CHM 504 : Supplementary Material
Reaction Mechanisms, Energetics, and Intermediates
Reaction Profile
The diagram shows how the energy (usually free energy) of a reaction changes as it progresses from
reactants to products.
Mechanism
This is the specific pathway the reaction takes. It is a sequence of steps leading, through various
intermediates, to the final products. The diagram above represents a two step reaction, going through
a single intermediate.
Intermediate
An intermediate is a highly reactive, unstable, and short-lived species which is high in energy, yet is
at a shallow energy minimum on the energy profile. In organic chemistry, typical intermediates
include carbocations, radicals, carbanions, and carbenes.
Transition states
A transition state occurs at every energy maximum in the energy profile. Sometimes known as an
activated complex, it is a high energy species, difficult to depict, in which bonds are being formed or
broken. Every step of the reaction has its own transition state.
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Activation energy
A certain amount of energy is required to carry out every step of the reaction, being the energy
difference between a given activated state and the preceding energy minimum. This may be thought
of as the free energy required to bring molecules or intermediates together in the proper orientation,
break bonds, etc., and may be thermal or some other kind of energy, such as light. The energy
difference between the reactants and the highest transition state is the activation energy of the
reaction, denoted G‡. This may be thought of as the energy barrier the reactants must overcome
in order to become products. The higher the activation energy, the slower the reaction (or
alternatively, the greater the energy, e.g., heat, that must be supplied in order to carry it out).
Free energy of reaction
Once the products form, there is a free energy change G as a result of the reaction.
G = Gproducts - Greactants
If G is negative, the products are more stable than the reactants, i.e., energy has been given off.
This is depicted in the diagram above. Sometimes the products are higher in energy than the
reactants. In that case, energy is absorbed, and G is positive.
G is negative:
reaction is exergonic.
G is positive:
reaction is endergonic.
Both enthalpy (heat) and entropy (randomness) contribute to G.
G = H - TS
In organic chemistry, the enthalpy change H is usually major contributing factor. This arises
mainly out of the difference in bond energies. Thus, many organic reactions occur because weaker
bonds are replaced by stronger bonds.
Reaction Intermediates
Carbocations
A carbocation is usually a planar (sp2 hybridized), trivalent, positively charged
C with an empty p orbital, as shown. R1, R2, and R3 may be H atoms or alkyl
groups. Depending on whether one, two, or all three are alkyl groups, the
carbocation is referred to as primary, secondary, or tertiary, respectively. The
greater the number of alkyl groups, the more stable the cation. Thus, the order
of stability is
tertiary > secondary > primary > methyl.
As discussed in class, the reasons for this are
(i) the positive inductive (electron-donating) effect of alkyl groups,
(ii) hyperconjugation.
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Carbocations may also be stabilized by resonance. For this to occur an adjacent atom must have a p
orbital or a  system, with which the cation's p orbital can overlap. Resonance is another way of
delocalizing the positive charge, and as a general principle, the greater the number of resonance
structures, the more stable the carbocation.
Radicals
A typical carbon radical is a planar (sp2 hybridized), trivalent, electrically
neutral C with its p orbital carrying an unpaired electron. Structurally, they are
similar to carbocations, and are stabilized by the same factors. The order of
stability is the same as for carbocations (3ry > 2ry > 1ry) and for the same
reasons.
Carbanions
A simple carbanion is a tetrahedral (sp3 hybridized), trivalent, negatively charged
C atom, with a lone pair on the fourth hybridized orbital. This is as predicted by
the VSEPR theory, according to which all 4 pairs of electrons, including the nonbonding pair, must be as far apart as possible.
Unlike a carbocation, a carbanion is de-stabilized by alkyl substituents on the C
atom. This is because alkyl groups are electron donors, whereas to stabilize a negative charge,
electron-withdrawing substituents are required. Thus, the order of stability for carbanions is the
reverse of that for carbocations and radicals: 1ry > 2ry > 3ry.
Like the previous intermediates, carbanions can be stabilized by resonance. In this case, however,
the C atom must be sp2 hybridized, with the lone pair on the p orbital, since resonance requires a p
orbital to overlap with the adjacent ones, as discussed earlier.
Carbenes
A carbene is a strange, rare intermediate containing a sp2 hybridized, divalent,
electrically neutral C atom. In addition to the two bonds, the C also has two
non-bonding electrons, usually as a lone pair on the remaining sp2 hybridized
orbital (leaving the p orbital empty), but sometimes as an unpaired electron on
each of the two non-bonding orbitals. Carbenes are extremely reactive.
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