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.

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

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).

 

[Blueshift5]

Hello Dan,

Naming organic compounds can be challenging at first, but with practice it becomes easier. In the first compound, you are correct that the longest carbon chain is 5 atoms, making it a pentane. However, the branch off the second carbon is a methyl group, which is considered a substituent. When naming compounds, we start by numbering the carbon chain from the end closest to the substituent. In this case, the methyl group is on the third carbon, so we name it 3-methylpentane. If the substituent was on the first carbon, it would be called 1-methylpentane. The number before the substituent name indicates the location of the substituent on the main carbon chain.

For the second compound, the brackets indicate a repeating unit. So in the first compound, there are two methyl groups attached to the second carbon, and in the second compound, there is a methyl group attached to the third carbon and a ethyl group attached to the sixth carbon. These repeating units can be simplified by using parentheses and a number to indicate how many times they repeat.

For the first compound, it would be written as (CH3)2CHCH2CH3 and for the second compound, it would be written as CH3(CH2)2C(CH3)2(CH2)2CH3. The numbers outside the parentheses indicate the location of the repeating unit on the main carbon chain.

I hope this helps clear up any confusion. Keep practicing and you will become more comfortable with naming compounds and handling brackets. Good luck with your studies!