Structure and Physical Properties

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Take two amines with the same formula, C three H nine N, so the same molar mass, fifty nine grams per mole. The first, propan one amine, boils at three hundred and twenty one kelvin. The second, trimethylamine, boils at only two hundred and seventy six kelvin. Same atoms, same mass, and yet forty five kelvin apart. So where does that gap come from? Questions like this come up often in JEE, so let's begin.

0:34Structure: a pyramid with a lone pair

Let us begin with the structure. In amines, nitrogen is s p three hybridised; the small three written high is a superscript, okay? That gives nitrogen four hybrid orbitals, but it makes only three bonds. So tell me, what fills the fourth orbital? The lone pair, yes, exactly! Three orbitals form bonds, and the fourth holds the lone pair of nitrogen. Right? Now, the three bonded groups sit like the base of a pyramid, with nitrogen at the top. So the shape is pyramidal, and the bond angles are a little below one hundred and nine point five degrees; the small circle written high means degrees. Take trimethylamine: C H three in brackets, and the small three after the bracket means three methyl groups on one nitrogen. Its angle C dash N dash C, at the nitrogen, is one hundred and eight degrees. Clear? And this lone pair is the hero of the whole unit. The double arrow on screen means therefore: because of the lone pair, amines are bases and nucleophiles. Got it?

1:40Inversion: chiral, yet not resolvable

Now here is something interesting. On screen, R, R prime and R double prime are three different groups on one nitrogen; the little strokes written high are primes. Such an amine has no plane of symmetry, so it is chiral in principle. With me? Chiral means it cannot be superimposed on its mirror image, so it has two mirror-image forms, the enantiomers. So, can we separate them? The answer is no, we cannot separate them. The nitrogen pyramid turns inside out very rapidly, like an umbrella flipping in the wind. This is called inversion. So the two mirror-image forms keep changing into each other, and the enantiomers cannot be separated. Okay? But now let nitrogen use its lone pair to bond a fourth group. That gives a quaternary ammonium ion. If all four groups are different, this ion cannot invert, so it can be resolved, which means separated into its enantiomers. Right?

2:38Physical state and hydrogen bonds

Now, the physical properties. The lower aliphatic amines are gases with a fishy smell. Clear? Aniline, C six H five N H two, is different; the small numbers written low count the atoms. It is a colourless liquid, but it darkens in air, because air slowly oxidises it. Okay? Now the big idea, hydrogen bonding. Here the small circle written high is read as degree: one degree means primary, and two degree means secondary. These amines have an N H bond. The dash is that bond, and the three dots show the hydrogen bond to the nitrogen of the next molecule. Right? Now, trimethylamine from our puzzle is a tertiary amine. Can its molecules hydrogen bond to each other? No, they cannot, and that is the key! Tertiary amines, three degree, have no N H bond at all. So there are no hydrogen bonds between their molecules. Got it?

3:34Boiling points: count the N–H bonds

So, here is our puzzle, solved. Propan one amine is primary; its molecules hold each other by hydrogen bonds, so it boils at three hundred and twenty one kelvin. Capital K stands for kelvin. Trimethylamine is tertiary, has no such bonds, and boils at only two hundred and seventy six kelvin. With me? So for isomeric amines, the boiling point goes primary, greater than secondary, greater than tertiary; the pointed sign means greater than. And here is a topper's trick: count the N H bonds. Primary has two, secondary has one, and tertiary has none. More N H bonds, more hydrogen bonding, and a higher boiling point. Clear? Now compare with alcohols. Ethanamine boils at two hundred and ninety kelvin, and ethanol at three hundred and fifty one kelvin, with nearly equal molar masses. Both form hydrogen bonds, so why is ethanol so much higher? It comes down to electronegativity, exactly! Nitrogen is less electronegative than oxygen, so the N H bond is less polar, and the hydrogen bonds between amine molecules are weaker. Okay? So, for comparable molar mass, the boiling point order is acid, greater than alcohol, greater than amine, greater than alkane. Amines sit right in the middle. Got it?

4:56Solubility in water

Next, solubility in water. Trimethylamine has no N H bond, so it cannot hydrogen bond with its own molecules. So it should not dissolve in water, right? Oh no, careful, that is a trap! Even with no N H bond, trimethylamine is very soluble in water. Okay? The reason is its lone pair. Water offers its hydrogens, and the nitrogen lone pair accepts these hydrogen bonds. So the lower amines of all three classes, primary, secondary and tertiary, dissolve in water. Clear? But as the hydrocarbon part grows, the solubility falls. That is why aniline, with its large benzene ring, is only slightly soluble in water. With me? And amines also dissolve in organic solvents. Got it?

5:46Example 1: boiling point order of three C4H11N isomers

Now, our first example. Arrange in order of decreasing boiling point: butan one amine, N ethyl ethanamine, and N N dimethyl ethanamine. The italic capital N in a name means that group sits on the nitrogen. Pause the video and try it first. Done with it? Okay, let's check. All three are C four H eleven N, so the molar mass is the same. That means the extent of hydrogen bonding decides the order. Okay? Butan one amine has an N H two group at the end, so it is primary. N ethyl ethanamine has two ethyl groups and one hydrogen on nitrogen, so it is secondary. And N N dimethyl ethanamine has no hydrogen on nitrogen at all, so it is tertiary. Right? So the boiling point order is butan one amine, greater than N ethyl ethanamine, greater than N N dimethyl ethanamine. Same mass, but different hydrogen bonding, and that decides it.

6:51Example 2: which amines can be resolved into enantiomers

Now a harder one. Which of these can be resolved into enantiomers? First, C H three N H, then C two H five in brackets. Second, in square brackets, one nitrogen carrying methyl, ethyl, propyl and phenyl, with a plus charge written high, and a chloride ion, C l minus. Third, butan two amine. Pause the video and try it first. Done with it? Okay, let's check. In the first, nitrogen carries a methyl, an ethyl, a hydrogen and the lone pair, four different things. Many students say resolvable here. Oh no, careful, that is the trap! It is chiral in principle, but the nitrogen pyramid inverts rapidly, so it is not resolvable. Clear? In the quaternary salt, nitrogen holds four different groups, and it cannot invert. So this one is resolvable. With me? Now butan two amine. Look at carbon two, marked with a star: it carries a hydrogen, a methyl, an N H two group and an ethyl group. So it is a chiral carbon, not an inverting nitrogen, and this one is resolvable too. Got it? So the answer is the quaternary salt and butan two amine. And remember, a chiral nitrogen with a lone pair does not count, because it inverts. Okay?

8:11Quick revision: inversion, boiling point order, solubility

So, quick revision. Nitrogen is s p three and pyramidal, and rapid inversion means an amine with three different groups cannot be resolved. For boiling point, primary, greater than secondary, greater than tertiary, and acid, greater than alcohol, greater than amine, greater than alkane. In water, the lone pair accepts hydrogen bonds, and a bigger hydrocarbon part means less soluble. Now go and try the practice problems on this page. All the best!