Activation Energy vs. Charge Transfer Energy

Activation Energy vs. Charge Transfer Energy

The activation energy for addition to This demonstrates the need to

1,3-butadiene is quite consistent with the also consider the electron affinity

correlation between activation energy for of the alkene, and not just its

the addition of peroxyl radicals to mono-alkenes ionisation energy, when examining

and the energy released by charge transfer its reactivity.

to the radical (EC).7

Appropriate Structure Activity Relationships for Radical Addition to Alkenes

Consideration of just the ionisation energy of The difference in electronegativities

the alkene can be misleading. The value for between the alkene and the attacking

1,3-butadiene is lower than, for example, radical controls the rate of addition, so

that for propene. peroxyl radical addition to 1,3-butadiene

has a similar activation energy to that for

However, the electron affinity of propene.

1,3-butadiene is also lower than that of

propene, so the electronegativities for both This is shown graphically here (the gradient

are comparable. for a zero charge transfer represents the

absolute electronegativity).6

Activation Energy vs. Alkene Ionisation Energy

The activation energy for addition of acetylperoxyl The activation energy for addition

radicals to 1,3-butadiene is higher than would be to 1,3-butadiene is in fact

expected from the relationship between alkene comparable to values for terminal

ionisation energy and activation energy for addition mono-alkenes, in spite of having a

to unsubstituted mono-alkenes. lower ionisation energy.

This is perhaps surprising, considering that the

resultant adduct radical is resonance stabilised.

Addition of Acetylperoxyl to Dienes

To examine how radical addition to dienes differs from addition to

unsubstituted mono-alkenes, Arrhenius parameters for the reaction of

acetylperoxyl radicals with three conjugated and one unconjugated

diene were determined (Table 1).

Transition State for Acetylperoxyl Addition to 1,3-Butadiene

Activation Energy vs. Radical Electonegativity

With this measurement, Arrhenius parameters The relationship between radical

are now available for a wide range of peroxyl electronegativity and activation energy for

radicals attacking the one alkene.1-3 addition to 2,3-dimethyl-2-butene is given

here.

The difference in electronegativity between

the radical and the alkene can be considered As a comparison, values for two other

to control the rate of the addition. oxygen centred species (ozone4 and the

nitrate radical5) are also given. They also

fall on the same correlation as the peroxyl

radicals.

Activation Energy vs. Alkene Ionisation Energy

This work on 2,3-dimethyl-2-butene now The addition shows no sign of steric

extends the reactions investigated to cover hindrance, in fact the pre-exponential

alkenes with ionisation energies ranging factor is slightly larger than for other

from 8.3 to 9.7 eV. peroxyl radical addition reactions.

The measured barrier for this reaction conforms

with the correlation between alkene ionisation

energy and the activation energy for addition of

acetylperoxyl to alkenes previously found.1

Addition of Acetylperoxyl to 2,3-Dimethyl-2-Butene

The first example of addition of oxygen However, the most polar of this class of

centred radicals to alkenes to be investigated reaction, the addition of acetylperoxyl

was for acetylperoxyl addition. eg.1 to 2,3-dimethyl-2-butene has not

The variation of rate of reaction with the previously been examined.

ionisation energy of the alkene identified the

reaction as an electrophilic addition.1 This reaction was studied here over the

temperature range 393 to 433 K, and

Transition State Arrhenius parameters found (Table 1).

Epoxidation of 2,3-Dimethyl-2-Butene, Conjugated Dienes and 1,5-Hexadiene by Acetylperoxyl Radicals

J. R. Lindsay Smith, D. M. S. Smith, M. S. Stark and D. J. Waddington

Department of ChemistryUniversity of York, York, YO10 5DD,

UK

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Alkene log10(A / dm3 mol-1 s-1) Eact / kJ mol-1

2,3-dimethyl-2-butene 9.1±0.6 13.3±5.0

1,3-butadiene 9.6±0.6 30.9±5.0

isoprene 8.7±0.9 23.3±6.7

2,3-dimethyl-1,3-butadiene 8.1±1.4 17±11

1,5-hexadiene 9.6±0.8 35.8±6.2

Table 1: Arrhenius parameters for the addition of acetylperoxyl radicals to

2,3-dimethyl-2-butene, selected conjugate dienes, and 1,5-hexadiene.




References

(1) Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chem. Soc. Perkin Trans. 2 1977, 360. (5) Atkinson, R. J. Phys. Chem. Ref. Data 1997, 26, 215.(2) Baldwin, R. R.; Stout, D. R.; Walker, R. W. J. Chem. Soc Faraday Trans. 1 1984, 80, 3481. (6) Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512. (3) Stark, M. S. J. Phys. Chem. 1997, 101, 8296. (7) Stark, M. S., J. Am. Chem. Soc. 2000, 122, 4162.(4)Wayne, R. P. et al. Atmos. Environ. 1991, 25A, 1.

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Activation Energy vs. Charge Transfer Energy