chemist Marcellin Berthelot had proposed a general theory of hydrocarbon
pyrolysis based on experiments with C 1 and C 2 hydrocarbons only. Now,
Haber’s path to Karlsruhe had been tortuous, but he had never lost ambition
or confidence in his ability and when he saw Bertholet’s academic jugular he
ripped it out, sharply criticising the great man’s work as arbitrary, confused
and insufficiently supported by wider experiment—proceeding of course to
prove it. The simple hydrocarbons studied by Bertholet were unrepresentative
of the decomposition of larger molecules, so therefore were his inferences.
Haber began his studies by adopting a ‘higher’ more typical molecule, the
‘straight chain’ n-hexane or C 6 H 14 . This choice was dictated by experimental
convenience, the molecule’s (then perceived) relationship to ‘benzene’
(C 6 H 6 ), and its importance in oil and gas manufacture. To Haber, very much
an orthodox organic chemist, the work was new yet he devised the experimental procedures involved and determined all of the decomposition products both qualitatively and quantitatively adapting and improving existing
methods. For one so impatient with laboratory minutia this was a triumph!
Haber was able to move beyond Bertholet and propose a more generalised
decomposition mechanism for higher straight chain hydrocarbons introducing for the first-time concepts of chemical thermodynamics influenced no
doubt by his introduction to physical chemistry by Abegg. He was able to
summarise the difference between ‘aromatic’ and ‘aliphatic’ decomposition in
terms of carbon-to-carbon (C–C) and carbon-to-hydrogen (C–H) bond
‘strengths’ (dissociation energies) through what became known as ‘Haber’s
Rule’, concluding that the thermal stability of the C–C bond was stronger
than that of the C–H bond in aromatic compounds and weaker in aliphatic
compounds, and unlike Bertholet, he properly accounted for the appearance
and role of ‘acetylene’ (ethyne) within the pyrolysis process.
1
The pyrolysis of hydrocarbons was Haber’s first independent investigation,
yet it rapidly became well regarded in a little understood area. The episode
also encapsulates Haber’s approach to his science at the time. He was
ambitious, but to avoid controversy, ambition had to be coupled with
accuracy and reproducibility. He was therefore thorough, intolerant of
vagueness, superficiality and theory that had no practical application, and he
always tried to address the root of the problem. Sensitive to criticism—to the
extent that it caused him physical illness—he developed an approach characterised by meticulous attention to detail thereby making it difficult for any
potential critic to break him down. At last, he was making sense of Bunsen’s
training. His work on pyrolysis together with later studies on gas combustion
was published in 1896 as a book entitled Experimental Studies on the
Decomposition and Combustion of Hydro-carbons,
24 and in the same year it led
48
D. Sheppard
pyrolysis based on experiments with C 1 and C 2 hydrocarbons only. Now,
Haber’s path to Karlsruhe had been tortuous, but he had never lost ambition
or confidence in his ability and when he saw Bertholet’s academic jugular he
ripped it out, sharply criticising the great man’s work as arbitrary, confused
and insufficiently supported by wider experiment—proceeding of course to
prove it. The simple hydrocarbons studied by Bertholet were unrepresentative
of the decomposition of larger molecules, so therefore were his inferences.
Haber began his studies by adopting a ‘higher’ more typical molecule, the
‘straight chain’ n-hexane or C 6 H 14 . This choice was dictated by experimental
convenience, the molecule’s (then perceived) relationship to ‘benzene’
(C 6 H 6 ), and its importance in oil and gas manufacture. To Haber, very much
an orthodox organic chemist, the work was new yet he devised the experimental procedures involved and determined all of the decomposition products both qualitatively and quantitatively adapting and improving existing
methods. For one so impatient with laboratory minutia this was a triumph!
Haber was able to move beyond Bertholet and propose a more generalised
decomposition mechanism for higher straight chain hydrocarbons introducing for the first-time concepts of chemical thermodynamics influenced no
doubt by his introduction to physical chemistry by Abegg. He was able to
summarise the difference between ‘aromatic’ and ‘aliphatic’ decomposition in
terms of carbon-to-carbon (C–C) and carbon-to-hydrogen (C–H) bond
‘strengths’ (dissociation energies) through what became known as ‘Haber’s
Rule’, concluding that the thermal stability of the C–C bond was stronger
than that of the C–H bond in aromatic compounds and weaker in aliphatic
compounds, and unlike Bertholet, he properly accounted for the appearance
and role of ‘acetylene’ (ethyne) within the pyrolysis process.
1
The pyrolysis of hydrocarbons was Haber’s first independent investigation,
yet it rapidly became well regarded in a little understood area. The episode
also encapsulates Haber’s approach to his science at the time. He was
ambitious, but to avoid controversy, ambition had to be coupled with
accuracy and reproducibility. He was therefore thorough, intolerant of
vagueness, superficiality and theory that had no practical application, and he
always tried to address the root of the problem. Sensitive to criticism—to the
extent that it caused him physical illness—he developed an approach characterised by meticulous attention to detail thereby making it difficult for any
potential critic to break him down. At last, he was making sense of Bunsen’s
training. His work on pyrolysis together with later studies on gas combustion
was published in 1896 as a book entitled Experimental Studies on the
Decomposition and Combustion of Hydro-carbons,
24 and in the same year it led
48
D. Sheppard