Resonance

[dat_student]

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How many resonance forms does acetate ion have?
A) 2
B) 3
C) infinite number
D) not enough information is given

Believe it or not, 3 of the 4 answer choices may be correct depending on what your source is. For the DAT test, how many resonance forms does acetate ion have?

 

[grmaster1]

How many resonance forms does acetate ion have?

You can draw two contributing Lewis structures (presuming you know what an acetate ion is ;-) ) that yield one resonance hybrid structure for the deprotonanted acetic acid product...

Which answers you thought were correct?

 

[dat_student]

I'd say 2 is the right answer. John McMurry (O'Chem, 4th edition) agrees with me.

Another reference says 3 resonance forms. They say there is a third resonance structure that forms when you have a central carbocation plus 2 negatively charged oxygen atoms.

Another reference says we have an infinite number of resonance forms but some forms are more stable than others. The more stable ones are the ones that are usually drawn for freshman students

 

[dat_student]

What does everybody else think? For the DAT which one should I choose as the right answer? (trying to read the mind of the DAT chemists) 😱

 

[QofQuimica]

What does everybody else think? For the DAT which one should I choose as the right answer? (trying to read the mind of the DAT chemists) 😱

For DAT purposes, you should pick the number of MAJOR resonance contributors. So I agree that the answer here would be two. The carbocation, while technically a resonance structure, will be a relatively minor contributor compared to the other two.

 

[71263]

Hey dat student? I probably sound like an idiot right now, but Im curious, is your GPA really 1.5??

 

[dat_student]

Hey dat student? I probably sound like an idiot right now, but Im curious, is your GPA really 1.5??

Should my GPA be lower than 1.5? 😉 I probably have a zero chance of getting accepted to any dental school but it's worth trying.

 

[cowsgomoo]

I would say two too.

 

[gatszu]

the number of resonances is not infinite. you can only draw three, with the one w/the carbocation being a very minor contributor.

 

[dat_student]

the number of resonances is not infinite. you can only draw three, with the one w/the carbocation being a very minor contributor.

I guess that means you'd pick 3 resonance forms as the right answer. Based on John McMurry's criteria, the third resonance form (the one with the carbocation) is not acceptable because it doesn't follow the octet rule (i.e. the carbocation is NOT surrounded by 8 electrons).

 

[psiyung]

I guess that means you'd pick 3 resonance forms as the right answer. Based on John McMurry's criteria, the third resonance form (the one with the carbocation) is not acceptable because it doesn't follow the octet rule (i.e. the carbocation is NOT surrounded by 8 electrons).

This is correct. There are 5 rules in resonance that must always be obeyed (yes even by you dat_student) 😛

Rule #1: Resonance structures must have the same number of electrons.
Rule #2: All resonance structures must be valid Lewis dot structures.
Rule #3: The hybridization does not change between resonance structures.
Rule #4: The positions of atoms are the same in all structures.
Rule #5: Resonance structures do not have to be equivalent.

rule #3 will be broken with the carbocation structure

 

[dat_student]

This is correct. There are 5 rules in resonance that must always be obeyed (yes even by you dat_student) 😛

Rule #1: Resonance structures must have the same number of electrons.
Rule #2: All resonance structures must be valid Lewis dot structures.
Rule #3: The hybridization does not change between resonance structures.
Rule #4: The positions of atoms are the same in all structures.
Rule #5: Resonance structures do not have to be equivalent.

rule #3 will be broken with the carbocation structure

Thanks. case is settled. two resonance structures. I mainly blame Dr. Bresnick for the confusion. This guy has two doctorate degrees but still writes books full of errors. 🙄 He may have a 3rd doctorate degree in the field of "misinformation." I should have paid more attention to other people's comments before reading parts of his book. Wasted precious time... 😡

 

[psiyung]

Thanks. case is settled. two resonance structures. I mainly blame Dr. Bresnick for the confusion. This guy has two doctorate degrees but still writes books full of errors. 🙄 He may have a 3rd doctorate degree in the field of "misinformation." I should have paid more attention to other people's comments before reading parts of his book. Wasted precious time... 😡

The only book you need for Organic is Wade 4th or 5th edition. Wehn I took the class 3-4 years back, that book along with my ak-47 were under my pillow 😀

 

[grmaster1]

that book along with my ak-47 were under my pillow 😀

Damn, that's a huge pillow...

 

[dat_student]

The only book you need for Organic is Wade 4th or 5th edition. Wehn I took the class 3-4 years back, that book along with my ak-47 were under my pillow 😀

Are you a soldier in Iraq?

 

[gatszu]

This is correct. There are 5 rules in resonance that must always be obeyed (yes even by you dat_student) 😛

Rule #1: Resonance structures must have the same number of electrons.
Rule #2: All resonance structures must be valid Lewis dot structures.
Rule #3: The hybridization does not change between resonance structures.
Rule #4: The positions of atoms are the same in all structures.
Rule #5: Resonance structures do not have to be equivalent.

rule #3 will be broken with the carbocation structure

how will a carbocation break rule #3??

the acetate ion structure w/a carbocation and negative charges on both oxygen is a valid resonance structure. but due to separation of charge and a positive charge on a carbon, this resonance is a minor contributor to the hybrid structure. it is possible for carbon to have less than 8 electrons around, though, this is not very favorable.

@ dat_student: i don't know who john mccurry is, but that rule is bogus. look at molecules where the central atom has expanded octet. do you think those structures are valid?? You can reference different text and so on, but if you understand electron pushing, then figuring out resonance is not all difficult.

i may suggest "electron pushing" by weeks to those who want to better master resonance.

the correct answer to the # resonance contributor is 3, though, dat or whatever book the question came from may be 2. that is because there are 2 major resonance contributor, and the contribution by the structure w/the carbocation is very very minor.

 

[dat_student]

how will a carbocation break rule #3??

the acetate ion structure w/a carbocation and negative charges on both oxygen is a valid resonance structure. but due to separation of charge and a positive charge on a carbon, this resonance is a minor contributor to the hybrid structure. it is possible for carbon to have less than 8 electrons around, though, this is not very favorable.

@ dat_student: i don't know who john mccurry is, but that rule is bogus. look at molecules where the central atom has expanded octet. do you think those structures are valid?? You can reference different text and so on, but if you understand electron pushing, then figuring out resonance is not all difficult.

i may suggest "electron pushing" by weeks to those who want to better master resonance.

the correct answer to the # resonance contributor is 3, though, dat or whatever book the question came from may be 2. that is because there are 2 major resonance contributor, and the contribution by the structure w/the carbocation is very very minor.

Ok, if that's the case how many resonance forms does a simple aromatic ring have? Would you say 2 or too many? You can get many resonance forms if you unevenly break double bonds such that each break results in one carbon getting both electrons and the other carbon becoming a carbocation. Once you get your carbocations and carbanions then you can move them around in the ring system to get more resonance forms. Then you can try more possibilities: you can unevenly break one bond, then two bonds, then three bonds and then you can combine the new structures in different ways. If you go down that path, in some cases, there seems to be endless possibilities and I am just not sure what the DAT expects: all the possible combinations (major and minor) that can be obtained as a result of moving the electrons in the pi system or just the major ones ?! Different people and different books seem to have different opinions about this.

 

[gatszu]

in a aromatic system, the pi electrons are not localized on any atom, they are ramdonly dispersed around all the carbons involved (benzene = 6), and because of electron repulsion, dispersion is quite even. however, at any point, transiently, one carbon may have more electron density around it than other, and has a slight negative charge. all in all, in a aromatic system, separation of charge is basically nonexistent because of the high instability.

also, you have to understand that the hydrid stucture is how the molecule really behaves. resonance stuctures are what we draw to illustrate what the molecule looks like.

on the dat, you draw/count the resonance structures are that stable, and majorly contribute to the hybrid structure. stable and major contributors are those w/the least separation of charge and those w/the most atoms that satisfy the octet rule.

i hope that helps, and may this topic rests in peace.

 

[dat_student]

..., and may this topic rests in peace.

Amen...