A Primer On Organic Reactions
7 Factors That Stabilize Positive Charge in Organic Chemistry
Last updated: August 23rd, 2022 |
Stabilization Of Positive Charge In Organic Chemistry: 7 Key Factors
Just to clarify: make sure you’re familiar with how formal charge can mislead before you read this. These factors determine the stability of “true” positive charges (i.e. low electron densities), not “formal” positive charges.
After discussing 7 factors that stabilize negative charge, it would make sense to talk about 7 factors that stabilize positive charge.
Table of Contents
- The Importance Of Stabilizing Positive Charge In Organic Chemistry
- Factor #1: With Increased Positive Charge Comes Increased Instability
- Factor #2: The Lower The Electronegativity Of The Atom, The Better It Can Stabilize Positive Charge
- Factor #3: The Greater The Polarizability Of The Atom, The Better It Can Stabilize Positive Charge
- Factor #4: Resonance Stabilization – Spreading Positive Charge Out Over A Greater Volume Is Stabilizing
- Factor #5: Increasing The Number Of Alkyl Substituents Adjacent To A Carbocation Will Stabilize Positive Charge
- Factor #6: Electronegativity Of An Atom Increases As The s-Character Of Its Hybridization Increases; This Destabilizes Positive Charge
- Aromaticity Can Also Stabilize Positive Charge
- Tying Together The Factors That Stabilize Positive Charge With The Factors That Stabilize Negative Charge
1. The Importance Of Stabilizing Positive Charge In Organic Chemistry
Anytime you have a reaction where positive charge is being formed, it’s useful to ask: how stable will this new positive charge be? What factors might help to stabilize it? The more stable that positive charge is, the more favorable the reaction will be.
Conversely, anytime you see a reaction where positive charge is being destroyed, it’s useful to ask the same question. If you see that a particularly unstable positive charge is being destroyed, this is going to be particularly favorable.
As with negative charge, there are several major factors worth keeping in mind:
- Opposite charges attract, so positive charges are stabilized through donation of electron density by neighboring atoms.
- High charge densities are unstable. So if charge can be “spread out” or “diffused” somehow, this is stabilizing. Also, if a neighboring group removes electron density from a positively charged atom, this is going to destabilize it (lead to higher charge density).
OK, here we go!
2. Factor #1: With Increased Positive Charge Comes Increased Instability
Simply put, the lower the charge density, the more stable the positive charge will be. For this reason the (ridiculous) case of H4O(2+) is far more unstable than H3O(+) which is more unstable than neutral H2O.
3. Factor #2: The Lower The Electronegativity Of The Atom, The Better It Can Stabilize Positive Charge
Just like high electronegativity stabilizes negative charge, the opposite is true: low electronegativity will result in a more stable positive charge. That helps to explain why cations like Na+, Li+, and K+ are quite innocuous and stable, whereas F+ and Cl+ are not.
4. Factor #3: The Greater The Polarizability Of The Atom, The Better It Can Stabilize Positive Charge
As we go down a column of the periodic table, the atomic radius will increase, which means that the charge density will decrease. (Higher volume = lower density). Since the charge is more “spead out”, the stability of the positive charge will increase.
5. Factor #4: Resonance Stabilization – Spreading Positive Charge Out Over A Greater Volume Is Stabilizing
Again, “spreading out” positive charge, which is possible when neighboring p orbitals can participate in resonance, is a stabilizing factor. Resonance stabilizes positive charge.
6. Factor #5: Increasing The Number Of Alkyl Substituents Adjacent To A Carbocation Will Stabilize Positive Charge
If you’re (electron) poor, it helps to have (electron) rich neighbors. Since opposite charges attract, positive charges will be stabilized by neighbors that can donate electrons. The classic example is carbocation stability, which increases as the number of adjacent carbon atoms increases. Another example is π donation, where neighboring atoms with lone pairs can donate their electrons to electron-poor species such as carbocations.
7. Factor #6: Electronegativity Of An Atom Increases As The s-Character Of Its Hybridization Increases; This Destabilizes Positive Charge
Negative charge is stabilized as we go from sp3 to sp2 to sp hybridization (alkyl to alkene to alkyne), since the negative charge is held in orbitals with increasing s-character – closer to the positively charged nucleus.
The opposite is true for positive charge! Having a positive charge in an sp orbital would mean that the positive charge is held more closely to the positively charged nucleus, which is bad! So the stability of positive charge will increase as we go from sp to sp2 to sp3.
8. Aromaticity Can Also Stabilize Positive Charge
This is a special case you usually meet in the early days of Org 2. There are certain types of molecules which possess a special stability called “aromaticity” (see post: Introduction to Aromaticity). Certain molecules that bear positive charge are aromatic: if that’s the case, the positive charge will be extraordinarily stable, like the cyclopropenium and tropylium carbocations. (you can even buy the tropylium carbocation!)
Conversely, there’s a related (but opposite) phenomenon called “antiaromaticity” (see post: Antiaromaticity) where certain molecules are particularly unstable.
If a molecule bearing a positive charge happens to be antiaromatic, it will be even more unstable than normal.
9. Tying Together The Factors That Stabilize Positive Charge With The Factors That Stabilize Negative Charge
Have you noticed how the factors that stabilize positive charge are related to the factors that stabilize negative charge?
1. “Spreading out” stabilizes both negative and positive charges. Resonance, polarizability, decreasing charge density and aromaticity are factors that can stabilize both positive and negative charge.
2. Negative charge is stabilized by adjacent positive charge – such as electronegativity (increasing), increasing s-character, and electron withdrawing groups, and destabilized by adjacent negative charge. (Opposite charges attract, like charges repel).
3. Positive charge is stabilized by adjacent negative charge – such as electron donating groups – and destabilized by adjacent positive charge such as electronegativity (increasing) and increasing s-character of orbitals. (Opposite charges attract, like charges repel).
These 7 factors will help us immensely toward understanding why certain reactions happen and others don’t.
12 thoughts on “7 Factors That Stabilize Positive Charge in Organic Chemistry”
For (CH3)+, shouldn’t the hybridization be sp2 instead of sp3 because the steric number of (CH3)+ is 3?
I think Stabilization of positive charge is not explained well, Positive charge means deficiency of electron, not any real positive charge near the nucleus. your statement “The opposite is true for positive charge! Having a positive charge in an sp orbital would mean that the positive charge is held more closely to the positively charged nucleus, which is bad! So the stability of positive charge will increase as we go from sp to sp2 to sp3. ”
is not satisfactory.
I am a bit confused, my book says that H2S is better acid than H2O because SH- is more stable than OH- similarly we can say that H2S is better base than H2O because H3S+ is more stable than H3O+.Now how is this possible that H2S is both better acid as well as better base.
I would not say that H2S is a better base than H2O. I have never seen a pKa value for H3S+
A positive charge is stabilized by adjacent electron donating groups (methyl, for example). But something like oxygen is more electronegative, so wouldn’t that be an electron withdrawing group, pulling more electron density away from an already electron-poor area, making it more positive and less stable? Or is this a case where resonance has a more stabilizing effect than induction, i.e. the lone pairs on oxygen, adjacent to a positive charge, can resonante and “redistribute” that positive charge?
I know aldehydes are more reactive than ketones because they have less CH/electron donating groups to stabilize the partial positive charge on the carbonyl C. But esters have two oxygens bonded to the carbonyl carbon; my mind tells me “Shouldn’t that ester oxygen destabilize/pull more charge away/make the carbonyl C more positive?”. Another part of my mind tells me “An O-CH3 is an electron donating group; shouldn’t it stabilize the partial positive on the carbonyl C?”. But I guess this is another resonance beats induction thing? A ketone only has one resonance structure, placing the positive charge fully on the carbonyl carbon, whereas the ester has two resonance structures, one where a positive charge is placed on the ester oxygen (thus making the positive charge less concentrated).
When in doubt, I always remind myself that “resonance beats everything”, and this is why, for example, halogen substituents on a benzene, while being deactivating through induction, are still ortho/para directors through resonance.
I am quite confused with the relationship between hybridisation and charge density. I our college our prof has taught us dat : “the more the s character, the more the destability or the stability decreases” . Can you also clarify the difference between the charge density and electronegativity and their effect on the carbocation?
As electronegativity increases, the stability of a negative charge (e.g. pair of electrons) will increase. You can think of going from sp3 to sp2 to sp hybridization as changing the effective electronegativity of the atom.
What’s the opposite of a lone pair of electrons? An empty orbital. As electronegativity increases, the *instability* of an empty orbital increases. That’s why we see carbocations, but almost never see nitrogen, oxygen, and (especially) fluorine with completely empty orbitals.
Applying the analogy above, you can think of going from sp3 to sp2 to sp as increasing the effective electronegativity of the atom, with the result that an empty orbital will be less stable.
Another way to look at it is from the perspective of potential energy. Think of an object 1 km above the surface of the Earth. It has a certain potential energy that is related to the gravitational force of the planet. If you keep that distance constant but increase the mass of the planet (say Earth -> Uranus -> Jupiter) you are increasing the potential energy of that object, just as increasing electronegativity correlates with increased potential energy.
When we’re talking about “destabilization of positive charge” due to hybridization (or electronegativity) you can really think of it as decreasing the ionization energy.
if electron having only negative charge so how is possible radicals.
Radicals are neutral because the negative charge from the electrons balances out the positive charge from the nucleus.
In summary any factor which reduces the charge will stabilise the molecule.What about the stability of phenyl cation?
I think phenyl cation would be the case of stabilization by aromaticity.
The pi-electrons in the ring can move the positive charge around the benzene ring, thus spreading out the charge, therefore conferring stability to the molecule.
That’s a common trap! In the phenyl carbocation [C6H5 (+) ] the empty p orbital is in the same plane with the C-H bonds, which is 90 degrees away from the p orbitals of the aromatic ring. So resonance stabilization is not possible.