Naming, Structure and Physical Properties

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Ethanoic acid, also called acetic acid, has a molar mass of sixty grams per mole. But measure it from the density of its vapour, just above the boiling point, and you get about one hundred. So did the molecule suddenly become heavier? That sounds strange, right? Carboxylic acids come up often in JEE, so let's begin.

0:27Naming acids: the carboxyl carbon is C-1

So, let us start with the names. Every carboxylic acid carries the carboxyl group, dash COOH; the short dash is the bond to the rest of the molecule. Its IUPAC name ends in oic acid, and we number the chain from the carboxyl carbon, which is always C one. Okay? You must also know the common names. Methanoic acid is formic acid, ethanoic is acetic, propanoic is propionic, and butanoic is butyric. Got it? Acids with two carboxyl groups end in dioic acid. Ethanedioic acid is oxalic, propanedioic is malonic, and butanedioic is succinic. Right? Then pentanedioic is glutaric, and hexanedioic is adipic. And benzene one two dicarboxylic acid is phthalic acid; the one comma two means the carboxyl groups sit on ring carbons one and two. With me?

1:20Naming acid chlorides, anhydrides, esters and amides

Now the derivatives, written in the heading as RCO dash L, where R is the rest of the molecule and L takes the place of the O H group. Acid chlorides are named oyl chloride, so C H three COC l, where C l is chlorine, is ethanoyl chloride; the small three written low counts hydrogen atoms. Okay? Anhydrides are named oic anhydride. Here C H three C O sits in brackets with a small two outside, so two such groups share one oxygen atom. That is ethanoic anhydride. Right? Next, an ester. Look at C six H five, COO, C H three; C six H five is the phenyl group, a benzene ring. So is this phenyl ethanoate? No, no, careful, that name is wrong! Esters are named alkyl oate. The methyl group sits on the oxygen, so it is the alkyl part, and it comes first, as a separate word. The rest comes from benzoic acid, so the name is methyl benzoate. Clear? And amides end in amide. In HCON, with C H three in brackets and a small two, the two methyl groups sit on the nitrogen, so the name is N comma N dimethylmethanamide; each italic letter N marks a methyl group on nitrogen. Got it?

2:39Structure: a flat group, and resonance

Now let us look at the structure. The carboxyl carbon is s p two hybridised, where the small raised two is part of the name, so the whole group is planar. Okay? Now, the carboxyl group contains a carbonyl group, C double bond O, just like aldehydes and ketones. So does a carboxylic acid form an oxime, or a two four DNP derivative? It does not, and here is why. The lone pair on the oxygen of the O H group is shared with the carbonyl group by resonance. So the carbonyl carbon carries less positive charge, and it is less electrophilic than in aldehydes and ketones. With me? And so acids do not give the addition reactions of the carbonyl group: no oxime, and no two four DNP derivative.

3:23Hydrogen bonds: acids exist as pairs

Clear? Now the most important physical property. Carboxylic acids form strong hydrogen bonds, and two molecules join into a ring, a cyclic dimer, which simply means a pair. Each pair is held by two hydrogen bonds. Okay? These dimers exist in the liquid, in the vapour, and in aprotic solvents such as benzene. So the pairs survive even in the vapour, and that is the clue to our puzzle; we finish it in Example two. Right? Now, boiling points, for compounds of comparable molar mass. An acid and an alcohol both form hydrogen bonds. So which one boils higher? It is the acid, correct! Its hydrogen bonds are stronger, and it exists as dimers. So the order is acid, greater than alcohol, greater than aldehyde or ketone, greater than ether or alkane; the pointed sign means greater than. Got it?

4:19Solubility and melting points

Next, let us look at solubility in water. The first four acids are miscible with water. But as the alkyl group grows, the solubility falls. Okay? And benzoic acid is nearly insoluble in cold water. Right? Now here is a strange one, about melting points. Butanoic acid has four carbons, and pentanoic acid has five. Which one melts higher? It is tempting to say pentanoic, the bigger one. But no, it is butanoic acid! Melting points alternate: an acid with an even number of carbon atoms melts higher than the odd numbered acids on either side of it. Clear?

5:03Example 1: IUPAC names of three carboxylic acids

Time for Example one, an easy one. Give the IUPAC names of these three acids. In part a, C H three is in brackets with a small two, so two methyl groups sit on one carbon. In part b, the two short lines between the carbons mean a double bond. Pause the video and try it first. Done? Okay, let's check. In part a, the chain with the carboxyl carbon has four carbons, so it is a butanoic acid. The methyl group is on the second carbon from the left, so two methylbutanoic acid, right? No, no, careful, that is the trap! The carboxyl carbon is always C one, so we number from the right. Then the methyl group sits on C three, and the name is three methylbutanoic acid. Clear? In part b, three carbons give the stem prop, and the double bond, C double bond C, starts at carbon two, counted from the carboxyl carbon. So the name is prop two enoic acid. Got it? And part c has four carbons in all, counting both carboxyl carbons, with two COOH groups. So it is butanedioic acid, the succinic acid from our list. That's it!

6:19Example 2: acetic acid dimer fraction from vapour density

Now Example two, a harder one, and it finishes our puzzle. The vapour density of ethanoic acid, just above its boiling point, is about fifty. What fraction of the molecules, counted as monomers, that is single molecules, is present as dimers? Pause the video and try it first. Done? Okay, let's check. M, the molar mass, is two times the vapour density, so the observed M is two times fifty, which is one hundred. But C H three COOH has M equal to sixty. Okay? So take one mole of monomer, and let a fraction alpha form dimers; alpha is the Greek letter on screen. Now, how many moles of particles do we have in total? Two monomers make one dimer, so we get alpha over two moles of dimer, plus one minus alpha moles that stay single. So the total is one minus alpha over two. Fewer particles, the same mass, so the observed molar mass goes up. With me? One mole of monomer weighs sixty grams, and moles of particles equal mass over the observed molar mass. So sixty over one hundred equals one minus alpha over two, and the double arrow leads to alpha over two equal to zero point four. So the answer is zero point four, right? Oh no, careful, zero point four is only alpha over two! Double it, and alpha is zero point eight, so eight out of every ten molecules are in dimers. And that solves our puzzle: the molecules never got heavier, they were travelling in pairs. That's it!

7:57Quick revision: naming, resonance and boiling point order

So, quick revision. Number the chain from the carboxyl carbon, which is C one, and the name ends in oic acid. Resonance means no oxime and no two four DNP derivative. And acids form cyclic dimers, so for comparable molar mass, acids boil higher than alcohols, then aldehydes and ketones, then ethers and alkanes. Now go and try the four practice problems on this page. All the best!