Aliphatic Amines

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Alkyl groups push electrons towards nitrogen and make an amine a stronger base. So trimethylamine, with three methyl groups, should be the strongest of the three methylamines, right? In the gas phase, it really is. But in water, it is the weakest of the three! How can one molecule be the strongest and also the weakest? Questions on the order of basic strength come up often in JEE, so let's find out.

0:34Measuring basic strength

So, how do we measure basic strength? An amine, RNH two, where R is an alkyl group, reacts with water to give RNH three plus, the ammonium ion, and O H minus. The raised plus and minus are charges, the small numbers written low count atoms, and the two half arrows mean an equilibrium. Okay? Its equilibrium constant is K b, with a small b written low, for base. K b equals RNH three plus, times O H minus, over RNH two, each inside square brackets, which mean concentration. Right? Next comes p K b, which is minus log of K b. The small p in front simply means minus log. Clear? Now let me ask you this: if K b gets larger, does p K b get larger, or smaller? It gets smaller, yes, exactly, because of that minus sign! So the larger the K b, or the smaller the p K b, the stronger the base. Students often flip this in the exam, so fix it now. Got it? And for the conjugate acid, RNH three plus, p K a equals fourteen minus p K b, at two ninety eight kelvin. We will use this in Example one. With me?

1:46Alkyl groups push electrons to nitrogen

Now, what makes one amine a stronger base than another? First, anything that makes the lone pair on nitrogen more available makes the base stronger. The double arrow on screen means implies. Okay? Second, anything that stabilises the ammonium ion formed also makes the base stronger. This whole video is a contest between these two. Right? Now, alkyl groups release electrons towards the nitrogen. This is the plus I effect, a plus sign and a capital I, for inductive. Clear? So compare an alkylamine with ammonia, N H three, which has no alkyl group at all. Which one is the stronger base? Yes, exactly, the alkylamine is the stronger base! Its alkyl groups push electrons onto nitrogen, so its lone pair is more available. So all alkylamines are stronger bases than ammonia. Got it? Now, in the gas phase there is no water, so only this inductive effect acts. The little raised circle is read as degree: one degree means primary, with one alkyl group on nitrogen, two degree means secondary, and three degree tertiary. So, in the gas phase, which one is the strongest base? The tertiary amine, yes, exactly, because three alkyl groups push the most! So in the gas phase, tertiary beats secondary, which beats primary, which beats ammonia; each greater than sign means is a stronger base than.

3:18In water: hydrogen bonding and crowding

With me? Now, water changes the story. In water, the ammonium ion formed is stabilised by hydrogen bonding to the water molecules; H bonding on screen is short for hydrogen bonding. Okay? The hydrogens that do this are on nitrogen, in the N H bonds; the small dash is a bond. So, of the ions from a primary, a secondary and a tertiary amine, which has the most N H bonds? The ion from the primary amine, correct, with three; the secondary gives two, and the tertiary only one. So for hydrogen bonding, primary is best, then secondary, then tertiary, the reverse of the gas phase order. Right? Second, bulky groups crowd the nitrogen and hinder solvation, so water molecules cannot get as close to stabilise the ion. Clear? So now three things compete: the push, the hydrogen bonding, and the crowding. Trimethylamine has the biggest push of all, so in water it should still be the strongest, right? That is the trap, and the answer is no! In water, dimethylamine, the secondary amine, is the strongest, then methylamine, then trimethylamine, then ammonia; the small two or three after a bracket counts the methyl groups. The ion of trimethylamine has only one N H bond and is crowded, so water stabilises it poorly, and that outweighs the push of its three methyl groups. So our puzzle is solved. Got it? And the p K b values agree: three point two seven, then three point three eight, then four point two two, and then four point seven five. Smaller p K b means a stronger base, as the less than signs show. With me? And remember this line: in water, the secondary amine is the strongest in both the methyl and the ethyl series. We will see the ethyl series in Example two. Okay?

5:10Example 1: pKa of methylammonium ion from pKb 3.38

Now Example one, an easy one. The p K b of methanamine is three point three eight. Find the p K a of the methylammonium ion. Pause the video and try it first. Done? Okay, let's check. Methanamine is C H three N H two, our methylamine. With a proton it becomes C H three N H three plus, the methylammonium ion, its conjugate acid. Right? So, which formula connects the p K a of the conjugate acid with the p K b of the base? Yes, exactly, p K a equals fourteen minus p K b, at two ninety eight kelvin! So the p K a equals fourteen minus three point three eight. Okay? Fourteen minus three point three eight gives ten point six two. So the p K a of the methylammonium ion is ten point six two. See the pattern: a small p K b for the base means a large p K a for its ion. Clear?

6:08Example 2: basicity order of ethylamines, gas vs water

And now Example two, a harder one. Arrange ammonia, ethanamine, N ethylethanamine, and N, N diethylethanamine in order of decreasing basic strength, part a, in the gas phase, and part b, in water. Pause the video and try it first. Done? Okay, let's check, and first sort them into classes. Ethanamine, C two H five N H two, is primary. The italic N means the ethyl groups named with it sit on nitrogen, so N ethylethanamine is diethylamine, secondary, and N, N diethylethanamine is triethylamine, tertiary. Right? Part a is quick. In the gas phase only the inductive effect acts, so more ethyl groups means a stronger base: tertiary, then secondary, then primary, then ammonia. Okay? Now part b, in water. For the methylamines we had secondary first, then primary, then tertiary. So for the ethylamines, we simply copy that order, right? Careful, that would be the second trap of the day! For the ethyl series in water, diethylamine is still first, but then comes triethylamine, then ethanamine, then ammonia. Here the tertiary amine beats the primary one, because the three factors balance out differently, so learn both orders. Clear? And the p K b values agree: three point zero zero, then three point two five, then three point two nine, and then four point seven five. And once again, the secondary amine is the strongest. Got it?

7:40Quick revision: pKb, pKa and basicity order, gas vs water

So, quick revision. The stronger base has the larger K b and the smaller p K b, and the p K a of its conjugate acid is fourteen minus p K b. In the gas phase, the inductive effect alone gives tertiary, then secondary, then primary, then ammonia. In water, hydrogen bonding and crowding join in, and the secondary amine is the strongest. Now try the practice problems on this page, and in every question, first check whether it says gas phase or water. All the best!