Nomenclature

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Allyl bromide is a three carbon chain, with a double bond at one end and a bromine atom at the other end. Bromine sits on an end carbon, so it should get the number one, right? But the correct IUPAC name begins with three bromo. So why does bromine get number three? Strange, isn't it? Naming questions come up often in JEE, and the options can differ by just one number. So let us learn where each number comes from. Let's begin.

0:34Two kinds of names

So, a halogen compound can have two kinds of names. The first is the common name: the name of the alkyl group, plus the word halide. Okay? For example, this one: C H three in round brackets with a small three written low, then C, a dash, and B r for bromine. That means three methyl groups on one carbon, joined by a single bond, the dash, to bromine. This group is tert butyl, with tert in italics, so this is tert butyl bromide. With me? The second kind is the IUPAC name. Here the halogen is not the main word at all. It is a substituent, written as a prefix on the parent hydrocarbon: fluoro, chloro, bromo or iodo. Clear?

1:18Numbering: the IUPAC rules

Now, the rules for the numbers. First, number the chain so that the substituents get the lowest locants. Locant just means the number of the carbon that carries a group. Okay? Second, write the prefixes in alphabetical order. So bromo comes before chloro, and chloro comes before methyl. Right? Third, sometimes both ends give exactly the same locants. Then alphabetical order breaks the tie: the prefix that comes first in the alphabet gets the lower number. Example one needs this. Got it? Fourth, multiple bonds. C, two lines, C is a double bond, and C, three lines, C is a triple bond. Either one takes the lower number in preference to a halogen. Clear? And here is the big idea. Halogens are always prefixes, so they never decide the suffix, the ending of the name. Once a multiple bond, or a principal functional group, fixes the direction of numbering, the halogen takes whatever number results. With me?

2:20From common names to IUPAC names

Now let us use these rules on the course's table. Tert butyl bromide has a longest chain of three carbons, so the parent is propane. Carbon two carries the bromine and one methyl group. So it is two bromo two methyl propane, bromo before methyl. The small dashes just join each number to its word. Okay? Isopropyl chloride is two chloro propane, and sec butyl chloride, with sec in italics, is two chloro butane. In both of them, the chlorine sits on carbon two. Right? Now, isobutyl chloride. It also has four carbons, and its name also says butyl. So is it two chloro butane as well? No, no, careful, that is exactly the trap here! Isobutyl is branched. Its longest chain has only three carbons, with chlorine on carbon one and a methyl group on carbon two. So it is one chloro two methyl propane. Common names hide the branching, but IUPAC names show it. Clear? And neopentyl bromide is one bromo, two, two dimethyl propane. The comma separates the two locants: both methyl groups sit on carbon two, and di says there are two of them. Got it?

3:32Halides of methane and ethane

Next, the small halides you will meet again and again. Methylene chloride is dichloro methane, and chloroform is trichloro methane. Okay? Iodoform is tri iodo methane, and carbon tetrachloride is tetra chloro methane. Now look at the question on screen: ethylidene chloride or ethylene dichloride. Which one has both chlorines on the same carbon? Take a guess. Ethylidene chloride, yes, exactly! It is one, one dichloro ethane, and the locants one and one say that both chlorines sit on carbon one. Right? And ethylene dichloride is one two dichloro ethane, with one chlorine on each carbon. See, the locants settle the confusion at once. With me?

4:17Vinyl, allyl, benzyl and the benzene ring

Now, halides with a double bond. Vinyl chloride is C H two, double bond, C H, single bond, C l, so the chlorine sits on a carbon of the double bond. Its IUPAC name is chloro ethene. Okay? And here is our puzzle from the start. Allyl bromide is C H two, double bond, C H, then C H two, with bromine at the end. Use the rules we just saw. Which one gets the lower number here, the double bond or the bromine? The double bond, yes, exactly! So we number from the double bond end. The double bond starts at carbon one, and the bromine lands on carbon three. So the name is three bromo prop one ene. Puzzle solved: bromine simply takes the number that results. Clear? Benzyl chloride is C six H five, the phenyl group, then C H two, then C l, the chlorine. Its IUPAC name is chloro phenyl methane: the parent is methane, carrying a chlorine and a phenyl group. Got it? Last, a benzene ring with two halogen atoms, a dihalobenzene. Ortho, in italics, means positions one and two, meta means one and three, and para means one and four.

5:27Example 1: breaking a locant tie, bromo before methyl

Right? Alright, Example one, an easy one. Give the IUPAC name of C H three, dash, C H with a methyl branch, C H three in round brackets, dash, CHB r, dash, C H three. Pause the video and try it first. Done? Okay, let's check. The longest chain has four carbons, so the parent is butane. It carries two substituents, a methyl group and a bromine. With me? Now number it both ways. From the left, methyl is on two and bromo on three. From the right, bromo is on two and methyl on three. Both ends give two and three, a tie! So which one gets the number two, bromo or methyl? Remember the tie rule? Alphabetical order decides. The letter b of bromo comes before the letter m of methyl, so bromo gets the lower number, two. So the name is two bromo three methyl butane. Got it?

6:26Example 2: double bond beats bromine in a bromopentene

Now a harder one. Give the IUPAC name of C H three, dash, C H, double bond, C H, dash, then a carbon carrying B r, and two C H three groups, shown in round brackets with a small two. Pause the video and try it first. Done? Okay, let's check. The longest chain runs through the double bond and ends in one of the two methyl groups on the last carbon, so it has five carbons. With the double bond, the parent is a pentene. Right? Now, the bromine and a methyl group sit on one carbon near the right end. From the right, they get carbon two, and the double bond gets carbon three. So the name is two bromo two methyl pent three ene, right? Oh no, careful, that is exactly the trap! The double bond takes the lower number in preference to the halogen. So we number from the left: the double bond starts at carbon two, and the bromine and the methyl group land on carbon four. Clear? So the name is four bromo four methyl pent two ene. Same idea as allyl bromide: the double bond fixes the direction, and the halogen takes the number that results. Got it?

7:40Quick revision: prefixes, lowest locants, double bonds first

So, quick revision. The halogen is always a prefix, and the prefixes go in alphabetical order. Give the substituents the lowest locants, and on a tie, alphabetical order gives the lower number. And double or triple bonds take the lower number before any halogen. Now go and try the practice problems on this page. All the best!