A level Chem4 Question (organic)

The rate determining step in the mechanism in part b (ii) involves attack by the nucleophile , suggest how the rate of reaction of propanone with HCN would compare with the rate of reaction of propanal with HCN

part b ii was just nu addition of hcn to propanal to form 2 hydroxy butanitrile .

I dont understand this question? any help

Part b (ii) gives you light on how the reaction normally works. The Nucleophile (HCN) attacks the C of the C=O group...

Now compare the two structures of propanone and propanal, what do you notice is different?





HINT

Tips for organic chemistry? I am really struggling :/

Is there something you're struggling specifically?

Perhaps the steps in certain reaction mechanisms? Remembering which mechanism is which? etc

Yes!
remembering Mechanisms and general revision for that module

+

I did the 2016 ocr paper (breath) as a school moc and found it extremely dificult, I didn;t get the calculation questions (they had percentages etc..) which were really different to the past paper questions :/

There's several ways you can go about remembering mechanisms... the one I did it and I think would really work is ONE BIG FAT MINDMAP/NETWORK

Lay out all the organic molecules you know: alcohols, amines, esters, ketones, benzenes, alkanes, allthatsht

You can then begin to draw links between the molecules to understand how to get from one to the other, e.g. alcohol --> aldehyde --> carboxylic acid

something like this
https://www.thestudentroom.co.uk/attachment.php?attachmentid=148500&d=1337182423


And then once you have that, I would just memorise that everyday. The map will also help in those questions where they're like "Describe a series of reactions that turn A into B"


When it comes to the actual steps of each mechanism, I think actually understanding it will help. the way I learned was
1. examine what the reacting agents look like (look for electron pairs, look for reactive groups to give you an idea of what would happen)
2. draw some arrows and just logically work out what should happen
3. compare to what it says in the pathway and if you got the end molecule, bravo
4. then you write the name

For instance.





^^ It's just that thought process which I think if you really understand helps to consolidate the mechanism. And then you could probably work it out yourself..


Once all that's in place, it's just a matter of practicing with a **** TON of questions. Seriously, organic chemistry is almost like maths: essentially the steps are the same, all that's changed is the species.
And it doesn't matter what exam board those questions came from. I just went on physicsandmathstutor, found so many questions on organic chemistry and did them all. They might have been the wrong exam board but it doesn't matter because the actual chemistry is the same

http://www.physicsandmathstutor.com/chemistry-revision/

It might not work for you. I know people who did songs to memorise what each of them was, some even acted out the mechanism by pretending to be the molecule.

Part b (ii) gives you light on how the reaction normally works. The Nucleophile (HCN) attacks the C of the C=O group...

Now compare the two structures of propanone and propanal, what do you notice is different?





HINT

Lol thanks man!
Do you need to know the equations of the reactions as well (or just the diagrams?)
How would you draw the sigma/pi bond in alkenes/alkanes if you were asked in a question?
And Why is there only 1[O] in alcohol to aldehyde reaction but 2[O] to form a carboxylic acid?
Do we need to know about hydrogenation/cracking in the new spec?

Yes that does sound familiar.
So the alkyl groups release electrons stabilising the centre carbon in propanone or is that less as effective as an ethyl on the centre carbon? And the more electron stable thr carbon the slower the reaction or?

Sorry im not so good with this atm.

You can derive the equations of the reactions from the diagram, IF you understand how structural formula works... when you do the mechanism, you tend to draw the structure in displayed formula which makes it easier to visualise. Hence, draw the structural formula from that.


So when they ask you to write the equation, the mechanism, and the mechanism name, the order to do things would be
1. the mechanism
2. the mechanism name
3. the equation
(at least how I see it)

Not all sigma bonds need to be drawn in a mechanism. For instance that propanone above, I didn't draw the sigma bonds between the C and the H in CH₃ because it doesn't add anything to the mechanism.
Basically, draw the sigma bonds around the reacting part, the part that's relevant, everything is just some group that tags along

In pi bonds, you should always draw the bond (at least that's how I learned it), for EVERYTHING, in your mechanisms, in your equation, because it removes any ambiguity whatsoever. That's why ethene should be CH₂=CH₂
Or if you wanna be super specific...
H₂C=CH₂



Regarding your question about oxidiation of primary alcohols, the easiest way to think of it is to think of the structures of the aldehyde and the carboxylic acid.






I had to learn about hydrogenation and cracking in my specification, I am not sure if it has changed so you'd need to look at your new spec, find it online, find any terms to do with cracking/hydrogenation and that'll answer it. If you do need to know, I suspect it's only minor, I only had to learn why you do cracking/hydrogenation and an overview of how to do it.

Thanks so much!
Could you explain the pi bond/sigma bonds, I dont get it. :/ If a question asked you to draw the formation of sigma and pi bonds in terms of s/p orbitals. How would you draw it?