Converting chemical structure to a molecular formula

[Aussiek]

Hi there,

I don't have a chemistry background, so am struggling a bit with this one. I am required to convert the (shown) molecule to a molecular formula, but i'm not understanding how to do this.

I thought maybe it was as simple as just adding everything up, i.e answer C3H9, but realise C3H9 can't be the right answer. Is it a matter of dropping a Hydrogen due to not having enough bonds on the Carbon, therefore the answer would be C3H8?

I'm just not sure of the process how on to look at the structure given and write the molecular formula for the molecule. Fortunately i'm not the only one in class that is stuck on this too, we really haven't been given clear information on what the process is, everyone else has just moved on from it, but i don't like to move on unless i understand something.

Any help given will be much appreciated.

[John Cuthber]

The good news is that you are right it is simply a matter of counting the hydrogens.

When drawing structures like that people don't always draw in all the hydrogens (because they get in the way).

There are 14 hydrogens altogether in that structure, but only 9 of them are written as such.

Each corner of the pentagon represents a carbon atom and it will have 4 bonds to it. if you can only see 2 then the other 2 must have hydrogens on them.

Similarly, if there are 3 bonds to carbons then there must be 1 hydrogen.

[Aussiek]

Hello John, Thank you so much for your explanation - it is making much more sense than anything i've come across in my study material!

I'm not sure where the 14 hydrogen bonds come from, i get 12? (does the top point not already have 2 bonds, therefore only 2 hydrogens, and the bottom right point have 3 bonds so would only have 1 hydrogen?). I thought the maximum bonds a pentagon could have was 12?

 

So if i am writing it into a molecular formula, do i add in the carbons and hydrogens i can't see - i.e c5h12 (pentane).

 

I really appreciate your help!!! i'm much happier with biology or finance!!

 

Edited April 3, 2016 by Aussiek

[John Cuthber]

Does this help?

[hypervalent_iodine]

Hello John, Thank you so much for your explanation - it is making much more sense than anything i've come across in my study material!

 

I'm not sure where the 14 hydrogen bonds come from, i get 12? (does the top point not already have 2 bonds, therefore only 2 hydrogens, and the bottom right point have 3 bonds so would only have 1 hydrogen?). I thought the maximum bonds a pentagon could have was 12?

 

So if i am writing it into a molecular formula, do i add in the carbons and hydrogens i can't see - i.e c5h12 (pentane).

 

 

You can see them, they're just written in a different way. You could also draw the same molecule in the following ways and it would be the same thing:

 

 

The reason we don't often write in every atom and bond is that, as you can probably see, it gets messy and convoluted.

[Aussiek]

Thank you both of you for assisting with this.

 

Last night when i saw your picture John, it didn't make sense - but this morning it does.

 

It finally clicked that i'm not just looking at a pentagon, i'm also looking at the bonds coming off the pentagon which has now been shown in the above picture (thank you).

I had just come on to say i realised because i need carbon on the ends and the bends, then the diagram would actually indicate C8H14 - which is what is shown in the extended diagram above.

Totally get now why everything isn't written it, it does look very messy and hard to read! Although it was also, initially, hard to make sense off without all that showing - but i finally get it!

Ahhhhh, glad i can move on to the rest of my chemistry work now understanding this. Unlike others in my class who have decided to bypass it, i knew it was something i had to know as i know it'll pop up again further down the track.

Thank you so much both of you.

[studiot]

i knew it was something i had to know as i know it'll pop up again further down the track.

 

 

 

 

This example brings up another point for down the track.

 

The pentagon is a closed shape, called a ring or cyclic structure for obvious reasons, however it is not the most common or important one.

You should always be aware which ring you mean, it is particularly, if your glance is too quick you can confuse the pentagon and hexagon.

 

The hexagonal benzene ring is the most common arrangement and the basis for a large branch of organic chemistry, the aromatic compounds.

 

 

 

http://www.2b1stconsulting.com/benzene/