Kinds of Isomerism; Structural Isomers

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Two bottles sit on a lab shelf, and both labels show the same molecular formula, C two H six O, okay? So it must be the same compound, right? Well, here is the surprise: not necessarily! One bottle can hold ethanol, and the other methoxymethane, two different compounds. How is that possible? The answer is isomerism, which comes up often in JEE questions. So, let's begin.

0:30Isomers: one formula, different compounds

So, what exactly are isomers? They are compounds with the same molecular formula, but different properties. The tree on the right splits isomerism into two branches, structural on the left and stereoisomerism on the right. Okay? The first branch is structural, or constitutional, isomers: the atoms are joined in a different order, so the connectivity is different. Its box lists six types: chain, position, functional group, metamerism, tautomerism and ring chain. Those six are today's lesson. Clear? The second branch is stereoisomers, with the same connectivity, but the atoms arranged differently in space. Its boxes, configurational, geometrical, optical and conformational, are for later lessons, with me?

1:17Chain and position isomers

Now, the six types, one by one. Type one is chain isomers, which have different carbon skeletons. Right? Look at these two. The dashes are single bonds, and the small numbers written low are subscripts. On the left, four carbons in a straight chain. On the right, three in a row, and the C H three in brackets is a branch on the middle carbon. So, are these two isomers? Count the atoms. Yes, exactly, they are isomers! Both are C four H ten, but one skeleton is straight and the other is branched. So butane and two methyl propane are chain isomers. Got it? Type two is position isomers. Same skeleton, same functional group, but the group, the multiple bond or the substituent sits at a different position. Okay? Take but one ene and but two ene. The equals sign is a double bond. In but one ene it starts at carbon one, and in but two ene at carbon two. Only its position changes. Clear?

2:21Functional-group isomers

Type three is functional group isomers, which have different functional groups. Propanal ends in CHO, the aldehyde group, and propanone has C O in the middle, the ketone group. Count them, and both are C three H six O, okay? Now a topper's trick: the general formula tells you which families can be isomers. Here the letter n is the number of carbons. C n H two n plus two O fits an alcohol and an ether, C n H two n O two an acid and an ester, and C n H two n minus two an alkyne and a diene. With me? And C n H two n O fits aldehydes and ketones, and also unsaturated alcohols and cyclic ethers. Propanal and propanone, with n equal to three, fit right here. Right? Three more sets to remember: cyanide and isocyanide, nitroalkane and alkyl nitrite, and primary, secondary and tertiary amines. Same formula, different group, every time. Got it?

3:21Metamers, tautomers and ring–chain isomers

Type four is metamers: the same functional group, but different alkyl groups on its two sides. Okay? Ethoxyethane and one methoxy propane are both ethers, C four H ten O, with the oxygen between two alkyl groups. Ethoxyethane has an ethyl, C two H five, on each side, while one methoxy propane has a methyl and a propyl, C three H seven. So they are metamers. Clear? You find metamers in ethers, thioethers, ketones and esters, and in secondary and tertiary amines. Keep this list in mind, okay? Type five is tautomers: isomers in rapid equilibrium that differ in the position of a hydrogen and a double bond. The two half arrows show that equilibrium, between the keto form and the enol form. With me? And type six is ring chain isomers, where one is an open chain and the other a ring. Propene, C H two double bond C H, C H three, is a chain, and cyclopropane is a ring, both C three H six. That completes the six types. Got it?

4:23Trap: check the formula first

Now, here is a trap that catches many students. Butan one ol is an alcohol and butanone is a ketone, four carbons each. So they must be functional group isomers, right? Oh no, careful, count the hydrogens first! Butan one ol has three, two, two, two, and one on the oxygen, ten in all. Butanone has three, two and three, just eight. So it is C four H ten O against C four H eight O, okay? Different formulae, so they are not isomers at all! So here is the rule: two compounds are isomers only if their molecular formulae are identical. Always count the atoms first. Clear?

5:02Example 1: propanols, then ethanol vs methoxymethane

Let us try the first example. What type of isomers are, in part a, propan one ol and propan two ol, and in part b, ethanol and methoxymethane? Pause the video and try it first. Done? Okay, let's check. Propan one ol is C H three, C H two, C H two, O H, and propan two ol has the O H in brackets on the middle carbon. Both are C three H eight O, so the formula check passes, okay? The three carbon chain is the same, and only the O H group moves, from carbon one to carbon two. So part a gives position isomers. Right? Now part b, and remember our two bottles? Ethanol is C H three, C H two, O H, and methoxymethane is C H three, O, C H three, both C two H six O, okay? So which type of isomers are they? Yes, they are functional group isomers! Ethanol is an alcohol and methoxymethane is an ether, two different functional groups. That is how our two bottles, with one formula, held two different compounds, and that solves our puzzle.

6:12Example 2: which compounds are isomers of butan-1-ol

Now a harder one. Which of these are isomers of butan one ol: butanal, two methyl propan two ol, ethoxyethane, butanoic acid, and but three en one ol? Here is a hint, count the hydrogens. Pause the video and try it first. Done? Okay, let's check. Butan one ol has four carbons, ten hydrogens and one oxygen, so it is C four H ten O, okay? Every candidate must match this formula. Butanal, the four carbon aldehyde, is C four H eight O, two hydrogens short. Butanoic acid has an extra oxygen, C four H eight O two. So both are out. Clear? Now, but three en one ol. It has an O H group and four carbons, an alcohol just like butan one ol. So it must be an isomer, right? No, no, wait, count the hydrogens! Two, one, two, two, and one on the oxygen, which makes eight. So it is C four H eight O, two short because of the double bond. Not an isomer, got it? Now two methyl propan two ol. The subscript three after the brackets means three C H three groups on the carbon that carries the O H group. That makes ten hydrogens, so it is C four H ten O, and ethoxyethane, our ether, matches too. Right? So the isomers of butan one ol are two methyl propan two ol and ethoxyethane. Counting the atoms did all the work. See how fast that was?

7:43Quick revision: formula check and the six structural types

So, quick revision. Isomers have the same molecular formula, so always check it first. Structural isomers come in six types: chain, position, functional group, metamers, tautomers and ring chain. And the general formula points to the families, like C n H two n O for aldehydes and ketones. Now go and try the practice problems on this page. All the best!