October 24, 2021, 03:24:47 AM
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Topic: Preparation of 4-methylcyclohexene by dehydration of 4-methylcyclohexanol  (Read 37093 times)

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Offline ch3m_boy

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Hi guys,

I'm new here so I'll introduce myself first.  You can call me charlie or chemboy.  I'm 19 and an sophomore at asu and i like chemistry... that's pretty much it :)

OK, i read the rules on posting and although I'm a noob, I'll try to not break the rules.  In my lab class, we used 4-methylcyclohexanol to prepare an alkene.  I working on some of the questions (we use the Pavia textbook if knows what that is) and there is a question that i am not sure about...

I attached a separation scheme that i made for this experiment so you know what we did. 

The question was..
In this experiment, 1.0 mL of saturated NaCl is used to rinse the Hickmen head after the initial distillation.  Why is saturated NaCl, rather than pure water, used for this procedure and the subsequent washing of the organic layer.

My answer was

5.  Saturated sodium chloride is used to rinse the Hickman Head rather than water since it is not polar and rinsing the Hickman Head with water would revert any 4-methylcyclohexene back to 4-methylcyclohexanol in the Hickman Head and thus lowering the percent yield; using water would shift the equilibrium towards the reactants.  Also sodium chloride removes the small amount of phosphoric acid and also a small amount of water.  If one were to add water, both 4-methylcyclohexene and phosphoric acid are partially soluble making difficult to remove the water later; sodium chloride makes the water less reactive so easier to remove by making the aqueous later more polar.

I have doubts because my professor (i didn't show him what i put) and he was telling me the NaCl makes the aqueous layer more ionic and therefore will remove the acid impurities without removing the alkene...   so i am a lil confused


Thank you all for the help, and you will be seeing alot more of me :)
-charlie

Offline ch3m_boy

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is there anyway i can edit my post?  I should have proof read it before submitting.. sorry

Offline Yggdrasil

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Saturated sodium chloride is used to rinse the Hickman Head rather than water since it is not polar and rinsing the Hickman Head with water would revert any 4-methylcyclohexene back to 4-methylcyclohexanol in the Hickman Head and thus lowering the percent yield; using water would shift the equilibrium towards the reactants. 

If you only expose your alkene to water for a brief time at low temperatures, you generally don't have to worry about water adding to the double bond.  However, the reason you cited above is why you must keep water out of the initial reaction mixture when dehydrating the 4-methylcyclohexanol.

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Also sodium chloride removes the small amount of phosphoric acid and also a small amount of water.  If one were to add water, both 4-methylcyclohexene and phosphoric acid are partially soluble making difficult to remove the water later; sodium chloride makes the water less reactive so easier to remove by making the aqueous later more polar.

I have doubts because my professor (i didn't show him what i put) and he was telling me the NaCl makes the aqueous layer more ionic and therefore will remove the acid impurities without removing the alkene...   so i am a lil confused

Phosphoric acid is more soluble in aqueous solution than organic solution because phosphoric acid is an ionic compound.  Therefore, washing your product with an aqueous solution will remove the phosphoric acid.  However, your product is slightly soluble in water and water is slightly soluble in your product; therefore, washing your product with pure water will remove some of your product from the organic layer and will introduce additional contaminants (water) into your product.

A general way around this is to add a salt to your wash water.  Adding NaCl to water raises the ionic strength of the solution, essentially making the water act as if it is a more polar solvent.  This decreases the amount of product you lose when washing your solution (since making water "more polar" decreases the solubility of organics in water) and it also helps remove residual water from your product (since saturated sodium chloride is fairly hygroscopic).


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Thank you all for the help, and you will be seeing alot more of me :)
-charlie

You are welcome, and I hope to see you around the forums. :)

P.S. use the modify button on the upper-right hand side of your message to edit your original post

Offline Borek

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is there anyway i can edit my post?  I should have proof read it before submitting.. sorry

Look for Modify button (next to Quote).
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Offline ch3m_boy

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is there anyway i can edit my post?  I should have proof read it before submitting.. sorry

Look for Modify button (next to Quote).

Hey thank you Borek and Yggdrasil for the advice and the nice welcome :) .

I am about to start some hwk (substitution and elimination rxns of alkyl halides) and i know i'll probably have some questions for you again later today if you don't mind :)

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