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Solvation and Solvent Effects

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					Reactive Intermediates                            -55-                            Prof. Bernhard Jaun


5. Solvation and Solvent Effects
5.1 Solvent polarity scales

Solvents are grouped according to their properties into the following main classes:

1. Protic polar solvents

Examples: Water, alcohols, acids, primary and secondary amides.

Properties: high dielectric constant, hydrogen bond donor and acceptor. Strong solvation of cations
and anions.

2. Aprotic polar solvents

Examples: HMPT, DMSO, DMF, acetonitrile, nitromethane, tert-amines

Properties: high to medium dielectric constants; good hydrogen bond acceptors; solvation of cations is
good; solvation of anions is very weak.

3. Aprotic nonpolar solvents

Examples: aliphatic alicyclic and aromatic hydrocarbons, halogenated hydrocarbons, ethers.

Properties: low dielectric constant, inefficient solvation of small hard ions (exception: polyethers such
as dimethoxyethane, glymes). Aromatic solvents are known to form donor-acceptor complexes with
suitable solutes. Halogenated solvents are quite good solvents for medium to soft cations such as
carbocations.

In view of the many different types of possible interactions between the solvent and individual solute
molecules/ions, no single polarity scale is suitable for all purposes.

Generic polarity scales

1. ET-scale

This polarity scale is based on the wavelength of the charge transfer band in the visible spectrum of A
(solvatochromicity; from 453 nm in H2O to 810 nm in diphenyl ether):




                           N                                     N
                                          hν



                           O                                    O


                           A
Reactive Intermediates                             -56-                           Prof. Bernhard Jaun


The ET-parameter is defined as the energy of this transition in kcal/mol. The rates of many types of
reaction, including SN2, obey a linear relation based on ET of the solvent
      log k = a•ET + b

The slope of such plots is an indicator for the change in the polarity upon going from the reactant
ground state to the transition state. It will be positive if the TS is more polar than the reactants and
negative in the opposite case.

2. Z-scale
This scale is based on the solvatochromicity of B and runs practically parallel to the ET scale (formula
for conversion: Z = 1.41 ET + 6.92).

                                                    COOMe


                                                              I
                                                    N
                                                    CH3

                                                     B

3. Y-scale

This polarity scale, which was introduced by Winstein, measures the influence of the solvent on the
rate of SN1 type solvolysis reactions.

Therefore, Y is only defined for protic polar solvents. Y is defined as the log of the ratio between the
rate of solvolysis of t-butyl chloride in a given solvent and the corresponding rate in 80% aqueous
ethanol.

                                  Y = log k(solvent) - log k(80% EtOH)

The sensitivity of the rate of a particular reaction to solvent polarity is expressed as the coefficient m
in:

                                                 k(solvent)
                                         log                      = mY
                                               k(80% EtOH-H2O)

By definition, solvolysis of t-butyl chloride has m=1. Typical SN1 reactions have 0.7 < m < 1.2;
SN2 reactions have m < 0.5.
Reactive Intermediates                                   -57-                              Prof. Bernhard Jaun


4. The Hildebrand solubility parameter δ

The internal pressure c of a pure solvent is defined as the energy (per molar volume) needed to
vaporize the solvent to a vapor pressure of 0 Pascal.

                          ΔHv - RT                     ΔHv =enthalpy of vaporisation to 0 pressure
                  c=
                              Vm
                                                       Vm = molar volume

Therefore, c reflects the energy needed to make a “hole” in the solvent that is large enough to ac-
                                                                                                     1/2
commodate a solute molecule. Hildebrand has defined his solubility parameter as δ = c                      and has
shown that substances are optimally miscible if they have the same δ value. If the difference between
the δ-values of solute and solvent differ by more than 3, one can no longer expect any solubili-
ty/miscibility.
                                                                                       ≠
Solvents with a high internal pressure (high δ) make reactions with positive ΔV more difficult. At least
for nonpolar reactions, where no special solvation effects are to be expected, variation of the solvent
                                                                ≠
and plotting log k vs. δ allows to crudely estimate ΔV and serves as a “poor man’s” substitute for the
                                ≠
correct measurement of ΔV with high pressure equipment.




5.2 Ion Pairs and Solvolysis

Ion pairs

From todayʼs perspective, we need to discriminate between contact ion pairs, solvent separated ion
pairs and free ions. Some authors also include solvent-shared ion pairs into the (actually somewhat
continuous) spectrum of interactions between ions of opposite charge in solution.




       contact            solvent sharing            solvent separated                     dissociated

Transitions between these species can be observed by physical and spectroscopic methods, such as
                  13      6         23
conductivity or        C-, Li and        Na-NMR. In his classical work during the 1960s, Winstein has demon-
strated convincingly that ion pairs are a reality and have to be taken into account in order to explain
the kinetics of solvolysis reactions.

In the following, we give a summary of the developments and follow the arguments of, first, Ingold and
then Winstein (for a contemporary review see: S. Winstein et al. Chem. Soc. Spec. Publ., No. 19,
1965, 109)
Reactive Intermediates                                 -58-                               Prof. Bernhard Jaun


1. The normal salt effect:

Reactions producing ions (or a charge separation in the transition state) show a rate increase if the
ionic strength of the medium in increased. At first approximation, this effect is usually linear:

                                                   k = ko + b [salt]

Coefficient b is a measure of the sensitivity of the reaction towards ionic strength.


2. The kinetics of SN1-reactions according to Ingold:

                           k1                          SOH
              R–X                    R+ + X–                      RO–S + H+ + X–
                           k-1                          k2'

If the first step, the dissociation into ions, is reversible we can apply the steady state approximation for
the concentration of the carbenium ion and the expression for the overall rate is:

                  d[RX]            k2 k1
              –            =                          with k2 = k2' [SOH]
                      dt         k2 + k-1 [X-]

                                                          –
Because, during the reaction, more and more X is generated, the rate decreases with increasing
                                                                                           +   –
turnover number. If, on the other hand, we deliberately add an excess of a salt M X , the concentration
    –
of X will be independent of turnover and one observes pseudo first order kinetics (with a reduced
apparent rate constant). This phenomenon is called “common ion rate depression” (CIRD).
                                                                       +   –
At the same time, however, addition of an excess of salt M X will also cause a normal salt effect
which accelerates the reaction:

                  d[RX]          k2k0(1+b[X-])
              –            =                         with k2 = k2' [SOH]       k1 = k0(1+b[X-])
                      dt         k2 + k-1   [X-]

Whether the normal salt effect or CIRD dominates depends on the reactivity of the carbenium ion: for
                  +                                                        –
highly reactive R , k–1 is comparable to k2ʼ. Because [SOH] >> [X ], the normal salt effect will dominate
and an increase of the overall rate is observed.
                      +                     –                                      –
For more stable R , k–1 >> k2ʼ (since X is charged while SOH is neutral, X is expected to be the better
nucleophile than SOH) and the overall rate decreases because the CIRD effect dominates.


3. Ion pairs. So far (up to Ingold), it was assumed that solvolysis generates free ions directly. The
results obtained with the following solvolysis reaction prompted Winstein to postulate the formation of
ion pairs as intermediates:
Reactive Intermediates                                   -59-                              Prof. Bernhard Jaun

                                                                                                 Cl
    H3C                             H3C                                                    H3C

                                                     +          Cl
           CH3        Cl                   CH2                                                   CH3
                                                     C
                  A                                                                                   B


                                         AcOH/AcO-                   OAc
                      H3C                                     H3C

                             CH3    OAc                               CH3

The overall rate of this solvolysis in aqueous acetic acid/acetate buffer was determined by acid/base
                                                                            st
titration (consumption of acetate). For the solvolysis of A, pseudo-1 order kinetics was observed and
              –
addition of Cl did not lead to a CIRD.

The rate of solvolysis of B was, at first, higher than that for A but then gradually decreased and
eventually approached the rate found for A. If B was isolated back at incomplete turnover, it was found
that, in parallel to solvolysis, B had also rearranged to A (which explains the change in rate with time).
                                                                       –
The crucial argument of Winstein was that, since external Cl did not show a CIRD, the rearrangement
of B into A could not have gone via the free dimethylallyl cation but that there must be an additional
intermediate which is not accessible for external chloride ions but allows 1,3 migration of chloride to
give A. This led to the postulate of a contact ion pair.



                                    H3C                                           Cl
    H3C                                                                     H3C
                                             Cl
           CH3        Cl                   CH3                                    CH3
                  A                contact ion pair                                    B
                                    H3C

                                                         +      Cl
                                           CH3
                                         free ions




                                      AcOH/AcO-                      OAc
                  H3C                                        H3C

                            CH3    OAc                               CH3
Reactive Intermediates                                                     -60-                                             Prof. Bernhard Jaun


4. Solvent separated ion pairs. In order to explain the phenomena observed in the solvolysis of
substrates that racemize in parallel to solvolysis, Winstein was eventually forced to introduce an
additional intermediate, termed Solvent separated ion pair, into the kinetic scheme:


                                        contact ion pair           solvent separated                           free ions
                                                                        ion pair
                                  k1                       k2                                    k3
                        RX                    R+X–                     R+ || X–                               R+ + X–
                                  k-1                        k-2                                 k-3

                                                                       k4         SOH                          k5     SOH


                                                                       products                                products


                                                              O
                                          Bs = Br             S        brosylate
                                                              O                             OMe
                         MeO

                                                                                                     OBs contact ion pair
                                           H CH            k1
                                                   3

                              H                              k-1/2                               H
          enantiomers




                                      OBs                                         Me                  H
                              H3C MeO
                                                                                       Me
                                                                   k-1/2

                                                                                       k2         k-2
                                                       CH3
                                                             H
                                   H3C                 OBs
                                          H                                                 OMe

                        MeO
                                                                                                                OBs


                                        CH3                          k4                                       solvent seperated ion pair
                                                                                                 H
                                          H                                       Me                   H
                                                                                        Me
                        H3C                                          AcO–
                                        OAc
                              H                                                             k3         k-3
                        MeO                                            k5
                                                                                             OMe
                                                                 AcO–
                                        H CH                                                              +
                                               3                                                                OBs

                          H             OAc
                          H3C                                                                     H           free ions
                                                                                  Me                   H
                         products of acetolysis                                         Me

                                                                           Scheme I

Of course, Winstein and coworkers first tried to analyze the kinetics of this system based on their
original scheme with only one intermediate, the contact ion pair, between reactant and free ions. This
amounts to leave out rate constants k2, k–2 and k4 from scheme I above.
Reactive Intermediates                                                 -61-                             Prof. Bernhard Jaun


Experimentally, two rates could be followed: the rate of racemization of the starting material kα , and the
overall rate (acetate consumption) kt.

Using the steady state approximation for both, contact ion pair and free carbenium ion gives:

                       d[AcO- ]                          k1 k3 k5
                   –                     =                                     [ reactant]
                           dt                k5 (k-1 + k3 ) + k-1k-3 [BsO– ]

                                                     k1 k3 k5
                           i.e. kt =
                                             k5 (k-1 + k3 ) + k-1k-3 [BsO– ]

                                                                                          –
Looking at this expression, one would expect that addition of BsO should lead to a CIRD effect. This
was, however, not observed. Therefore, we have to assume that
                                                     –
                  k5(k-1 + k3) >> k-1k3[BsO ]

                           k1 k3                                                kα        k-1
          i.e. kt =                                  k1 = kα        and               =            +1
                          k-1 + k3                                               kt           k3

In other words: the ratio between kα and kt should vary if the ratio between k–1 and k3 changes. Addi-
tion of an “innocent” salt such as LiClO4 should decrease k-1 and increase k3 through a normal salt
effect (higher ionic strength favors the direction of dissociation). In fact (see curve C in the plot below)
the ratio between kα and kt did decrease when LiClO4 was added. But a plot of [LiClO4] vs. rate
revealed that this increase was far from linear (as would be expected for a normal salt effect) and
leveled off at quite low [LiClO4], with kα/kt becoming constant again at higher [LiClO4].



                                                     kα
                                                                                      Salt effects in SN1 solvolysis
      k
                                                           LiClO4                     A= normal salt effect
              special salt effect
                                                     kt                               B= CIRD
                            C

                                   induced CIRD
                                                                                      C= special salt effect

                                                  normal salt effect
                                                                                              and induced CIRD

                            A                        kt

                                                         CIRD
                            B                         kt



          0                     [salt]               0.05 M




Winstein interpreted this puzzling result as follows: there must be an additional intermediate between
the contact ion pair and the free carbenium ion. In contrast to the contact ion pair, this second inter-
mediate must be accessible for counter ion exchange: brosylate can be exchanged by external
Reactive Intermediates                                -62-                                    Prof. Bernhard Jaun


perchlorate which makes the backwards reactions k–2 and k–1 impossible and increases the overall
                                                                                      –                     –
rate kt. Winstein also realized that this hypothesis can be checked: if ClO4 can replace BsO in this
                                        –                                                               –
new intermediate, addition of BsO should reverse this effect. Indeed, addition of BsO after the
special salt effect had first been induced with LiClO4 cancelled the latter (downwards curve in the plot).
This CIRD, which is only seen in the presence of LiClO4, was termed the induced CIRD.

The complete scheme with contact ion pair and solvent separated ion pair as intermediates gives the
following analytical expression for the overall rate kt:

                                             k3k5
                         k1k2    k4 +
                                        k5 + k-3[BsO–]
              kt =
                                             k3k5
                     (k-1 + k2 ) k4 +                  + k-1k-2
                                        k5 + k-3[BsO–]

Analyses of this equation for conditions compatible with the absence of a normal CIRD leads to four
limiting cases:

Case I : k4 >> k3                            product formation from the solvent seperated ion pair
                                             is much faster than dissociation into free ions.
                                             No free carbenium ions are formed at all.

                                                            k1k2k4
                                             kt =
                                                    (k-1 + k2) k4 + k-1k-2



Case II : k5 >> k-3 [BrO- ]                  product formation from free ions is so fast that the
                                             solvent seperated ion pair is never reformed
                                             from the free ions

                                                            k1k2(k4 + k3 )
                                             kt =
                                                    (k-1 + k2)( k4 + k3)+ k-1k-2



Case III : k-3 [BrO- ] >> k5 and k3 ≈ k4     products are exclusively formed from
                                             the solvent separated ion pair, never from
                                             the the free carbenium ion.

                                                          k1k2k4
                                             kt =                            (as in case I)
                                                    (k-1 + k2) k4 + k-1k-2




Case IV : k-1 ≈ 0 and k-2 ≈ 0                No re-association to contact ion pair or
                                             starting material from the solvent seperated ion pair
                                                     k1k2
                                             kt =
                                                    k-1 + k2

                                             (this is equivalent to the case without the solvent
                                             seperated ion pair as an additional intermediate)
Reactive Intermediates                                     -63-                              Prof. Bernhard Jaun


Only cases I and III make it necessary to include the solvent separated ion pair into the kinetic
scheme. Case IV can be checked for by back-isolation of starting material after having added either
labeled brosylate or e.g. tosylate instead of brosylate. Case II requires that the free carbenium ion is
very reactive. Winstein excluded it on the basis of the reactivity-selectivity principle by showing that
acetate and azide were reacting not according to their concentrations (statistically) but according to
their nucleophilicities. See however the comments on the RSP earlier in chapter 3).

For the reactant discussed here, Winstein concluded that only cases I or III could apply making the
solvent separated ion pair a necessary component in the kinetic scheme. Winstein and others investi-
gated many other substrates. Sometimes case IV applies, whereas other cases showed the normal
CIRD as described by Ingold. In essence, the more stable the carbenium ion, the more likely it is that
solvent separated ion pairs and free ions are formed before the carbenium ion is trapped by the
buffer/solvent.

5. The rates of dissociation and racemization are not necessarily equal. In the example shown below
it was demonstrated that kα< k1:



                                                      18
                                                       O
                                                                            NO2
                                                                                            18
                  H 18O                                    O                                 O
                                     k1                                                                            NO2
            C                                                                                CH
                                                            CH
                  O                                                                            O
                                  retention                            18O-exchange

                                  NO2                                             Cl
                                                 Cl
    Cl

                                   contact ion pair                 inversion
                                      still chiral                                                     inversion



                  H 18                                                 18O-exchange
                      O              k1
            C                                                CH                                  CH
                  O               retention                  18 O                                18
                                                                                NO2                O
                                  NO2                                                                                NO2
                                                              O                                    O
                                                 Cl                                    Cl
    Cl



The rearrangement in the contact ion pair to give inversion requires an “anion above the cation→anion
below the cation” movement, whereas the oxygen exchange requires only a small translation of the
                                            18
carboxylate group in the plane. Since            O exchange requires only a minimal movement but can occur
only when the bond to carbon is broken, it is probably the experimentally accessible rate that comes
closest to the true dissociation rate k1.
Reactive Intermediates   -64-   Prof. Bernhard Jaun

				
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