How Do I Correctly Name Organic Compounds and Handle Isomers?

[danago]

Hey. I've just started organic chemistry in class, and I am currently up to naming compounds, and isomerism.

Ive been going through some excersizes, and came across a few questions I am having trouble with.

The first one:
http://img136.imageshack.us/img136/870/3methylpentanech5.gif

The answer book says its called 3-methylpentane. I can't understand why though. The longest carbon chain consists of 5 atoms, therefore making it a pentane, but from what i can understand, there is a branch off the second carbon, so i would have thought it would be called 2-ethylpentane, but apparently not. Where am i going wrong?

The next two I am very confused with. How do i draw them? I am a little lost as to how to handle the brackets.

$$\begin{array}{l}<br /> (CH_3 )_2 CHCH(CH_3 )_2 \\ <br /> CH_3 CH_2 C(CH_3 )_2 CH(CH_2 CH_3 )CH_2 CH_2 CH_3 \\ <br /> \end{array}$$

All help greatly appreciated.

Thanks,
Dan.

 

[3trQN]

Yeah your correct, the longest being 5 C's long is pentane, but the branch is just a single methyl group at the thrid carbon of that branch (attached at the CH ).

to draw it another way:

       CH3
        |
CH3-CH2-CH-CH2-CH3

For the brackets, you can see that in the first one you have all methyl groups within the brackets and you can deduce that they can only be singly bonded. Use the bonding rules to predict the shape, remembering how many bonding electrons Carbon has. There should be only one result (unless it is an isomer).