Factors That Affect Base Strength In Organic Chemistry

Basicity In Organic Chemistry: Some Basicity Trends

In this article we go through some important basicity trends in organic chemistry. (Previous article – Acidity Trends)

• The stronger the acid, the weaker the conjugate base; any factor which stabilizes the conjugate base has the effect of increasing acidity.
• OK, great. But what if we’re asked to determine which base is stronger? How do we do that?
• By flipping our trends around!
• One way of thinking about base strength is “instability” of a pair of electrons.
• So any factor which destabilizes the conjugate base…. increases basicity.
• For instance, removing resonance… increases basicity; removing electron-withdrawing groups…increases basicity; decreasing electronegativity of the atom… increases basicity.
• In other words, basicity will be increased by the opposite of any factor which increases acidity.
• Avoid the common misconception that “weak acids are strong bases”. Not true! Methane (CH₄) is a weak acid, but cannot act as a base since it lacks a lone pair of electrons. The proper phrasing is that “weak acids have strong conjugate bases”.  The conjugate base of CH₄, CH₃(-) is an extremely strong base.

After reading this article, you should be better at spotting basicity trends. You can test yourself with lots of quizzes on this topic at the bottom.

1. “Basicity” Is Just Another Word For “Stability Of A Lone Pair Of Electrons”
2. First Things First: Basicity Requires a Lone Pair of Electrons
3. Basicity Increases With Increasing Negative Charge
4. Basicity Increases With Decreasing Electronegativity and Size Of Atom
5. Basicity and Resonance
6. Basicity and Inductive Effects
7. Basicity Increases As s-orbital Character Decreases
8. One Approach To Solving “What’s The Strongest Base” Problems
9. A Final Word of Caution
10. Notes
11. Quiz Yourself!

1. “Basicity” Is Just Another Word For “Stability Of A Lone Pair Of Electrons”

We’ve learned 5 key factors to look at that determine acidity.

The natural next question is, well,  what about factors that the affect basicity?

Good news: they are the same factors. But they work in opposite directions.

Recall that acidity is determined by the equilibrium:

• The further this equilibrium goes from left to right, the stronger the acid.
• Any factor which stabilizes the conjugate base A(-) increases acidity.

So what does this mean for basicity?

If we want to think about factors that increase basicity, then we need to look at this equilibrium from right to left. 

• The further this equilibrium goes from right to left, the stronger the base.
• Any factor which destabilizes the conjugate base A(-) increases basicity.

That’s right. We can really just think of basicity as “instability of a lone pair of electrons”. The more unstable a pair of electrons is, the more basic it will be.

So here are the two sides of the same coin:

• The stronger the acid, the weaker the conjugate base, and
• The weaker the acid, the stronger the conjugate base.

In this article, let’s look again at the five factors that influence acidity, but we will look at it through the lens of basicity instead.

2. First Things First: Basicity Requires A Lone Pair

Let’s start with a simple point.

Recall that one definition of basicity (Lewis basicity) states that a base is an electron-pair donor.

If a molecule or atom can’t donate a lone pair of electrons, it can’t be a base.

(For our purposes here, this means “it must have a lone pair of electrons”. Later on you might see examples like BH₄(-) and AlH₄(-) which are basic since they can readily donate H(-) but we’re not going to deal with that right now). 

CH₄ is not a base since there is no lone pair of electrons on carbon. Same for ethane, hexane, cyclopentane and other alkanes.

Similarly, NH₄+ is not a base since there is no lone pair on nitrogen.

So any discussion about the “base strength” of molecules like this just ends with – no lone pair, not a base, discussion over.

3. Basicity Increases With Increasing Negative Charge

If acidity increases as the positive charge is increased, then it follows that basicity increases as negative charge is increased. 

Increasing the density of negative charge is destabilizing and we would predict this to increase basicity.

This is another way of saying that a conjugate base is always a stronger base than its parent acid, since a conjugate base is what’s left over after a molecule loses H+ (that is, its charge becomes more negative by 1).

I realize this may fall into the “water is wet” category of obvious observations for some people, but sometimes it’s helpful to have this clearly spelled out. :-)

In Gen Chem you might have done some titrations of phosphate ion and learned that (PO₄)3- is more basic than [HPO₄]2- which is more basic than [H₂PO₄]- which is more basic than phosphoric acid. Same idea.

Click to Flip

4. Basicity Increases Depending On Electronegativity and Size Of The Atom

If acidity increases as the atom becomes more electronegative going from left to right across the periodic table, then we should predict that basicity increases as electronegativity decreases going from right to left along the periodic table.

This means that CH₃(-) is a stronger base than NH₂(-) which in turn is a stronger base than HO(-) which is stronger than F(-).

It also applies to neutral compounds; it helps to explain why amines are more basic than alcohols which in turn are more basic than alkyl halides; the electron pair isn’t held as tightly!

Try this quiz:

Click to Flip

We also saw that acidity increases going down a column of the periodic table (H-F, H-Cl H-Br H-I) since the negative charge is spread out over a much larger volume as the size of the atom increases (greater polarizability). Negative charge in the iodide ion (I-) is more stable than it is in the fluoride ion (F-) since the same charge of -1 is spread out over a larger volume.

Conversely, basicity should increase going up a column of the periodic table since the negative charge is spread out over a smaller volume.

So F(-) is more unstable (more basic!) than Cl(-), which is more basic than Br(-), which in turn is more basic than I(-).

This also helps to explain why alkoxides (RO(-) ) are more basic than thiolates (RS(-) and also why neutral alcohols (ROH) are more basic than thiols (RSH). Same idea.

Think you understand the concept? Try this quiz:

Click to Flip

5. Basicity and Resonance

We’ve seen that acidity increases if the conjugate base can be stabilized through resonance, since that lone pair of electrons can be distributed over a greater volume (“delocalization”).

What happens if you take resonance away? That would make the lone pair less stable – more basic, in other words.

Compare the conjugate bases of phenol and cyclohexanol:

It also helps to explain why amines adjacent to aromatic rings are less basic than amines that can’t be delocalized through resonance. When lone pairs can be delocalized through resonance, that makes the lone pair more stable, and therefore less basic.

Click to Flip

6. Basicity and Inductive Effects

Hopefully you’re starting to get the picture by now that any factor which increases acidity, decreases the basicity of the conjugate base.

So all we have to do to increase basicity is to remove this factor.

Let’s look at inductive effects now.

Remember that strongly electronegative atoms (e.g. fluorine, chlorine) draw electron density away from a  negative charge and help to stabilize it.

That’s why 2,2,2-trifluoroethanol is a stronger acid than ethanol.

If we know this, then we would predict that if the electron withdrawing groups are removed, then we should increase basicity (spoiler: this is true)

Basicity is increased even more if there are electron-donating groups nearby that make the negative charge even less stable.

Click to Flip

7. Basicity Increases As s-orbital Character Decreases

Here’s the last item on our list: orbitals.

s-orbitals are closer to the nucleus than p-orbitals. When a pair of electrons is held more closely to the nucleus, it’s effectively increasing the electronegativity – that lone pair feels a greater pull from the nucleus than it would when it’s in an orbital that’s farther away.

That’s why alkyne C-H bonds (sp-hybridized, 50% s-character) are more acidic than alkene C-H (sp²-hybridized, 33% s-character) and alkane C-H bonds (sp³-hybridized, 25% s-character).

So let’s apply this in reverse.

Alkyl anions [e.g. CH₃(-) ] should therefore be more basic than alkenyl anions, which in turn are more basic than alkynyl anions.

This also works with atoms that are not carbon, as in the quiz below:

Click to Flip

8. One Approach To Solving “What’s The Strongest Base” Problems

If you already understand the factors that affect acidity, then you need no new information to solve problems that ask you questions like, “rank the compounds here in order of increasing basicity”.

Since acidity increases with decreasing basicity (stability) of the conjugate base, all you need to do is invert the problem.

As Charlie Munger wisely said, “Invert, always invert”.

One way to do it is to redraw all the molecules as their conjugate acids.

Then ask yourself, “What’s the weakest acid here”.

The weakest acid will have the strongest conjugate base.

Try it:

Click to Flip

9. A Final Word of Caution

A common misconception students have is that “weak acids are strong bases”.

Not true! Methane (CH₄) is a weak acid, but it can’t act as a base – it doesn’t have a lone pair.

The proper way to say it is that “weak acids have strong CONJUGATE bases“. So the conjugate base of CH₄, CH₃(-) is an extremely strong base.

That’s it!

Notes

Quiz Yourself!

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.