VCE Chemistry Question Thread

[stone1997]

ok so i'm really confused about functional groups
If a question asks to name the principal functional group do you name all functional groups or just the most important one? How do you choose the principal functional group?
Would alkyl, alkenyl and halo groups be considered a functional group are a substituent group? would they be the principal functional group if they are the only functional groups in the molecule?
Is the functional group in amines called amino or amine?

[wunderkind52]

Can someone please explain why homonuclear molecules such as O2 and N2 don't absorbed infrared radiation?

Thankyou!

I assume it is because they have the same electronegativity? So there won't be an overall dipole moment? Not sure if van der waals can though.

ok so i'm really confused about functional groups
If a question asks to name the principal functional group do you name all functional groups or just the most important one? How do you choose the principal functional group?
Would alkyl, alkenyl and halo groups be considered a functional group are a substituent group? would they be the principal functional group if they are the only functional groups in the molecule?
Is the functional group in amines called amino or amine?

This is a handy website.
http://www.masterorganicchemistry.com/2011/02/14/table-of-functional-group-priorities-for-nomenclature/

[RazzMeTazz]

Would this molecule have 4 carbon environments?

Thanks

[Sundal]

The theory of NMR is puzzling me.

So I thought that for nuclei with an overall spin, when an external magnetic field is applied they will either align themselves with the external magnetic field or against it. Then when those nuclei aligned with the external magnetic field absorb a particular energy of light (in the radio frequency of the EMS) they resonate and move to a higher energy level, thus being aligned against the external magnetic field.

However, my textbook describes this but in terms of nucleons not nuclei?
"When an external magnetic field is applied, the tiny magnetic field of a spinning nucleon aligns either with the field or against the field. A nucleon can absorb a quantum of energy and be promoted to a higher spin energy level."

Yet a few other sources I've read about online describe this process in terms of nuclei.

Can someone please help! Which is it, nuclei or nucleons? Both?

Cheers for any input.

[RazzMeTazz]

Can someone please help me with this question (below) ?

I wasnt quite sure what the carbon environment would be for the peak with a chemical shift just above 200ppm.

Thanks!

[grannysmith]

Can someone please help me with this question (below) ?

I wasnt quite sure what the carbon environment would be for the peak with a chemical shift just above 200ppm.

Thanks!

Most likely a C=O. Because there are 3 C environments, it can't be propanone so it must be propanal.

Edit: Thushan has a nice simple table which helps identify the type of carbon bond:
C-C (0-50 ppm)
C-O (50-100 ppm)
C=C (100-150 ppm)
C=O (150+ ppm)

[Sundal]

In an organic molecule, if there is an oxygen in the backbone, will it only have two bonds attached to it?

[stone1997]

would isomers such as propan-1-ol and propan-2-ol have any difference in their mass spectrum?

[Sundal]

Most likely a C=O. Because there are 3 C environments, it can't be propanone so it must be propanal.

Edit: Thushan has a nice simple table which helps identify the type of carbon bond:
C-C (0-50 ppm)
C-O (50-100 ppm)
C=C (100-150 ppm)
C=O (150+ ppm)

Does knowledge of different homolgous series such as alkanone and aldehydes help with this? How did you know the options could be propanone and propanal?

[grannysmith]

Does knowledge of different homolgous series such as alkanone and aldehydes help with this? How did you know the options could be propanone and propanal?

Yeah I guess it helps to know the structure of a ketone (R-CO-R', where Rs are hydrocarbons) and aldehydes (R-CHO, where R is any combination of hydrocarbons), both of which have C=O.

So if we infer that there must be a C=O, it can either be a carboxyl, ester, ketone or aldehyde. But there's only one O so we can ignore the first two options. Drawing out the structures, we find that a ketone would give 2 carbon environments while an aldehyde 3. So it must be an aldehyde i.e. propanal

[Sundal]

If you had a peak in your C-13 NMR spectra corresponding to a C=O and another peak corresponding to C-O, could you say that both of these peaks arise from the C in a carboxyl group?

[Sundal]

Question: Would different low resolution proton NMR spectra be observed for C3H6O(l) as opposed to C3H6O(aq)? Give a reason to support your answer.

I assume this is something to do with Hydrogen bonding, but wouldn't Hydrogen bonding exist in both the liquid and aqueous states of this substance?

Thanks

[keltingmeith]

Question: Would different low resolution proton NMR spectra be observed for C3H6O(l) as opposed to C3H6O(aq)? Give a reason to support your answer.

I assume this is something to do with Hydrogen bonding, but wouldn't Hydrogen bonding exist in both the liquid and aqueous states of this substance?

Thanks

Hint: In aqueous form, the organic compound C3H6O is surrounded by water - essentially making the aqueous form a mixture of C3H6O + H2O. The liquid form is just pure C3H6O.

[cosine]

The number of possible isomers with side chains increases as the number of carbon aroms increases. The side chain can be considered as an alkane molecule that has lost a hydrogen atom. The fragment of the alkane that forms the side chain is called an alkyl group.


What does this even mean???

[wunderkind52]

The number of possible isomers with side chains increases as the number of carbon aroms increases. The side chain can be considered as an alkane molecule that has lost a hydrogen atom. The fragment of the alkane that forms the side chain is called an alkyl group.


What does this even mean???

The number of possible isomers with side chains increases as the number of carbon aroms increases.
The more carbon atoms you have, the greater possible side chains, or branching. Consider CH4. How many structural isomers are there? Just CH4.
Consider butane - you can have CH3CH2CH2CH3 (straight chain C), but you can also have a side chain, which will make it 2-methylpropane, CH3CH(CH3)CH3.
So here you have two possible isomers.
If you take pentane - there is CH3CH2CH2CH2CH3, but also 2-methylbutane with 1 side chain CH3CH(CH3)CH2CH3, and also 2,2-dimethylpropane CH3C(CH3)2CH3.

The side chain can be considered as an alkane molecule that has lost a hydrogen atom.
Look at the bits in brackets - this is the side chain. Note that it is not CH4, but CH3. That's because it's bonded to a carbon atom in the backbone.
The fragment of the alkane that forms the side chain is called an alkyl group.
Basically, the bits in the brackets, CH3, or C2H5 etc are just called alkyl groups.