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Author Topic: ALL CIE CHEMISTRY DOUBTS HERE !!  (Read 122905 times)

Offline Amii

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #360 on: February 05, 2011, 12:23:50 pm »
Quote from: Ari Ben Canaan on February 04, 2011, 11:00:59 am
Questions 23,26,29 and 30

26) B-CH3CHBRCH=CH2 has identical atoms attached to one of the carbon atoms in the double bond- so no cis-trans in this compound.

A,B,D - exhibits cis-trans isomerism.

But only D has a Carbon atom which has 4 different atoms/groups attached to it - hence only D will can show optical isomerism.

I hope it makes sense.  :-[

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Offline Amii

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #361 on: February 05, 2011, 01:03:38 pm »
Quote from: Ari Ben Canaan on February 04, 2011, 11:00:59 am
Questions 23,26,29 and 30
29)D (CH3)3CBr

-Tertiary halogenoalkane.
-Undergoes hydrolysis by SN1 mechanism.
-Reaction takes place in two steps.
-Since the rate determining step doesn't involve the nucleophile,changing the concentration of OH- doesn't affect the rate of the SN1 reaction.

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Amelia

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #362 on: February 05, 2011, 01:15:05 pm »
Quote from: Ari Ben Canaan on February 05, 2011, 11:45:44 am
I've identified the chiral centres by drawing a red square next to them.

Am I correct ?

Yes.


The answer to 23 is C.
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elemis

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #363 on: February 06, 2011, 03:31:43 pm »
In the first one, why isnt the answer D ?

Reduction is the GAIN of Hydrogen, right ? In step 2 a massive molecule is added that has several hydrogens. Isnt this reduction ?

Can you explain why the answer is C ?

In the second one, I have marked the chiral centre. Is it correct ?

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Amelia

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #364 on: February 07, 2011, 08:23:18 pm »
 ^ There are three methyl groups in 2,2dimethylbutane. Available for Chlorine radicals to attack on. So free radicals are formed from each Ch3's.


Quote
In the second one, I have marked the chiral centre. Is it correct ?

Yes, it is.
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elemis

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #365 on: February 08, 2011, 02:41:52 pm »
I know the carbon with the red square is DEFINITELY chiral. Why is the one with the PINK square  NOT chiral ?
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elemis

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #366 on: February 08, 2011, 02:44:27 pm »
Quote from: Amelia on February 07, 2011, 08:23:18 pm
^ There are three methyl groups in 2,2dimethylbutane. Available for Chlorine radicals to attack on. So free radicals are formed from each Ch3's.


I dont understand. Are you talking about the right question ?
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Offline ashish

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #367 on: February 08, 2011, 02:57:25 pm »
Quote from: Ari Ben Canaan on February 06, 2011, 03:31:43 pm
In the first one, why isnt the answer D ?

Reduction is the GAIN of Hydrogen, right ? In step 2 a massive molecule is added that has several hydrogens. Isnt this reduction ?

Can you explain why the answer is C ?

In the second one, I have marked the chiral centre. Is it correct ?



it is not reduction as we are nor adding hydrogen directly to the molecule!
the alcohol is acidic (releases H+) creating CH3CH2O- which then attack the CH3C+HO- creating an nucleophile which is then attacked by the proton
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elemis

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #368 on: February 08, 2011, 02:59:17 pm »
Quote from: ashish on February 08, 2011, 02:57:25 pm
it is not reduction as we are nor adding hydrogen directly to the molecule!
the alcohol is acidic (releases H+) creating CH3CH2O- which then attack the CH3C+HO- creating an nucleophile which is then attacked by the proton

Thanks.

Could you answer my other questions, please ?
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Offline EMO123

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #369 on: February 08, 2011, 03:08:39 pm »
Best of luck Twinkle Charms for your chem exams
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Amelia

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #370 on: February 08, 2011, 03:34:52 pm »
Quote from: Ari Ben Canaan on February 08, 2011, 02:41:52 pm
I know the carbon with the red square is DEFINITELY chiral. Why is the one with the PINK square  NOT chiral ?

It actually has three chiral's. The pink one you marked is a chiral as well. And also, the first Carbon. (CH-CH3)

Quote from: Ari Ben Canaan on February 08, 2011, 02:44:27 pm
I dont understand. Are you talking about the right question ?

Mistook your question. I apologize.
« Last Edit: February 08, 2011, 03:43:47 pm by Amelia »
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elemis

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #371 on: February 17, 2011, 04:37:02 am »
Last question, (e) part.

I drew the following compound, yet the markscheme shows something else altogether.

Would my answer be accepted ?
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Offline Deadly_king

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #372 on: February 17, 2011, 05:15:36 am »
Quote from: Ari Ben Canaan on February 17, 2011, 04:37:02 am
Last question, (e) part.

I drew the following compound, yet the markscheme shows something else altogether.

Would my answer be accepted ?

Sorry for lateness but I had to do the whole number to get the required answer.

So your answer will NOT be accepted. Tests 5 and 6 eliminates your answer completely. Now I'll explain :

For the compound you drew, upon test 5, it would get oxidised to a carbon dioxide and a carboxylic acid as the C=C will break. So the resulting carboxylic acid will give a positive result with the following test.(test 6)

Hence your answer is rejected.

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elemis

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #373 on: February 17, 2011, 05:25:48 am »
Quote from: Deadly_king on February 17, 2011, 05:15:36 am
Sorry for lateness but I had to do the whole number to get the required answer.

So your answer will NOT be accepted. Tests 5 and 6 eliminates your answer completely. Now I'll explain :

For the compound you drew, upon test 5, it would get oxidised to a carbon dioxide and a carboxylic acid as the C=C will break. So the resulting carboxylic acid will give a positive result with the following test.(test 6)

Hence your answer is rejected.



2,4-DNPH gives a positive result only with Aldehydes or Ketones not Carboxylic acids.
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Offline Deadly_king

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Re: ALL CIE CHEMISTRY DOUBTS HERE !!
« Reply #374 on: February 17, 2011, 05:42:10 am »
Quote from: Ari Ben Canaan on February 17, 2011, 05:25:48 am
2,4-DNPH gives a positive result only with Aldehydes or Ketones not Carboxylic acids.

That's true but before turning into a carboxylic acid......the compound will first be converted into an aldehyde. We need to oxidise much more for it to get converted to carboxylic acid.
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