AS Chemistry P2 Prep.

[angelgirl:)]

sorry but either u understood it wrong or u wrote it wrong. reflux, i simple words means heating under pressure. for oxidation of alcohol, reflux produces ann ACID!! for aldehyde u have to heat it and distill off the product as soon as it is formed.
this is the definition:

my teacher told me reflux means strong heating....

 

[Gémeaux]

my teacher told me reflux means strong heating....

your teacher told you the right thing., in simplest of words.

 

[oldfashionedgirl]

wht does aldehyde/ketone give in reaction to DNPH

orange-red precipitate

 

[Saad (سعد)]

orange-red precipitate

Orange or yellow... preferably orange. beware of red; I haven't seen it being used by the Examiners. In book Cambridge-endorsed books and Mark Schemes, they've used orange, or in rarer cases yellow. We have to be careful to use the terms they like; orange is the safest.

 

[BazD]

Did I just read reflux means really strong heating? Guyz, no!

Reflux is when you have to distill something off before the reaction proceeds. FOr instance, while oxidizing an alcohol to get an aldehyde and not an acid,
one should reflux the aldehyde as it is formed so that it doesn't stay in there with the agent and get oxidized further.
Similarly, here, for conversion of nitriles to -COOH acids, you gotta reflux the acid as it's formed!

I'm happy that you noticed that the meaning of reflux as strong heating was wrong, but it's sad to know that even you haven't got it right... Your exam is true to the fact that when you distill , aldehydes form rather then then carboxylic acids. But reflux aids the formation of carboxylic acids ! Basically reflux is the exact opposite of distillation where a condenser is attached on top of the reacting mixture to prevent volatile mixtures from escaping.

 

[angelgirl:)]

condition and reagent for hydrolysis of ester?

 

[angelgirl:)]

condition and reagent for hydrolysis of ester?

can we just write H2O under reflux?

 

[oldfashionedgirl]

Orange or yellow... preferably orange. beware of red; I haven't seen it being used by the Examiners. In book Cambridge-endorsed books and Mark Schemes, they've used orange, or in rarer cases yellow. We have to be careful to use the terms they like; orange is the safest.

nah, orange is safest to write if ur not sure, but i've seen in many mark schemes that u can write orange-red as well. NOT just red, but orange-red.

 

[oldfashionedgirl]

condition and reagent for hydrolysis of ester?

there are 2 conditions, acid hydrolysis and basic hydrolysis. In acid hydrolysis with HCl for example, after hydrolysis, simply the corresponding carboxyllic acid and alcohol are formed.
But in basic hydrolysis, initially sodium salt and alcohol are formed, but after you reflux it, the sodium salt is also converted to the corresponding carboxyllic acid.

 

[angelgirl:)]

there r 2 diff. hydrolysis of esters...acid hydrolysis...eg warm with acid catalyst, produces alcoholes and -cooh...
but alkaline hydrolysis forms propane-1,2,3-triol and the salts of fatty acids...

 

[aaditya menon]

I'm happy that you noticed that the meaning of reflux as strong heating was wrong, but it's sad to know that even you haven't got it right... Your exam is true to the fact that when you distill , aldehydes form rather then then carboxylic acids. But reflux aids the formation of carboxylic acids ! Basically reflux is the exact opposite of distillation where a condenser is attached on top of the reacting mixture to prevent volatile mixtures from escaping.

Okay, first, I KNOW that for acids to form u need to reflux.
And a condenser is attached anyhoo (distillation or Reflux!). And no reflux is not the exact opposite of distillation. Here, check this image
http://en.wikipedia.org/wiki/File:Fractional_distillation_lab_apparatus.svg

And see what it reads "Laboratory apparatus using reflux to supply energy to chemical reactions. An Erlenmeyer flask is used as a receiving flask. Here the distillation head and fractionating column are combined in one piece."

 

[Gémeaux]

but alkaline hydrolysis forms propane-1,2,3-triol and the salts of fatty acids...

those salts of fatty acids are further reacted with H+ because it is hydrolysis so we need to get to the acid in the end.

 

[aaditya menon]

"Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture"

Thats what it means. More or less, similar to distillation.

 

[angelgirl:)]

"Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture"

Thats what it means. More or less, similar to distillation.

then y sometimes even if the condition is to supply heat...ms just write heat/reflux/warm

 

[angelicsuccubus]

orange-red precipitate

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)

 

[aaditya menon]

Because placing the condenser above doesn't matter a difference if the liquids are not volatile. So the student won't be penalized for writing reflux because heating is accompanied with reflux.

 

[angelicsuccubus]

then y sometimes even if the condition is to supply heat...ms just write heat/reflux/warm

because heating and reflux heating is not the same thing. Reflux is important when you're tryna make sure, no gases and stuff are lost... but sometimes it's just heating.. providing heat... isn't the process of heat refluxing... so don't think of them the same at all.

 

[SuperGirl123]

"Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture"

Thats what it means. More or less, similar to distillation.

Preventing loss of volatile substances would be reflux. Distillation would be something you do after the reaction is complete to separate the reactants and products. No? At least that's what I thought.

I'm asking not telling btw. I'm pretty confused right now.

 

[angelgirl:)]

because heating and reflux heating is not the same thing. Reflux is important when you're tryna make sure, no gases and stuff are lost... but sometimes it's just heating.. providing heat... isn't the process of heat refluxing... so don't think of them the same at all.

so if u just write reflux... then wat does it mean?

 

[angelgirl:)]

Preventing loss of volatile substances would be reflux. Distillation would be something you do after the reaction is complete to separate the reactants and products. No? At least that's what I thought.

I'm asking not telling btw. I'm pretty confused right now.

same here...i never thought that it will become this much confusing?