Naming Alkenes - Video Tutorials & Practice Problems
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1
concept
Rules for Naming Alkenes
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2m
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Now before we get into naming alkenes, recall that alkenes possess a carbon carbon double bond. And the set of rules for naming alkenes is unique. Here we're going to say that the modification is, we're gonna modify the ending from -ane to -ine. So we're thinking of an alkane, but because there's a presence of a double bond it becomes an alkene. This naming convention will have things called cis and trans, which we'll go into. We still have to give the location of our substituents. We have to give the location of our parent in terms of the double bond, and then we have our modifier where we change the ending from ane to ene. Now here recall that geometric isomers have a different spatial orientation around a double bond. And remember that rotation is not possible around a double bond, which is a pi bond. Now, we need to indicate which side of the double bond both groups, where both or each group lies. And we do this by saying cis or trans. Now, this only applies when there are 2 groups around a Pi bond. So if we take a look here, we have our 2 double body carbons here, and we have our 2 groups which happen to be these methyl groups. We create a border, this border dissects right through the double bonded carbons. And you're either here on the bottom side, or you're here on the top side, so you're different sides of the of the pi bond. Both methyl groups happen to be on the same side. When they're on the same side, we say that this is cis. Now, here if we take a look at this alkene, this is a geometric isomer with the same molecular formula, same connections, just a slight difference in spatial orientation. Again, here are our 2 double bonded carbons. We create a border by cutting straight through them. Now we have 1 methyl group up here, and 1 methyl group down here. They're on different sides of this border that we've created. So if they're on different sides, that means that they are trans. So here we have a Cisalkyne on the left, and a Transalkene on the right. Now, if we're dealing with a cyclic Alkene, we're gonna say there are no Cis or Trans within the ring. So we don't have to worry about this type of notation. So again, for us to be cis or trans, we need to have 2 groups that are connected to our double bonded carbons. If they're on the same side it's cis, if they're on opposite sides it's trans.
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example
Naming Alkenes Example
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3m
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Here it says, determine the systematic name of the following alkene. So to name this alkene, we're gonna follow the given steps. Here it says, find the longest carbon chain which represents our parent chain, and assign a name according to the prefix and the modifier. The parent chain should include a double bond, and have greater number of carbons. Now, if it's cyclic, we add the prefix of cyclo to the name of the alkene chain. If a tie between longest chains, choose chain with more substituents. So if we take a look here, what's our longest chain? Our longest chain appears to be this here. This carbon up here which is a methyl is our substituent. So step 2 says, assign name to all substituents. So this would be methyl. Start numbering the chain from the end closest to the double bond. So the double bond is here on the left, so we'll start numbering on this side. 123456. Assign location to the 1st double bonded carbon. If double bond in a ring, no location is not necessary. Repeat from, previous naming topics. So go back and take a look at my videos on naming alkanes with substituents, because it talks about how we give the numerical location for each of the substituents, how we name them alphabetically. And here we have step 7, where we add Sys or Trans designation to the front of the name if applicable. Alright. So first, let's just name this before we worry about cis and trans. We're going to say here our substituent is methyl, and it's on carbon number 4, So it'd be 4 methyl. We then have to talk about the first double bonded carbon. The double bond starts on carbon number 2. So we'd say the location is 2, it's a 6 carbon chain which is hexane as an alkane, but here it's an alkene so we change the -ane ending to -ine. So this would be 2 hexene. Now, we said that if we have 2 groups attached to our PI Bond, then we use Sys and Trans. So here is a group attached to our double bonded carbon, which contains the Pi bond, and here is another group attached. So we do have 2 groups attached to our Pi bonds, in terms of those 2 Alking carbons. Remember we create a border here between the double bonded carbons, and we can see that the 2 groups that I, basically put in within those blue boxes, they're on different sides of this border. Because they're on different sides of this border, that means that they are trans to one another. So we put trans at the beginning of the name. So the name of this particular Alkina would be, 'trans 4methyl 2hexene'.
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Problem
Problem
Provide systematic name for given alkene.
A
trans-2,7-dimethyl-3-octene
B
2,7-dimethyl-3-octane
C
trans-2,7-dimethyl-5-octene
D
2,7-dimethyl-5-octane
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Problem
Problem
Provide systematic name for given alkene.
A
3-ethyl-5-hexene
B
4-isopropyl-1-hexene
C
4-ethyl-1-hexene
D
4-isopropyl-2-hexene
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Problem
Problem
Draw a structure for cis-5-methyl-2-hexene.
A
B
C
D
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Problem
Problem
Determine the structure of the following alkene: 3-isopropylcyclohexene.