Naming Alkenes - Online Tutor, Practice Problems & Exam Prep

• Alkenes have a carbon, carbon double bond.
• Alkene naming convention modifies the ending from A to E.
• Rule for naming alkenes is unique.
• Geometric isomers have different spatial orientation around a double bond.
• Rotation is not possible around a double bond.
• Cis and trans indicate which side of the pi bond groups lie.
• Cis means both groups are on the same side.
• Trans means groups are on opposite sides.
• Cyclic alkenes have no cis or trans notation.
• Two groups are needed to determine cis or trans.

Rules for Naming Alkenes

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 going to modify the ending from Ain to in. So we're thinking of an alkane, but because of the presence of a double bond, it becomes an alkene. This naming convention will have things called sis and trans which will 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 we'll call 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 π 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 two groups around a double bond. So if we take a look here, we have our two double Bonded Carbons here and we have our two 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 your different sides of the of the π 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. Same molecular formula, same connections, just a slight difference in spatial orientation. Again, here are our two double bonded carbons. We create a border by cutting straight through them. Now we have 1 methyl group up here and one 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 sis alkene on the left and a transalkine on the right.

Now if we're dealing with a cyclic alkene, we're going to 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 sis or trans, we need to have two groups that are connected to our double bonded carbons. If they're on the same side, it CIS. If they're on opposite sides, it's trans.

Naming Alkenes Example

Here it says determine the systematic name of the following alkene. So to name this alkene, we're going to 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 start numbering on this side 1, 2, 3, 4, 5 and 6. Assign location to the first double bonded carbon if double bonded in a ring. Note location is not necessary repeat from previous naming topics.

So go back and take a look at my videos on naming out canes 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 CIS or trans designation to the front of the name if applicable. Alright, so first let's just name this before we worry about Sis and trains. We're going to say here our substituent is methylants on carbon #4, so it'd be 4 methyl. We then have to talk about the first double bonded carbon. The double bond starts on carbon #2, so we'd say the location is 2.

It's a six carbon chain, which is hexane as an alkane, but here it's an alkene, so we change the ane ending to ENE. So this would be two hexene. Now we said that if we have two groups attached to our π bond then we use CIS and trans. So here is a group attached to our double bonded carbon which contains the π bond, and here is another group attached. So we do have two groups attached to our Pi bonds in terms of those two alkene carbons. Remember we create a border here between the double bonded carbons and we can see that the two 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 alkene would be trans 4, methyl 2, hexane.

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

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

Draw a structure for cis-5-methyl-2-hexene.

• A.
• B.
• C.
• D.

Determine the structure of the following alkene: 3-isopropylcyclohexene.

• A.
• B.
• C.
• D.