VCE Chemistry Question Thread

[sweetiepi]

Ok, so this question is with ref to naming hydrocarbons in the Chem checkpoints book. For 1-chloroprop-2-ene, which is the way its written in the answers, shouldn't it be 3-chloroprop-1-ene, because the double bond has a higher priority compared to the halogen, and so numbering begins there? I understand the book is trying to get the lowest overall numbers, but im confused. Please help, cheers

The -ene functional group is definitely higher than the chloro- group. Therefore, I do believe that Checkpoints is wrong in this case. (As you do number from the highest priority group )

(Apologies for my late weigh-in )

[usernameincorrect]

The -ene functional group is definitely higher than the chloro- group. Therefore, I do believe that Checkpoints is wrong in this case. (As you do number from the highest priority group )

(Apologies for my late weigh-in )

Thanks so much! In fact, checkpoints has done this on a number of occasions, which is rather annoying. Luckily there's atarnotes...

[Gogo14]

I am really finding it hard to understand the principles of infrared and nmr spectroscopy. Can someone please explain? What i mainly dont understand is the principles on why they work and how to interpret/produce the graphs. Sorry if my question is really big, literally been stuck on this for 2 weeks

[VanillaRice]

I am really finding it hard to understand the principles of infrared and nmr spectroscopy. Can someone please explain? What i mainly dont understand is the principles on why they work and how to interpret/produce the graphs. Sorry if my question is really big, literally been stuck on this for 2 weeks

Hi Gogo14,
I'll try my best to provide a brief overview of the principles behind these techniques

IR spec: let's say that all bonds and atoms vibrate, and different types of bonds, and by extension, functional groups, have different ways of vibrating. IR spec uses the fact that different functional groups will absorb IR radiation at different wavelengths (frequencies).  When absorption of the IR radiation is detected, a signal (peak) is produced, which we can see on an IR spectrum (the graph). Functional groups will have specific wavelength ranges at which they absorb the IR radiation. Therefore, we can take a peak given on a spectrum and gain an idea of what functional groups may be present in the molecule.

NMR spec: in essence, NMR spec is similar to IR spec in that it analyses a change in a state - however in NMR, we are dealing with radio waves under a magnetic field (not IR) and changes in spin (not vibrational state). You will be using NMR to analyse the hydrocarbon backbone of organic molecules. VCE covers carbon NMR, which uses properties of the carbon-13 isotope, and proton (hydrogen) NMR, which analyses the hydrogen-1 isotope. Due to the internal structures (i.e. arrangement and interaction of protons, neutrons and electrons) of these isotopes, they have a 'spin'. When they spin, they produce a magnetic field. Without going into too much detail, NMR spec is able to analyse changes in spin in carbon-13 and hydrogen-1 atoms when they are exposed to specific ranges of radio frequencies. Similarly to IR spec, when such a change is detected, it will be indicated by a peak on the NMR spectrum. Different arrangements of hydrogen/carbon atoms will give different signals with respect to the standard (commonly, TMS).

This is probably a bit more than is required, but I hope it provides some context. Note you do not need to know how the instrumentation works (for VCE). Hope this helps 

With respect to interpreting the spectra (graphs), is there anything in particular you were stuck on?

[peanut]

For the addition reaction between alkenes and hydrogen halides (e.g. HCl), is a catalyst required? In my textbook, there's no mention whether there is or isn't, but my teacher said there is (AlCl3). Can someone confirm?

[MisterNeo]

For the addition reaction between alkenes and hydrogen halides (e.g. HCl), is a catalyst required? In my textbook, there's no mention whether there is or isn't, but my teacher said there is (AlCl3). Can someone confirm?

There is no catalyst for hydrogen halide addition because the alkene's double bond is already very reactive.

I think your teacher is referring to the addition of hydrogen halides to benzene. This would use aluminium chloride as a catalyst.

I found this on google images too.

[Gogo14]

Hi Gogo14,
I'll try my best to provide a brief overview of the principles behind these techniques

IR spec: let's say that all bonds and atoms vibrate, and different types of bonds, and by extension, functional groups, have different ways of vibrating. IR spec uses the fact that different functional groups will absorb IR radiation at different wavelengths (frequencies).  When absorption of the IR radiation is detected, a signal (peak) is produced, which we can see on an IR spectrum (the graph). Functional groups will have specific wavelength ranges at which they absorb the IR radiation. Therefore, we can take a peak given on a spectrum and gain an idea of what functional groups may be present in the molecule.

NMR spec: in essence, NMR spec is similar to IR spec in that it analyses a change in a state - however in NMR, we are dealing with radio waves under a magnetic field (not IR) and changes in spin (not vibrational state). You will be using NMR to analyse the hydrocarbon backbone of organic molecules. VCE covers carbon NMR, which uses properties of the carbon-13 isotope, and proton (hydrogen) NMR, which analyses the hydrogen-1 isotope. Due to the internal structures (i.e. arrangement and interaction of protons, neutrons and electrons) of these isotopes, they have a 'spin'. When they spin, they produce a magnetic field. Without going into too much detail, NMR spec is able to analyse changes in spin in carbon-13 and hydrogen-1 atoms when they are exposed to specific ranges of radio frequencies. Similarly to IR spec, when such a change is detected, it will be indicated by a peak on the NMR spectrum. Different arrangements of hydrogen/carbon atoms will give different signals with respect to the standard (commonly, TMS).

This is probably a bit more than is required, but I hope it provides some context. Note you do not need to know how the instrumentation works (for VCE). Hope this helps 

With respect to interpreting the spectra (graphs), is there anything in particular you were stuck on?

Thnks heaps, made it a lot clearer. I still don't really understand the concept of change in spin and chemical shift in nmr. Also don't understand how to interpret the structure of the molecule from the peaks in the graph.
But thanks a lot for the explanation, helped me understand it much better

[Bri MT]

Thnks heaps, made it a lot clearer. I still don't really understand the concept of change in spin and chemical shift in nmr. Also don't understand how to interpret the structure of the molecule from the peaks in the graph.
But thanks a lot for the explanation, helped me understand it much better

NMR:
I'm going to in terms of H-NMR because it tells you more than C-NMR

• The amount of energy you need to change spin state depends on shielding by electrons on neighbouring atoms
• Based on this, the data booklet has values for if a Hydrogen is around certain atoms, what the chemical shift may be
• The size of the peak (area contained by it) indicates how many hydrogens are in that "environment"
• (if the peak has an area of 3, and another peak has an area of one, then there are 3 times more hydrogens in the environment represtented by the first peak, than in the one represented by the second peak)

In high resolution H-NMR, a peak may also split into a cluster of peaks. The number of peaks in the cluster - 1 equals the number of hydrogens on neighbouring carbons.
In low resolution HNMR, there is no splitting.

For example, a H-NMR of propane would show:
-a peak due to the -CH3 groups   (bigger) which would be located between 0.9-1.0
-a peak due to the -CH2 group     (smaller) which would be located between 1.3-1.4
If this was high resolution we would see:
-the CH3 peak split into  3 peaks   (2 from the CH2 group +1)
-the CH2 peak split into 7 peaks   (3 from the 1st CH3 group + 3 from the second CH3 group +1)

Hope this helps

[j.wang]

hey

can someone please explain why a molecule is called 2-fluoro-3-bromopentane and another molecule is called 3-chloro-2- fluoro-2-iodohexane?

From the first molecule, i hypothesised that the functional groups were listed in numerical order (2, then 3)

But the 2nd molecule's name contradicts this??? (3, 2,2) The 2nd molecule seems to based on alphabetical order?

I noticed both of the molecules' functional groups are haloalkanes (if that makes a difference)

So is it numerical order or alphabetical order?

thanks

[VanillaRice]

hey

can someone please explain why a molecule is called 2-fluoro-3-bromopentane and another molecule is called 3-chloro-2- fluoro-2-iodohexane?

From the first molecule, i hypothesised that the functional groups were listed in numerical order (2, then 3)

But the 2nd molecule's name contradicts this??? (3, 2,2) The 2nd molecule seems to based on alphabetical order?

I noticed both of the molecules' functional groups are haloalkanes (if that makes a difference)

So is it numerical order or alphabetical order?

thanks

By IUPAC nomenclature, substituents should be listed in alphabetical order. The fact that they are haloalkanes doesn't affect this.

Hope this helps 

[j.wang]

By IUPAC nomenclature, substituents should be listed in alphabetical order. The fact that they are haloalkanes doesn't affect this.

Hope this helps 

Thanks so much

If it's in alphabetical order, why is the name of the first molecule 2-fluoro-3-bromopentane then? Shouldn't it be 3-bromo-2-fluoropentane?

[sweetiepi]

Thanks so much

If it's in alphabetical order, why is the name of the first molecule 2-fluoro-3-bromopentane then? Shouldn't it be 3-bromo-2-fluoropentane?

It should be 3-bromo-2-fluoropentane, I'd most likely put "2-fluoro-3-bromopentane" down to error, as I cannot see a logical reason as to why it should be the former, due to IUPAC nomenclature rules.

[j.wang]

It should be 3-bromo-2-fluoropentane, I'd most likely put "2-fluoro-3-bromopentane" down to error, as I cannot see a logical reason as to why it should be the former, due to IUPAC nomenclature rules.

Ahh thanks heaps, got it! 

[rg_123]

Hi Guys,

Quick question. We had our SAC earlier this week on the nomenclature, drawing and reactions section of the course and on it there was a question that asked us to draw three structural isomers of C3H5Cl. There was a bit of contention surrounding the naming of some of these molecules. Per VCAA rules does it matter whether the name is 3-Chloroprop-1-ene or 3-Chloropropene?

Thanks

[sweetiepi]

Hi Guys,

Quick question. We had our SAC earlier this week on the nomenclature, drawing and reactions section of the course and on it there was a question that asked us to draw three structural isomers of C3H5Cl. There was a bit of contention surrounding the naming of some of these molecules. Per VCAA rules does it matter whether the name is 3-Chloroprop-1-ene or 3-Chloropropene?

Thanks

As there is a chloro substituent, you would go with 3-chloroprop-1-ene, otherwise the term would be "chloropropene", which appears ambigious (as the chloro group could be anywhere on the molecule).

(Feel free to correct me here )