Benzene Derivatives

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. In a carbon chain, we always number from one end, right? Now think of benzene. Six carbon atoms in a ring, and all six exactly alike. There is no end at all! So which carbon is carbon number one? By the end of this video, you will pick carbon one correctly, every time. And naming benzene compounds comes up often in JEE, so let's begin.

0:27One group: just a prefix on benzene

So, the simplest case first. With just one group on the ring, we write that group as a prefix, in front of the word benzene. Chlorine on the ring gives chlorobenzene, and a nitro group gives nitrobenzene. Now tell me, does chlorobenzene need a number, like one chloro benzene? No, it does not, that is right! All six ring carbons are alike, so with one group, no number is needed. Okay? Now, some names are so common that IUPAC keeps them. Toluene, phenol, aniline and anisole. Right? And four more: benzaldehyde, benzoic acid, acetophenone and benzonitrile. Learn these eight by heart, because each can also be a parent name. Clear?

1:14Two groups: ortho, meta and para

Now, two groups on the ring. Here the positions get special names. One comma two, with a hyphen after it, is ortho, written as a small italic o, and the two groups sit on neighbouring carbons. Got it? Next, one comma three is meta, written as a small italic m, with one carbon in between the two groups. With me? Now you tell me. If the two groups sit on opposite carbons of the ring, which positions are those, and what do we call them? Yes, exactly, it is one comma four, and we call it para, written as a small italic p. Ortho, meta, para: one two, one three, one four. Okay?

1:56Principal group: its ring carbon is C-1

So, here is the first answer to our puzzle. If the ring carries a principal group, or the compound has one of those retained names, then the ring carbon carrying that group is carbon one. C dash one, on screen, means carbon number one. Careful here: the carbon inside a CHO or COOH group is not numbered at all, only the ring carbons are. Clear? Then we number around the ring so that the other groups get the lowest locants, and we list them in alphabetical order, just like in chains. Right? Let's try salicylic acid, which is ortho hydroxy benzoic acid. Benzoic acid is a retained name, so the ring carbon with the acid group is carbon one. Ortho means next door, so the hydroxy group is on carbon two. Two hydroxy benzoic acid. Simple, isn't it? Now vanillin. It is a benzaldehyde, with an OCH three group meta, and an O H group para. The small numbers written low are subscripts that count atoms, and the dash in front of each group is its bond to the ring. Benzaldehyde is the parent, so the ring carbon carrying the aldehyde group is carbon one. Meta could be carbon three or carbon five. Which one do we take? It is carbon three, yes, exactly! We go around the ring in the direction that gives the lower locants. So the ring carbon with the CHO is carbon one, the OCH three is on carbon three, and the O H, para to carbon one, is on carbon four. With me? Now the name. Hydroxy starts with the letter h, and methoxy with the letter m, so hydroxy comes first, even though its number is bigger. Four hydroxy three methoxy benzaldehyde. Three methoxy four hydroxy is a common mistake, so be careful. Got it?

3:43No principal group: the lowest set of locants

Now, what if there is no principal group at all? Then no carbon is fixed as carbon one in advance. We simply number the ring to give the lowest set of locants. Okay? And how do we compare two sets? Same as in chains: term by term, and the first point of difference decides, not the sum. Clear? Only when two numberings give exactly the same set does the alphabet come in, and then the group that comes first in the alphabet gets the lower number. Remember that word, only. Using the alphabet first is the trap in Example two. Right?

4:16Groups made from benzene

Sometimes the ring itself is the substituent. C six H five, with a free bond shown by the dash, is phenyl. Add a C H two, and C six H five C H two is benzyl. Phenyl and benzyl are not the same, so do not mix them up. Okay? Next, C six H five C H, with the two short lines of a double bond, is benzal, also called benzylidene, the name in brackets. And C H three C six H four, a ring that carries a methyl group, is tolyl. Got it?

4:50Example 1: naming o-xylene, mesitylene, m-dinitrobenzene

Alright, now Example one. Name three compounds. Part A is ortho xylene. Part B is mesitylene, which has three methyl groups on alternate carbons. And part C is meta dinitro benzene. Pause the video and try it first. Done? Okay, let's check. Xylene is benzene with two methyl groups. Ortho means one comma two, so part A is one, two dimethyl benzene, with di for two. Okay? Now mesitylene, with three methyl groups on alternate carbons. You could number them one, three, five, or two, four, six. Which set do we take? One, three, five, of course, because it is the lowest set. So mesitylene is one, three, five trimethyl benzene, with tri for three. Clear? And part C is easy now. Meta means one comma three, and there are two nitro groups, so it is one, three dinitro benzene, and that is all there is to it. With me?

5:54Example 2: 4-ethyl-1-fluoro-2-nitrobenzene, alphabet trap

Now a harder one. This benzene ring carries three groups: a fluorine atom, written as capital F, then N O two, the nitro group, ortho to the fluorine, and C two H five, the ethyl group, para to the fluorine. The dash before each one is its bond to the ring. Name the compound. Pause the video and try it first. Done? Okay, let's check. Ethyl starts with the letter e, so it comes first in the alphabet. So the ethyl carbon should be carbon one, right? No, no, wait, that is the trap! There is no principal group here, so the lowest set of locants decides first. The alphabet only breaks a tie. Okay? So let us test each start. The curly brackets hold the set of locants. Ethyl at carbon one puts the fluorine opposite, at four, and the nitro next to it, at three. That gives one, three, four. Nitro at carbon one puts fluorine at two and ethyl at five, which gives one, two, five. Right? Now fluorine at carbon one. The nitro goes next door, at two, and the ethyl opposite, at four. That gives one, two, four. Compare term by term. Against one, three, four, our second term is smaller. Against one, two, five, our third term is smaller. So one, two, four is the lowest set. Got it? Now the order in the name: ethyl, then fluoro, then nitro, in alphabetical order. So ethyl is written first in the name, even though its locant is four. Clear? So the answer is four ethyl one fluoro two nitro benzene. And see, our puzzle is solved again. With no principal group, carbon one is simply where the lowest set of locants begins. With me?

7:46Quick revision: ortho meta para, C-1 rule, lowest locants

So, quick revision. One group is just a prefix on benzene. For two groups, ortho is one two, meta is one three, and para is one four. With a principal group or a retained name, the ring carbon carrying it is carbon one, and the other groups go in alphabetical order. With no principal group, take the lowest set of locants, and use the alphabet only to break a tie. Now go and try the practice problems on this page. All the best!