It is therefore evident

that

desolvation

in the transition

state

depends at the same time on

116 the

electrophile, In

the in

concl usi on,

simplicity

of

solvent

the

behaves

solvent spite

model as

and the of

a true

the

with

The nucleophile-electrophile

sarily the in

electrophilic

accompanied molecules

which

takes

of

the

part

molecules order

to

of

the

plays

the

reaction

be taken the

the

solvent,

Because

of

its

of

electrophile

weakening

the

between

a much more co-ordinate.

transition

of

Tine solvent

process

into

the

combination

sol vent.

but

must locate

the

of

represent

character

chemical

place,

ming

by

limitations

to

reactant.

modifies it.

the

adopted

nucleophile.

account states

is

active This

MIND0/3

not

between only

its

imp1 ies

together and other

on the

is 1 ic

the

in

the

character

it

is

combined is

necesand

physical

surroundings

and

electrophile

the

parameters

reactants

potenti

stationary

the

that

electrophile

geometric that

it

solution

the

and of

clear

when

nucleophile part,

method

it

nucleophi

reaction

links

the

the

al

for-

and solvent

hypersurface

in

points.

REFERENCES 1 2

9

;: 12 13 14

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