[kylatee]

So I just started studying organic chemistry as a part of my general chemistry nursing class, and I understand the basics of it, but what i don't understand is how to numbers on the Hydrogen atoms are rearranged.

For example:

in propene its : CH2 = CH2 and 1-Pentene its CH2=CH2 - CH2- CH2- CH3, why isn't CH3 first? or why isn't there another CH3 and then just 2 CHs? i understand how the total number of hydrogens are obtained, but how they are placed is mind boggling to me.

Any comments?

[kylatee]

I'm guessing that's a no to any comments..

[Arkcon]

Ugh.  Sometimes it takes more than 50 minutes for people to come up with answers. Especially on Sunday, in summer time.

OK.

So I just started studying organic chemistry as a part of my general chemistry nursing class, and I understand the basics of it, but what i don't understand is how to numbers on the Hydrogen atoms are rearranged.

in propene its : CH2 = CH2 and 1-Pentene its CH2=CH2 - CH2- CH2- CH3, why isn't CH3 first? or why isn't there another CH3 and then just 2 CHs? i understand how the total number of hydrogens are obtained, but how they are placed is mind boggling to me.

Hrm.  Can you draw out, what you're thinking it could also be, and see if there's anything wrong with your proposed structures?

[kylatee]

like for example, I did 1-pentene has CH3=CH2-CH2-CH2-CH2, but its the reverse in the book and done by the professor...is there any reason for this? and also why isn't it like CH4=CH2CH2CH2CH or something..

[se7en47]

it's fine if it's reverse in the book or the way your prof taught you since you always will start at where the double bonded C is. That's the rule btw, always start at the double bond for alkenes. So either way it will be 1-pentene. Yours is correct too so don't worry.CH4=CH2 etc does NOT work cause look at the C in the CH4. It has 6 bonds which equals a total of 12 valence e's, that's not possible because you only want 8 and you can ONLY have 8 e's in an octet. Hope that clears everything up.

[ufalynn88]

well i think the most fundamental thing in organic chemistry is that there are NEVER more than 4 bonds to carbon...and for now it is safe to say there are usually not less than 4 (unless youre dealing with a cation or anion..)thus a carbon in the middle of a structure will have at least 2 of those bonds already "taken", since they are bonded to the adjacent atoms..similarly a carbon double bonded to an atom then has only 2 more possible bonds...

[kylatee]

ok, thanks for the input, i guess i have to check for valence electrons too? -wondering why the prof. didn't mention this- here is another one that annoyed the hell outta me...it was for an Alcohol:

Book:                                                                Me:
2-methy-2-butanol                                                     CH3
                                                                     CH2 -  CH  - CH - CH2
        OH                                                                   OH
CH3 - C - CH2 - CH3
        CH3

Is there a difference? How do you know when to remove the H from one of the Hydrogens and place it onto another Carbon? and in the books method why isn't it CH2 then C then CH3? Why is there a Carbon atom alone?

[se7en47]

look at your carbon. do you see anything out of the ordinary?

[kylatee]

Well comparing it the books' version, i have a hydrogen atom on there, soo i want to know why it's wrong?

and the last one uhmm 3-ethylhexane is supposed to have 14 hydrogen atoms right? and is this counting the Hydrogens on the ethyl branch or no?

[sjb]

I think this is quite difficult to explain without being there in the flesh, but we'll have a go.

Carbon likes to bond with 4 things, in the case of single bonds. In the case of your structure from left to right it is bonding to 3, 5, 3 and 3 (assuming single bonds). The "carbon atom alone" is bonded to two methyl groups, an ethyl group and the OH group, there is no more to share out and bond to a hydrogen.


3-ethylhexane would have a total of 18 hydrogens, but let's take a step back - where have you got the 14 figure from?

[kylatee]

uhmm you lost me when you said 14 figure?

[kylatee]

oh nvm, i see what your saying. I got the 14 Hydrogens because its an alkane, and the alkane formula, which is CnH2n+2, so hexane would be C6H2n+2 which is 14...right?

[sjb]

oh nvm, i see what your saying. I got the 14 Hydrogens because its an alkane, and the alkane formula, which is CnH2n+2, so hexane would be C6H2n+2 which is 14...right?

That's fine, yes. Hexane is C₆H₁₄ Now, what's the structure of 3-ethylhexane and how does it differ from hexane - can you draw the two molecules out?