Chemistry help (1 Viewer)

[may22]

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]

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]

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]

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]

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]

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]

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/

 

[icycledough]

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/

I guess you can also look at it like as if it were algebra. For example, you would typically tend to write 2x^2 instead of x^2 * 2

 

[may22]

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]

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:

 

[may22]

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]

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.

 

[jazz519]

What is the difference between these?
View attachment 32815

The bond exists between the two carbons. The hydrogen are then bonded to the top carbon. They are not connected to the CH carbon

Therefore it is incorrect to draw the 2nd one because it implies there is a bond either with the carbon or the 3H

 

[CM_Tutor]

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.