Classification and Nomenclature

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Two methyl propan two ol is a tertiary alcohol: its O H group sits on a carbon joined to three other carbons. Now swap that O H for N H two, the amino group. So we get a tertiary amine, right? But chemists call it a primary amine! How can that be? Classifying and naming amines comes up often in JEE, so let's begin.

0:28Amines: replace the hydrogens of NH₃

So, what is an amine? Take ammonia, N H three, in the heading; the small three written low means three hydrogens. Replace them with alkyl or aryl groups to get an amine. Replace one, and it is a primary amine, RNH two, where R is the carbon group and N is nitrogen. The small circle written high is the degree sign, so one degree means primary. Okay? Replace two, and you get a secondary amine, two degree, R two N H, with one hydrogen left on the nitrogen. Right? Replace all three, and you get a tertiary amine, three degree, R three N, with no hydrogen left on the nitrogen. Clear? The nitrogen can even carry four groups. Then it has a positive charge, the small plus written high, so R four N plus is a quaternary ammonium ion. With me?

1:19The trap: look at the nitrogen, not the carbon

Now, the most important rule of all. The class of an amine depends on the number of carbon groups on the nitrogen itself, not on the carbon that carries the nitrogen. Got it? Back to our puzzle: C H three in round brackets with a small three, then C, then a dash, a single bond to the O H group. Alcohols are classified by the carbon carrying the O H, and here it is joined to three carbons, so two methyl propan two ol is tertiary. Now the box puts N H two in its place. Is that a tertiary amine too? No, no, careful, that is exactly the trap here! The nitrogen is joined to only one carbon. The three methyl groups belong to that carbon, not to the nitrogen. So two methyl propan two amine is a primary amine. Puzzle solved, okay?

2:08Aliphatic, aromatic or aralkyl?

Next, a second way to sort amines. In aliphatic amines, the nitrogen carries only alkyl groups, like methyl or ethyl. Okay? In aromatic amines, or arylamines, the nitrogen is bonded directly to a benzene ring. Like aniline, C six H five N H two, where the N H two sits right on the phenyl ring, C six H five. Right? Now, benzylamine. It also has a benzene ring. So it is an aromatic amine, isn't it? Not quite, look at the formula: C six H five, C H two, N H two. So the nitrogen sits on the C H two, not on the ring. Benzylamine is an aralkyl amine, aryl plus alkyl, and it behaves as an aliphatic amine. Got it?

2:50Naming amines: common and IUPAC names

Now let us name them. Common names are simple: the alkyl group plus the word amine, as one word. Ethylamine, diethylamine, trimethylamine. Easy, right? For the IUPAC name, drop the final e of the alkane and add amine, so ethane becomes ethanamine. With the nitrogen on carbon two of propane, it is propan two amine; the small dashes join the number to the words. Okay? Now suppose a methyl group sits on the nitrogen of ethanamine. Which number does that methyl group get? It gets no number, because it is not on a carbon of the chain. Its locant, the label for where it sits, is the letter N, in italics. So it is N methyl ethanamine. With me? Two methyl groups on the nitrogen? Write N twice, with a comma: N, N dimethyl ethanamine. And two N H two groups on ethane give ethane one, two diamine. Notice, here the e of ethane stays, because diamine begins with a consonant; we drop the e only before a vowel, as in ethanamine. Clear?

3:55Aromatic amines and the prefix amino

For aromatic amines, the parent is aniline, C six H five N H two. In the round brackets is its IUPAC name, benzenamine. Okay? Put a methyl group on the ring, and these methylanilines are called toluidines. Put the methyl on the nitrogen, C six H five NHCH three, and it is N methyl aniline. Right? Last rule: when a group that ranks higher than the amine is present, like the alcohol in ethanol or the acid in benzoic acid, the N H two, with a dash for its bond, becomes the prefix amino. So, two amino ethanol and four amino benzoic acid. Got it?

4:32Example 1: primary, secondary or tertiary for five amines

Alright, Example one, an easy one. Classify the five amines on screen as primary, secondary or tertiary, using our one rule: count the carbon groups on the nitrogen. Pause the video and try it first. Done? Okay, let's check. The first one has two methyl groups on a C H that carries the N H two, so the nitrogen has one carbon group, and it is primary. The second has three methyl groups on one carbon. So it is tertiary, right? No, no, careful, this is the same trap as our puzzle! The three methyl groups sit on the carbon, not on the nitrogen. The nitrogen carries one group, so it is primary again. Got it? The third, C H three, N H, C two H five, has a methyl and an ethyl on the nitrogen, so it is secondary. The fourth, C two H five in brackets with a small three, has three ethyls on the nitrogen: tertiary. Clear? And the last is C six H five, N H, C H three. The phenyl and the methyl are both on the nitrogen, so it is secondary. Okay?

5:37Example 2: the 8 amines of C4H11N and which one is chiral

Now a harder one, JEE style. How many amines have the formula C four H eleven N? How many are primary, secondary and tertiary, and how many are chiral? Here is a hint: sort them by class. Pause the video and try it first. Done? Okay, let's check. In a primary amine, all four carbons form one group on the nitrogen. On the straight chain, the N H two sits on carbon one or two: butan one amine and butan two amine. On the branched chain, it gives two methyl propan one amine and two methyl propan two amine. That makes four primary amines, right? Now the secondary amines, where the four carbons split into two groups on the nitrogen. Let me ask you this: in how many ways can we split them? In two ways: three plus one, or two plus two. Three plus one is a methyl on the nitrogen, with a propane chain joined at carbon one or carbon two: N methyl propan one amine and N methyl propan two amine. Two plus two gives N ethyl ethanamine. So three secondary amines. With me? For tertiary, we need three groups from four carbons, and the only split that works is one plus one plus two. Two methyl groups and an ethyl give N, N dimethyl ethanamine, the only tertiary amine. Okay? So the total is eight: four primary, three secondary and one tertiary. The trap here is to stop at the four primary amines. Now, for these amines, being chiral needs a carbon with four different groups. Which of these eight has one? Yes, exactly, it is butan two amine! Its carbon two carries a hydrogen, a methyl, an ethyl and the N H two, four different groups. In the other seven, every carbon has at least two identical groups. Got it?

7:27Quick revision: classifying, aromatic or not, IUPAC names

So, quick revision. To classify an amine, count the carbon groups on the nitrogen, not on the carbon. It is aromatic only when the nitrogen sits directly on the benzene ring. In IUPAC names, drop the e, add amine, and groups on the nitrogen take the locant N, in italics. Now go and try the practice problems on this page. All the best!