Strengths of Amines as Bases — Overview & Examples

What are Amines?

Amines are a type of functional group. What are functional groups? If you study chemistry in college, you'll take organic chemistry. Functional groups are combinations of atoms within a molecule. They differentiate the types of organic molecules. They dictate a molecules' physical and chemical properties. For now, just know that some functional groups are acids or bases.

Let's return to amines. Consider nitrogen. We know that it can form three covalent bonds. Amines always have at least one carbon-nitrogen bond. But, it may have up to three carbon-nitrogen single bonds. They only have multiple bonds in cyclic (ring) compounds. If the bond is not to carbon, it's to hydrogen.

Let's look at some examples.

$CH_{3}NH_{2}$ , methylamine
$(CH_{3})_{2}NH$ , dimethylamine
$(CH_{3})_{3}N$ , trimethylamine

In each case, the nitrogen has carbon-nitrogen single bonds to any of the methyl groups ( $-CH_{3}$ ). Any remaining bonds are nitrogen-hydrogen single bonds.

Why are Amines Basic?

Amines have a tetrahedral molecular geometry. Why? They have three bonds plus an electron lone pair. An amine's basicity comes from the lone pair. We also know that nitrogen is a fairly electronegative element. So, amines are polar molecules. The nitrogen atom will pull electron density toward itself. The electrons act as a Lewis base. So, they get donated to a hydrogen ion. The ion's charge is stabilized by the nitrogen's electron density. Various factors affect nitrogen's ability to contribute its electrons. These factors alter the basicity of the amine. Let's take a look at each aspect!

Factors Affecting Amine Basicity 1: Alkyl Substitution

The first factor is the degree of alkyl substitution. What does that mean? Nitrogen has decent electronegativity. So, it can pull electron density towards itself. The electron density helps stabilize nitrogen when it donates its lone pair. How would we increase the available electron density? By having more valence electrons!

Let's consider an example. The formula for methylamine is $CH_{3}NH_{2}$ . Nitrogen has bonded to one methyl group and two hydrogens. Nitrogen is the more electronegative element compared to carbon or hydrogen. So, it can draw electron density from these groups. But, hydrogen's only electron is in the covalent bond. So, it doesn't have extra electron density to give. Now let's consider dimethylamine, $(CH_{3})_{2}NH$ . Each of the two methyl groups has some electron density to give. The electrons move toward the nitrogen and stabilize the ammonium ion. More alkyl groups increase basicity.

The benefits of alkyl substitution extend to the size of the alkyl group. For example, ethylamine, $CH_{3}CH_{2}NH$ , has higher basicity than methylamine. The ethyl group has more electron density to share. Larger alkyl groups also increase basicity.

Factors Affecting Amine Basicity 2: Steric Hinderence

In the previous section, why didn't I use trimethylamine? Trimethylamine is a weaker base than methylamine or dimethylamine. But, why? It should have more alkyl substitution. But, the molecule's size impedes any reactions. Hydrogen is very small. So, there is an easy route for acid and nitrogen to collide. But, with big bulky groups, the path is blocked. The result is fewer collisions. So, we see a significant drop in basicity with a third alkyl group.

Factors Affecting Amine Basicity 3: Other Polar Groups

Many amines contain a halogen. For example, consider dichloromethylamine, $CHCl_{2}NH_{2}$ . So, carbon has two bonds to chlorine, one to hydrogen, and one to the amine. How does chlorine affect the basicity? It reduces it! Chlorine is slightly more electronegative than nitrogen. So, nitrogen and chlorine compete for electron density. Now, nitrogen has less electron density to stabilize itself. So, it is less basic. Other polar functional groups lower basicity.

Factors Affecting Amine Basicity 4: Resonance

Way back, we investigated valence bond theory and resonance. Sometimes, a compound has many lewis dot structures. We learned that it doesn't flip between the resonance forms. Instead, the electron density is the average of the different resonances.

How does resonance affect amines? An amine's basicity depends on its ability to donate its lone pair of electrons. What would happen if the lone pair got distributed across multiple resonances? The electron density becomes dispersed across the whole molecule. So, the molecule is more stable. It doesn't need to balance the lone pairs with a hydrogen ion. The basicity decreases!

What amines experience resonance? The most common are amines with benzene rings. As a side note, we call benzene rings aryl groups. An $-NH_{2}$ group bound to benzene is aniline. The three double bonds in benzene are movable. So, aniline has four possible resonance structures. The lone pair's electron density distributes across the four resonance structures.

Have you forgotten about resonance structures? You can practice by trying to draw the four resonance structures of aniline. You can look up the answer after you've attempted it. Do you need a hint? Start by moving the nitrogen lone pair down. You'll create a carbon-nitrogen double bond.

How do we Compare Amine Basicity?

Now we know what affects amine basicity. But, how do we measure it? We use the $pK_{a}$ 's of the conjugate acid! Remember, a large $pK_{a}$ , indicates a weak acid. But, the weaker the acid, the stronger its conjugate base. So, very large $pK_{a}$ 's indicate more basic amines.

Let's look at the $pK_{a}$ changes caused by alkyl substitution and steric hindrance.

Alkyl Substitution

$CH_{3}NH_{2}$ - $pK_{a}=10.62$
$(CH_{3})_{2}NH$ - $pK_{a}=10.64$

By adding an extra methyl group, the $pK_{a}$ increased. So, the amine is more basic.

$CH_{3}NH_{2}$ - $pK_{a}=10.62$
$CH_{3}CH_{2}NH_{2}$ - $pK_{a}=10.63$

Lengthening the alkyl group caused the $pK_{a}$ to increase. So, the amine is more basic.

Steric Hindrance

$(CH_{3})_{2}NH$ - $pK_{a}=10.64$
$(CH_{3})_{3}N$ - $pK_{a}=9.76$

Adding a third methyl group significantly lowered the $pK_{a}$ . So, steric hindrance decreased the basicity.