Jul 5, 2009
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Dear SDN member,

I am really confused with this question, especially option B



I understand when the solution tries to do the first switch to make the H into dashes..

But why can't that be it?
I would just go ahead and assign priority and get a R configuration.
But instead, there is a switch 2 and made the configuration into S..

Why...:confused:
 

joonkimdds

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You don't have to switch like destroyer did, I think that will confuse me even more.

If it's fischer projection, meaning horizontal line is always 100% coming towards you, then if 4th priority is at horizontal, we switch R to S or S to R.

But unlike fischer projection, this has horizontal line going away from you, and vertical line coming toward you.

Try my method and see if this works for you.

Assign Br #1, C of C=C #2, CH3 #3, and H #4.
Try this as if you are doing PAT, this is like super simple PAT question if this is actually in PAT so you should know how to do this.

Grab #4, make it go away from you. now how does it look like?
Br is on right, C of C=C is at the top, and CH3 is at the left. Do you agree?
Br--> C of C=C --> CH3
this is S.

If above method sounds confusing try this.
Use your right hand fingers.
Your thumb is Br, ur second finger is C of C=C, and ur middle finger is CH3, and your pinky is H.
Now, grab your pinky and pull it away from you.
Your thumb is still on right side, ur 2nd finger is pointing upward, and ur middle finger is on the left side.
Got it?
 
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Jul 5, 2009
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Yes! Thank you so much! joonkimdds!

This is irrelevant to this question, but have you come across to Destroyer Orgo # 67?

The problem says..

Which compound below will absorb UV light at the longest wavelength?
a) Benzene
b) Anthracene
c) Naphthalene
d) 1,3-butadiene
e) 1,3,5,-octatriene

The book states "The more conjugated the molecule is, the more stable, more likely to have color, and the longer will be the UV wavelength absorption. Thus the answer is B"

So my question is, since the question is asking the longest UV wavelength absorb, it's asking for the smallest energy absorb right?
If a molecule is very stable, doesn't it need lot more energy to get excited it?

If the question is asking for which molecule would release the least amount of energy, then I would go ahead and pick the most stable molecule.
But since it's asking for the most energy absorb...

The question seems a little confusing to me
Please explain the concept for me if you can, thank you!!
 
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No the equation is E=hv, E=hc/lamda. So the more conjugated the less energy so the longer the wavelength. Frequency and energy are directly related and wavelength is inverse of energy
 

joonkimdds

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Yes! Thank you so much! joonkimdds!

This is irrelevant to this question, but have you come across to Destroyer Orgo # 67?

The problem says..

Which compound below will absorb UV light at the longest wavelength?
a) Benzene
b) Anthracene
c) Naphthalene
d) 1,3-butadiene
e) 1,3,5,-octatriene

The book states "The more conjugated the molecule is, the more stable, more likely to have color, and the longer will be the UV wavelength absorption. Thus the answer is B"

So my question is, since the question is asking the longest UV wavelength absorb, it's asking for the smallest energy absorb right?
If a molecule is very stable, doesn't it need lot more energy to get excited it?

If the question is asking for which molecule would release the least amount of energy, then I would go ahead and pick the most stable molecule.
But since it's asking for the most energy absorb...

The question seems a little confusing to me
Please explain the concept for me if you can, thank you!!
I don't know if I am understanding your question right because it sounds like you already know what you are doing by coloring longest wavelength = smallest energy.

Usually more stable the structure, the less energy it has....wait am I wrong?
Think about newman projection, the more stable structure one has lowest energy. I don't know if I am relating it to the right example.
 

dl9006

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Jul 2, 2008
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Yes! Thank you so much! joonkimdds!

This is irrelevant to this question, but have you come across to Destroyer Orgo # 67?

The problem says..

Which compound below will absorb UV light at the longest wavelength?
a) Benzene
b) Anthracene
c) Naphthalene
d) 1,3-butadiene
e) 1,3,5,-octatriene

The book states "The more conjugated the molecule is, the more stable, more likely to have color, and the longer will be the UV wavelength absorption. Thus the answer is B"

So my question is, since the question is asking the longest UV wavelength absorb, it's asking for the smallest energy absorb right?
If a molecule is very stable, doesn't it need lot more energy to get excited it?

If the question is asking for which molecule would release the least amount of energy, then I would go ahead and pick the most stable molecule.
But since it's asking for the most energy absorb...

The question seems a little confusing to me
Please explain the concept for me if you can, thank you!!

good question..
you are correct that a long wavelength equals a small amount of energy. (E=Hv=Hc/lambda)

Just because a molecule is stable doesn't mean that you need more energy. You are not trying to atomize it.

i think the explanation as to why more conjugated system absorb lower energies requires the use of molecular orbital diagrams to properly explain it.


my understanding is that color arises when an electron which was excited from its pi bonding orbital to its pi antibonding orbital comes back down to its pi bonding orbital, releasing its energy as a photon.

If the energy difference between the pi bonding and pi antibonding becomes larger, a larger amount of energy is required to excite the electron and thus the shorter the frequency of the light emitted(bigger energy).

I don't know what the MO diagram of a conjugated system looks like but it seems that as you add more conjugation, the energy difference between the pi bonding orbital and pi antibonding orbital becomes smaller.

As the difference gets smaller, the less energy is required to excite the electron and thus a more conjugated system absorbs the longest wavelenght.

It says in wikipedia that conjugated systems of less than 8 only absorb uv light which means that the energy difference between the pi and pi* orbtitals are much larger. As you add more conjugations, the difference becomes smaller.

hope this makes sense lol i think i rambled a bit

cheers
 
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Jul 5, 2009
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Yes. The more stable the structure is, the less energy it has. That's for sure.

And that's also why I am confused, because the question is asking which molecule will absorbed the least amount of energy..

So I was thinking, a stable molecule should absorb ALOT more energy compare with the unstable one right?

Since the unstable one ALREADY has lots of energy, thus it can only absorb a little more...

Is my logic completely wrong?
Or am I just thinking too much...haha..:rofl:

I don't know if I am understanding your question right because it sounds like you already know what you are doing by coloring longest wavelength = smallest energy.

Usually more stable the structure, the less energy it has....wait am I wrong?
Think about newman projection, the more stable structure one has lowest energy. I don't know if I am relating it to the right example.
 

joonkimdds

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conjugation = stable = less energy

Question asked which absorbs the least energy = less energy = stable = conjugation.


what you said
"So I was thinking, a stable molecule should absorb ALOT more energy compare with the unstable one right?"

should be
"a stable molecule should have a lot less energy"
 

dl9006

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conjugation = stable = less energy

Question asked which absorbs the least energy = less energy = stable = conjugation.


what you said
"So I was thinking, a stable molecule should absorb ALOT more energy compare with the unstable one right?"

should be
"a stable molecule should have a lot less energy"

i think he was just asking about why conjugation absorbs at lower energy.
 

joonkimdds

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i think he was just asking about why conjugation absorbs at lower energy.
isn't it because conjugation = stable = low energy?
and also because UV light = high conjugation = longest wavelength = low energy?
 
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Jul 5, 2009
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I would understand perfectly if the question is, which of the following molecule has the least amount of the energy release..then the answer will have to be the most stable molecule.

But I'm just really confused when the word "release" is changed to "absorbed" because I would think the answer should be the opposite...that more reactive molecule would absorb less energy since it already posses lots of energy, so it can't take as much as a stable molecule could..



isn't it because conjugation = stable = low energy?
and also because UV light = high conjugation = longest wavelength = low energy?
 

dl9006

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I would understand perfectly if the question is, which of the following molecule has the least amount of the energy release..then the answer will have to be the most stable molecule.

But I'm just really confused when the word "release" is changed to "absorbed" because I would think the answer should be the opposite...that more reactive molecule would absorb less energy since it already posses lots of energy, so it can't take as much as a stable molecule could..
if the question was re-worded to release instead of absorb, the answer would still be the same. if you absorb x amount of energy then you must emit that same amount right?

also, i don't think it's correct to think that a reactive molecule absorb less energy since it is already at a high energy level.

Completely different thing.


an unstable molecule has electrons in molecular orbitals that are high in energy which is why it's unstable in the first place

when you excite these electrons (ie light or heat), the electrons move up to higher molecular orbitals. (remember homo ->lumo ? )

when the electron in the highest molecular orbital comes back down, light is emitted.


what i was trying to say in my earlier post is that when you have conjugation the difference between the lowest occupied molecular oribtal and the highest unoccupied molecular orbital becomes smaller.

Thus the energy that is required or absorbed is smaller.

hope this clarifies things for you.

cheers
 
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Jul 5, 2009
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Yes. Perfectly! Thank you so much!!

if the question was re-worded to release instead of absorb, the answer would still be the same. if you absorb x amount of energy then you must emit that same amount right?

also, i don't think it's correct to think that a reactive molecule absorb less energy since it is already at a high energy level.

Completely different thing.


an unstable molecule has electrons in molecular orbitals that are high in energy which is why it's unstable in the first place

when you excite these electrons (ie light or heat), the electrons move up to higher molecular orbitals. (remember homo ->lumo ? )

when the electron in the highest molecular orbital comes back down, light is emitted.


what i was trying to say in my earlier post is that when you have conjugation the difference between the lowest occupied molecular oribtal and the highest unoccupied molecular orbital becomes smaller.

Thus the energy that is required or absorbed is smaller.

hope this clarifies things for you.

cheers
 
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