Laboratory for Chemical Technology, Ghent University

http://www.lct.UGent.be

A Systematic Study of Radical Addition Reactions to Substituted Benzenes and

the Reverse -Scission Reactions

Hans-Heinrich Carstensen, Kevin Van Geem, Marie-Francoise Reyniers and Guy B. Marin

1

International Conference on Chemical Kinetics, Ghent, Belgium, 29/06/2015

Chemistry of Aromatics is Important for Steam Cracking

Aromatic molecules play an important role

in pyrolysis chemistry

Molecular weight grows chemistry

Source for carbon black

Coking of reactors

Sooting in combustion systems

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ICCK, Ghent, 28.06-02.07.2015

Pilot steam cracker

Steam cracking

Base Chemicals

EthenePropene

ButadieneAromatics

Light fractions from oilor NGLEthane

PropaneNaphtha

Thermochemical Conversion Of Biomass => Aromatics

Thermochemical conversion of biomass

produces large amounts of aromatic products

Subsequent chemistry has a significant

impact on the product quality (bio-oils)

Large amounts of benzene and toluene

at higher temperatures has been related

to initial pool of aromatics

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ICCK, Ghent, 28.06-02.07.2015

OH OHH3CO OH

H3CO OH

OCH3

Phenol Benzaldehyde Guaiacol

Syringol Vanillin

OCH3

OH

OH

ferulic acid

O

OCH3

CO

CH3

C6H7

Anisole pyrolysis 1000 K (Pecullan et al. 1997)

Biomass Bio-oil

pyrolysis

Addition-Elimination Reactions Are Important in (subst.) Syringol Pyrolysis

4

Conference, City, Date (adjust through header and footer)

Flash vacuum pyrolysis experiments indicate importance of addition/elimination chemistry(Britt et al. J. Org. Chem. 2000, 65, 1376-1389)

possible/likely mechanism involves H,CH3 addition; note: 30% of the products

Kinetic modeling of biomass pyrolysis Radical requires the inclusion of addition reactions to aromatic species and their reverse reactions

Long-Term Objective: Let Computers Create the Models

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ICCK, Ghent, 28.06-02.07.2015

Automated Mech Gen.

(RMG or Genesys)

Conditions Reactants T P Reaction time Reactor model Termination criteria

Reaction mechanism complete consistent error free quickly generated

Thermo databases(Group Additivity)

kinetic databases(Group Additivity)

C2H5

CH2

CH3

CHO

OH

OCH3

GAV/NNI for • 143 closed-shell

subst. benzenes• > 300 radicals

G4BAC - GA Closed-shell

Long-Term Objective: Let Computers Create the Models

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ICCK, Ghent, 28.06-02.07.2015

Automated Mech Gen.

(RMG or Genesys)

Conditions Reactants T P Reaction time Reactor model Termination criteria

Reaction mechanism complete consistent error free quickly generated

Thermo databases(Group Additivity)

kinetic databases(Group Additivity)

24 GAV/NNI for • 104 C6-cyclic

radicals

∆fH298 [kJ/mol]

CBS-QB3BAC – GACH

R

CH

R

CH

R

Long-Term Objective: Let Computers Create the Models

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ICCK, Ghent, 28.06-02.07.2015

Automated Mech Gen.

(RMG or Genesys)

Conditions Reactants T P Reaction time Reactor model Termination criteria

Reaction mechanism complete consistent error free quickly generated

Thermo databases(Group Additivity)

kinetic databases(Group Additivity)

� = � �� ∙ ����,�

������������������

�

+ � �� ∙ ����,�

���������������

�

Thermo:

Objective of this study:1. Systematic investigation of reaction family2. Impact of X,Y, Z on reactivity 3. Will ∆GAVo approach work ?

One of the missing reaction families is

Kinetics: database for kinetics Group Additivity parameters (∆GAVo) is incomplete

Group Additivity To Predict Rate Parameters

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8

R3 R2 R1Z2

Z1

Z3

Y1

Y2

X1

X2

N2

N1

N3

N1

N2

N3

N3

N2

N1 M3

M2

M1

M1 M3

M2

L1

L3

L3 L2

L1

L2

�� � = ��,��� � +�∆�����,�� ��

�

���

+�∆�����,�,��

�

���

log�� ��(�) = log�� �����(�) + �∆��������,�� ��

�

���

+�∆��������,�,��

�

���

���� = �� ∙���

�∙

��

�

∆�∙ ��(∆

‡���∆

‡��)/�� = �� ∙ �� ;�� =

����,‡

∏ ����,�������

∙∏ s�������

s‡

�� � = ��� ∙�ln �

��= ∆‡� + (1+ ∆�)��

�� � =���

ℎ∙��

�

∆�

∙ �∆"

‡��

� ∙ ���∆�

GAV is well-known to work for H, S, Cp

Similar GA approach must work for ��, ��

Introducing a referencereactions finally leads to

No of single events

R1,R2,R3 => GAV0

Xi,Yj,Zk => NNI0

R + olefins

Methodology

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QM calcsCBS-QB3 level

Statistical Mech.Atomization Method

TSTdGEckart tunneling

Geometry Frequencies Electronic EnergyHindrance potentials for internal rotors (B3LYP/6-31G(d)

∆fH0 , S0 and cp(T)

NASA polynomials

k(T) Arrhenius expressions

����(�) = �(�) ∙���

�∙

��

�

∆���∙ ��

∆�ǂ

��

Just relative energies needed => no BAC

all internal rotors replaced

Methodology chosen to be consistent with previous work of this lab

���� = �� ∙ �� ;�� = ����,‡

∏ ����,�������

∙∏ s�������

s‡Single event rate coefficient ��:

Intuition: reactivity should depend on X,Y,Z

But only X is involved in bond breaking/making

Y, Z only indirectly (non-nearest neighbor interaction NNI)

involved

Reactive Center of the Title Reaction

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Y X

Z

Y

Z

+ X

X

Z

+ Y

�� � = ��,��� � + ∆�����,�� � +�∆�����,�,�

�

�

���

Expectation is that

“X” affects reactivity most

Suitable reference reaction for

carbon-centered radical addition

would be

CH3 + benzene or

CH3 + toluene

�� � = ��,��� � +�∆�����,�� ��

�

���

log�� ��(�) = log�� �����(�) + �∆��������,� ��

�

���

log�� ��(�) = log�� �����(�) + ∆��������,� � +�∆��������,�,�

�

���

Systematic Investigation of the Impact of X

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Y X

Z

Y

Z

+ X

X

Z

+ Y

How does the type of the adding radical (X) change the reactivity?

X + PhH

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X = H+ X

X H

H + Benzene → cyclohexadienyl

k [

cm

3m

ol-

1s

-1]

Nicovich 1984

Gordon 1978

Hoyermann 1975

Ackermann 1990

Nicovich 1984

Gordon 1978

Hoyermann 1975

Ackermann 1990

Berho 1999

x Mebel 1997 (calc)

+ Baulch 1992 (review) k [

cm

3m

ol-

1s

-1]

X + PhH; X = Alkyl

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X = HCH3

C2H5

i-C3H7

t-C4H9

CH2OHCCHOCC(=O)CHCOCH3C(=O)PhC(=O)

+ X

X H

C-centered radicals are > factor 100 slower than H

Surprise: HCO and even resonantly stabilized radicals react similarthan alkyl radicals

X + PhH; X =Alkyl

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X = HCH3

C2H5

i-C3H7

t-C4H9

CH2OHCCHOCC(=O)CHCOCH3C(=O)PhC(=O)

PhH+H

PhH+CH3

PhCH3+H

PhH+CCHO

PhCCHO+H

PhH+CHO

PhCHO+H

PhH+H

The PES are different but the entrance barriers are similar for this group of C-centered radicals Addition reactions group reasonably well.

X + PhH; X = Vinylic Radicals

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X = H”Alkyl”C=CCC=CC2C=CC=CC=CC=CCC=CC=C2-MePh2,6Me2Ph

+ X

X H

X + PhH: fully oxidized C-centered Radicals

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X = H”Alkyl”“Vinyl”C(=O)OHC(=O)OMe

+ X

X H

X + PhH: Allylic Radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR”C=CCCC=CCC=CCCC=CCC2

+ X

X H

X + PhH: Propargylic Radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR””p-allyl””s,t-Allyl”C#CCCC#CCC#CCCC#CCC2

+ X

X H

X + PhH: X = Benzylic Radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl”PhCPhCC (- -)PhCC2 (…)

+ X

X H

X + PhH: X = Allenenic Radials

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl””benzyl””C=C=C”

+ X

X H

X + PhH

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl””benzyl””Allenyl”

+ X

X H

X + PhH: O-centered radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl””benzyl””Allenyl”

OH

+ X

X H

Note: OH forms a pre-complex

with benzene the barrier is below

the reactants vTST for complex

formation stepstill needs to be done

current rate coefficientsnot reliable

X + PhH: O-centered radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl””benzyl””Allenyl”

OH

CH3O

+ X

X H

Note: • Almost no interaction

with the aromatic ring• Rate coefficient

should be okay

X + PhH : O-centered radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl””benzyl””Allenyl”

OH

CH3O

C=CO

+ X

X H

X + PhH : O-centered radicals

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X = H”Alkyl”“Vinyl”“C(=O)OR””allyl”“propargyl””benzyl””Allenyl”

OH

CH3O

C=CO

PhO

+ X

X H

Resonantly stabilizedO-centered radicalsare among the least reactive ones

X + PhCH3: H and C-centered Radicals

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+ X

CH3 X CH3

Same observations for addition to toluene.

X + PhCH3: plus O-centered Radicals

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+ X

CH3 X CH3

X + PhCH3 : Evans-Polanyi Plot

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OH

RO

RC=CO

PhO

+ X

CH3 X CH3

vinyl

phenyl

alkyl

carbonyl

C(=O)OR

allyl

benzyl

propargyl

Wide range of heats of reactionand activation energies

Loss of aromaticity leads to high endothermicity for resonantly stabilized radicalsdespite cyclohexadienylresonance

Low activation energiesof OH and RO oxygenated radicals

Vinoxy radicals roughly in line with C-centered radicals

Nature of X clearly influences the reactivity

-Scission of Substituted Cyclohexadienyl Radials

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+ X

X H

H and CH3 are

slowly released

CH3 and C2H5

differ clearly

OH release is

faster than alkyl

C-O bond breaks

easier in -scission

reactions than C-C

Allyl vs vinoxy

supports conclusion

that C-O bond

breaks easily

No steric effect for

alkoxy radicals

-Scission of Substituted Cyclohexadienyl Radials

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+ X

CH3X CH3 Toluene case confirms

a strong steric impact on

-scission kinetics:

=> Branched substituents

are more quickly released

vinyl

propargyl

benzyl

alkyl

The roles of Y and Z

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How important are Y and Z for the reactivity ?

Y X

Z

Y

Z

+ X

X

Z

+ Y

Rate Coefficients Depend Little On Y

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ICCK, Ghent, 28.06-02.07.2015

Y

+ H

Y H

blue Cb-(C)red Cb-(O)green Cb-(Cd)orange Cb-(CO) brown Cb-(Cb)black Cb-(H)

Y leads to different Cb GAV groups in reactant

Observations:

1. As expected, rather

small changes in

reactivity

2. Rate coefficients for

same reactant type

(GAV) fall together

3. Deviations for factor 2 is

within uncertainty of

calculations

4. Y = H and Ph are

special:

CBS-QB3 has problems

with phenyl radical

y y

x1E-3 1/T [K-1]

kad

dit

ion

[cm

3m

ol-

1s

-1]

Sin

gle

eve

nt

-Scission Barriers Independent Of Most Y-Substituent

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X = HCH3

CCHOHCO

PhH+H

PhH+CH3

PhCH3+H

PhH+CCHO

PhCCHO+H

PhH+CHO

PhCHO+H

PhH+H

-scission barriers for Y = H, CH3, CCHO are very similar

-Scission Barriers Independent Of Most Y-Substituent

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X = HCH3

CCHOHCO

PhH+H

PhH+CH3

PhCH3+H

PhH+CCHO

PhCCHO+H

PhH+CHO

PhCHO+H

PhH+H

-scission barriers for Y = H, CH3, CCHO are very similar

CHO substituent has slightly lower barrier because of resonance stabilization

CH3+PhY: Narrow Range of A, Ea, ∆RH298 Values

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Very narrow range in Ea (1000K)

Small range in ∆RH298 values range is

comparable with earlier discussed

substituent effects

Oxygenated Y might alter reactivity

somewhat due to H bonds.

At 1000K: Single event pre-exponential factors ��:

o 1.9E11 +/- 0.7E11 cm3mol-1s-1 without H

Similar data obtained for CH3

Y

+ H

Y H

∆fH0 [kJ mol-1]

Ea

[kJ m

ol-

1]

Rate Coefficient Depend Little On Z

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H

+ H

H H

Z Z

ka

dd

itio

n[c

m3m

ol-

1s

-1]

At relevant temperatures,

reactivity differences

are not significant

Radical addition reaction

to subst. benzenes is

mainly affected by

the adding radical

Note: polarized radicals

and substituents not

considered

x1E-3 1/T [K-1]

Relation to X+Olefins

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Same trend in ∆RH298

Higher values for ethylene expected+ stability gain due to aromaticity in benzene

cannot be compensated by resonancestabilization of cylohexadienyl radical Same trend in Ea

Higher barriers for benzene correlate with lower exothermicity (Evans-Polanyi)

Conclusions

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ICCK, Ghent, 28.06-02.07.2015

A systematic study of radical addition to substituted benzenes and the reverse reaction was performed at the CBS-QB3 level of theory

The reactivity is mainly determined by the adding (or leaving) radical

For the radicals considered, the activation energies vary by ~ 100 kJ/mol and the heats of reactions by some 200 kJ/mol.

Substituents at the ipso, ortho, meta or para positions influence the reactivity very little

The ∆GAV0 methodology can easily applied (with one major contribution) and thus this reaction family can easily be implemented into automated mechanism generation codes.

The -scission reactions releasing H and CH3 from cyclohexadienyl radicals are slower compared to most other radicals. Combined with the fact that H and CH3 radicals are generally readily available in pyrolysis reactions, this explains the observation that substituted monocyclic aromatic species are converted with time to benzene and toluene

Acknowledgement

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European Research Council ERC grant agreement n° 290793.(MADPII)

IWT-SBO project “Bioleum”

Financial support from

STEVIN Supercomputer Infrastructure at Ghent University

Computational support from

Thank you for your attention !