[madscientist]

I need to name the structure below:

CH3
 l
C=CH-CH3
 l
CH2-CH3

MY CHOICES ARE:

A.) 2-ETHYL-2-BUTENE

b.) 3-METHYL-3-PENTENE( not this because of carbon double bond position)

C.) 3-METHYL-2-PENTENE

D.) 3-ETHYL-2-BUTENE ( not this because of ethyl position)

im lost to find a difference in the structure between a and c, my gut tells me its c.

ANY hints would be appreciated.

cheers,

madscientist

[movies]

Suppose there was no double bond in this structure.  How would you name that compound and why would you name it that way?

[madscientist]

if no double bond i would name it like this:

3-methyl-pentene

"3-methyl" because of the position of methyl ( CH3) being on the third carbon

and

"pentene" because the longest chain consists of five carbons

[arnyk]

There are many types of hydrocarbons, but early on the simple types you'll learn about are the aliphatic compounds.  These are divided into 4 main groups, the alkanes, alkenes, alkynes, and the alicyclics.  Now the distinguishing features between the first 3 groups would be the type of bonds (single, double, triple).  The fourth is easy to distinguish from its ring structure.  

Examine the suffixes of these groups, alk-ane, alk-ene, alk-yne.  Match these up to the bond types they have.  

As for nomenclature: (tried to narrow the steps down as it applies to your question...there are so many more rules I can't remember right now )

1) Identify the parent chain (the longest chain).  The chains are not necessary always linear.  In your question I would count the carbons from one carbon to the other until you reach a place where there is no where else to go.  Follow the chain all the way to an endpoint, not just where the straight chain ends.
2) Check for double bonds/triple bonds, if they are present be sure to indicate which carbon the bond exists on.  Number this to give it the lowest possible number, it takes precedence over substituent groups.
3) Identify substituent groups, name them accordingly and number them in the direction which gives the lowest possible number (but remember #2 takes precedence).

I am assuming you know how to name the parent chains and substituent groups along with the proper prefixes and suffixes.  There are alot of rules to remember, but generally at this level they aren't that bad at all.  

[arnyk]

PS: Are you doing this in school or on your own time?

[movies]

"pentene" because the longest chain consists of five carbons

Good.  So how would this particular point apply to the case with the alkene?  Which of the two choices you have left would satisfy this rule?

[madscientist]

Does it matter which side of the double bond you count to?
i know that you have to give it the lowest possible number, its hard to put into words what im stuck on so ill just number the structure and name it, please tell me if im right.

CH3
l3  2     1
C=CH-CH3
l4     5
CH2-CH3

Therefore: 3-methyl-2-pentene

[arnyk]

Excellent.

Remember if when the double bond is already the lowest number from whichever end you count it from, then pick the end which will give the substituent group the lowest number since it is the same for the double bond either way.

[movies]

CH3
l3  2     1
C=CH-CH3
l4     5
CH2-CH3

Therefore: 3-methyl-2-pentene

Looks good!