Chemistry 3/4 2013 Thread

[Sapphire]

Hey guys,
I was doing a question that asked to describe the H NMR spectrum of ClCH2CH2Cl, and I thought it would consist of a single triplet. But reading the solution, it said that protons that are chemically equivalent do not split their own signal. So, does that mean the spectrum would consist of a single, singlet?
Does this apply exclusively to molecules where all the hydrogens are chemically equivalent and have the same chemical shift? Because, when two protons that are chemically equivalent are in a molecule that has other proton environments, the NMR signal still shows splitting... I hope this makes sense, I'm just a bit confused right now

[thushan]

Splitting only occurs with nearest neighbours that are in a DIFFERENT chemical environment

[Sapphire]

Splitting only occurs with nearest neighbours that are in a DIFFERENT chemical environment

Thanks T.Rex yeah, I'm aware of that but the question I was doing just threw me off a bit. So just for verification,the H NMR of ClCH2CH2Cl would consist of a single, sharp peak, right?

[clıppy]

If someone could please help me on this redox titration question.
"When analysing the Fe²⁺ content of fertiliser by redox titration, Fe²⁺ (very pale green solution) is oxidized by MnO₄^- (intense purple solution) to Fe³⁺ (very pale yellow) and the MnO₄^- is reduced to Mn²⁺ (colourless solution)"

A primary standard solution of KMnO₄ is made up in 0.5M H₂SO₄. Why is this the solvent, rather than water?

The only thing i could think of is that water would not oxidize the Fe2+ to Fe3+ but I'm not sure if that is correct. Any help?

[teletubbies_95]

noclip:
I dont know if I'm answering the question right or not! This was in our first sac , i think!

So the full balanced equation is:
MnO4- + 8H+ + 5Fe2+ ----> Mn2+ + 4H2O + 5Fe3+

The H2SO4 is needed to supply H+ ions for the reaction to occur. For the reaction to occur , an acidic medium is needed for the KMn04- is a better reductant in this sort of environment compared to basic or nuetral( in your case, water) environment
The reason why H2SO4 is added is because it is a strong acid and will give up H+ ions at a faster rate compared to H+ ions in water.

Hope that helps !

Eclipse:Yesh! because all the hydrogens belong to the same environment.

[clıppy]

noclip:
I dont know if I'm answering the question right or not! This was in our first sac , i think!

So the full balanced equation is:
MnO4- + 8H+ + 5Fe2+ ----> Mn2+ + 4H2O + 5Fe3+

The H2SO4 is needed to supply H+ ions for the reaction to occur. For the reaction to occur , an acidic medium is needed for the KMn04- is a better reductant in this sort of environment compared to basic or nuetral( in your case, water) environment
The reason why H2SO4 is added is because it is a strong acid and will give up H+ ions at a faster rate compared to H+ ions in water.

Hope that helps !

So when the question is asking about the solvent, it's not talking about the KMnO4 it's talking about the solution the KMnO4 is in?

[hardworker]

Does anyone know why acid is used has a catalyst for addition reactions of alkenes.

[teletubbies_95]

noclip: From what I understand from your question , yes!

hardworker : The acid is needed to transfer the hydrogen proton and the OH- ion  when the double bond is broken to form an alkanol(I'm using the alkene + H20 example) . So , for example, if you use water , it is way to weak to transfer the proton to the alkane , therefore strong acids are used to transfer the proton .
I dont know if this is right or not.

[lzxnl]

If someone could please help me on this redox titration question.
"When analysing the Fe²⁺ content of fertiliser by redox titration, Fe²⁺ (very pale green solution) is oxidized by MnO₄^- (intense purple solution) to Fe³⁺ (very pale yellow) and the MnO₄^- is reduced to Mn²⁺ (colourless solution)"

A primary standard solution of KMnO₄ is made up in 0.5M H₂SO₄. Why is this the solvent, rather than water?

The only thing i could think of is that water would not oxidize the Fe2+ to Fe3+ but I'm not sure if that is correct. Any help?

Yes, sulfuric acid is needed as the redox reaction requires H+ ions, but don't forget that "sulfuric acid" is merely a solution of sulfur trioxide. We DO still have water in the 0.5 M sulfuric acid and THAT's where the H+ comes from. Theoretically, if we had pure sulfuric acid, we wouldn't actually have any H+ because for starters, H+(aq) implies that there is water and secondly, I highly doubt sulfuric acid can auto-dissociate like water can solely because it's a much stronger acid than base.

Does anyone know why acid is used has a catalyst for addition reactions of alkenes.

It will make more sense if we look at the mechanism. In an alkene, we have a carbon-carbon double bond, which contains a sigma bond and a pi bond. Regular C-C bonds are sigma bonds and the extra bonds (in the case of doubles and triples) are pi bonds, which are weaker. Therefore, they are easier to break. Now, the pi bond contains electrons. Therefore, something which is capable of grabbing electrons will attract the electrons towards them. If we tried to react an alkene without H+, then the oxygen on water would try to attack the pi bond. However, oxygen is negative and the electrons are negative, so we have a high activation energy.
If we use H+(aq), however, then the extra proton on the H3O+ can attack the pi bond as it is a positive charge and the electrons are negative. One of the carbons forms a bond with the proton and we have an intermediate which has a positive charge on a carbon. This is now much better for the oxygen on the water molecule to attack as the oxygen is negative and the intermediate formed is positive. Therefore, the activation energy is much lower. At the end, one of the hydrogens is grabbed by a water molecule as there are too many bonds to a carbon, reforming the catalyst.
Strong acids just provide more H3O+.

[Edward21]

Question, write an equation for the fermentation of ethanol (that's just glucose (occurring in yeast) to ethanol and CO2 yeah?)
But secondly, "The fermentation of ethanol stops when the %w/v reaches 20%, what is the maximum amount of sugar that can be used when producing ethanol in a 10L vessel?" I don't fully understand what is going on here, the %w/v of what? 

[Chazef]

how do the two reactions for producing aspirin differ (ethanoic anhydride vs ethanoic acid) because apparently in the reaction with ethanoic anhydride reactant, water produced pushes the reaction backwards. What I don't understand is that, surely there is already plenty of water to push the reaction backwards from the very beginning, considering that there will be water that comes with the sulfuric acid, or is all of that just hydronium. Help a brother out pl0x

[thushan]

how do the two reactions for producing aspirin differ (ethanoic anhydride vs ethanoic acid) because apparently in the reaction with ethanoic anhydride reactant, water produced pushes the reaction backwards. What I don't understand is that, surely there is already plenty of water to push the reaction backwards from the very beginning, considering that there will be water that comes with the sulfuric acid, or is all of that just hydronium. Help a brother out pl0x

Concentrated sulphuric acid. H2SO4 (l). Hardly any water present.

[hardworker]

Can anyone explain to me the reaction pathway and give me an example please.

Thanks in advance

[jgoudie]

Hardworker: The reaction parthway depends on what your starting chemicals are and what your products are.  There is no 'one reaction pathway' so to speak.  In general you will looking at a formation of esters from alkanes/alkenes, these take a few different steps.  If you have a certain reaction in particular post it and we can work through it.

Edward: This is one of my questions, the 20%w/v is the percentage of ethanol that is produced.  If we have a 10L vessel, the maximum amount of ethanol to be produced is 20%w/v of that 10L ie. 2kg. (once it hit ~20% the yeast dies and thus no more ethanol is produced).  You need to find out how much sugar is required to create the 2kg of ethanol.  Perhaps I should have been a little more specific.

Can anyone explain to me the reaction pathway and give me an example please.

Thanks in advance

[09Ti08]

Can someone please tell me how to study for reaction pathways? Somehow I am not so good at this and I keep messing things up. Thank you very much!