Overview

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Tollens' reagent gives a shining silver mirror with aldehydes. So if a compound gives the silver mirror, it must be an aldehyde, right? Well, not always. One carboxylic acid gives the silver mirror too! And here is a second puzzle. Two different esters can have exactly the same molar mass, one hundred and two. So how does a question tell us which ester it means? Today, from this unit problem bank, we will crack both puzzles with two solved questions. This is a common JEE question pattern, so let's begin.

0:39How to use this chapter

So, first, how to use this chapter. This unit problem bank on organic compounds containing oxygen has three parts. Part A is at the JEE Main level, so start there. Okay? Part B is at the JEE Advanced level, so expect harder questions. Right? And Part C is the concept checks: it tests the ideas behind the reactions, not only what happens, but why. Clear? Now, the most important line. Attempt each part as a test, and read the solutions only after that. Today our examples come from Part A and Part B, with one question from Part C as well.

1:19Esters: from an acid and an alcohol

Got it? Now, the idea behind both our questions: esters. Here, a carboxylic acid, RCOOH, and an alcohol, R prime O H, give an ester, RCOOR prime, and water. R prime, with the small dash written high, is a second alkyl group, and the small numbers written low count the atoms, so H two O is water. The two half arrows mean the reaction is reversible, and it needs concentrated sulphuric acid, H two S O four, or dry H C l, okay? Now look closely. The acid and the alcohol join, and one water molecule leaves. So tell me, how is the molar mass of the ester related to the other two? Yes, exactly! Add the molar masses of the acid and the alcohol, then subtract eighteen, for the water that left. Capital M is the molar mass, and the small word written low says whose molar mass it is. With me? Now the reverse. Treat the ester with sodium hydroxide, N a O H, and the arrow, which means gives, leads to the sodium salt, RCOON a, and the alcohol back. This is saponification. Right? Look at the equation again: one mole of sodium hydroxide for each mole of ester. And it is irreversible, because the acid leaves as its salt. So the alkali used tells you exactly how many moles of ester there were.

2:43From Part C: why esters boil lower

Clear? Now a question straight from Part C, the concept checks. Take an ester and a carboxylic acid of the same molar mass. The ester boils lower. Why is that? Yes, it is all about hydrogen bonding! Look at the acid first. Its O H bond, and the dash here is the bond, lets the molecules form strong hydrogen bonds. Acid molecules even pair up as cyclic dimers. Okay? Now the ester. It has no O H bond at all, so its molecules cannot hydrogen bond with one another. Less attraction, so a lower boiling point. Simple, right?

3:20The acid that gives a silver mirror

Now back to our first puzzle. Tollens' reagent is reduced by aldehydes, all of them, and it gives the silver mirror. Okay? So, which carboxylic acid could behave like an aldehyde? Think about its structure. Methanoic acid, HCOOH, yes, exactly! It has no alkyl group at all. Its carbonyl carbon carries a hydrogen, so it contains an aldehyde unit: hydrogen, single bond to carbon, double bond to oxygen. The single dash is a single bond, and the double dash is a double bond. With me? So methanoic acid reduces Tollens' reagent and gives the silver mirror, although it is an acid. That is our first puzzle, solved! Keep this in mind, because we need it in Example two. Got it?

4:06Example 1: alcohol of a propanoate ester with molar mass 102

Now Example one, a JEE Main level question from Part A, okay? An ester of propanoic acid has a molar mass of one hundred and two. Identify the alcohol from which it is made. Pause the video and try it first. Done? Okay, let's check. Propanoic acid is C H three C H two COOH: three carbons, six hydrogens and two oxygens. Thirty six plus six plus thirty two gives seventy four. Okay? So the alcohol is just one hundred and two minus seventy four, which gives twenty eight, right? No, no, wait, that is the trap here, because a water molecule left when the ester formed! So seventy four plus M, minus eighteen, equals one hundred and two. That gives M equals forty six for the alcohol. With me? Now, which alcohol has a molar mass of forty six? C two H five O H: twenty four plus six plus sixteen gives forty six. So the alcohol is ethanol, and the ester is ethyl propanoate. Clear?

5:10Example 2: ester from NaOH used, Tollens test and chirality

Now Example two, from Part B, and it brings back both our puzzles. Two point zero four grams of an ester needs twenty millilitres of one molar sodium hydroxide for saponification. On screen, g is grams, m L is millilitres, and capital M is molar, which means moles per litre. The acid obtained reduces Tollens' reagent, and the alcohol is optically active. Identify the ester. Pause the video and try it first. Done? Okay, let's check. Saponification uses one mole of sodium hydroxide for each mole of ester, remember? Twenty millilitres of a one molar solution is zero point zero two moles, so we have zero point zero two moles of ester. On screen, m o l is short for mole. Okay? So the molar mass is two point zero four over zero point zero two, and the fraction bar means divided by. That gives one hundred and two. This fits C five H ten O two: five carbons give sixty, ten hydrogens give ten, and two oxygens give thirty two. Look, the same molar mass as in Example one! Right? Now the first clue. The acid from this ester reduces Tollens' reagent. Which acid can do that? Methanoic acid, HCOOH, exactly! That is our silver mirror puzzle again. Clear? Now count the carbons. The ester has five, and methanoic acid brings only one, so the alcohol has four: C four H nine O H, a butanol. With me? So it is simply butan one ol, the straight chain alcohol, right? No, no, careful, because the question says the alcohol is optically active! So it needs a chiral carbon, which is a carbon with four different groups. Butan one ol has no such carbon, and of the four butanols, only butan two ol has one. In butan two ol, carbon two, marked with the little star, carries a hydrogen, the O H group in brackets, a methyl and an ethyl. Four different groups, so it is chiral. Got it? So the ester comes from methanoic acid and butan two ol: butan two yl methanoate, HCOO, then C H with a methyl group in brackets, then C H two C H three. Two esters, both with molar mass one hundred and two, told apart by the clues. That is our second puzzle, solved!

7:35Quick revision: ester molar mass, HCOOH, chiral alcohols

So, quick revision. An ester's molar mass is the acid plus the alcohol, minus eighteen, and saponification uses one mole of sodium hydroxide for each mole of ester. Methanoic acid gives Tollens' test, although it is an acid. And for an optically active alcohol, find a carbon with four different groups. Now open Part A, and take it as a test before you read the solutions. All the best!