Chemistry help (1 Viewer)

may22

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Okay so my teacher started year 12 content with mod 7, but I'm confused about something.
When drawing alkanes/alkynes/alkenes etc, we have to include the hydrogens with the carbons right? hydrocarbons and all that...
I know how many hydrogens to have with each carbon when doing alkanes, but I'm confused with alkenes and alkynes, particularly with the double and triple bonds.

I'm confident with naming, it's just drawing them that throws me off
 

CM_Tutor

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Each C atom will have 4 bonds... any that are vacant, fill with H atoms.

So, if I have a compound with a carbon backbone of C-C=C-C-C=C=C-C=C, then filling in H's, I'd get H3C-CH=CH-CH2-CH=C=CH-CH=CH2
 

may22

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Each C atom will have 4 bonds... any that are vacant, fill with H atoms.

So, if I have a compound with a carbon backbone of C-C=C-C-C=C=C-C=C, then filling in H's, I'd get H3C-CH=CH-CH2-CH=C=CH-CH=CH2
Ah okay that made it sound so much simpler than I thought

Also, how do you know if you have to write it as H3C or CH3?
 

Life'sHard

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Ah okay that made it sound so much simpler than I thought

Also, how do you know if you have to write it as H3C or CH3?
Carbon always before the hydrogen. More about learning how to draw them. You'll eventually learn the general structure of an alkane/ene/yne. Once you do, it should be pretty simple.
 

may22

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Carbon always before the hydrogen. More about learning how to draw them. You'll eventually learn the general structure of an alkane/ene/yne. Once you do, it should be pretty simple.
Okie cool thank you!
 

icycledough

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Ah okay that made it sound so much simpler than I thought

Also, how do you know if you have to write it as H3C or CH3?
As @Life'sHard mentioned, convention is to write it as CH3. However saying that, I don't think you would be penalised for writing H3C. DON'T TAKE MY WORD FOR IT THOUGH; you should always clarify with your teachers how they will be marking, as you should never assume anything you aren't 100% confident about.
 

Life'sHard

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As @Life'sHard mentioned, convention is to write it as CH3. However saying that, I don't think you would be penalised for writing H3C. DON'T TAKE MY WORD FOR IT THOUGH; you should always clarify with your teachers how they will be marking, as you should never assume anything you aren't 100% confident about.
You're right. Though it looks better when the C is infront lol plus it's also how the HSC formats their solutions. https://www.physicsforums.com/threads/whats-the-difference-between-h3c-and-ch3.574027/
 

may22

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Yeah that makes sense
I always write it as CH3 because like @Life'sHard said it looks better lol. I was just confused because some of the worksheets we got had it with the H in front so I wasn't sure
Thanks guys :)
 

CM_Tutor

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Carbon always before the hydrogen. More about learning how to draw them. You'll eventually learn the general structure of an alkane/ene/yne. Once you do, it should be pretty simple.
Ummm... not quite.

You should always show bonds connecting to the atom that is bonded. So, I might write methanol as

CH3OH

or

CH3-OH

or

H3C-OH

(to emphasise that the bond is between the C and the O)

or even

HOCH3

or

HO-CH3

but never as

CH3-HO

or

OH-CH3

for example, as these last suggest a bond from C to H then to O

Similarly, while propane can be

CH3CH2CH3

or even

C3H8

I might also draw it as

H3C-CH2-CH3

If I was drawing something like 2-methylpropane

CH3CH(CH3)CH3

when I draw the central methyl above the chain, I'd be very careful to show the bond going to the C, not to the H or vaguely to the CH3 somewhere. To illustrate:

methylpropanes.png
 

may22

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Ummm... not quite.

You should always show bonds connecting to the atom that is bonded. So, I might write methanol as

CH3OH

or

CH3-OH

or

H3C-OH

(to emphasise that the bond is between the C and the O)

or even

HOCH3

or

HO-CH3

but never as

CH3-HO

or

OH-CH3

for example, as these last suggest a bond from C to H then to O

Similarly, while propane can be

CH3CH2CH3

or even

C3H8

I might also draw it as

H3C-CH2-CH3

If I was drawing something like 2-methylpropane

CH3CH(CH3)CH3

when I draw the central methyl above the chain, I'd be very careful to show the bond going to the C, not to the H or vaguely to the CH3 somewhere. To illustrate:

View attachment 32813
Oh wow I didn’t realise how specific it was meant to be!
Thank you! The diagram made it pretty clear as well 😊
 

icycledough

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What is the difference between these?
View attachment 32815
I believe that with the first one, the single bond connecting the methyl group to the chain is directly pointing on the Carbon atom, which it should be as carbon forms 4 bonds. However, with the second one, it's kind of pointing in the middle of the C and H. If you are going to use condensed form, which I believe should be fine OTHERWISE they state to draw the full diagram, then it has to be pointing precisely at the Carbon atom.

Again, I think that's what @CM_Tutor is trying to say, but I'm not 100% on that.
 

CM_Tutor

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I believe that with the first one, the single bond connecting the methyl group to the chain is directly pointing on the Carbon atom, which it should be as carbon forms 4 bonds. However, with the second one, it's kind of pointing in the middle of the C and H. If you are going to use condensed form, which I believe should be fine OTHERWISE they state to draw the full diagram, then it has to be pointing precisely at the Carbon atom.

Again, I think that's what @CM_Tutor is trying to say, but I'm not 100% on that.
This is indeed what I mean. Drawing a bond to the middle of a group, like to between the O and the H in an OH group, is bad practice... you might get away with it, or not, depending on the marker and whether a decision has been made to be strict.
 

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