Compounds with One Functional Group

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Take an ester written as C two H five, COO, C H three. Reading from the left, you first meet C two H five, the ethyl group. So its name should begin with ethyl, right? Well, not so fast, because esters follow their own rule. We will name it properly in Example two. Naming questions come up often in JEE, so let's begin.

0:28One group, one suffix: the priority table

This table lists the functional groups in decreasing order of priority. That order matters when a molecule has two groups, our next lesson. Today each compound has one group, and it gives the ending, the suffix. At the top, the carboxylic acid, COOH, ending in oic acid. The dash before a group is its bond to the chain; the dash before an ending joins it to the root. Then the sulphonic acid, S O three H, ending in sulphonic acid, where the small three, written low, is a subscript. Okay? Next, the acid anhydride, C O, O, C O, ending in oic anhydride. Then the ester, COOR, where R stands for an alkyl group. Its name has two words, alkyl alkanoate; that is our puzzle. Right? Then the acid halide, COX, where X stands for a halogen, like chlorine, ending in oyl halide. And the amide, CONH two, ending in amide. Got it? Now the nitrile: the three lines between C and N are a triple bond, and it ends in nitrile. The aldehyde, CHO, ends in al. In the ketone, the open angle shows two more bonds of the carbon, and the two lines are a double bond to oxygen. It ends in one, spelt o, n, e. Clear? Lower down, the alcohol, O H, ends in ol, and the amine, N H two, in amine. Now tell me, where are the halogens, or the nitro group, in this table? They are not in the table, and that is the point! Halogens on their own, written X, nitro, N O two, alkoxy, O R, and alkyl groups have no ending. They are always prefixes. With me?

2:20The parent chain and its numbering

Now, how do we build the name? First, the parent chain: the longest chain containing the carbon that carries the functional group. Okay? Second, that carbon gets the lowest number. Take this alcohol: C H three, C H with O H in brackets, C H two, C H three. The brackets mean the O H hangs on that carbon. From which end do we number? From the left, yes! The O H carbon is then number two: butan two ol, not butan three ol. The double arrow means so, and the number between hyphens is the locant. Right? Now a special rule. In these six groups, the group itself has a carbon, always carbon number one; that is what C dash one on screen means. So, is that carbon counted in the chain, or not? Yes, it is counted! Many students miss this and lose a carbon. And since it is always number one, we do not write its locant. Got it?

3:23Keep or drop the final e?

Now a spelling rule students often get wrong. Alkane names end in e, like ethane. Add ol, which starts with a vowel, and the e is dropped, giving ethanol. The single arrow means becomes. Okay? Now ethane with the ending nitrile. Does the e stay, or does it go? It stays! Nitrile starts with n, a consonant, so the e is kept: ethanenitrile. Same with diol: ethane one two diol, where the locants one and two, with a comma, show the two O H positions. Clear?

3:59Esters, ethers and amines

Three more rules. First, esters, our puzzle family. An ester is named in two words: first the alkyl group from the alcohol, then the acid part, as alkanoate. Okay? Next, an ether, like C H three, O, C H two C H three. The smaller group, together with the oxygen, becomes the alkoxy prefix. So which group is that here? The methyl, yes! C H three with the oxygen is methoxy, and the larger ethyl part gives the parent, ethane. So the name is methoxyethane. Right? Last rule. In an amine or amide, a group on the nitrogen gets the locant N, the capital N in italics, instead of a number. With me? For example, C H three C H two, N H, C H three. The larger group, ethyl, is the parent, so it is an ethanamine, and the methyl on the nitrogen takes the locant N, giving N methyl ethanamine. Clear?

5:01Example 1: 2-methylbutanal and pentan-2-one

Now, Example one. Name these two compounds. The first is C H three, C H two, then C H with C H three in brackets, then CHO at the end. The second is C H three, C O, C H two, C H two, C H three. Pause the video and try it first. Done? Okay, let's check the first one. Count the carbons in the parent chain. Is it three, or four? Four, very good! The CHO carbon is counted, as carbon one. So the chain is a butanal, with the methyl in brackets on carbon two. If you said three, you left out the CHO carbon; that is the trap. Okay? So the name is two methyl butanal. There is no locant for the al ending, because the CHO carbon is always number one. Right? Now the second one, a ketone with five carbons. From the left, the C double bond O carbon is number two; from the right, number four. Lowest number wins, so we number from the left. Got it? So the answer is pentan two one, where the last word is the ketone ending, spelt o, n, e. And the e of pentane is dropped, because that ending starts with a vowel. Clear?

6:24Example 2: naming three esters, the methyl propanoate trap

Now, Example two: three esters. The first is HCOOCH three. The second is C H three COO, then C H with C H three in brackets; the small two means two of them. And the third is our puzzle, C two H five, COO, C H three. Pause the video and try them first. Done? Okay, let's check. Find the acid part, ending in COO, and the alkyl group on the oxygen, from the alcohol. In the first, HCOO has one carbon, so methanoate, and the C H three on the oxygen is methyl. So, methyl methanoate. Okay? In the second, C H three COO has two carbons, so ethanoate. The alkyl group is C H with two methyls. Its carbon one, joined to the oxygen, carries a methyl, so it is one methyl ethyl, also called isopropyl. With me? So the name is one methyl ethyl ethanoate, or isopropyl ethanoate. Both names are fine. Clear? Now the third, our puzzle. C two H five sits on the left, and C H three plus the COO carbon looks like two carbons. So is it ethyl ethanoate? No, no, careful, this is exactly the trap! The C two H five is joined to the COO carbon, so it belongs to the acid part: two carbons plus that carbon make three, so propanoate. The C H three sits on the oxygen, so it is the methyl, from the alcohol. Got it? So, alkyl group of the alcohol first: methyl propanoate. Puzzle solved: it begins with methyl, though ethyl is written first. Always split the ester before naming it. Right?

8:20Quick revision: suffix vs prefix, C-1 rule, esters, ethers

So, quick revision. One group gives the suffix; halo, nitro and alkoxy are always prefixes. The chain runs through the group's carbon, numbered lowest, and an acid or aldehyde carbon is carbon one. Esters: the alcohol's alkyl first, then alkanoate; in an ether, the smaller side is alkoxy. Now go and try the practice problems on this page. All the best!