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Unformatted text preview: lower bound for detection, what is the lowest temperature that
would be needed to see the minor isomer? 8 Radioastronomy confirms that both hydrogen cyanide and hydrogen isocyanide are
present in interstellar clouds. The interesting observation is that they occur in similar
amounts. Speculate why.
Protonation of Dimethylamine and Aziridine: Molecules contained the high vacuum
environment of an ion cyclotron resonance spectrometer can be protonated and ionmolecule equilibria established. The relative abundance of the ions involved in the
equilibrium can be measured which, together with knowledge of the relative amounts of
the neutral molecules, allows highly accurate determination of relative proton affinities.
Use the HF/6-31G* model to calculate the equilibrium geometries of the four molecules
involved in the proton-transfer equilibrium between dimethylamine and aziridine.
dimethylamine-H+ + aziridine = dimethylamine + aziridine-H+
Which molecule has the higher proton affinity and by how much? Assuming a 1:1
mixture of the two amines, and that the limit of detection of the “minor” ion is 5%, is it
possible to directly establish the relative proton affinities of dimethylamine and
aziridine? 9 Classifying Chemical Reactions
Chemical reactions may be conveniently be divided into one of three
categories depending on the extent that overall bonding is maintained.
Reactions that Change the Total Number of Electron Pairs
The most common reactions of this type are homolytic bond dissociation
reactions. Hartree-Fock models are expected to provide bond energies
that are too large. To see why this is so, consider bond dissociation in H2.
H–H → H• +H• While the Hartree-Fock energy for the product (two hydrogen atoms) is
exact (each contains only a single electron), that for the reactant
(hydrogen molecule) is too high. This means that the bond dissociation
reaction will not be sufficiently endothermic. In reality, homolytic bond
dissociation reactions are not very important as they are only infrequently
encountered. However, they serve as “absolute standards” with which to
judge the performance of different theoretical models.
An important possible “exception” involves comparisons between reactants and
transition states (activation energies), whic may also lead to a change in the number
of electron pairs. These will be discussed in Chapter P4. Reactions that Conserve the Total Number of Electron Pairs
This category separates reaction into two classes, depending on whether
or not total bond count in addition to total electron-pair count is
maintained. Protonation of trimethylamine (defining its absolute proton
affinity) and association of trifluoroborane and carbon monoxide (leading
to trifluoroborane carbonyl) are examples of the latter.
(CH3)3N: + H+ (CH3)3NH+
BF3 + :CO BF3CO
Here the product contains one more bond than the reactants but the same
number of electron pairs. Examples of reactions that conserve bonds as
well as electron pairs are more common. Among the most important are
comparisons of structural isomers, for example, comparison of allene and
propyne. Here, the reactant incorporates two double bonds and the
product one single and one triple bond.
CH2=C=CH2 → CH3CH≡CH2
10 Some structural isomer comparisons, for example, that between 1-butyne and 2butyne, not only maintain overall bond count, but also maintain the numbers of
individual bond types involving (in this case, two single bonds and one triple bond).
HC CCH2CH3 → CH3C≡CCH3
In this case, only the nature of the “atomic hybrids” (environments) are altered. One
of the carbon-carbon single bonds in 1-butyne made from sp and sp3 hybrids and the
other from two sp3 hybrids, whereas both of the carbon-carbon bonds in 2-butyne are
made from sp and sp3 hybrids. This and other reactions like it will be put into a third
class (see next section). Addition reactions such as cycloaddition of 1,3-butadiene and ethylene to
form cyclohexene (a Diels-Alder reaction) and of addition of molecular
bromine to cyclohexene to yield trans-1,2-dibromocyclohexane, also
conserve total bond count (but not individual bond counts). In the first
case, there is a loss of two π bonds but a gain of two new carbon-carbon
σ bonds, while in the second case a π bond and a Br-Br bond are traded
for two C-Br bonds.
+ Reactions that conserve the total number of electron pairs would be
expected to benefit from error cancellation to a greater extent than
reactions that do not. Because of this, it might be anticipated that their
energies will be described even by quantum chemical models that do not
properly take electron correlation into account, specifically Hartree-Fock
models. We will explore this later in the chapter.
Reactions that Conserve Individual Bond Counts
Processes that conserve not only the total number of bonds and electron
pairs but also the numbers of each kind of chemical bond (and each kind
of non-bonded lone pair) represent another very important and very
diverse class of chemical reactions. Some structural isomer compariso...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.
- Spring '09