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AS Chemistry P2 Prep.

angelicsuccubus

angelicsuccubus

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angelgirl:) said:
ok check m/j 2005 Q4 ai...for 2nd stage...its nucleophllic subs.. of halogeno alkne with KCN to from nitril...condtion they wrote heat in ethanol...not reflux...so is wrong?
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no that's okay, they tend to write incomplete stuff in the mark scheme.. but if you wrote heat with reflux or heat it would be right. The process in real life would use a refluxing column.
 
angelgirl:)

angelgirl:)

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angelicsuccubus said:
no that's okay, they tend to write incomplete stuff in the mark scheme.. but if you wrote heat with reflux or heat it would be right. The process in real life would use a refluxing column.
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in the b part of that Q to change -CN to -COOH they wrote heat/reflux...iso it doesnt matter write?
 
angelicsuccubus

angelicsuccubus

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angelgirl:) said:
in the b part of that Q to change -CN to -COOH they wrote heat/reflux...iso it doesnt matter write?
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probably not here, but its best to remember which use reflux and which don't because sometimes writing reflux is incorrect and sometimes heat only is incorrect. In november 07, for the KCN reaction they wrote heat under reflux... so for that reaction, stick to heat under reflux. For the oxidation of an alcohol to an aldehyde is the one reaction you can never write reflux! (because if you refluxed it.. it would give a carboxylic acid, not an aldehyde)
 
angelgirl:)

angelgirl:)

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angelicsuccubus said:
probably not here, but its best to remember which use reflux and which don't because sometimes writing reflux is incorrect and sometimes heat only is incorrect. In november 07, for the KCN reaction they wrote heat under reflux... so for that reaction, stick to heat under reflux. For the oxidation of an alcohol to an aldehyde is the one reaction you can never write reflux! (because if you refluxed it.. it would give a carboxylic acid, not an aldehyde)
Click to expand...
ya about -OH to -CHO i know...but anyways thnks alot...
 
angelgirl:)

angelgirl:)

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ok ppl leaving the thread now... have to go...c u tomo..withpaper discussion....hope all to well...
InshaAlllah He the Almighty will help us and wont waste our hardwork...Allah hafiz..(y)
 
angelicsuccubus

angelicsuccubus

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iKhaled said:
need to know what reactions involve heating with reflux and which ones only needs heating?
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Organic chem reactions that use heat reflux:
  1. Nucleophilic substitution of alcohols with a mixture of red phosphorus and Bromine/Iodine
  2. Oxidation of primary alcohols to carboxylic acids
  3. Oxidation of secondary alcohols to ketones
  4. Nucleophilic substitution of halogenoalkanes with NaOH to form alcohols
  5. Nucleophilic substitution of halogenoalkanes with KCN to form nitriles
  6. Nitriles forming caboxylic acids using dil. HCl (Hydrolysis)
Everything else just uses heat.
 
Jaf

Jaf

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angelicsuccubus said:
that means you're giving it heat by radiation or a bunsen burner or a reactor.. or anything.. but not refluxing it, and so if the condition is meant to be reflux, its important that you write reflux.

Organic chem reactions that use heat reflux:
  1. Nucleophilic substitution of alcohols with a mixture of red phosphorus and Bromine/Iodine
  2. Oxidation of primary alcohols to carboxylic acids
  3. Oxidation of secondary alcohols to ketones
  4. Nucleophilic substitution of halogenoalkanes with NaOH to form alcohols
  5. Nucleophilic substitution of halogenoalkanes with KCN to form nitriles
  6. Nitriles forming caboxylic acids using dil. HCl (Hydrolysis)
If I missed anything.. sorry
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You know... it'd be easier to list reactions that don't require heat under reflux. :p There are several you missed (elimination, hydrolysis of esters, hydrolysis of sodium alkanoates, oxidation of aldehydes to acids, nucleophilic substitution when preparing HCl/HBr in situ)

BTW, what is the first reaction? :S
I mean I know how the red phosphorus reacts with iodine to form PI3 which reacts with the alcohol. But does this happen with bromine too? :O
 
iKhaled

iKhaled

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angelicsuccubus said:
Organic chem reactions that use heat reflux:
  1. Nucleophilic substitution of alcohols with a mixture of red phosphorus and Bromine/Iodine
  2. Oxidation of primary alcohols to carboxylic acids
  3. Oxidation of secondary alcohols to ketones
  4. Nucleophilic substitution of halogenoalkanes with NaOH to form alcohols
  5. Nucleophilic substitution of halogenoalkanes with KCN to form nitriles
  6. Nitriles forming caboxylic acids using dil. HCl (Hydrolysis)
Everything else just uses heat.
Click to expand...
Jaf said:
You know... it'd be easier to list reactions that don't require heat under reflux. :p There are several you missed (elimination, hydrolysis of esters, hydrolysis of sodium alkanoates, oxidation of aldehydes to acids, nucleophilic substitution when preparing HCl/HBr in situ)

BTW, what is the first reaction? :S
I mean I know how the red phosphorus reacts with iodine to form PI3 which reacts with the alcohol. But does this happen with bromine too? :O
Click to expand...
haha..this is funny..memorizing all these reactions for just 3 marks or somethin :O:O dats insane :D
 
Jaf

Jaf

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angelgirl:) said:
no u r write...look at the m/j 2011 p21 Q3b....when exp..r carried out in school labs...we cant write heat under reflux...:)
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Dude that's a question about inorganic chemistry. Why would you heat inorganic substances under reflux?

I'm 100% sure you SHOULD write heat under reflux when there's a college/school laboratory mentioned. Most schools (not talking about Pakistan here :p ) with a chemistry lab have this facility available. I did a question a couple of hours ago from the past paper and I remember very well it said heat under reflux for a similar question.
 
RGBM211

RGBM211

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can sumone tell me when we react phosphorous with oxygen when is it P205,P203 AND P406/P4010
 
angelicsuccubus

angelicsuccubus

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Jaf said:
You know... it'd be easier to list reactions that don't require heat under reflux. :p There are several you missed (elimination, hydrolysis of esters, hydrolysis of sodium alkanoates, oxidation of aldehydes to acids, nucleophilic substitution when preparing HCl/HBr in situ)

BTW, what is the first reaction? :S
I mean I know how the red phosphorus reacts with iodine to form PI3 which reacts with the alcohol. But does this happen with bromine too? :O
Click to expand...

Yeh it happens with Bromine too.. and I said earlier when I posted these, I'm sorry if I missed any so just fix the list. I didn't go through esters or carboxylics so I probably missed stuff from there.. and I thought aldehyde--> carboxylic was pretty obvious but oh well.. and I didn't have heat reflux for nucleophilic substitution with HBr in my notes, so..
 
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oldfashionedgirl

oldfashionedgirl

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angelicsuccubus said:
umm cause that's the colour of 2,4-dinitrophenylhydrazone (it's the product of the reaction between an aldehyde/ketone and 2,4-dinitrophenylhydrazine)
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yeah....errr, that's what she asked :p the colour of the product of reaction between aldehydes/ketones and 2-4 DNPH
 
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