Bicyclo and Spiro Compounds; Common Names

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. There is a molecule called norbornane, with two rings joined together. Its IUPAC name is bicyclo two, two, one heptane. Now, hept means seven carbons. But two plus two plus one is only five! So where are the other two carbons hiding? Something is strange here, right? By the end of this video, you will find them. And names like these come up often in JEE, so let's begin.

0:30Bicyclo: two rings sharing two carbons

So, let us start with the bridged rings. When two rings share two carbon atoms, those two carbons are called the bridgeheads, okay? The name is bicyclo, then three numbers, x, y and z, inside square brackets and separated by full stops, and then the alkane name. With me? Between the two bridgeheads there are three paths, called bridges. x, y and z are the numbers of carbons in these three bridges. And this sign means greater than or equal to, so the largest always comes first. Clear? Now here is the key question. Add up the carbons in the three bridges. Does that count every carbon in the molecule? No, it does not! The bridges join the two bridgeheads, so the bridgeheads themselves are in none of the bridges. So the total is x plus y plus z, plus two for the bridgeheads, and this total gives the alkane name. Okay? Now for the numbering, start at one bridgehead, go along the longest bridge to the other bridgehead, continue along the next longest bridge, and finish with the shortest. With me?

1:37A bridge with zero carbons

Now a famous one, decalin. Two cyclohexane rings share one bond, so the two bridgeheads are bonded directly. Four carbons go round one side, and four round the other. So only two bridges, and only two numbers in the name, right? No, no, wait, that is the trap! The shared bond is also a bridge, a bridge with zero carbons. So the bridges are four, four and zero. Four plus four plus zero, plus two, gives ten, so decane. Decalin is bicyclo four, four, zero decane. Got it?

2:11Spiro: two rings sharing one carbon

Now spiro compounds. Here the two rings share just one carbon, the spiro carbon. The name is spiro, then small a and small b in square brackets, and then the alkane name. Okay? Small a and small b count the carbons in each ring, leaving out the shared one. And look at the sign. It is less than or equal to, so here the smaller number comes first, the opposite of bicyclo. Don't mix them up! With me? Now let us find the total. In bicyclo we added two for the bridgeheads. So what do we add here? Just one, yes, exactly! There is only one shared carbon. So the total is small a plus small b, plus one. And the numbering starts in the smaller ring, at the carbon next to the spiro carbon. Clear? Let's try one. Take a cyclopentane ring and a cyclohexane ring sharing one carbon. Without that carbon, the rings have four and five carbons. Four plus five, plus one, gives ten. So it is spiro four, five decane. Got it?

3:15Common names still used in JEE

Now, some common names are still used in JEE questions, so know their IUPAC names. The common name comes first, and the IUPAC name after the colon. Formic acid is methanoic acid, and acetic acid is ethanoic acid. Okay? Next, four acids with two acid groups each, so the names end in dioic acid; the short dashes stand for that ending. Oxalic is ethanedioic, malonic is propanedioic, succinic is butanedioic. But careful, adipic jumps to six carbons, hexanedioic acid. Right? Then lactic acid is two hydroxy propanoic acid, and acrylic acid is prop two enoic acid. In these names, the hyphens join the numbers to the words. Clear?

4:02More common names

Now some carbonyl compounds. Acetone is propanone, acetaldehyde is ethanal, and acrolein is prop two enal. With me? Chloroform is trichloromethane, and isobutane is two methyl propane. Four carbons, but only three in the main chain. Okay? Ethylene glycol is ethane one, two diol, and the comma separates the two locants. Glycerol has three alcohol groups, so it is propane one, two, three triol. Got it? Now watch this one. Isooctane has eight carbons, but its parent is not octane. It is two, two, four trimethyl pentane, with five carbons in the chain and three methyl groups. Right?

4:42Gem and vic dihalides

One more pair of old names, for dihalides, with two halogen atoms. Geminal, or gem for short, means both halogens on one carbon, and that is called an alkylidene dihalide. Clear? Vicinal, or vic, means the halogens are on adjacent carbons, and that is an alkylene dihalide. Now a quick test for you. Ethylene dichloride and isopropylidene chloride: which is gem, and which is vic? Ethylene is an alkylene name, so it is a vic dihalide. So it is one, two dichloro ethane. Isopropylidene is an alkylidene name, so it is gem, with both chlorines on the middle carbon of propane. So it is two, two dichloro propane. With me?

5:26Example 1: naming norbornane as bicyclo 2.2.1 heptane

Now Example one, which is our puzzle from the start. Name the bridged hydrocarbon C seven H twelve; the small numbers written low are subscripts that count the atoms. Its bridgeheads are joined by bridges of two, two and one carbons. This is norbornane. Pause the video and try it first. Done? Okay, let's check. Largest first, x, y and z are two, two and one. Now some students just add these. Two plus two plus one is five, so it is pentane, right? No, no, wait, that is the trap! The two bridgeheads are in none of the bridges, so we add two more. Two plus two plus one, plus two, gives seven. Seven carbons means heptane, and it matches C seven H twelve. Clear? So the answer is bicyclo two, two, one heptane. And that solves our puzzle! The two missing carbons were the bridgeheads all along. Got it?

6:27Example 2: carbons and DU of bicyclo 2.2.2 and spiro 2.2

Now a harder one, JEE level. How many carbons do bicyclo two, two, two octane and spiro two, two pentane have? And what is the degree of unsaturation of each? Pause the video and try it first. Done? Okay, let's check. For the bicyclo, three bridges of two carbons, plus two bridgeheads, so two plus two plus two, plus two, gives eight. And octane says eight too. Okay? Now spiro two, two. Many students add two again, the bicyclo way. Two plus two plus two gives six carbons, right? Oh no, careful, that is the trap! Spiro rings share only one carbon, so we add one. Two plus two plus one gives five, and pentane says five too. Clear? Now the degree of unsaturation, D U for short, is the number of rings plus the number of pi bonds; pi is the Greek letter on screen. Both molecules have two rings and no pi bond, so the D U is two plus zero, which is two for both. With me? Let's confirm with the formulas. Each ring removes two hydrogens from the open-chain alkane formula, C n H two n plus two, so two rings remove four, and the arrow means becomes. So C eight H eighteen becomes C eight H fourteen, and C five H twelve becomes C five H eight. Got it?

7:54Quick revision: bicyclo plus 2, spiro plus 1, gem vs vic

So, quick revision. Bicyclo: largest first, the carbons are x plus y plus z plus two, and a shared bond is a zero bridge. Spiro: smallest first, the carbons are small a plus small b plus one, numbered from the smaller ring. Gem means one carbon, alkylidene, and vic means adjacent carbons, alkylene. And learn the common names. Now go and try the practice problems on this page. All the best!