Lecture 5 Flashcards
What are the 3 types of pericyclic reactions?
- electrocyclic reactions
- cycloaddition reactions
- sigmatropic rearrangements (not covered)
What are electrocyclic reactions?
An electrocyclic reaction is an intramolecular reaction in which a new
sigma bond is formed between the ends of a conjugated p (pi) system (shown in digital notes). This reaction is easy to recognize—the product is a cyclic compound that has one more ring and one less pi bond than the reactant. These reactions are reversible
What are cycloaddition reactions?
In a cycloaddition reaction, two different pi-bond-containing molecules react to form a cyclic compound. Each of the reactants loses a pi bond, and the resulting cyclic product has two new sigma bonds. The Diels–Alder reaction is the best-known
example of a cycloaddition reaction.
What are the common features of all pericyclic reactions?
■ They are all concerted reactions—that is, all the electron reorganization takes place in a single step. Therefore, there is one cyclic transition state and no intermediate.
■ They are highly stereoselective.
■ They are generally not affected by catalysts or a change in solvent.
What does the configuration of the product formed in a pericyclic reaction depend on?
Before we answer, these have yet to be discussed. Why the product is dependent on them will be discussed in the upcoming flashcards:
■ the configuration of the reactant;
■ the number of conjugated double bonds or pairs of electrons in the reacting system;
■ whether the reaction is a thermal reaction or a photochemical reaction.
What is a thermal and photochemical reactions?
- A thermal reaction is a reaction that takes place without the absorption of light.
- A photochemical reaction is a reaction that takes place when a reactant absorbs light.
Despite its name, a thermal reaction does not necessarily require more heat than what is available at room temperature. Some thermal reactions do require additional heat in order to take place at a reasonable rate, but others readily occur at, or even below, room temperature
What was so puzzling about pericyclic reactions?
The question should be what wasnt so puzzling about this reaction. They broke all the rules that organic chemists set. How come some pericyclic reactions happened under photothermal conditions and others under thermal conditions? And what do you mean that a reaction that can happen under both conditions yields different configurations????
What was the answer that solved the puzzle of pericyclic reactions?
Simply it was the introduction of the “conservation of orbital symmetry theory” which states that a pericyclic reaction requires the overlap of in-phase orbitals. This means the symmetry of a molecular orbital controls both the conditions under which a pericyclic reaction takes place and the configuration of the product that is formed.
Because the behaviour of pericyclic reactions is so precise, it is not surprising that everything about their behaviour can be explained by one simple theory
What are the important points to remember about the MO theory?
■ When two in-phase p atomic orbitals interact, a covalent bond is formed. The interaction of two out-of-phase p atomic orbitals subtracts from bonding because a node is created between the two nuclei.
■ Electrons fill molecular orbitals according to the same rules that govern how they fill atomic orbitals (Section 1.2)—that is, an electron goes into the available MO with the lowest energy (the aufbau principle); only two electrons can occupy a particular MO and they must be of opposite spin (the Pauli exclusion principle); and an electron will occupy an empty degenerate orbital before it will pair up (Hund’s rule).
■ Each carbon that forms a p bond has a p atomic orbital, and the p atomic orbitals of the carbons combine to produce a set of MOs. Thus, a MO can be described by the linear combination of atomic orbitals (LCAO). In a MO, each electron that previously occupied a p atomic orbital belonging to an individual carbon now occupies the entire part of the molecule that is encompassed by the interacting p orbitals.
■ Orbitals are conserved: two atomic
orbitals combine to produce two
MOs, four atomic orbitals combine
to produce four MOs, six atomic
orbitals combine to produce six
MOs, and so on.)
What is a node again?
a node is a place in which there is zero probability of finding an electron
What is the general trend found when the energy of a MO increases?
as the energy of the MO increases, the number of bonding interactions decreases and the number of nodes between the nuclei increases
When is a MO bonding and when is it anti-bonding?
a MO is bonding if the number of bonding interactions is greater than the number of nodes between the nuclei, and a MO is antibonding if the number of bonding interactions is fewer than the number of nodes between the nuclei.
What happens when light is shown on a molecule, and what does that tell us about how it reacts?
If a molecule absorbs light of an appropriate wavelength, the light will promote an electron from its ground-state HOMO to its LUMO. The molecule is
then in an excited state where the HOMO is now the previous LUMO and the LUMO is now the MO above the previous LUMO.
This now tells us that a molecule reacts from its ground state in a thermal reaction
and from its excited state in a photochemical reaction.
What are symmetric and anti-symmetric MO?
if the lobes at the ends of the MO are in-phase (both have blue lobes on the top and green lobes on the bottom), then the MO is symmetric; if the two end lobes are out-of-phase, then the MO is antisymmetric
What is the trend observed in symmetry when the MOs increase in energy and what does this tell us about the ground state and excited state HOMO?
as the MOs increase in energy, they alternate from being symmetric to being antisymmetric. Therefore, the ground-state HOMO and the excited-state HOMO always have opposite symmetries: if one is symmetric, the other is antisymmetric. Now we can use our brains and see why some pericyclic reactions only happen in photothermal conditions bas the book likes to be nonchalant and wants to explain it slower so gl ;).
What are the frontier orbitals?
They are the HOMO and the LUMO and even though a molecule is defined by all of its MOs, examining these two tells us a lot about what will happen in pericyclic reactions
Where does the position of equilibria lie in electrocyclic reactions?
It lies towards the product if a six-membered ring is formed, if not it will lie towards the open-chain compound. This is because of the angle strain associated with non-6-membered rings.
a question should have been asked about what happens if a 7-member ring is formed
How does a new sigma bond in electrocyclic reactions (Specifically what happens to the p orbitals at the end of the conjugated system)?
For the new sigma bond to form, the p orbitals at the ends of the conjugated system must rotate in order to (in phase) overlap head-to-head as they rehybridize from sp2 to sp3. Rotation can occur in two ways: conrotatory and disrotatory
What is conrotatory and disrotatory ring closure?
Conrotatory - If both orbitals rotate in
the same direction (both clockwise or both counterclockwise) RING CLOSURE is conrotatory. (lowkey away from each other as shown in notes)
Disconrotatory - if the orbitals rotate in opposite directions (one clockwise, the other counterclockwise), ring closure
is disrotatory. (lowkey towards each other as shown in notes)
Whether conrotatory or disrotatory ring closure occurs is determined by?
Whether conrotatory or disrotatory ring closure occurs depends only on the symmetry of the HOMO of the compound undergoing ring closure. The symmetry of the HOMO determines the course of the reaction because this is where the highest energy electrons are. These are the most loosely held electrons and, therefore, the ones most easily moved during a reaction.
