Alkenes, Alkynes and Rings

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. You may know this rule: when a chain has a double bond and a triple bond, the double bond gets the lower number. Now take C H three, C H double bond C H, and C triple bond C H at the end. By that rule, we number from the left, right? But the correct IUPAC name is pent three en one yne. The triple bond got number one! So, did the rule break? By the end of this video, you will know exactly why it did not. Questions like this do appear in the JEE exam. So, let's begin.

0:39The parent chain must hold the multiple bond

So, now our chain gets a multiple bond. On screen, C joined to C by two lines is a double bond, and by three lines, a triple bond, okay? Rule number one: the parent chain must contain the multiple bond, even if a longer chain exists. Clear? Rule number two: number the chain so that the multiple bond gets the lowest number. That locant goes just before ene for a double bond, or yne for a triple bond. Right? So we get names like but one ene, and pent two yne. The small hyphens just join the number to the name. Got it? Now try this one. The small numbers written low are subscripts counting the hydrogens, and the short dashes are single bonds. Five carbons, so the root is pent. Now tell me, which end do we number from? From the left, yes, exactly! Then the double bond starts at carbon two, written C dash two on screen. From the right, it would start at carbon three. Lower wins, so the name is pent two ene. With me?

1:41Many double bonds, or ene with yne

Now, two double bonds make a diene, and three make a triene. And we add the letter a to the root, so but becomes buta, as in buta one three diene. The comma just separates the two locants. Okay? Here is isoprene. The C H three in round brackets hangs from carbon two. Double bonds start at carbons one and three, so it is two methyl buta one three diene. Right? Now, a double bond and a triple bond together. First, give the lowest numbers to both multiple bonds as a set. The name takes the form alk en yne. Clear? And only if there is a choice, which means a tie, does the double bond get the lower number. Remember this word, choice. Got it? Look at this one. From the left, the double bond is at one and the triple bond at four. The curly brackets hold this set, one and four. From the right, the triple bond is at one and the double bond at four. The same set! So, which bond gets number one? The double bond, correct, because this is a tie. So the name is pent one en four yne. Okay?

2:51Rings: the prefix cyclo

Now, rings. Rings of carbons take the prefix cyclo. With just one substituent on a cycloalkane, a ring with only single bonds, no number is needed. Right? With more substituents, give the lowest locants, and on a tie, follow the alphabet. With me? So, ethyl and methyl on adjacent carbons of a cyclohexane. Either way round the ring, the locants are one and two, which is a tie. So, which group gets carbon one? Ethyl, yes! Ethyl comes before methyl in the alphabet. So the name is one ethyl two methyl cyclohexane. Clear? Now a cycloalkene, a ring with a double bond. The two doubly bonded carbons are always C one and C two, numbered in the direction that gives the substituents the lowest numbers. So, a methyl group on the carbon next to the double bond. Is that carbon three, or carbon six? Three, exactly! Continue round the ring from C two, and the methyl carbon becomes C three. The other way, it would be C six. So, three methyl cyclohexene. Got it?

4:00Ring or chain? And the unsaturated groups

Now, a ring joined to a chain. If the chain has more carbons than the ring, the ring becomes a substituent, like cyclopropyl or cyclohexyl. Okay? So, a cyclopropane ring on carbon two of a pentane chain. Five chain carbons beat three ring carbons, so the name is two cyclopropyl pentane. Right? But a propyl group on a cyclohexane? Now the ring is larger, six against three. So it is propyl cyclohexane, with no number, since there is only one substituent. Clear? Finally, some groups with multiple bonds have common names. C H two double bond C H, where the last dash is the free bond, is vinyl, or ethenyl. Add one C H two, and it is allyl, or prop two enyl. Okay? With a triple bond, H C triple bond C, with its free bond, is ethynyl. Add one C H two, and it is propargyl, or prop two ynyl. Got it?

4:59Example 1: longest chain trap, 2-ethylpent-1-ene

Alright, now Example one. Name the compound on screen. The C two H five in brackets is an ethyl group on carbon two. Pause the video and try it first. Done? Okay, let's check. Longest chain first. Start in the ethyl group, pass through carbon two, and run to the right end. That is six carbons! So that is our parent chain, right? No, no, wait, that is the trap! The double bond is not inside that six carbon chain; the C H two at the left end hangs off it. And the parent chain must contain the double bond. Okay? So, take the longest chain that does hold the double bond. From the C H two end, through carbon two and down the propyl side, we get five carbons. Through the ethyl side, only four. Number from the C H two end, so the double bond gets locant one. Right? And the ethyl group sits on carbon two. So the name is two ethyl pent one ene: a parent of five carbons, not six, because it must hold the double bond. Clear?

6:04Example 2: ene or yne first? Why pent-3-en-1-yne

Now, remember our puzzle from the start? Let's finish it. Name the compound on screen. Pause the video and try it first. Done? Okay, let's check. Many students say this: the double bond gets the lower number, so we number from the left. That gives pent two en four yne, right? Oh no, careful, that is the trap! That rule works only when there is a choice. First, we compare the sets. From the left, the double bond is at two and the triple bond at four, so the set is two and four. With me? From the right, the triple bond starts at carbon one, and the double bond at carbon three, so the set is one and three. Compare the sets term by term. One is lower than two, so the right end wins. Okay? So there is no tie, and no choice for the double bond rule to settle. The triple bond takes carbon one, and the name is pent three en one yne. And that solves our puzzle! The rule never broke; it simply never got a chance to act. Got it?

7:10Quick revision: parent chain, en-yne locants, rings

So, quick revision. The parent chain must hold the multiple bond, and the multiple bond gets the lowest locant. With ene and yne together, give the lowest set first; the double bond gets the lower number only on a tie. And rings take the prefix cyclo, but if the chain has more carbons, the ring becomes a substituent. Now go and try the practice problems on this page. All the best!