w2 Flashcards
Arrhenius Equation relates to
energy, collision frequency, temperature, orientation
Arrhenius Equation
k = rate constant = Ae ^-Ea/RT
Frequency factor (units L mol -1 s-1)
- combines collision frequency and probability of correct orientation
- specific to each reaction
- temp dependent
fraction of molecules with minimum energy for reaction
always less than 1, changes significantly with temp
Effect of catalysts on RR
provides a mechanistic pathway with a lower activation energy, can be regenerated, very effective, specific, accelerates reactions.
Homogeneous catalysis: interconversion of but-2-ene without catalysts. Ea: 264 kJ mol-1
- large activation energy
- because C=C must be broken
- low frequency
- requires high temps
mechanism of iodine-catalysed isomerization of cis-but-2-ene (learn the circle)
- converted to trans-but-2-ene with iodine
- emphasizes the catalytic cycle
- rate = [cis-C4H8][I2]^1/2
heterogeneous catalysis: ethylene hydrogenation
- H2 molecules collide with platinum surface and form Pt-H bones, weakening H-H bones and leaves H bound to surface
- C2C4 also attaches to surface through bones from each C atom to a Pt by breaking the C=C bond in each molecule
- Rxn with adjacent H atoms first leads to formation of C2H5 and then makes ethene C2H6
- New C-H bonds are formed as Pt-C bonds are weakened and C2H6 molecules leaves the surface
- In their place, other C2H4
and H2 molecules can bond to the surface and undergo reaction.
Rxn mechanisms
- sequence of bond-making
and bond-breaking steps
that occurs during the
conversion of reactants to
products. - can’t be proved but shown through experiments
sequence of steps: rxn of Br2 (g) and NO (g)
Nitric oxide is a free radical, i.e., it has an unpaired electron. It is sometimes
denoted by a dot in its formula, i.e., ·NO. (Ozone depletion). - unlikely to have a singe step reaction
Br2 molecules and NO molecules combine to produce molecules of an
intermediate species, Br2NO.
Br2NO then reacts with another NO molecule to form two molecules of the
reaction product.
Elementary steps
Each step.
The equation for an elementary step describes particular collisions:
1. which particles collide,
2. which bonds are broken or formed,
3. and which particles are formed.
Molecularity of an elementary step
the number of molecules, ions, or atoms that collide and undergo change.
– A unimolecular step involves reaction of only one particle,
– a bimolecular step involves two particles,
– termolecularstep involves three particles (atoms, molecules or ions).
Rate equations for elementary steps
Determined directly from stoichiometric equations
1. A -> product, unimolecular, Rate = k[A]
2. A + B -> product, bimolecular, Rate = k[A][B]
3. A+A -> product, bimolecular, rate = k[A]^2
4. 2A + B ->product, termolecular, rate = k[A]^2[B]
not expected to have the same value
What controls the rate at which products are produced
the slowest elementary step (rate limiting step
Nucleophilic substitution rxns
involves the substitution of one electron-rich nucleophile (such as a chloride or a hydroxide ion) in molecules of a substance (referred to as the substrate) for an atom, group of atoms, or an ion, called the leaving group.
A good nucleophile…
needs to be able to donate an electron pair to an electron deficient (electrophilic) site on the substrate. The leaving group must accept an electron pair.
S(n)2 Mechanism
As the two particles react, the C-Br bond weakens simultaneously with the gradual formation of a C-OH bond.
In the maximum-energy transition state, the C-H, C-CH3 and C-CH2CH3 bonds are planar. As the rearrangement proceeds, the C-OH bond formation is completed. The product CH3CH9CH(OH)CH3 molecule
has stereo-chemistry inverted at the C atom. The Brion is released with the electron pair from the former C-Br bond.
Based on:
* experimental observation of
second-order kinetics
* stereochemical evidence: the
stereochemistry of chiral
substrates is inverted in the
product
These reactions proceed by a single
bimolecular step during which the
nucleophile interacts with the
substrate on the side opposite the
leaving group.
Substitution nucleophilic (unimolecular) S(n)1
There is one species involved in the rate-determining step. rate is first order with respect to the conc of the substrate (CH3)3CBr (SN1), but independent of the conc of the OH nucleophile. (OHis not in the rate equation) A rate-determining, bond-breaking elementary step is
consistent with first-order kinetics.
- Infrequent (and rate determining)
dissociation of Br ions creates a
carbocation. - Carbocations react quickly with
nucleophilic water molecules. - This produces protonated alcohol.
- Loss of a proton from the protonated
alcohol ion, gives molecules of the
alcohol product.
Racemate formation in SN1 reaction mechanism from a
chiral substrate.
As the carbocations are planar, there is a 50% probability nucleophiles will
collide with them on either side of the plane of the ion. If chiral substrate: product is racemic.
The SN1 Mechanism of Nucleophilic Substitution Reactions
The SN1 mechanism is based on the experimental observation of firstorder kinetics and the stereochemical evidence that a racemic mixture of
the product is formed from a chiral substrate.
There is one reacting species in the rate limiting step.
These reactions proceed by infrequent unimolecular dissociation of the
substrate molecules to form planar carbocations, with which the
nucleophiles can react on either side with 50% probability.
Enzymes
polypeptides that catalyse
chemical reactions.
They have well defined amino acid
sequences and structures - in biology
structure is almost synonymous with
function.
There are four levels of structure: primary, secondary, tertiary and quaternary.
increase the reaction rate by a
factor of between 10^7 and 10^14
CO2 Equilibria in Water
Carbon dioxide dissolves in water to a small extent to produce carbonic acid,
which ionizes to give H+
(aq) and HCO3-(aq) ions
Carbonic anhydrase catalyst for rxn 1 and 2.
In blood, as hemoglobin loses O2, it picks up H3O+ (aq) ions produced by ionization of H2CO3 (reaction 3). HCO3 - (aq) ions are sent to lungs.
When hemoglobin takes on O2 in the lungs it releases H3O+. H3O+ (aq) and HCO3 (aq) re-form H2CO3 (aq), from
which CO2 (g) is liberated and exhaled.
Enzyme changes shape
Hexokinase catalyzes the initial step of glucose metabolism:
The transfer of a phosphate group from adenosine tri-phosphate (ATP) to
glucose, forming glucose 6-phosphate and adenosine diphosphate (ADP).
The hexokinase structure has an active site that firmly accommodates
a 1 glucose and ATP molecule pair in a reacting orientation.
Acids and Bases background
- common in nature
- pH of lakes, rivers, oceans, and rains are controlled by dissolved acids and bases
- lots of bodily functions depend on acids and bases (stomach, blood)
