VCE Chemistry Question Thread

[I am a unicorn]

Hi
According to my textbook, generally only liquid or solid samples (that can be vapourised) can be analysed using mass spec.
Why can't gaseous samples be analysed via mass spec??

Thankyou!!

[cosine]

Draw the structural formula for:

2-chloro-1-ethylpropane

[sunshine98]

Draw the structural formula for:

2-chloro-1-ethylpropane

Prop-suggests that there are three carbons.
ethyl - means that there is a CH2CH3 group attached at the first carbon
2-chloro--> there is a chloride attached at the 2nd carbon

Write in your carbon backbone , then place in those substitutes(the one's mentioned above) then fill the rest in with H.

[keltingmeith]

Draw the structural formula for:

2-chloro-1-ethylpropane

Where'd you get this one from? Because it sorta doesn't follow IUPAC naming rules... In fact, if named correctly, it should be 2-chloropentane.

[cosine]

Where'd you get this one from? Because it sorta doesn't follow IUPAC naming rules... In fact, if named correctly, it should be 2-chloropentane.

Heinemann ..

What do you mean, what's wrong with the name of it? Can you show me how it's drawn please, the answers do not show structural formulas for some reason...

[Splash-Tackle-Flail]

Why can cyclohexane undergo complete combustion but cyclohexene can't?

[lzxnl]

Why can cyclohexane undergo complete combustion but cyclohexene can't?

Erm...I'm fairly sure if you ignite almost any hydrocarbon it'll undergo complete combustion, given enough time, oxygen and maybe heat input.

[Splash-Tackle-Flail]

Ah ok, so how would I explain burning cycloethene and observing a blackish flame (when lighted with a match, whereas the cyclohexane had a clean flame (this is for a Prac)

[lzxnl]

Ah ok, so how would I explain burning cycloethene and observing a blackish flame (when lighted with a match, whereas the cyclohexane had a clean flame (this is for a Prac)

I'm assuming you mean cyclohexene not cycloethene which doesn't exist.
This has probably got to do with the rates of reaction and the reaction pathway. Cyclohexene has a double bond which will be broken first in the reaction with oxygen. You'll likely form an epoxide (3 membered ring containing C, O, C) first which reacts further with more oxygen.

Reacting cyclohexane, however, proceeds via a different pathway. As cyclohexane is saturated, there aren't any really weak bonds to break. The reaction pathway here is definitely going to be different to the first one (probably some complicated radical mechanism).

In any case, the different reaction pathways will produce different products at different rates. Given enough time, you SHOULD eventually get CO2 and H2O from either hydrocarbon. It's a kinetics issue.

[Splash-Tackle-Flail]

I'm assuming you mean cyclohexene not cycloethene which doesn't exist.
This has probably got to do with the rates of reaction and the reaction pathway. Cyclohexene has a double bond which will be broken first in the reaction with oxygen. You'll likely form an epoxide (3 membered ring containing C, O, C) first which reacts further with more oxygen.

Reacting cyclohexane, however, proceeds via a different pathway. As cyclohexane is saturated, there aren't any really weak bonds to break. The reaction pathway here is definitely going to be different to the first one (probably some complicated radical mechanism).

In any case, the different reaction pathways will produce different products at different rates. Given enough time, you SHOULD eventually get CO2 and H2O from either hydrocarbon. It's a kinetics issue.

Thank you! Makes sense!

[Sense]

When you're asked for ratio of peak areas on a chromatogram, how do you know what to round it off to?

Ex.
Peak Area of ethanol = 418,530
Peak area of propanol = 1,561,900

418530/418530 = 1
1561900/418530 = 3.73187107256

Would I just go 1:3.75, or is there a rule to how many significant figures you go to?

[scarletmoon]

For the Aspirin SAC what exactly do you need to know? (other than the esterification process, structure of the molecules involved)

[keltingmeith]

For the Aspirin SAC what exactly do you need to know? (other than the esterification process, structure of the molecules involved)

There's a few different ways to make it (consider acetic anhydride versus acetic acid), make sure you know the advantages/disadvantages of each. How do you know you made aspirin? And then just general pathways and shit.

[Macka598]

i'm not sure i need to know this, but i'm wondering why is the OH on COOH acidic and the OH on hydroxy groups such as on alcohols not acidic?

[lzxnl]

i'm not sure i need to know this, but i'm wondering why is the OH on COOH acidic and the OH on hydroxy groups such as on alcohols not acidic?

Consider the C=O bond. The negatively charged oxygen makes the carbon quite positive. Hence the carbon then attracts electrons from the neighbouring oxygen a bit more than normal. That oxygen loses some of its electron density, so it tugs on the hydrogen. The hydrogen now has even less electron density than normally, corresponding to a weaker bond. Hence it's deprotonated more readily.

Also, there's something called resonance which stabilises a carboxylate but not an alkoxide (conjugate base of an alkanol). Look it up if you want; essentially in COO-, the negative charge is averaged over the two oxygens and this makes the conjugate base more stable which means the conjugate base strength decreases and the acid strength increases.