How To Determine Homo And Lumo

HOMO and LUMO in Diels’ older reactionToday we will go into the mechanism of the Diels-Alder reaction from a molecular orbital perspective. Using our previous articles on the construction of molecular orbitals, we will show the Diels-Alder result from the construction orbital overlap between the highest occupied molecular orbital (HOMO) of the diene. with the lowest molecular orbital (LUMO) of dienophile. Read: how to determine homo and lumoTable of contents

A quick summary of the Diels-Alder . reaction

Bond formation requires overlap between the HOMO of one molecule (Nucleophile) with the LUMO of another molecule (Electrophile).

Reaction with interest: When two bonds form at the same time, many orbitals must overlap

The molecular orbitals in [2+2] Cyclic transition between Ethene and Ethene reveals why the reaction is unfavorable under “thermal” conditions

Molecular Orbitals in Diels-Alder Reaction: The interaction of Diene HOMO with Dienophile LUMO is favorable at both bond-forming sites

Under “Photochemical” Condition, [2+2] Actually works pretty well

Summary – The main role of orbital symmetry in the related reactions of the Pi . coefficient

Note

1. A quick summary of the Diels-Alder . reaction

Let’s recap our position with Diels-Alder so far:

• The Diels-Alder reaction combines a diene with a dienophile to form a new hexagonal ring [see: Introduction to the Diels-Alder reaction]
• forming three bonds (two sigma bonds and one pi bond) and three broken bonds (three pi bonds)
• The stereochemistry of the product can be reliably predicted from stereochemistry analysis of diene and dienophile. [see: Stereochemistry of the Diels-Alder Reaction]
• under certain circumstances, mixtures of non-deodorant isomers (exo- and endo-) are obtained. In general endo- is preferred over exo. [see this post on exo and endo]

What we haven’t really covered is why Diels-Alder actually works. After all, we’ve seen plenty of examples of things not working; for example, two alkenes do not combine to form four memorized rings when heated in the same way that a diene and a dimenophile combine to form a six-membered ring. The two dienes also do not readily combine when heated to form eight-sided rings.Why is Diels-Alder so easy, and so many seemingly involved reactions so difficult?The answer to this question lies in the arrangement of the pi molecular orbitals in the two components of this reaction. We’ll scratch the surface of the orbital symmetry rules here and use them to show why the reaction of dienes with alkenes (Diels-Alder) occurs readily on heating, but the reaction of alkenes with alkenes (also known as [2+2] cycloadditions) no.

2. Bond formation requires overlap between the HOMO of one molecule (Nucleophile) with the LUMO of another molecule (Electrophile).

Most of the reactions we’ve seen involve a nucleophile (an electron pair donor) reacting with an electrophile (an electron pair acceptor) to form ONE new bond. fill orbitals on the nucleophile containing the electron pair must be in contact (overlapped) with empty orbital on the electrophile that can accept electron pairs.[Perfect orbital overlap between nucleophile and electrophile.]

• The electron pairs on the nucleophile almost always derive from the highest energy-occupying molecular orbital (HOMO) of the nucleophile. Why? Since these are the least tightly held electrons.
• The orbital on the electrophile that receives the electron pair is almost always the lowest energy non-molecular (LUMO) orbital, as this will result in the lowest energy (and fastest) transition state.

In most reactions (such as SN2), only one bond is formed at a given center at any given time:One small note. In SN2, we assume that HOMO and LUMO have the same phase. This is perfectly valid – as long as we only deal with one bond that is formed at a time.

3. Reactions of interest: When two bonds form at the same time, many orbitals must overlap

Things get more complicated when we have a reaction where two or more bonds are formed at the same time. This is called a coordination reaction (as opposed to a “step-by-step” reaction). Take the combination of two alkenes to make a cyclobutane ring. (This is often called [2+2] cyclic.) Since we have two bonds forming at the same time, we have two interacting orbitals to consider.What are nucleophiles and electrophiles here? Ethylene and ethylene. 🙂 More specifically, the nucleophile is the HOMO of one ethylene molecule, and the electrophile is the LUMO of another ethylene. HOMO of an ethylene . molecule right combined with the LUMO of another ethylene molecule. [We can’t combine two occupied orbitals – Nature has a strict 2-electron occupancy limit per orbital. And since we can’t form a bond without electrons, combining two LUMOs would be silly]

4. Molecular orbitals in [2+2] Cyclic transition between Ethene and Ethene reveals why the reaction is unfavorable under “thermal” conditions

Read more: how to remove the evil eye with camphor Consider the π molecular orbitals of ethylene. HOMO has no buttons and LUMO has a single button. [We learned how to build up molecular orbitals of ethene in this post].For the reaction to occur in a coordinated fashion, we must have constructive overlap between each lobe where the bonds are being formed. [If the phases are opposite, there is destructive interference between the orbitals and therefore zero electron density between the atoms]Now let’s bring the two ethylene molecules together:Note that only one interactions between lobes have interacting phase (linking) lobes. The other interaction has the lobes of the opposing phase interacting with each other, which will not lead to bonding. [Note 1]This helps us understand why [2+2] the cycle change does not usually occur under “thermal” (i.e. heating) conditions. The orbitals do not overlap! [Note 2][2+2] However, the cycle change occurs under photochemical conditions. Plus in a moment.

5. Molecular Orbitals in Diels-Alder Reaction: The interaction of Diene HOMO with Dienophile LUMO is favorable at both bond-forming sites

Now let’s do the same kind of analysis for the Diels-Alder reaction Since we’ve seen the molecular orbitals of ethylene, let’s consider butadiene. [Relevant post: The Pi Molecular Orbitals of Butadiene].Now let’s see what happens when we try to stack the HOMO of butadiene with the LUMO of ethylene.[Why not the other way around, with the LUMO of butadiene and the HOMO of ethene? See Note ]Here we have the diene (in the green plane) approaching the dienophile (the orange plane) from above, since a helicopter can approach the landing pad. A new bond forms between C1-C6 and C4-C5. Note that the phases of the lobes for each interaction pair match, and so there is constructive orbital overlap.[Also note that although the diene is depicted as being on “top” here, it works equally well if it’s on the bottom hover here for a pop-up image -(link to file)]. Symmetry works in both cases – as it does for two Lego blocks, although the phases of the “lobes” on each face are opposite]This helps us understand why the Diels-Alder reaction works. dynamic – the orbital interactions are favorable.We’ll stop with Diels-Alder, but [Note 4] continue the discussion [nerds only].

6. Under “Photochemical” Condition, [2+2] Actually works pretty well

Above, I said [2+2] cycloaddition does not work under “normal” conditions, by “heating” I mean. [Organic chemists usually use the term “thermal” conditions]However, it is observed that if one is exposed to ultraviolet (UV) light, the reaction can go quite well. [These are called, “photochemical conditions”].Why? Ultraviolet light drives an electron from the HOMO to the LUMO, creating a “new” HOMO. [sometimes called HOMO-prime, or SOMO (for “singly occupied molecular orbital”)]. [Here is a previous post on UV spectroscopy].Now there are two interactive links between the lobes. And react really works! Read more: how to make a cosplay mask | Top Q&ANot that one wants to mess with perf, but it should at least be briefly noted that promotion of the Diels-Alder reaction is accomplished through heating, not through photochemical means. Attempts to run Diels-Alder under “photochemical conditions” will encounter an error similar to [2+2] periodic change in thermal conditions, and for the same reasons – because the orbital symmetry is wrong.

7. Summary – Main role of orbital symmetry in related reactions of the Pi . coefficient

In “thermal conditions” (heating, no UV rays), [2+2] is “forbidden” and Diels-Alder is “allowed”. [Note 4]The situation is reversed in the presence of ultraviolet light, where an electron can be promoted to generate new HOMO with different orbital symmetry. [2+2] The cyclic change between two alkenes is “allowed” and Diels-Alder is “forbidden”. We can aggregate all this into a simple table:As we continue to explore this topic, we will revisit this table and update, because there is a wide range of reactions in which orbital symmetry plays an important role.

Note

Note 1. Here we are making the assumption that one ethylene molecule approaches the other ethylene molecule the way we put two Lego pieces together. The bottom face of one component is linked to the top of another component. Each pair of lobes participating in the binding are on the same side of the molecule. This arrangement is called a superface. It is similar to “syn”.There is another possibility. What if instead of the “shaded” lobe of HOMO ethene combined with the “white” lobe of LUMO, instead it combined with the “shaded” lobe on the other side of LUMO. Since both lobes have the same phase this will be a bonding interaction! There is a name for the situation where lobes on opposite sides of a reactant participate in a reaction: it is called “antifacial” (similar to “anti”).You might ask why this doesn’t happen in [2+2] cyclic variation between alkenes. However, if you build a model, you will quickly find that the answer is that it is not so easy! Transition state for a [2+2] between two alkenes with a single high-stress component.[There are examples of [2+2] cyclic substances operating under thermal conditions, such as those involving ketones, have an anti-surface component. That is not a topic for today. ]Further reading: How to beat a player (11 ways to beat a player in his match) Note 2. The success of this analysis implies that in these types of reactions, counting the symmetry of the molecular orbitals is preserved – in other words, we can consider the relative phases of the lobes on the orbitals to be constant on the transition state time scale. This is why these rules are named “Orbital Symmetry Conservation”. Note 3. The interaction between the dienophile HOMO and the diene LUMO is also favorable from the point of view of “orbital symmetry”. However, the reaction rate will be fastest in cases where the energies of the HOMO/LUMO pair are close together. Most of the Diels-Alder reactions you will see will be electron-rich dienes (high energy HOMO) with electron-poor dienophile (low energy LUMO). energy LUMO) with electron-rich dinophile (high energy HOMO). This is called the Diels-Alder-demand-reverse reaction. Read more: How to hurt a player (11 ways to beat a player in his game) Note 4 – The pattern continues to alternate as additional pi links are added; the [4+4] is “forbidden” and [6+4] is “heat allowed”. The [6+6] is again “thermally forbidden”, etc. The cyclic mass with the largest number of pi electrons that I know of is a [14+2] cycle change. This is only thermodynamically permissible because one of the reactive components (heptafulvalene) reacts in an anti-reactive fashion. To beat a player at his game)

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