Past JEE Questions: Structure and Properties

Transcript

0:00Introduction

Hello everyone! Let us 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. Propan one amine boils at three hundred and twenty one kelvin. Trimethylamine boils at only two hundred and seventy six kelvin. Same formula, same mass, and still forty five kelvin apart! So why is that? Hold that thought, we will come back to it soon. Today we solve two JEE Main questions, from twenty twenty one and twenty twenty three, on the structure and properties of amines. Questions like these do appear in JEE, and each needs only a few ideas from this chapter. So, let's begin.

0:46Idea 1: count the groups on nitrogen

So, idea number one. Amines come from ammonia, with alkyl or aryl groups in place of its hydrogens, and R on screen stands for such a group. One group on nitrogen gives a primary amine, RNH two, where the small two written low means two hydrogens. The little circle written high after the one is read as degree, so this is a one degree, or primary, amine. And N dash H is the bond between nitrogen and hydrogen, so a primary amine has two N H bonds. Okay? Now put two groups on nitrogen. How many N H bonds are left? Yes, exactly, only one is left. That is a secondary amine, R two N H, a two with the small circle. Right? And with three groups, R three N, we get a tertiary amine, with no N H bond at all. Remember these counts: two, one and zero. Everything today depends on them. Clear?

1:38Idea 2: hydrogen bonds and boiling points

Now idea number two. For a hydrogen bond between two amine molecules, we need a hydrogen on the nitrogen of one, and the nitrogen of the other. So which classes of amine can do this? Very good, it is the primary and the secondary amines. They have N H bonds, so they form the hydrogen bond you see on screen: N dash H, three dots, N, where the three dots are the hydrogen bond. Tertiary amines have no N H bond, so they cannot do this with each other. Okay? And more N H bonds per molecule means more hydrogen bonding, so more association between the molecules. Primary amines, with two N H bonds, associate the most. Got it? So for isomeric amines, the boiling point falls from primary, to secondary, to tertiary; the pointed sign means greater than. Right? Now our puzzle is solved. Propan one amine is primary: two N H bonds, strong association, three hundred and twenty one kelvin. Trimethylamine is tertiary: no N H bond, so no hydrogen bonds between its molecules, and it boils at just two hundred and seventy six kelvin. Same formula, but very different association. With me?

2:51Idea 3: basic strength of alkylamines

Now idea number three, basic strength. Alkyl groups release electrons towards nitrogen, and make its lone pair more available. So every alkylamine is a stronger base than ammonia, N H three. Okay? So here is a question. Tertiary amines have three alkyl groups, all pushing electrons. So the tertiary amine is always the strongest base, right? Careful now, that is true only in the gas phase, where the inductive effect acts alone. There the order is tertiary, then secondary, then primary, then ammonia. But in water, the story changes. With me? In water, two more factors act. The ammonium ion formed is stabilised by hydrogen bonds with water, and bulky groups get in the way of this. So for the methyl amines in water, the order is dimethylamine, then methylamine, then trimethylamine, then ammonia. The brackets with a small two mean two methyl groups on nitrogen; with a small three, three methyl groups. Right? And for the ethyl amines, C two H five being the ethyl group, the order is diethylamine, then triethylamine, then ethylamine, then ammonia. With me? Look, the secondary amine comes first in both series. So in water, primary amines are less basic than secondary amines. Clear?

4:10Idea 4: two reactions of primary amines

Now idea number four, two reactions. First, the Gabriel phthalimide synthesis. It gives pure primary amines, free of secondary and tertiary amines. But it does not work for aromatic amines, because aryl halides do not react this way. So it gives only primary aliphatic amines. Right? Second, a primary aliphatic amine with nitrous acid, HNO two. The nitrous acid comes from sodium nitrite and hydrochloric acid, shown in the brackets. So RNH two plus nitrous acid gives ROH, the alcohol, with nitrogen gas, N two, and water; the arrow means gives. The alcohol is the main product. Got it?

4:52Example 1: why stored aniline turns coloured (2023)

Time for Example one, from JEE Main twenty twenty three. The Roman numerals on screen number the two statements. Statement one says pure aniline and other arylamines are usually colourless. Statement two says arylamines get coloured on storage because of atmospheric reduction. Comment on the two statements. Pause the video and try it first. Done? Okay, let's check, starting with statement one. Pure aniline, C six H five N H two, is a colourless liquid. So statement one is correct. Okay? Now statement two. It is true that a stored sample gets coloured. But is the reason reduction? Tell me, what does the air do to aniline: does it oxidise it, or reduce it? Yes, exactly, the air oxidises it. On storage, aniline darkens, because it is slowly oxidised by air to coloured products. Right? So the colour comes from oxidation, not reduction. The colour change in statement two is real, but the reason given is wrong, so statement two is incorrect. With me? So the answer is that statement one is correct, and statement two is incorrect. Watch the small words in these questions: the single word reduction made the whole statement wrong.

6:10Example 2: wrong statement on primary amines (2021)

Clear? Now the harder one, Example two, from JEE Main twenty twenty one. Which statement is not correct for primary aliphatic amines? Statement one says the intermolecular association in primary amines is less than that in secondary amines. Statement two says primary amines with nitrous acid form the corresponding alcohols, except methylamine. Statement three says primary amines are less basic than secondary amines. And statement four says primary amines can be prepared by the Gabriel phthalimide synthesis. Pause the video and try it first. Done? Okay, let's check, starting with statement one. Secondary amines have two alkyl groups, so they are bigger molecules. Bigger molecules, more association, so statement one looks correct, right? Careful now, association comes from N H bonds, not from size. Each primary amine molecule has two N H bonds, and a secondary amine has only one. So primary amines associate more than secondary amines. Statement one has the order the wrong way round, so statement one is incorrect. Okay? Statement two is the nitrous acid reaction from idea four. RNH two with nitrous acid gives ROH, the corresponding alcohol, as the main product. The words except methylamine only set aside methylamine as a special case; they do not make the statement wrong. So statement two is correct. Right? Statement three comes from idea three. In water, secondary amines are stronger bases than primary amines, and in the gas phase too. So statement three is correct. Right? And statement four is from idea four: the Gabriel phthalimide synthesis gives pure primary amines. So statement four is also correct. Clear? So the only statement that is not correct is statement one, and the answer is option one. And notice the method: when a question asks which is not correct, check every statement before you mark the answer. Got it?

8:12Quick revision: boiling point, basicity, aniline colour

So, quick revision. More N H bonds means more association, so for isomeric amines the boiling point goes primary, then secondary, then tertiary. In water, the secondary amine is the strongest base. And aniline is colourless when pure, and gets coloured on storage by oxidation in air. Now go and try the practice problems of this chapter, and use these ideas there, you are well prepared for them now.