# Homework Help: Naming of hydrocarbons

1. Jan 5, 2006

### PhysicBeginner

Hi, my question is when naming Alkanes like CH3CH(CH3)CH3CH(CH2CH3)CH(CH3)CH3
Do I name it as 3-ethyl-2,5-dimethylpentane?

Also does this naming rules also apply to alkene and alkyne?

2. Jan 6, 2006

### Mattara

Here are the rules for naming alkanes, alkenes, alkynes:

Alkanes:

Start with the longest unbroken chain of carbon atoms it doesn't matter how it is twisted as long as it isn't a part of a circle (then cyclo) or broken.

Use the normal methane, ethane series to name it. This is the suffix of the alkane.

(In your case:6 carbon atoms = hexane)

Then look at the branches. Assign the carbon atoms numbers from 1 - the highest in the chain (in your case 6). Start from the end of the molecule where the carbon atom of the first branch has the lowest number.

After that, you go to the first branch. See how many carbon atoms the branch compose of. Name this with the methane ethane series aswell but the words change a bit just here:

methane --> methyl
ethane --> ethyl
propane --> propyl etc.

This is to distinquise between the main chain and the branshed
Do not only write down the name of the branch but also the number of the carbon atom from the main chain on which it is positioned on.
Go to the next in line and repeat until you are done.

Let us take an example:

$$CH_3CH_2CH_2(CH_2CH_3)CH_2CH_2(CH_2)CH_3CH_3$$

The longest unbroken carbon chain = 7 = -Heptane
The number of the first branch carbon atom = 2 (from the right)
The name for the 1st branch = methyl (1 carbon)
The number of the second branch carbon atom = 4 (from the right)
The name of the 2nd branch = ethyl (2 carbon)

The name of that molecule would be:

2-methyl-4-ethylheptane

if there are 2 methyl groups in a molecule it is dimetyl

Lets say that the ethyl group had been a methyl group:

2,4-dimethylheptane

Naming alkenes and alkynes are done in excatly the same order, but the carbon chain picked must contain the doubble/tripple bond.

Now i think you can name your molecule by yourself :)

Last edited: Jan 6, 2006
3. Jan 8, 2006

### bomba923

"3-ethyl-2,5-dimethylpentane" is correctly named as 2-methyl-3-ethylhexane

4. Jan 8, 2006

### Mattara

No that is not correct :S

Shouldn't it be
2-methyl-4-ethylhexane
?

5. Jan 9, 2006

### bomba923

No, the ethyl group is on the 3rd carbon,
hence 2-methyl-3-ethylhexane.

When PhysicBeginner mentioned "3-ethyl-2,5-dimethylpentane,"
-Draw a pentane molecule.
-Add a methyl group to the 2nd & 5th carbons.
-And an ethyl group to the 3rd carbon.

What results:
-Adding a methyl to the fifth carbon of pentane results in hexane.
-The 2-methyl group retains its name and location.
-The 3-ethyl group name retains its name and location.

Thus, you have 2-methyl-3-ethylhexane.

Last edited: Jan 9, 2006
6. Jan 9, 2006

### Mattara

here is the forumla that was given:

$$CH_3CH(CH_3)CH_3CH(CH_2CH_3)CH(CH_3)$$

which can be simplified to

$$CH_3CH(CH_3)CH_3CH(CH_2CH_3)CHCH_3$$

$$CH3CH(CH3)CH3CH(CH2CH3)CHCH3$$
1____2________3____4_____________5__6

2-metyl-4-ethylpentane :)

Last edited: Jan 9, 2006
7. Jan 9, 2006

### bomba923

Yes,
$$CH_3CH(CH_3)CH_3CH(CH_2CH_3)CH(CH_3)$$ is 2-methyl-4-ethylhexane. I never said it wasn't.

But if you read what I wrote carefully,
I said,
Clearly, I refer only to the name--"3-ethyl-2,5-dimethylpentane", ----not whatever
$$CH_3CH(CH_3)CH_3CH(CH_2CH_3)CH(CH_3)$$ was.

And yes, my statement is correct. Please read more carefully next time .

Last edited: Jan 9, 2006
8. Jan 9, 2006

### Mattara

Aha, i missed the last $$CH_3$$