OCR Chemistry A F324 Rings, Polymers and Analysis Tue 19 June 2012

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was the the reaction between hcooh and cr2o7 or cr2o7 and hcho?
because i first did hcho + cr207 because the emf of the hcho went to the left because it was more negative.. vali khob age ro emf hesab kony mishe cr207 + hcho. pas did both of the half equations go to the right?

I did Cr2O7 + 3HCHO + ____ => 3HCOOH

I don't know what you mean 'did both go to the right' :/

Reply 221

niaghez

Original post by Bright

I did Cr2O7 + 3HCHO + ____ => 3HCOOH

I don't know what you mean 'did both go to the right' :/

ohhh damnnn i did that but i crossed it out because i thought it said in the question hcooh to FORM hcho :frown:

thats another two marks gone! OMG :frown:

Reply 222

Bright

Original post by niaghez

ohhh damnnn i did that but i crossed it out because i thought it said in the question hcooh to FORM hcho :frown:

thats another two marks gone! OMG :frown:

Ahh maybe you were right, I was wrong..
don't worry about it, it's done :]

What did you apply for at uni?

Reply 223

Cloudmoshi

Original post by ChrisJ

Hello,

For polymers, do we need to remember the monomers that make Kevlar, terylene etc ?

So could we get a question:

Describe the polymerisation reaction that forms Kevlar, including an equation with the repeat unit.

(without it providing information on the monomers)

These question are normally testing if you know how to connect two monomers to form a polymer rather than you remembering the two monomers. So I believe they will tell you don't take my word for it though.

Reply 224

pinkcherrytart

im just dreading the NMR questions!

Reply 225

hetty123

hey guys here are some questions i predict will probably come up:
compare structure/reactivity of benzene compared with that of phenol/alkane/alkene (didn't come up in jan 2012)
electrophilic sub mechanism...
products/reagents for reduction/oxidation/polymerisation
observations/reaction type/products of using tollens and bradys reagents
drawing optical isomers (didn't come up in transition metals part of f325 june 2012)
+'s and -'s of drug synthesis
definitions of sterioisomers/GC/GCMS, how it seperates...
identifying chiral carbons
mass from %yield
NMR/predictions/MS for identifying: usually an ester

Reply 226

hetty123

Original post by pinkcherrytart

im just dreading the NMR questions!

yeh i really hate them too, in a mock i got all of my marks from BOD because i didn't use any actual nmr values, just ratios and the n+1 rule with %mass and a whole load of guessing

Reply 227

hetty123

can anybody explain to me about bromine water being added to an aromatic arene... i know i may sound really stupid but do the double bonds get taken away, how does in work, why does bromine water get decolourised....i just dont get it :s-smilie:

Reply 228

ChrisJ

Original post by Cloudmoshi

Thanks, thought so, but I think I will learn them anyway. Won't take too long :smile:

Reply 229

ChrisJ

Original post by hetty123

Bromine water has an orange colour because of the Br2, so when the Br2 reacts with an amine, the water won't contain Br2 any more and it is colourless. You can react it with 3 different aromatic amines, but I guess you're referring to cyclohexene from your mention of double C=C bonds? The other two are phenol and benzene and they react quite differently.

Cyclohexene has a C=C bond with localized electrons and a high electron density. It is able to induce a dipole in the Br-Br molecule and break it by heterolytic fission to form the Br+ electrophile, which then attacks the C=C bond.
This process is electrophillic addition, can you remember it from AS chemistry?

check out the bottom of this page: http://www.chemguide.co.uk/mechanisms/eladd/symbr2.html
(Only look at the simplified mechanism)

Reply 230

Bright

Original post by hetty123

If a double bond is present e.g. in cycloalkenes then the high pie electron density polarises the bromine molecule and electrophillic substitution takes place. So the double bonds react with the bromine and bromine water goes from orange to colourless (is decolourised). :]

Reply 231

VQG

Need to start revising for this - got 79 UMS in January. Trying to make up 11 - grade boundaries were stupid in Jan though :/

Reply 232

hetty123

if i use the example below- safranal as it came up in the jan 2011 paper

why is it that if Br2 is added to this it loses its double bonds, i thought it just replaces the hydrogens so why does it lose its double bonds????????
this is really :confused:

Reply 233

NaeemChoudhury

When drawing the nitration of benzene mechanism where does the curly arrow come from, im confused because my teacher says it comes from the center of the ring but in the mark schemes and the ocr book it doesn't come from the center of the ring any help? Thank you :smile:

Reply 234

bencwbrown

Original post by hetty123

if i use the example below- safranal as it came up in the jan 2011 paper

why is it that if Br2 is added to this it loses its double bonds, i thought it just replaces the hydrogens so why does it lose its double bonds????????
this is really :confused:

The safranol undergoes electrophilic addition, just like a normal alkene does. So the pi-bond of the double bond breaks, and the two bromine atoms join onto the two carbons involved in the double bond (so no hydrogens are lost).

This might help as it shows the reaction on a simple alkene (but identical principles) http://www.chemguide.co.uk/mechanisms/eladd/symbr2.html#top (you should notice that each carbon has the same number of hydrogens before and after the reaction)

(edited 11 years ago)

Reply 235

andrew_c

Original post by VQG

Need to start revising for this - got 79 UMS in January. Trying to make up 11 - grade boundaries were stupid in Jan though :/

I heard that too, apparently it was 50 or 60 % for an E? :confused:

Reply 236

freakymonkey

Original post by andrew_c

I heard that too, apparently it was 50 or 60 % for an E? :confused:

Yup. 57/60 for full marks (the equivalent paper in Physics is never more than 50 or so), and then 4 mark grade boundaries (49 for an A :O) down to 29/30 for an E. Ridiculous.

I blame this, not my total lack of preparation, for my C in jan :P

Reply 237

mack94

Original post by racheatworld

It has 5, as shown by my diagram below:

Carbon 1 in the benzene ring does not count as a proton environment as there are no H's attached to carbon 1.

Hi whats the difference between D and E aren't they ion the same environments?

Reply 238

tomkeys

Original post by hetty123

if i use the example below- safranal as it came up in the jan 2011 paper

why is it that if Br2 is added to this it loses its double bonds, i thought it just replaces the hydrogens so why does it lose its double bonds????????
this is really :confused:

It is not an arene, just a cyclic hydrocarbon with two alkene groups thrown in. As has been mentioned above, it undergoes electrophilic addition, rather than electrophilic substitution, which is what I believe you are describing. A good add-on question would be "Why does this compound react differently to an arene?" :tongue: