The
Simplest Mechanistic Step: STAD
All chemical
reactions can be broken down into "transfer" steps that involve
a defined chemical species moving from one environment to another. Sometimes
these steps are referred to as substitutions, sometimes as transfers, sometimes
as abstractions and sometimes as displacements (even though they are mechanistically
identical), hence the general term STAD.
Introduction
The language used by chemists
obscures the simplest mechanistic step. Indeed, the various chemistry
sub disciplines are prone to describe the same process from different
perspectives and to use different terminologies. Consider the hypothetical
"X, Y, Z reaction".

To describe this process we
can say:
- X
substitutes Z at the Y centre
- Y transfers
from Z to X
- X abstracts
Y from Z
- X displaces
Z from Y
- X
exchanges with or replaces Z
from
Y
By way of example, Lewis acid/base
chemistry usually discusses mechanisms in terms of nucleophilic and electrophilic
substitution, while radical chemists prefer to use the term abstraction.
Single electrons are transferred. There are no logical reasons for such
differences.
The simplest possible
mechanistic step occurs when a defined chemical species transfers
from one environment to another.
This mechanistic process
can be described in in terms of Substitution, Transfer,
Abstraction or Displacement.
We shall use a single
term, the acronym STAD.
We shall discuss come common
mechanistic processes in terms of the STAD mechanistic step. Notice that
each example is also defined with respect to one or more of the five reaction
chemistries, here.
Click
here to enlarge the table of STAD reactions. These are discussed one-at-a-time
over the rest of this page:

Electron Transfer

If zinc metal is added to a
solution containing copper(II) ions, Cu2+, a redox reaction
takes place and electrons transfer from the zinc to the copper to give
a precipitate of copper metal. The zinc metal goes into solution as Zn2+.
In this reaction:
The zinc metal is oxidised
to zinc(II) ions by the copper(II) ions.
The copper(II) ions are reduced
to copper metal by the zinc metal.
The reaction proceeds via a
double SET (single electron transfer) process.
Proton (Lewis Acid) Transfer

The first point to note is
that hydrogen chloride, HCl or HCl(g), is a gas.
Hydrogen chloride, the gas,
is readily absorbed into water to give an acidic solution of hydrochloric
acid.
The hydrogen chloride is a
proton donating Brønsted acid. When hydrogen chloride gas dissolves
in water the hydrogen chloride's proton transfers to water, H2O:,
to give a solution consisting of oxonium ions, [OH3]+
and chloride ions, Cl.
In this reaction:
Hydrogen chloride is the
Brønsted acid
Water is the Brønsted
base
Oxonium ion, [OH3]+,
is the conjugate Brønsted acid
Chloride ion is the conjugate
Brønsted base
The oxonium ion, [OH3]+,
is the Brønsted acidic species in all aqueous Brønsted
acid solutions.
When aqueous Brønsted
acids are described as "H+ donors", the actual
proton donor species is always [OH3]+.
Water will continue to absorb
hydrogen chloride until it is 37% HCl by weight, a material is called
concentrated hydrochloric acid. If concentrated hydrochloric acid is cooled
to 15°C, crystals of the ionic salt [H3O]+/Cl
will precipitate.
Note that oxonium chloride
OH3Cl is the oxygen analogue of ammonium chloride,
NH4Cl. Both are type 11 Lewis acid/base complexes,
here.
The transfer of H+
from Cl to H2O: is a STAD process.
Hydrogen Radical Transfer

During the radical chlorination
of methane, here, one of
the propagation steps involves a hydrogen radical (a hydrogen atom) transferring
from a methyl radical to a chlorine radical.
This process is commonly referred
to hydrogen abstraction; it is also a STAD mechanistic process.
Hydride Ion (Lewis Base) Transfer

Sodium borohydride, NaBH4,
can be prepared by reacting sodium hydride, NaH, with borane, BH3.
(Actually, diborane is used.)
The crucial step of this reaction
involves a hydride ion transferring from sodium ion Lewis acid, Na+,
to a borane Lewis acid.
This reaction could
be regarded as an electrophilic substitution, with the borane electrophile
substituting the sodium ion electrofuge (Lewis acid leaving group) from
a hydride ion centre.
Or it can just be regarded
as a STAD process.
Carbenium Ion (Lewis Acid) Transfer

Cyanide ion, CN,
reacts with methyl bromide, CH3Br, to give methyl
cyanide, CH3CN and bromide ion, Br.
This reaction is commonly referred to as a nucleophilic substitution.
The argument is:
The cyanide ion is an electron
rich Lewis base. The cyanide ion is also nucleophilic (nucleus or positive
charge seeking) in that it is attracted to and "attacks" delta+
carbon centres, such as the delta+ carbon of methyl bromide.
The cyanide ion engages with
the methyl bromide and forms a five centre (bipyramidal triangular)
transition state. This TS rapidly collapses with the ejection of the
nucleofugal (nucleus fleeing) bromide ion Lewis base. The reaction proceeds
as a concerted, single-step process.
The product is methyl cyanide
and bromide ion.
It is also quite reasonable
to regard the mechanism as proceeding as a carbenium ion, H3C+,
transferring between a pair of Lewis bases.
The reaction proceeds via a
STAD mechanism.
Chlorine Radical Transfer

During the radical chlorination
of methane, here, one of
the propagation steps involves a methyl radical abstracting a chlorine
radical from chlorine, Cl2, to give methyl chloride,
CH3Cl, and leaving a chlorine radical.
This process can also be regarded
as a chlorine radical transferring from a chlorine radical to a methyl
radical. Another example of a STAD mechanistic step.
Chloride Ion (Lewis Base) Transfer

The first step of Friedel-Crafts
chlorination of benzene involves adding chlorine to a solution (or a slurry)
of anhydrous aluminium chloride, AlCl3, a powerfully
halophilic (affinity for halogen anion) Lewis acid.
A reaction takes place whereby
the aluminium chloride abstracts a chlorine anion from chlorine, Cl2,
to form chlorenium aluminium tetrachloride, [Cl]+ [AlCl4].
The reaction can be regarded
as electrophilic substitution, with the electrophilic Lewis acid AlCl3
displacing the electrofugal chlorenium ion Lewis acid, Cl+,
from the chloride anion Lewis base centre.
Alternatively, the reaction
can be viewed as chloride ion, Cl, transferring between
two Lewis acids, a STAD mechanism.
Note that the same mechanistic
process occurs with Friedel-Crafts acylation, when aluminium chloride
abstracts as chloride anion from an acyl chloride, RCOCl, to give an electrophilic
acyl cation, RCO+, and an aluminum tetrachloride anion.
Benzene Ring (Lewis Base) Transfer

The second step of Friedel-Crafts
chlorination of benzene involves the chlorenium ion (chlorine cation,
Cl+) electrophilic Lewis acid substituting a proton at a benzene
ring carbon.
The argument is that the
electron poor (and hence electrophilic) chlorine cation, Cl+,
is attracted to the electron rich π-system
of the benzene ring.
A "Wheland" intermediate
is formed in which the chlorenium ion complexes with the benzene.
The Wheland intermediate
collapses and a hydrogen cation (proton) Lewis acid electrofuge is ejected
leaving the chlorobenzene product, C6H5Cl.
This reaction can also be regarded
as a benzene ring Lewis base (actually phenyl anion, [C6H5],
Lewis base) transferring between a pair of Lewis acids, Cl+
and H+; a STAD mechanistic step.
Phase Transfer

When a benzene molecule evaporates
it transfers from an association with other benzene molecules in the liquid
phase, C6H6(l), to an association
with the gas phase.
There are a number of points
to make about evaporation and similar processes which are of relevance
here.
When discussing chemical
equilibria systems, chemists are prone to discuss "physical"
systems and processes and "chemical" systems and processes.
However, on close examination the idea of the "physical" process
is found to be misleading.
Throughout the Patterns in
Reaction Chemistry project, we take the broad view of what it is that
constitutes a chemical reaction:
Any process which can
be represented with a chemical equation is regarded as being a chemical
reaction.
This broad definition has
many advantages. For example, chemical equations have the property that
they can be balanced in terms of mass (stoichiometry) and energy. Certainly,
the evaporation of benzene can be discussed in these terms.
Chemists have the propensity
to define things with respect to standard states. For example, chemical
thermodynamics defines heat of formation, H°f,
and heat of reaction, H°r, with respect
to the elements in their standard states which are all defined as having:
H°f = 0.
In a similar way, chemists
regard the vacuum (the gas phase) as being null and without effect.
(Remember, when a species
is in the gas phase it only occasionally encounters other species
or the sides of the container. For most of the time the species behaves
as if it is alone and in a vacuum.)
The chemist's vacuum is referred
to as a classical vacuum. And the view is encouraged by the way
we draw molecules. Unless we explicitly include solvent molecules or
solid phase neighbours, we are assuming that all species to be in the
gas phase. This includes drawing and computer manipulating species in
silico.
However, particle
physicists regard the quantum vacuum as a seething ocean of virtual
particles which form and disappear on very short time scales. Thus,
in an absolute sense the quantum vacuum is just as active as any solvent.
Thus, if we take the physicists
view point we can regard the seething quantum vacuum of the gas
phase abstracting a benzene molecule from the liquid phase.
It follows that with
respect to a defined chemical species we can regard all the
following phase changes:
liquid <>
gas evaporation condensation
solid <>
gas sublimation condensation
solid <>
liquid melting solidifying
as transfer processes.
Two more phase transfer
examples:
liquid <>
liquid extraction
solid <>
liquid dissolution
are not nearly so controversial.
Liquidliquid phase
transfer extraction is used to transfer non-polar organic substances
from aqueous solvents (water) into organic solvents. Alternatively,
water is used to extract polar entities out of immiscible organic solvents.
Solid-liquid phase transfer
occurs whenever a species dissolves into a solvent or precipitates from
a solvent.
To sum up: phase transfers
are chemical reaction processes which proceed via the STAD mechanism.
Photon Transfer

The system described above
where an excited state sodium atom transfers a photon to a remote
ground state sodium atom which is excited is exploited in the technique
of atomic absorption spectroscopy (AA or AAS).
Atomic absorption spectroscopy
is a common analytical technique that uses the absorption of light to
measure the concentration of gas-phase atoms. Light from sodium atoms
are used to measure the concentration of sodium atoms in an analytical
sample.

A sodium hollow cathode
lamp is used to excite sodium atoms into emitting light at 589nm,
the sodium D-line.
Since analytical samples
containing sodium ions are usually liquids or solids, the analyte sample
must be vaporised and atomised in a flame or a graphite furnace.
The AA instrument is arranged
so that the sodium D-line light from the hollow cathode lamp passes
through the flame or graphite furnace containing analyte atoms.
The analyte sodium atoms
absorb the 589nm light and make transitions to higher electronic energy
levels.
The concentration of sodium
atoms is determined from the amount of absorption: the more sodium in
the flame, the more the 589nm light from the hollow cathode lamp is
attenuated.
Concentration measurements
are usually determined by calibrating the instrument with standards
of known concentration and determining the range over which the Beer-Lambert
law can be assumed.
Photon transfer is exploited
by all visual systems which detect photons emitted by excited state species,
see here.
Radical Coupling is "Back-to-Back" Transfer

Consider again the radical
coupling of nitrogen dioxide to dinitrogen tetroxide (introduced here).
Each of the nitrogen dioxide
molecules transfers from an association with the [seething, classical]
vapour phase to an association with a nitrogen dioxide molecule.
Thus, two paired or "back-to-back"
transfers are occurring, and this is true whenever two species couple
together.
The reverse argument is also
true whenever a single species fragments into two species.
It follows that all couplings,
complexations, decouplings and fragmentaions can be considered as paired
or back-to-back STAD processes.
However, whether this level
of mechanistic analysis is necessary or useful will be discussed in the
next section, here. But a couple
more examples follow.
Lewis Acid/Base Complexation is "Back-to-Back" Transfer

Silver ions in aqueous solution,
Ag+(aq), complex with aqueous chloride
ions, Cl(aq), and precipitate as
solid silver chloride, AgCl(s).
In this reaction the silver
ion, Ag+, transfers from an association with water, H2O:,
to an association with chloride ion, Cl, in a solid
silver chloride matrix, AgCl(s).
The chloride ion, Cl,
transfers from an association with water, H2O,
to an association with silver ion, Ag+, in a solid silver
chloride matrix, AgCl(s).
As stated above, all couplings,
complexations, decouplings and fragmentaions can be considered as paired
or back-to-back STAD processes. But whether this level of analysis is
strictly necessary will be discussed in the next section, here.
Cycloaddition is "Back-to-Back" Transfer

Furan reacts with maleic anhydride
to give a Diels-Alder cycloaddition adduct. This type of reaction is usually
carried out in a non-polar solvent such as toluene.
The furan molecule transfers
from the toluene liquid phase to an association with the maleic anhydride,
and the maleic anhydride transfers from the toluene liquid phase to
an association with furan.
Again, a pair of back-to-back
STAD processes.
Comparing Electron Transfer and Proton Transfer
There are two very important
STAD processes which involve the transfer of low mass species, e
and H+.
These low mass entities, e
and H+, transfer so readily the reactions have very
low activation energies that the thermodynamic equilibrium states
are quickly reached. The two processes are:
Single Electron Transfer
(SET) redox reactions
Proton transfer or Brønsted
acidity
The similarity of these two
STAD processes is such that both can be described by:
Full reaction equations
Net ionic equations
Paired half-reactions
Half-reactions in standard
form
The half reactions in standard
form can quantified and related to the deltaG, the Gibbs free-energy
of the process.
Typical Redox and
Typical Brønsted Acid Reactions
We will compare and contrast:
- Zinc metal reacting
with aqueous copper sulfate: a typical, simple SET redox process
- Hydrogen chloride
reacting with sodium hydroxide, a typical, simple Brønsted acid
reaction
First the full equations with
all species represented:

- Zinc metal is a solid
and it reacts with aqueous copper sulfate solution to give copper metal
and an aqueous solution of zinc sulfate. The copper sulfate solution
is blue and the zinc sulfate solution is colourless. The reaction can
be followed by the disappearance of the blue colour.
- Hydrogen chloride
is a proton donating Brønsted acid and sodium hydroxide is a
proton accepting Brønsted base. Hydrogen chloride will react
with sodium hydroxide to give sodium chloride and water. If equimolar
quantities of acid and alkali are used, the resulting mixture of sodium
chloride and water will have a neutral pH of 7.0.
Please note that "hydrogen
chloride", the compound HCl, is a gas. Hydrogen chloride dissolves
in water to give an acidic aqueous solution called hydrochloric
acid. Very confusingly, both hydrogen chloride gas and aqueous
solutions of hydrochloric acid are referred to as "H Cl".
Net Ionic Equations
Both the redox reaction and
the Brønsted acid reaction can be simplified to the "net ionic
equation" in which spectator counter ions are removed:

- In the redox reaction
the sulfate ion, SO42, is the
spectator ion. When this species is removed, the reaction is simplifies
to Zn + Cu2+ > Zn2+ + Cu. It now becomes
clear the oxidation state of the zinc increases from 0 to +2 and so
the zinc is oxidised. Likewise, the oxidation state of the copper reduces
from +2 to 0 and so the copper is reduced. We can go further:
The copper is reduced
from +2 to 0 by the zinc metal, so the zinc metal is the reducing
agent.
The zinc metal is oxidised
from 0 to +2 by the copper(II) ion so the Cu2+ is the oxidising
agent.
- The hydrogen chloride
plus sodium hydroxide reaction can also be viewed in net ionic form.
The sodium cation, Na+, is the spectator ion. The deconstruct
net ionic equation takes the form:
HCl +
HO -> Cl
+ H2O
HCl is the proton donating
Brønsted acid
Hydroxide ion is the proton
accepting Brønsted base
Water, H2O,
is the conjugate Brønsted acid
Chloride ion is the conjugate
Brønsted base
Paired Half Reactions
The redox and Brønsted
acid reactions can be further deconstructed into half reactions. It is
in this form that the transferred species appears for the first time:

- In the redox reaction
the zinc is oxidised from Zn° to Zn2+. To balance this
reaction in terms of charge two electrons must be added to the right
hand side of the equation.
In a similar manner, the
copper is reduced from Cu2+ to Cu° and this half reaction
must be balanced with two electrons on the left hand side of the equation.
Note that if the two half
reactions are added together the electrons which appear on
both sides are cancelled out and the net ionic equation is
obtained.
- The HCl + HO
reaction can also be deconstructed into a pair of Brønsted acid
half reactions. Again, the transferred species, H+, appears
for the first time in the half reactions.
The HCl is deemed to dissociate
to H+ and Cl.
The H+ reacts
with the HO to give water.
Half Reactions
in Standard Form
Half reactions are useful because
they can be arranged in a standard form so that large numbers of similar
reaction processed can be compared, contrasted and most importantly, quantified.

- By convention, SET
redox processes are arranged as reduction reactions with the oxidised
species plus the electrons on the left hand side of the equation and
the reduced species on the right.
Reactions are all compared
with the hydrogen redox reaction
2H+ +
2e >
H2
over a platinum electrode
which is defined has having an E° of 0.00V.
- Brønsted acid
reactions are all compared as proton donations. Thus, the Brønsted
acid appears on the left hand side of the equation and H+ plus the conjugate
[Brønsted] base on the right hand side.
All reactions are quantified
by standardising the proton acceptor as water, H2O:,
and having the conjugate Brønsted acid as the oxonium ion,
[OH3]+.
The standard measure is
the equilibrium constant, Ka
Or its "minus log"
form, pKa.
Electron Transfer

Proton Transfer

  
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© Mark R. Leach 1999-
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