Molecular Orbital for Sn2 reaction

In summary: It's also important to note that the thiolate anion is acting as a nucleophile and attacking the carbon center. The attack is happening simultaneously with the C-Br bond breaking. This is why it's helpful to draw a sketch showing the attack happening at the same time as the bond breaking.In summary, the equation shown in the picture indicates an Sn2 reaction with a polar aprotic solvent and strong nucleophile. The question regarding drawing the orbitals is best answered by considering both the orbital diagram and the molecular orbital diagram, and focusing on the bonding and antibonding interactions. The attacking orbital formed by the thiolate anion is a combination of the lone pair sulfur orbital and the C-Br antibonding orbital,
  • #1
Yokoko
15
1
Homework Statement
Question as attached to this thread. I'd like confirmation whether or not my solution to a) is correct. But I am mainly confused with b)
Relevant Equations
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IMG_20200802_151912__01.jpg


The equation can be seen in the picture. I was assuming it was an Sn2 reaction given the polar aprotic solvent and strong nucleophile.

For b), I assumed that the question asked me to draw the orbitals as shown above (as opposed to orbital diagram) because it asked me to identify the HOMO and LUMO.

I'm confused as to where SEt (-) attacks/how to draw the attack i.e. the reaction itself as both steps occur at the same time. Also I'm assuming that the orbital around S is the HOMO and the orbital around C the LUMO. Is that correct?

Thanks a lot!
 

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  • #2
Yokoko said:
I was assuming it was an Sn2 reaction given the polar aprotic solvent and strong nucleophile.
Yes.
Yokoko said:
For b), I assumed that the question asked me to draw the orbitals as shown above (as opposed to orbital diagram) because it asked me to identify the HOMO and LUMO.
Honestly, it’s probably best to do both.
Yokoko said:
I'm confused as to where SEt (-) attacks/how to draw the attack i.e. the reaction itself as both steps occur at the same time.
Well, you got the stereochemistry inversion correct, so where do you think the thiolate attacks?
Yokoko said:
Also I'm assuming that the orbital around S is the HOMO and the orbital around C the LUMO. Is that correct?
It’s probably more useful to only draw the portions of the orbitals that are important for the changes that occur during the reaction.

It’s kind of tough to give you more help without seeing you put in some more effort first.
 
  • #3
15964345860285810145332600895344.jpg

15964349434356802911227774981391.jpg

Thanks for your input!
I know it's not a lot but I started working upon your prompts. In the first one is the orbital attacking the C centre with the positive charge after Br has left. Maybe it would be good to but Br- on the side too?

The second one is my attempt at the orbital diagram. Again, I'm not sure how this question wants me to answer this. This one shows the c centre bonding to a hydrogen and another sp3 hybridised C (again unsure how to draw in the second one as both methyl and ethyl are bonded by a sp3 hybridised C, right? Are they all of equal energy?), The one orbital bonding with SEt is missing too as of now...

Thanks again
 
  • #4
Yokoko said:
after Br has left.
It’s SN2. Should Br have left?
Yokoko said:
The second one is my attempt at the orbital diagram. Again, I'm not sure how this question wants me to answer this.
I agree, the question isn’t particularly clear. I’ll try to focus on the most important conceptual takeaways. Instead of focusing on the hybridization of the orbitals, it’s probably better to focus on whether the interacting orbitals are bonding, non-bonding, or antibonding. Bring those ideas into your answer and see what you can come up with.
 
  • #5
15965384040621560329406741974335.jpg

15965384787463571689068384547697.jpg

I think I've come a bit farther this time. The bonding/anti bonding hint really helped! :)

Two questions: is there only one or two e- in the orbital of SEt- ? It is a lone pair, so there would be 2 e-, correct?

And would the orbital diagram drawn enough to answer the question combined with the sketch?

Thanks again!
 
  • #6
Getting closer. Look at the MO diagram in your second picture. Is the sigma antibonding orbital higher or lower in energy than the sigma bonding orbital?
Also, a Lewis structure should give you an idea of whether the thiolate anion has a lone pair or an unpaired electron.
 
  • #7
Oh you are right, the C would be lower in energy, therefore under the SEt orbital.

And since the bond with sodium breaks, it seems like it only has an unpaired electron left
 
  • #8
Yokoko said:
Oh you are right, the C would be lower in energy, therefore under the SEt orbital.
I meant the MOs that are formed by the interaction of the sulfur with the carbon. You have the antibonding orbital lower in energy than the bonding orbital in your picture.
Yokoko said:
And since the bond with sodium breaks, it seems like it only has an unpaired electron left
Look closer. The S has a negative charge. It really might help to draw out the Lewis dot structure and actually count the electrons around the sulfur.
 
  • #9
IMG_20200804_140228__01.jpg

So, this should be right now. I forgot about the negative charge on the sulfur, I think that was my mistake.
 
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  • #10
Better. I might have confused you before: the occupied sulfur HOMO is definitely lower in energy than the C LUMO. Otherwise it would be energetically favorable simply to transfer electrons to the lower energy empty orbital than to form a new set of MOs.

The biggest conceptual takeaway is this: the orbital that’s being created by the interaction of the thiolate with the carbon centered nucleofuge is a combination of the lone pair sulfur orbital and the C-Br antibonding orbital. The “bonding” orbital that ends up being formed by this combination has strong bonding character with the sulfur and strong antibonding character with the bromine. This is ultimately the reason that the C-Br bond breaks and the C-S bond forms.
 
  • #11
That makes sense! Thank you so much for help, I know it took a while!
 

Related to Molecular Orbital for Sn2 reaction

1. What is a molecular orbital?

A molecular orbital is a mathematical function that describes the wave-like behavior of an electron in a molecule. It is formed by the linear combination of atomic orbitals from the atoms in the molecule.

2. How does a molecular orbital affect an Sn2 reaction?

Molecular orbitals play a crucial role in determining the reactivity of a molecule in an Sn2 reaction. In this type of reaction, the nucleophile attacks the electrophilic carbon atom, and the stability of the molecular orbitals involved in the bond formation determines the rate of the reaction.

3. What is the significance of the HOMO and LUMO in a molecular orbital for Sn2 reactions?

The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) are important in Sn2 reactions because they are the molecular orbitals involved in the bond formation and breaking. The HOMO of the nucleophile interacts with the LUMO of the electrophile, resulting in the formation of a new bond.

4. How can the molecular orbital theory be used to predict the outcome of an Sn2 reaction?

By analyzing the molecular orbitals of the reactants and products, one can predict the outcome of an Sn2 reaction. If the HOMO-LUMO energy gap is small, the reaction will proceed quickly, whereas a large energy gap will result in a slower reaction. Additionally, the stability of the resulting molecular orbitals can also be used to predict the outcome of the reaction.

5. Can the molecular orbital theory be applied to other types of reactions?

Yes, the molecular orbital theory can be applied to many different types of reactions, including other types of nucleophilic substitution reactions, as well as reactions involving other types of functional groups. It is a useful tool for understanding the reactivity of molecules and predicting the outcomes of chemical reactions.

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