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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ICCK, Ghent, 28.06-02.07.2015
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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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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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 !
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