Basic Organic Chemistry Lab Question

[The Red Dragon]

I used a dehydration reaction to convert cyclohexanol to cyclohexene using 2 acid catalysts, phosphoric acid and sulfuric acid. After the reaction I had to assume the cyclohexanol was converted to cyclohexene. I distilled the assumed cyclohexene out of the reaction mixture. At this point I added about 10 mL of water (to 10 mL of cyclohexene), 2.00 g of NaCl and 2.00 of sodium carbonate (base). Now it says the NaCl saturates the solution and that's about it. We had to check the pH in the bottom aqueous phase. What was the purpose of doing this (adding the salt and sodium carbonate)?

Unfortunately I'm in the first lab wave so I can't get help from my peers and the lectures haven't caught up to this point.

 

[milo hobgoblin]

Im pretty sure the Sodium Bicarbonate was added as a buffer.

The NaCl... probably as another buffer because of the high acidity of the solution.

That formula should yield a lot of by-product. I loved Ochem.. fun class.

 

[GoldenDelicious]

if you wanted to add a buffer you would normally use a solution of a weak acid and a weak base

all of this should be explained in your prac procedure, but so you think that perhaps the base (sodium bicarb) was added in excess to remove any remaining acid catalyst, and the sodium chloride to perhaps chelate something or other?

what is the stated purpose (aim) of the practical?

 

[bran987]

this sounds illegal be careful bor

 

[The Red Dragon]

The phosphoric and sulfuric acid would distill out along with the cyclohexene? When I was doing the lab I originally thought it was added for neutralization but the boiling points of the acid way higher than that of the cyclohexene so I didn't think the acids would be able to find its way into the distillate.

It was not explained in the procedure oddly enough. It just says:

"Add 10 mL of water to the distillate. Two separate phases should be observed. Add solid sodium chloride (2g) in small amounts to the distillate to saturate the water layer (lower) and then add sufficient solid sodium carbonate (2 g) to make the water layer distinctly alkaline to pH paper."

 

[GoldenDelicious]

yeah the two acids arent going anywhere, theyll go into the aqueous phase, and the last sentence above "then add sufficient solid sodium carbonate (2 g) to make the water layer distinctly alkaline to pH paper." supports the idea that the addition of the sodium carbonate is to neutralise the acids.

i still dont see hte point of adding the sodium chloride to saturate the aqueous layer unless you wanted to chelate some molecule or other. something is missing. if i could see your prac sheet i could tell you (probably)

i was fairly good at this stuff, but it was a long time ago, and...i hated it. boring crap.

except the time i burnt down a bit of that lab. that was a good day

 

[The Red Dragon]

Prac sheet? Oh, when I said aqueous phase, I was talking about the distillate + water mixture. The upper phase is the cyclohexene and the bottom phase is the aqueous phase. The acid most likely did not escape the roundbottom flask being distilled (and was thrown out)
Thanks man

 

[GoldenDelicious]

i get it, ive done lots of those types of pracs. hated collecting all of the damn aqueous phase (the bottom, heavier layer) and then doing titrations on the damn stuff.

anyway, if you write out your whole prac procedure, someone (maybe me) should be able to help you out. i dont see why you would bother adding sodium chloride to the aqueous phase if you were just going to react the acid with sodium carbonate and then throw it all out, unless there was some sort of distribution of something or other back into the non-aqueous phase, and you wanted to stop that from happening.

 

[milo hobgoblin]

Probably to stop any secondary reactions.

 

[GoldenDelicious]

yep thats what i figured

 

[The Red Dragon]

Mix 15 mL of cyclohexanol with 5 mL of phosphoric acid and 5 drops of concentrated sulfuric acid. Boil it gently, making sure the temp doesn't rise above 110 (or the reaction will distll), heating it gently is just to promote the reaction. After 10 minutes, increase the heat for distillation. Distill until 1/3 of the original volume is left.

About 10 mL of distillate should be obtained. Add 10 mL of water to the distillate. Two separate phases should be observed. Add solid sodium chloride (2g) in small amounts to the distillate to saturate the water layer (lower) and then add sufficient solid sodium carbonate (2 g) to make the water layer distinctly alkaline to pH paper. Stir with a glass rod to mix the layers and remove the aqueous layer using a separatory funnel.

With the cyclohexene phase, add anhydrous calcium chloride to dry up the cyclohexene (remove H2O). Distill a second time, making sure the temperature does not go above 89 degrees (83 is the theoretical boiling point of cyclohexene).

Then do potassium permanganate testing and bromine testing on the cyclohexene to test for double bonds.

 

[GoldenDelicious]

its to maximise recovery of cyclohexene from the aqueous layer.

 

[milo hobgoblin]

If I wont the lottery.. I would go back and take all the ochem I could for a hobby. Those were the coolest classes I had. Awesoem instructor.. he had a real passion for it and really made it interesting.

 

[The Red Dragon]

The base was used to prevent further side reactions? Yeah the salting makes sense.
Thanks again guys

Yeah, I find that its the teacher that increases my interest in a particular class
and...how did you burn down a section of the lab?

 

[GoldenDelicious]

well if i won the lottery i would go to greece, buy a nightclub with a private beach, have an attached apartment with a jacuzzi, and break julio inglesias record for picking up chicks

i would also buy a gun with only one bullet, to be put in reserve in case i was ever forced by rival nightclub owners/ chick pickeruperers to do anything remotely like this chemistry shit again

 

[GoldenDelicious]

The base was used to prevent further side reactions? Yeah the salting makes sense.
Thanks again guys

Yeah, I find that its the teacher that increases my interest in a particular class
and...how did you burn down a section of the lab?

no the base was to get rid of the acid catalysts, and the salt was to saturate the aqueous layer and decrease how much cyclohexene the aqueous layer could hold...meaning that pretty much all the cyclohexene would end up in the non aqueous layer (the one you keep, and react with bromine etc) while you throw the salty, basic water away.

i burned down a couple of labs actually. one was a huuuuuge fireball with naphtheline (hehehehehehehe) but the damage was limited to some scortch marks and a bit of water damage from putting the fire out, and the other one was accidentally on purpose setting up a bunson burner in a way taht i knew was going to totally fuck everything up, and burning one of those benches that arent supposed to burn.

i hated chemistry, and chemistry hated me. or at least the teachers did. when they didnt love me for being one of the most entertaining students there (i love grad students who run prac classes the older, married guys...well, they can go to hell in a burette)

 

[The Red Dragon]

The acid catalysts can distill out the initial mixture?

Oh we did shit with heating naphthaleine, don't ever recall accidents beyond breaking glass. I hope you didn't have to reimburse the damages.

 

[GoldenDelicious]

looks like it.

hint for the future: be buddies with the young prac guys. not only will they explain everything, but tehy will put you in the same groups as all the hot chicks

 

[GoldenDelicious]

oh and no, i didnt pay for anything. i have a very honest face

(hehe )

 

[milo hobgoblin]

I would never want the headache of running a nightclub.. hell a gym was bad enough.. I cant imagine the legal nightmares a club owner must go thrugh.

Id probably buy a whorehouse in vegas and pu myself in charge of HR though.

 

[GoldenDelicious]

thatd work

MUCH better than the chemistry idea