Basicity of Aliphatic Amines Explained |ChemFam #91|

Hello my dear STEM enthusiasts. Are you the guy who can’t feel the morning without a freshly brewed coffee and you love the smell of your freshly prepared coffee. Ain’t you? And do you love pancakes? If the savoring aroma makes you cozy, well mostly a special class of organic compounds called ‘amines’ are responsible for the fragrance of your coffee to the taste of those lovely pancakes. This tiny molecules are behind the curtain of many other daily life things that gives you pleasure in some or other way. These amines are basic in nature. For simple reason, the basicity could be accounted for the lone pair of electrons they carry in their molecular structure. Yes, they do not actually have a negative charge to show the basic character. They are neutral compounds but basic in nature.

The saturated amines, could also be called as aliphatic amines in a more advanced way, are more basic than ammonia. I’m pretty sure you know what ammonia is! Yes, the fertilizer that is seen used in agriculture. As I have mentioned aliphatic amines, so let me further discuss the three types of amines with regards to the number of alkyl group attached to the N atom. There are three different aliphatic amines and how could I forget the parent ammonia compound. I will discuss and elaborate their basicity order in this blog and we will eventually get them arranged in an increasing or decreasing order at the end. Remember, there will be some twists to the order, so relax and you’ll understand everything by the end of the blog.

The pK_b values of aliphatic amines lie in the range 3.0 to 4.22 while that of ammonia is 4.75. The lower the pK_b value the higher is its basic nature. So from that we can easily conclude that all the three classes of aliphatic amines are more basic than ammonia. The reason is mainly the electron donating nature of the alkyl groups attached in them. As a result, the electron density around the N atom increases and they can donate the lone pair of electrons more easily than ammonia, resulting in the increase of their basic nature. I hope you do remember the Lewis acid-base concept. Just to be sure, an acid is an electron pair acceptor and the base is an electron pair donor.

As more the R groups in the aliphatic amines, the better the donation ability and hence the stronger the base is. So by following this rule, the order of basicity should potentially be like this,

So, we got the answer, right? Well actually No! The twist begins.

It has been found experimentally that a 2° amine is more basic than 1° and 3° amines in an aqueous solution. But why? Let me explain. Your friend Mr. ChemFam is here to help you (wink).

The reason why the order is actually changed could be explain by the following reasons.

Inductive effect (+I) of the alkyl group.
Extent of H-bonding with water molecules.
Steric effects of the alkyl group.

I will discuss each of them now.

+I effect

As the number of alkyl groups attached directly to the N atom increases, the extent of donor ability increases. Similarly the dispersal of positive charge on the ammonium cation by the +I effect of the alkyl group increases accordingly. Thus, on the basis of +I factor alone, the basicity of amines should decrease in the order:

Solvation effect

The stability of the ammonium cation due to H-bonding depends upon the number of H atoms present on the N atom. Obviously greater the the number of H atoms on the N atom, more stable is the ammonium cation. Thus the ammonium cation derived from a 1° amine is the most stable since it has three H atoms which can form H bonds with H₂O. The ammonium cation derived from the 2° amine is less stable since it has 2 H atoms while of the 3° amine is the least stable since it has only 1 H atom which can form H-bond with H₂O.

Amine solvation.png

This is in regard with the fact that, the more water molecule get associate with H atom of amines to form H-bonds, the more energy of that compound is going to get released and thereby the stability will increase. Thus, the stability order with respect to the solvation effect follows the order:

Amine 3.png

Steric Effect

Overcrowding of the alkyl groups around the N atom of amine hinders the attack of proton (H⁺) on the lone pair of amine and decreases the basic strength. The more bulky it gets, the less space is available for attack of an incoming proton. Thus, due to steric effect, the order should be as:

Amine 3.png

From the discussion above, we may conclude that it is a combination of +I effect of the alkyl groups, H-bonding and steric effect which determines the stability of the ammonium cations in solution. All these three factors are in favor for the increasing basicity of 2° amines and hence 2° amines are the strongest base among other aliphatic amines. If the alkyl group is small, i.e., CH₃ (methyl) then there is no steric hindrance/overcrowding to H-bonding. In other words, the stability due to H-bonding predominates over the stability due to +I effect of CH₃ group and hence 1° amine, i.e., CH₃NH₂ is a stronger base than 3° amine i.e., CH₃)₃N. In other words, the overall decreasing basic strength of methylamine is :

If, however an alkyl group bigger than CH₃ group, i.e., ethyl, propyl etc. there will be some steric hindrance to H-bonding. As a result, stability due to +I effect predominates over the stability due to H-bonding and hence 3° amines become more basic than 1° amines. Thus, the overall basic strength of ethylamine follows the following order:

I really hope that I was able to completely cover the topic and answered the questions that may arise when arranging the basic order of aliphatic amines. Exploring their basicity not only sheds light on their behavior in chemical reactions but also unveils the intricacies of molecular interactions.

Until next time, Keep STEMtastic!