Compounds with Several Functional Groups

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Citric acid is the acid that makes lemons sour. Its molecule has six carbon atoms and three acid groups. And its IUPAC name is two hydroxy propane one, two, three tricarboxylic acid. But propane is a parent with only three carbons! So where did the other three carbons go? Something is strange here, right? By the end of this video, you will know exactly where they went. And naming compounds with several groups comes up often in JEE, so let's begin.

0:36One principal group gives the suffix

So, a compound can carry two or three different functional groups. But its name can end in only one suffix, right? So the group highest in the priority order wins. We call it the principal group, and it gives the suffix, the ending of the name. Okay? Every other group goes in front of the name as a prefix, and the prefixes are written in alphabetical order. With me? Now the order itself. The greater than sign here means ranks above. At the top is acid, then sulphonic acid, anhydride, ester, acid halide and amide. Clear? Then nitrile, aldehyde, ketone, alcohol and amine. And right at the bottom, the double bond, and then the triple bond. Right? Let's use it. C H three, then C H with O H in round brackets, then C H two, and CHO at the end. The small numbers written low are subscripts that count the hydrogens, the short dashes are single bonds, and the group in brackets hangs from that carbon. So this is an alcohol and an aldehyde together. Which one gives the suffix? The aldehyde, yes, exactly! So the CHO carbon is carbon one, written C dash one on screen, and the O H sits on carbon three. Four carbons, so butanal, with hydroxy in front, and the hyphen joins the number to the name. So it is three hydroxy butanal. Got it?

2:03Parent chain and numbering

Now, which chain is the parent? It must contain the principal group, and as many double and triple bonds as possible. Okay? And the numbering has its own order. The principal group gets the lowest number first, then the multiple bonds, and only then the other substituents. Clear? Let's test it. C H two double bond C H, where the two short lines make a double bond, then C H two, then C H with an O H, and C H three. From the left, the double bond would get number one. From the right, the O H gets number two. So, which end do we number from? From the right, that is correct! The O H is principal, so it gets the lower number, two. Then the double bond starts at carbon four. So the name is pent four en two ol, with en for the double bond and ol for the alcohol. Pent one en four ol is the trap, so don't fall for it. With me?

3:01Oxo or formyl? A common trap

Now, a common trap. When the C double bond O group, the carbonyl, is not the principal group, it is always the prefix oxo. That holds for a ketone, and also for an aldehyde whose carbon is counted in the chain. Right? Look at this one. OHC, which is just an aldehyde group written backwards, then C H two, C H two, and COOH, the carboxylic acid. Acid outranks aldehyde, so the acid is principal. Many students write formyl here. Formyl or oxo, what do you say? Yes, it is oxo, exactly! The acid carbon is carbon one, and the CHO carbon is carbon four, inside the chain. Its carbon is counted, so the name is four oxo butanoic acid. Okay? So when do we use formyl? Only when the CHO carbon is not counted in the chain. Remember this, and the trap is gone. Got it?

4:00When the group's carbon stays outside the chain

Now, the rule behind our lemon puzzle. Sometimes the carbon of the group cannot be part of the chain. When does that happen? When the group sits on a ring, or when one chain carries three or more such groups. Okay? Then we use these special suffixes, and the dash in front shows they are endings: carboxylic acid, carbaldehyde, carbonitrile and carboxamide. Clear? The key point is this: the carbon of the group is then not counted in the root at all. Right? So, a cyclohexane ring of six carbons, carrying a CHO group. The dash before CHO is its bond to the ring. Now tell me, can the CHO carbon be one of the ring carbons? No, it cannot, because it sits outside the ring. So the ring stays cyclohexane, and we add carbaldehyde. The name is cyclohexane carbaldehyde. With me?

4:54Example 1: naming CH3COCH2COOH, 3-oxo not 2-oxo

Alright, now let's do Example one. Name this compound. C H three, then C O, a ketone carbonyl in the middle of the chain, then C H two, and COOH at the end. Pause the video and try it first. Done? Okay, let's check. Some students number from the left, so the C double bond O lands on carbon two. So the answer starts with two oxo, right? No, no, wait, that is the trap! The acid outranks the ketone, so the acid is the principal group, and its own carbon is carbon one. The principal group always gets the lowest possible number. Okay? Count along: four carbons, so the parent is butanoic acid. And the ketone carbon is now carbon three. It is not principal, so it becomes the prefix oxo. Clear? So the answer is three oxo butanoic acid. One suffix, the acid, and the ketone in front as oxo. Got it?

5:55Example 2: propane-1,2,3-tricarboxylic acid and citric acid

Now, remember our lemon puzzle? Let's finish it. Name two compounds. The first is HOOC, then C H two, then C H carrying a COOH in brackets, then C H two, and COOH at the end. The second is citric acid, the same skeleton, but the middle carbon also carries an O H group. Pause the video and try them first. Done? Okay, let's check the first one. The longest chain has five carbons, and it holds two of the acid groups. So we take that chain, and write the third acid group as a prefix, right? Oh no, careful, that is the trap! All three acid groups are principal groups, so all three belong in the suffix. And the three cannot all be part of one chain. Okay? So none of the acid carbons is counted. What is left? Only the three middle carbons: C H two, C H, and C H two. So the parent is propane. Clear? Each COOH becomes a carboxylic acid suffix, on carbons one, two and three, and the commas separate these locants. So the name is propane one, two, three tricarboxylic acid, with tri for three. With me? Now citric acid. The O H is on the middle carbon, carbon two, and the acid is principal, so the O H is the prefix hydroxy. So the name is two hydroxy propane one, two, three tricarboxylic acid. And that solves our puzzle! Six carbons in the molecule, but three of them sit inside the acid groups, so the parent is just propane. Got it?

7:39Quick revision: priority order, numbering, oxo, carbaldehyde

So, quick revision. The highest group in the order gives the suffix, and all the others are prefixes, in alphabetical order. Number from the principal group first, and a carbonyl that is not principal is always oxo. And for a group on a ring, or three or more on a chain, use carboxylic acid or carbaldehyde, and do not count that carbon. Now go and try the practice problems on this page. All the best!