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Topic: Where do the extra H atoms go in structural formulas?  (Read 3585 times)

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Offline OrionEmpire

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Where do the extra H atoms go in structural formulas?
« on: June 01, 2016, 07:34:28 PM »
For instance 3-methylhexane

                           CH2-CH3
                            |
CH3-CH2-CH2-CH-CH3



Hexane has 14H, hence C6H14

So why in the above formula does either chain only have 13 H,

OR 11H?

totally lost. Am I missing something?

Offline Arkcon

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Re: Where do the extra H atoms go in structural formulas?
« Reply #1 on: June 01, 2016, 07:50:54 PM »
For instance 3-methylhexane

                           CH2-CH3
                            |
CH3-CH2-CH2-CH-CH3

OK, you haven't drawn it correctly, for starters.  You have simply drawn a linear chain of 7 carbons, not a six carbon chain with a methyl group at the 3 position.  SO fix that. 

Quote
Hexane has 14H, hence C6H14

Yup.

Quote
So why in the above formula does either chain only have 13 H,

OR 11H?

totally lost. Am I missing something?


Well,one spot has only H.  But you drew it that way, so carbon isn't charge balanced.  You need to put 'H''s where you didn't put a carbon-carbon (or carbon other atom) bound, until it adds up to 4.  That's all up to you.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline OrionEmpire

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Re: Where do the extra H atoms go in structural formulas?
« Reply #2 on: June 01, 2016, 08:40:30 PM »
For instance 3-methylhexane

                           CH2-CH3
                            |
CH3-CH2-CH2-CH-CH3

OK, you haven't drawn it correctly, for starters.  You have simply drawn a linear chain of 7 carbons, not a six carbon chain with a methyl group at the 3 position.  SO fix that. 

Quote
Hexane has 14H, hence C6H14

Yup.

Quote
So why in the above formula does either chain only have 13 H,

OR 11H?

totally lost. Am I missing something?


Well,one spot has only H.  But you drew it that way, so carbon isn't charge balanced.  You need to put 'H''s where you didn't put a carbon-carbon (or carbon other atom) bound, until it adds up to 4.  That's all up to you.



Maybe it's the textbook I'm using. It's organic chemistry : the basis of life, by miller. From like the 1980s.

That's how they had it drawn out. But it's not just them.

Some UK chemistry website is doing the same thing for their structural formulas. After they draw it all out they're "missing H".

Offline OrionEmpire

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Re: Where do the extra H atoms go in structural formulas?
« Reply #3 on: June 01, 2016, 08:42:23 PM »
And they are counting 1 2 3 etc from the methyl group right to left on carbons. So the 3 is at the intersection.

Offline Arkcon

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Re: Where do the extra H atoms go in structural formulas?
« Reply #4 on: June 01, 2016, 08:58:54 PM »
Oh, I see.  The line got shifted by formatting.  Here it is in smiles format: CCCC(C)CC  But its up to you to just "know" that hydrogens are needed, in a hydrocarbon, to balance carbon's 4 charges.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline OrionEmpire

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Re: Where do the extra H atoms go in structural formulas?
« Reply #5 on: June 01, 2016, 09:15:06 PM »
Oh, I see.  The line got shifted by formatting.  Here it is in smiles format: CCCC(C)CC  But its up to you to just "know" that hydrogens are needed, in a hydrocarbon, to balance carbon's 4 charges.


Right but all the carbons are balanced. From left to right...

C to 3 h, and a c,
C to 2 h, and 2 c
Same as above
C to an h, and 3 c
C to 3 h and a c

And for the group
C to 2 h and 2 c,
C to 3h and a c.

Aren't they all balanced then? 4 bonds for each c?

But the whole molecule is still missing H.

Offline OrionEmpire

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Re: Where do the extra H atoms go in structural formulas?
« Reply #6 on: June 01, 2016, 11:21:46 PM »
Sorry if there's something I'm missing that is basic. I'm not being officially taught any of this. This is all me reading a college grade book and slowly making sense of it. Never even took the act or any chemistry classes in any type of school.

I've had a lot of people tell me to screw off because like God for I'd I don't have an official education and I'm wasting people's time or something, but also God forbid I want to learn this s#*$ cuz it interests me.

So if there is something I'm overlooking, and your thinking wtf is wrong with this guy, chances are I haven't learned it yet. My chemistry knowledge is not strong. I'm actively studying general chemistry as well or biochemistry organic chemistry so I'm still getting the hang of everything, and not having a physical teacher does not help the case.

That's why I come here for help.

Offline kriggy

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Re: Where do the extra H atoms go in structural formulas?
« Reply #7 on: June 02, 2016, 01:25:47 AM »
Ok basicaly the way those formulas are written is that you need to "think" the hydrogens. You dont draw them but one assumes that at each carbon there is enough hydrogens to make the carbon have 4 bonds.
So in the compound Arkcon drew:
3x CH3 = 9 H
3x CH2 = 6 H
1x CH = 1H
Totaly 16 H

Im not sure where you got the number 11 or 13

Offline Borek

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Re: Where do the extra H atoms go in structural formulas?
« Reply #8 on: June 02, 2016, 03:03:11 AM »
That's the "full" molecule, with the "missing" hydrogens marked. Trick is - it is pretty obvious where they should be, so we don't draw them to save the space and actually make the picture much more clear. It is just a shorthand notation.
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Offline Babcock_Hall

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Re: Where do the extra H atoms go in structural formulas?
« Reply #9 on: June 02, 2016, 09:26:19 AM »
Alkanes follow the formula CnH2n+2, as long as no rings are present.  So for 3-methylhexane, n = 7 and 2n+2 = 16.  I hope that this is not a tangent.

Offline OrionEmpire

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Re: Where do the extra H atoms go in structural formulas?
« Reply #10 on: June 02, 2016, 09:59:16 AM »
The way this 30 some year old book explains it, is basically like

The Hexane by itself would be
                         
                           
CH3-CH2-CH2-CH2-CH2-CH3

6 carbon, 14 hydrogen.

But then it starts talking about when adding groups you want to make the longest continously chain of carbon equal to 6.
                                        CH2(2)-CH3(1)
                                         |
CH3(6)-CH2(5)-CH2(4)CH(3)-CH3

And that's where I get lost. Why does the chain on the bottom lose a "CH2", wouldn't that kill the Hexane? Because wouldn't it be Hexane PLUS an methyl group molecule bonding with the original Hexane molecule?

My book doesn't explain this. So I figured, count the longest continuousted chain of carbons, right? But when I do, counting from to 6 above,

That equals 13 H.
If I count just the bottom chain it's 11 H.


I'm more than a little confused here.

Offline AWK

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Re: Where do the extra H atoms go in structural formulas?
« Reply #11 on: June 02, 2016, 10:14:44 AM »
Addition of any alkyl substituent to hydrocarbon will exchange 1 H atom with this group (eg CH3) since each C-atom shows tetravalency.
CH3-CH2-CH2-CH(CH3)-CH2-CH3
I do hope now all is more clear. I remove 1 H atom at third C-atom (lower locant number fom the right side)  and replaced it by a methyl group. Hexyl group has 1 hydrogen atom less than hydrocarbon hexane. Grup replacing H-atom is paranthesized in linear formula.
AWK

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