The Preparation of Pentan-2-one

[KaosuHamoni]

Hey there,

Chemists among you, I need a little bit of help. AS Level (UK) Evaluative coursework next week, and I need someone to explain to me the method used in preparing Pentan-2-One. I've looked online, searched plenty of different combinations, and have gone through my books and I've found nowt.

Any form of help would be greatly appreciated. =]

-Thanks

 

[Palfreyfish]

I do A2 Chemistry, I did this last year. Basically you have to oxidise the alcohol Pentan-2-ol and it goes to Pentan-2-one.

The equipment will probably look like this:

Primary Alcohol---->Aldehyde---->Carboxylic Acid

Secondary Alcohol---->Ketone

Tertiary Alcohol--X-->(Cannot oxidise)

You can reduce a Ketone or an Aldehyde into their original alcohols, and for AS Chemistry you can't reduce a Carboxylic Acid.

Notification

 

[SckizoBoy]

snip

... what [user]Palfreyfish[/user] said.

However, I'll add that for AS theory, knowledge of the K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub] oxidising agent should be enough. The mechanism is rather annoying and I can't even remember it myself. Products include your ketone, potassium hydroxide and CrO[sub]3[/sub].

For extra points, most people these days use the Swern oxidation with oxalyl chloride and dimethyl-sulphoxide, 'cos it's a lot cleaner, the reactants & products aren't (as) toxic and the reaction vessel can be a lot smaller, since the dichromate solution tends to be very dilute (though that's more of a necessity for primary alcohols).

 

[Palfreyfish]

snip

... what [user]Palfreyfish[/user] said.

However, I'll add that for AS theory, knowledge of the K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub] oxidising agent should be enough. The mechanism is rather annoying and I can't even remember it myself. Products include your ketone, potassium hydroxide and CrO[sub]3[/sub].

For extra points, most people these days use the Swern oxidation with oxalyl chloride and dimethyl-sulphoxide, 'cos it's a lot cleaner, the reactants & products aren't (as) toxic and the reaction vessel can be a lot smaller, since the dichromate solution tends to be very dilute (though that's more of a necessity for primary alcohols).

Yeah, probably should have mentioned the H[sup]+[/sup]/K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub]. It's been a while since I used it, everything's about ligands and whatnot now, sorry about that. The extra points bit is also correct, but not necessary, so only memorise it if you think you'll use it.

 

[SckizoBoy]

Yeah, probably should have mentioned the H[sup]+[/sup]/K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub]. It's been a while since I used it, everything's about ligands and whatnot now, sorry about that. The extra points bit is also correct, but not necessary, so only memorise it if you think you'll use it.

Oooohh... metal complexes, electron configs/shapes and all that stuff, eh... loved the theory behind it (group priorities etc.), but hated the practical.

@OP - Do you need spectroscopic data as well?

 

[Palfreyfish]

Yeah, probably should have mentioned the H[sup]+[/sup]/K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub]. It's been a while since I used it, everything's about ligands and whatnot now, sorry about that. The extra points bit is also correct, but not necessary, so only memorise it if you think you'll use it.

Oooohh... metal complexes, electron configs/shapes and all that stuff, eh... loved the theory behind it (group priorities etc.), but hated the practical.

@OP - Do you need spectroscopic data as well?

Yeah, the theory's great, but the practicals currently consist of put compound A into compound B and watch the pretty colours form... Ah well, I'd better make the most of chemistry practicals, I won't get a chance to do them for free again.

 

[KaosuHamoni]

-Snip-

-Snip-

Thanks guys. I'd suspected that it was a simple oxidization, but I wasn't entirely sure as to the apparatus and method used. As for spectroscopic data, I doubt I'll need it. It's just a little assessed practical, I just wanted to be prepared for it is all, seeing as the exam board graced us with the title before we've done it. ;]

Also, will I need to know any other dichromate mechanisms & reactions, or just K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub]? As far as I know, when used in distillation the solution will go a bright, bright orange to a green so dark that it's almost black, and that's what the exam board like, and I know the mechanism...

 

[SckizoBoy]

Thanks guys. I'd suspected that it was a simple oxidization, but I wasn't entirely sure as to the apparatus and method used. As for spectroscopic data, I doubt I'll need it. It's just a little assessed practical, I just wanted to be prepared for it is all, seeing as the exam board graced us with the title before we've done it. ;]

Also, will I need to know any other dichromate mechanisms & reactions, or just K[sub]2[/sub]Cr[sub]2[/sub]O[sub]7[/sub]? As far as I know, when used in distillation the solution will go a bright, bright orange to a green so dark that it's almost black, and that's what the exam board like, and I know the mechanism...

One, you're welcome. Two, best of luck.

Three... I don't think you do need to know the mechanism, because it is quite complicated as it involves the cleaving of an oxygen bridge in the dichromate ion to generate water & the two chromate products, as well as the 'kick-start' that the acid provides.

However, one thing that just came to mind... oxidation states. It'd be good to point out that the oxidation state of the chromium metal centre goes from +8 to +6 (i.e. reduced, and this change in oxidation state is what causes the colour change - electron count causes the complex to refract light differently etc. etc.). Thus, the redox cycle is complete... the alcohol is oxidised, the dichromate is reduced and it fits quite neatly.