Edexcel - Chemistry Unit 2 - 4 June 2013

Can anyone explain how this question is done please... Couldn't find stuff about it in the textbook or CGP revision guide...
It's from the January 2012 paper btw



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Add a hydrocarbon solvent like hexane and it turns purple


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Thank-you very much !

Anything else I need to know about hexane/hydrocarbons? In terms of group 7???

Oh and also guys, are any of you going to memorize group 7 reactions?? Such as halogens in alkali...
Cl2 + 2NaOH ---> NaCl + NaOCl + H2O

Just seems like a handful to remember

im pretty certain this is to do with sn1 and sn2 mechanisms. as one is primary and the other tertiary.

why do you put water into reflux condenser?

hey! does anyone understand this one:

how many molecular ion peaks occur in the massspec of 1,2 dibromoethane, CH2BRCH2BR? Assume the only isotopes present are 1H, 12C 79BR and 81BR?

A:1
B:2
C:3
D:4

The answer is D, anyone know why??

One of the examiner's report suggested creating a reaction database with reactant, reagent(s), conditions, (organic) product, the type and mechanism of the reaction.

But I mean who's bothered to do all that when only about 10% is questioned in the exam.

hey! does anyone understand this one:

how many molecular ion peaks occur in the massspec of 1,2 dibromoethane, CH2BRCH2BR? Assume the only isotopes present are 1H, 12C 79BR and 81BR?

A:1
B:2
C:3
D:4

The answer is D, anyone know why??

I don't know what those are... Could you please explain or cite where in the cgp book or Internet it is explained... Thanks


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Yh sure
S - substitution
N - nucleophile
'no.' - how many species involved in first step

Well usually associate the Sn2 mechanism with primary halogenoalkanes (for this particular unit anyway).
2 species are involved in the initial step... the nucleophile & the halogenoalkane. The delta - element in the nucleophile moves towards the delta + carbon in the halogenoalkane.
While it does this, The C-Br is in the process of breaking (electrons given to bromine).
So a bond is forming & breaking at the same time... this is called the transition state.
Here I've found an example of Sn2 reaction (ignore equilibria signs):
Moving onto Sn1 ...

In tertiary halogenoalkanes.... there are 3 methyl groups, these pushed electrons towards the carbon atom and make it less delta positive. This makes the C-X bond weak therefore it break. No nucleophile is required to approach it. The first step only contains one species... the halogenoalkane, hence the one in Sn1.
Here's an example of an Sn1 mechanism:


(Nu = nucleophile)

Mechanisms for secondary alcohols never comes up I believe... but they can undergo both Sn1 & Sn2. Though Sn1 is the most likely mechanism.

Hope that helps

hey! does anyone understand this one:

how many molecular ion peaks occur in the massspec of 1,2 dibromoethane, CH2BRCH2BR? Assume the only isotopes present are 1H, 12C 79BR and 81BR?

A:1
B:2
C:3
D:4

The answer is D, anyone know why??

I think you've copied this wrong, the answer is C is it not? I think im doing this paper right now. But in all honesty, I have no idea how to get to that answer and got it wrong just now.

Posthumus and Igcse are correct for the question above.

This is what happens when you don't read the question correctly

Hey James, have you memorized all the group 7 reactions??
From past papers it seems mainly they want you to show group 1 & 2 reactions.

I don't know many group 7 reactions

They have asked about molecular ion peaks, so the positive species that causes the peaks being considered are all [CH2BRCH2BR]⁺.

So you should consider all the possible isotopic configuration. Mathematically you can just multiple the possible isotope for each atom.
But luckily they have asked us to make an assumption about all the carbon and hydrogen atoms present in the ion. So they all can be only one isotope. 1*1*1*1...=1 so you don't need to bother.But you should bother about the bromine atoms.

There are two bromine atoms and each atom can be of two given isotopes,
So 2*2=4

Still got some more stuff to remember, I haven't learnt all.

Out of these which ones do you think are unnecessary to remember:
Halogen reactions with:
metals
phosphorus
other halides
water
alkali (hot & cold)
reducing agents
sodium thiosulfate

Then there's quite a few for halogen salts as well, I haven't come across hexane yet