Displayed Formula And Name Needed, PLEASE HELP!?

Question

1) CH3CHC(CH3)CH2CH3
2) CH2CHC(CH3)CH2

Could someone please also explain what those brackets mean?? Thats where im getting stuck at the moment.

Answer 1

Sure. The brackets mean that the group in brackets (in each case here a methyl group) is a side chain off the primary carbon chain.

In the first case, you have a five carbon chain (a pentane). The central (or "3" carbon--you count from either end to get the number of the carbon to which the side chain is attached and pick the smallest--in this case it's the same from either end). So this methyl group is attached in the 3 -position.

However, remember that all carbons must have four bonds attached to them. If you've written the formulae correctly, the 2-carbon is bonded to the terminal methyl, one hydrogen, and the 3-carbon. That's only three bonds, so the bond between carbons 2 and 3 must be a double bond, as C-C=C-C-C.

Thus this isn't a pentane but a pentene. (The "ene" clues you into the presence of a double bond). To give the double bond thre lowest position number we can, we start counting from the left. It's in the 2-position.

Thus our structure can be named as 3-methyl-2-pentene.

Try the samer reasoning with structure 2. Note that counting the bonds tells you there are TWO double bonds in this puppy. You should get (DON'T PEEK YET!) 2-methylbutadiene.

The basic structure is a butane, but with two double bonds that becomes a butadiene (di-ene, two "enes", i.e. two double bonds). You don't need to number the locations of the double bonds 'cause there's no other place they could be in this small molecule.

Not too tough, eh?

Answer 2

1) 3-methylpent-2-ene (Some people would call this 3-methylpentene-2, as this is easier to pronounce).

2) 2-methylbuta-1,3-diene.

It's difficult to display the formulae on this format. It can be done in one dimension only, rather than the usual two. The best I can do in one dimension is:
1) CH3---CH===C(CH3)---CH2---CH3

2) CH2===CH---C(CH3)===CH2

The brackets mean that the group enclosed in them (the methyl group, in this case) is a side-chain and not part of the main chain.

Incidentally, if you manage to write out the structural formula of 3-methylpent-2-ene in two dimensions, you'll see that it exists in a cis-form and a trans-form. (Or maybe you haven't covered cis-trans isomerism yet ?)

Note to Jon K: A good, well explained answer. But there's one slight mistake - you DO need the indices for buta-1,3-diene, because there is such a thing as buta-1,2-diene. It is one of a group of somewhat unstable hydrocarbons known as allenes. I remember, many years ago, working in a research lab and the director of the lab ordered a very small (½-litre) cylinder of propa-1,2-diene. I can't remember what he wanted it for.

Answer 3

1. CH3CHC(CH3)CH2CH3

This is iso-hexene or 3-methyl-2-pentene, the bracket means that the methyl group is attached to the middle carbon of the chain. The double bond comes between carbons 2 & 3 (from the left).

2. CH2CHC(CH3)CH2

This is 2-methyl-buta-diene. The bracket in this case means that the methyl group is attached to the 2nd carbon (from the right).
(Can't go at the end of a chain. The two double bonds 'diene' go between the carbons 1 & 2 and 3 & 4 counting from the left).

(I believe I'm right - if not please correct me).

Answer 4

It means that there is a CH3 side group attached to the carbon right before it.

Answer 5

i would suppose it would mean a branch.
C can create four bonds, since there are 3 hydrogens bonded to the C, therefore the other bond ought to be the C-C bond.
Beside the (CH3) is only C, therefore suggests a double bond with CH2(consider number2) a single bond with C and another bond with (CH3) thus it suggests branching.