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

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Reply 380

Smiley Face :)

Hi guys, this exam is in the afternoon right??

Reply 381

otrivine

Original post by kingam

electrophillic , replaced and more reactive?

Perfect just the last one i think is acceptable but i was looking for stable :wink:

my turn

Reply 382

kingam

Original post by Smiley Face :)

Hi guys, this exam is in the afternoon right??

Yh mines at 1

Reply 383

kingam

Original post by otrivine

Perfect just the last one i think is acceptable but i was looking for stable :wink:

my turn

Alright

Erm , the reagents needed to convert nitrobenzene into an amine, and the structure of a benzene molecule

Reply 384

otrivine

Original post by kingam

Alright

Erm , the reagents needed to convert nitrobenzene into an amine, and the structure of a benzene molecule

structure of benzene i would draw it out in the exam with the P orbitals overlap , and its a trigonal planar and is 120 and is a planar molecule , and the P orbitals are the adjacent overlaps of adjacent P orbitals. delocolised (how do u spell it again lool ) ?

to convert the nitrobenzene into amine u need to have conc HCL/Sn and <10 ?

Reply 385

kingam

Original post by otrivine

Delocalised *, and benzene stuff looks fine , for the reagants tin concentrated hcl and reflux

(edited 11 years ago)

Reply 386

otrivine

Original post by kingam

Delocalised *, and benzene stuff looks fine , for the reagants tin concentrated hcl and reflux

thanks

so which one was the <10 ? keep getting it confused ?

ok

Describe what is meant by optical isomer and state 2 advantages and disadvantages (5)

Reply 387

kingam

Original post by otrivine

thanks

so which one was the <10 ? keep getting it confused ?

ok

Describe what is meant by optical isomer and state 2 advantages and disadvantages (5)

That's forming a dizonium ion from phenyl amine , and an optical isomer are mirror images of each other and I think they had to be attached to 4 differs groups (chiral), they form two mixtures and only one in needed racemix mixture , so they are separated by natural enzymes and chiral Catalyts

Reply 388

otrivine

Original post by kingam

hmm yes i think its excellent 4/4 Just memorize the mark scheme for the advantages and disadvantages :wink:

my turn

Reply 389

Phenylethylamine_

Need 79UMS on this for an A*! What was it for Jan 12? Around 53/60 for 81UMS, right? 79/90 doesn't seem too bad but I'm really prone to mistakes with this exam! Hate it so much! Know all the content/understand it, just find the qs hard! Like the cyclic dimer q in Jan 12 :s-smilie:

(edited 11 years ago)

Reply 390

kingam

Original post by otrivine

hmm yes i think its excellent 4/4 Just memorize the mark scheme for the advantages and disadvantages :wink:

my turn

Cool, what's needed for acid hydrolosis of polypeptides ,reagents to nitrate benzene

Reply 391

otrivine

Original post by kingam

Cool, what's needed for acid hydrolosis of polypeptides ,reagents to nitrate benzene

aic hydrolysis you would need to heat under reflux and (24hours and 1 mol dm-3) with an acid like conc h2so4 and for the regents to make the nitrate benzene you need 50 and conc h2so4

Reply 392

kingam

Original post by otrivine

aic hydrolysis you would need to heat under reflux and (24hours and 1 mol dm-3) with an acid like conc h2so4 and for the regents to make the nitrate benzene you need 50 and conc h2so4

Acid hydrolosis you need to heat under reflux 6moldm-3 hcl 24 hours , first one it's concentrated hno3 and acid catalyst h2s04

Reply 393

bencwbrown

Original post by Smiley Face :)

Hi 2 questions ... 1) Why have the grade boundaries gone up soo much in jan 2012?
2) In jan 2012 Q4ii... How are we supposed to know the retention time values, why do alkanes interact more and alchols interact less??
Btw, this is a PM exam right?

In Q4bii, the stationary phase of the GC is a liquid alkane, which is a non-polar solvent.

Non-polar molecules interact more stongly with non-polar solvents (and don't interact strongly with polar solvents). Hence the alkane has a long retention time as it interacts strongly with the alkane stationary phase (both are non-polar).

Polar molecules interact strongly with polar solvents (and interact weakly with non-polar solvents). Hence the alcohol (which is polar due to the -OH) doesn't interact much with the alkane stationary phase and hence passes through the GC quickly ie the alcohol has a short retention time

Hope it helps :smile:

Reply 394

darkxangel

Rings, acids and amines:
arenes:
-an arene is an aromatic hydrocarbon containing a benzene ring,
-the simplest one is benzene, C6H6,
kekule and p-orbital overlap model:
-alternate double bond-interchanging all the time,

delocalised model shows a benzene molecule when in hybrid state, each carbon contributes one electron from it's p-orbital to form pi-bonds, the pi-bonds are spread out or delocalised over the whole ring,
-the double bond in an alkane is a localised pi bond between two carbon bonds, in the delcoalised structure of benzene, each bond is identical, with electron density spread out to encompass and stabilise the whole ring,
-each pi-orbital overlaps with adjacent p-orbital, so that the de-localisation is extended over all six carbon atoms,

evidence:
-the kekule structure suggests two bond lengths with the C-C bond being longer, however experiments show that the actual bond lengths are somewhere in between, an intermediate so to say, 0.139nm,
-hydrogenation shows that benzenes enthalphy change is -208kJmol, whereas cyclohexane
is -200kJmol, the predicted value for the kekule structure was 3 * -120+=-360kJmol, since benzene can accept 3H2 instead of just one H2. But the real value is -360 - (-208)+-142, so around 150kJmol less than expected,
-therefore benzene has more stable bonding than kekule model (cyclohexal 1,3,5 - triene)
-compounds that contain C=C double bonds should readily de-colourise bromine. However benzene only reacts with bromine when boiled and exposed to UV light, which makes it doubtful that it has any double bonds,

nitration of benzene:
-reagents: HNO3 and catalyst concentration H2SO4,
-conditions: 60C,
-C6H6+HNO3--->C6H5NO2+H20,
-reaction mechanism of concentration H2SO4 is to generate the nitronium ion, NO2+ as the active electrophile,
-HNO3+H2SO4--->H2NO3+ =HSO4-
-H2NO3+--->NO2+ +H2O
-The H+ formed regenerates a molecule of H2SO4,
-H+ + HSO4- ->H2SO4,

Bromination of benzene:
-electrophilic substitution,
-only takes place with the presence of a halogen carrier,
-Al(Cl3/Br3/I3) or Fe halides,
-reagents: Cl2 and AlCl3 catalyst,
-conditions: anydrous,
-C6H6+Cl2--->C6H5Cl + HCl,
-generation of electrophile,
-Cl2 + AlCl3----->Cl+ + AlCl4-,
-Al+ +FeBr4- ---> FeBr3 + HBr/H+ + AlCl4- ----> AlCl3+ HCl,

bromination of alkenes/arenes:
-cyclohexene reacts readily with bromine, in the absence of sunlight, and undergoes electophilic addition reactions
-cyclohexene has a C=C, which has a high electron density,
-the electron density in cyclohexane is sufficient enough to induce a dipole in the Br-Br bond, generating an electrophile,
-the electrophile is attracted to cyclohexene, because of the latters high electron density,

bromination of phenols
-doesn't require a halogen carrier, which is explained by the activation of the ring due to:
-de-localisation of the lone pairs of e-s on O2 atoms into the ring, which increases electron density which in turn polarises the halogen, which increases attraction for the halogen,
-this results in an increased reactivity for phenol compared to benzene,

reactions of Phenol:
-they are weak acids, but alcohol aren't acidic-ethanol ionises less than water, which is because of the relative inductive effect of the aryl and the alkyl groups and the relative stability of the phenoxide and -ethoxide ions,
-in an alcohol like ethanol, the alkyl groups release electrons along the sigma bond and have a positive inductive effect,
-which increases the electron density in the OH bond, and hence ethanol is unlikely to donate a H+ ion,
-the benzene ring (C6H-) pulls electrons into the ring and have a negative inductive effect, which weakens the OH bond in phenol and stabilises the phenoxide ion.
-it also activates the ring, particularly at the 2, 4 and 6 positions. Consequently phenol behaves a weak acid and undergoes electrophillic substitution reactions much more readily than benzene,

salt reactions:
-phenols act as weak acids and form salts called phenoxides,
-reagents: NaOH and Na,
-C6H5OH + NaOH---->C6H5O-Na+ + H2O (neutralised)
-C6H5OH + Na ------> C6H5O-Na+ +0.5H2,

-so basically C6H5OH<====>H+ +C6H5O-

Bromination:
-bromine is de-colourised and white crystals of 2,4,6 tribromo phenol are formed,
-phenol can undergo multiple substitutions with bromine, whereas benzene is mono-substitution only,

uses of phenol:
-dilute solution of phenol are used as disinfectants and it was used as the first antiseptic,
-used in production of resins and plastics,
-more complex phenols are used as flavourings and aromas,