IIT-JEE Questions (1986–1994)

Transcript

0:00Introduction

Hello everyone! Let us start with a small trap. You want butanenitrile, and butan means four carbons. So you heat butyl chloride, also four carbons, with potassium cyanide, right? The examiners put that option there for a reason, and we will catch it in Example one. Today we solve some IITJEE questions from nineteen eighty six to ninety four on nitro compounds, cyanides and amides. So let's begin.

0:29Nitriles: in from halides and amides, out to acids

So, first the nitriles, mapped in the course's figure on the right. R X is an alkyl halide, where X stands for a halogen atom. With potassium cyanide, KCN, the arrow gives RCN, a nitrile; the dash is a single bond and the three lines are a triple bond. The lower branch, with silver cyanide, gives the isocyanide instead. Okay? Now the naming rule. In the name of a nitrile, we count the carbon of the C N group too. So the cyanide adds one carbon to the chain. Keep this in mind for Example one. Right? Nitriles also come from amides. RCONH two is an amide, and the small two written low counts the hydrogens. Heat it with phosphorus pentoxide, P two O five, written over the arrow, and the amide loses water to give the nitrile. Clear? Now IITJEE nineteen ninety two. Benzamide is heated with P two O five, and the product is then heated with H plus and water; the raised plus is a charge, and H plus means acid. What are the two question marks? Yes, exactly, the first product is benzonitrile. Then acid hydrolysis, the top branch of the figure with H three O plus, turns a nitrile into the acid with the same carbons. So the second product is benzoic acid. Got it?

1:56Silver nitrite: nitro or nitrite?

Now the nitrite ion, which is ambident: it can attack through the nitrogen or through an oxygen. With silver nitrite, A g N O two, an alkyl halide gives mainly the nitroalkane, RNO two, as the brackets say. Okay? So, IITJEE nineteen ninety one. Bromoethane is treated with alcoholic silver nitrite. What are the products? Yes, the main product is nitroethane, with some ethyl nitrite as well. The word major tells you the nitro compound wins, but the nitrite also forms. With me?

2:31On the benzene ring, the order decides

Now the benzene ring, and IITJEE nineteen ninety two. Nitrobenzene, C six H five N O two, with bromine and ferric bromide, F e B r three, the slash meaning with, gives mainly meta bromonitrobenzene, where the small m means meta. The question gives four statements and asks which ones explain this. So, why does the nitro group send the bromine to meta? Of the four statements, the correct ones are a and b. The meta carbon has more electron density than the ortho and para carbons, and the cation formed by attack at meta is the least destabilised. Clear? Next, IITJEE nineteen ninety three. Make four bromonitrobenzene from benzene in two steps, with the bromine and the nitro group para to each other. Which group should go on first? Yes, exactly, the bromine goes on first. Bromine directs ortho and para, so nitric and sulphuric acids then put the nitro group ortho or para, and we separate the para isomer. Nitro first would send the bromine to meta, as we just saw. Got it? Same idea in IITJEE nineteen eighty seven: toluene into meta nitrobenzoic acid. We must oxidise the methyl group and add a nitro group. Which comes first? Oxidation comes first, because the methyl group directs ortho and para. Hot alkaline potassium permanganate, K M n O four, then acid, gives benzoic acid, and now the meta-directing carboxyl group controls the nitration with nitric and sulphuric acids. So the rule is to put on first the group that directs where you want. With me?

4:19Example 1: butanenitrile from propyl chloride and KCN

Now, remember our trap from the start? Here it is, Example one, from IITJEE nineteen ninety two. By heating which pair can butanenitrile be prepared? Propyl alcohol, butyl alcohol, butyl chloride or propyl chloride, each with KCN? Pause the video and try it first. Done? Okay, let's check. Butanenitrile has four carbons, and butyl chloride has four carbons, so the answer is butyl chloride, right? No, it is not, and that is exactly the trap here. The name counts the C N carbon too, so one of the four carbons comes from the cyanide. Okay? So the alkyl group must bring only three carbons, and that is propyl. Butyl chloride would give a nitrile with five carbons. Right? And the two alcohols? Our reaction needs the alkyl halide with potassium cyanide, so the alcohol options go out. Clear? So the answer is propyl chloride with KCN, and our trap from the start is solved. Got it?

5:24Example 2: propanamide from percentages and silver salt

Now a harder one, Example two, from IITJEE nineteen eighty eight. Compound A contains forty nine point three two percent carbon, nine point five nine percent hydrogen, and nineteen point one eight percent nitrogen, the rest being oxygen. Boiled with sodium hydroxide, it gives ammonia and a salt, and acidifying the salt gives B, a monobasic acid with no nitrogen. The silver salt of B contains fifty nine point six seven percent silver. Can you identify A and B? Pause the video and try it first. Done? Okay, let's check. Oxygen is not given, so find it first. One hundred minus the sum of the other three gives twenty one point nine one percent oxygen. Right? Now divide each percentage by its atomic mass, so carbon by twelve, hydrogen by one, nitrogen by fourteen and oxygen by sixteen. We get four point one one, nine point five nine, one point three seven and one point three seven. Divide by the smallest, and the ratio is three to seven to one to one. Clear? So the simplest formula of compound A is C three H seven N O, which is not yet the full molecular formula. And it gives ammonia, N H three, when boiled with sodium hydroxide, which is the course's test for an amide. With me? Now the acid. Monobasic means one COOH group, so its silver salt has one silver atom, of mass one hundred and eight, and that is fifty nine point six seven percent of the salt. So M, the molar mass of the salt, is one hundred and eight over zero point five nine six seven, which gives one hundred and eighty one. Okay? So the acid itself has a molar mass of one hundred and eighty one, right? No, careful, that is the salt, not the acid. In the salt, silver took the place of one hydrogen. So one hundred and eighty one minus one hundred and eight, plus one, gives seventy four. An acid with molar mass seventy four is C two H five COOH, propanoic acid. Clear? And compound A gave this acid and ammonia, so it is the amide of propanoic acid, propanamide, C two H five CONH two. Its formula, C three H seven N O, matches the simplest formula we found, so both clues agree. Got it?

7:56Quick revision: nitrile naming, amide to nitrile, ring order

So, quick revision. Potassium cyanide adds a carbon, and the nitrile's name counts it. Amides lose water to give nitriles, and acid hydrolysis turns a nitrile into the acid. And on a benzene ring, first put on the group that directs where you want. Now try the other past questions on this page, including the ones we skipped. All the best!