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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: joyb on February 19, 2013, 08:46:27 AM

Title: How do you hydrolyse dicyanoethene?
Post by: joyb on February 19, 2013, 08:46:27 AM: The question asks to identify the products and mechanism of the following reaction:
CH₃CH=C(CN)₂ + H₂O → C₂H₄O + C₃H₂N₂
The problem is on a sheet about addition to a C=C bond. I can't work out how you get two products with those molecular formulae. If anyone could help it would be much appreciated!
Title: Re: How do you hydrolyse dicyanoethene?
Post by: sjb on February 19, 2013, 08:54:23 AM: Quote from: amberbarton on February 19, 2013, 08:46:27 AM
The question asks to identify the products and mechanism of the following reaction:
CH₃CH=C(CN)₂ + H₂O → C₂H₄O + C₃H₂N₂
The problem is on a sheet about addition to a C=C bond. I can't work out how you get two products with those molecular formulae. If anyone could help it would be much appreciated!

Can you format the numbers using [sub][/sub] tags (they're barely readable)? In any event, where are your nucleophiles / electrophiles? Is the first compound ?
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 19, 2013, 09:06:08 AM: Yes, that is the reactant. I imagine that as the nitrile groups are electron withdrawing the dicyanoethene is the electrophile and water the nucleophile.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: orgopete on February 19, 2013, 09:48:55 AM: Hint, what structure is C3H2N2?
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 19, 2013, 10:02:03 AM: I would guess NCCH₂CN but really I don't know.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: Dan on February 19, 2013, 10:19:05 AM: Good guess...

Have a look at your starting malononitrile. Is it a nucleophile/electrophile/acid/base?

What kind of reagent is water - nucleophile/electrophile/acid/base?
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 19, 2013, 03:26:30 PM: My original thought was that water would act as a nucleophile and that conjugate addition would occur due to the electron withdrawing nature of the CN groups. That doesn't give me the correct molecular formulae though, which is why I'm stuck.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: Dan on February 19, 2013, 03:53:10 PM: Quote from: amberbarton on February 19, 2013, 03:26:30 PM
My original thought was that water would act as a nucleophile and that conjugate addition would occur due to the electron withdrawing nature of the CN groups.

Good. Half way there.

Hint: The pKa of H₂(CN)₂ is around 11
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 19, 2013, 06:57:51 PM: I don't really understand how you get from the reactants to the products.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: Dan on February 20, 2013, 02:55:53 AM: Quote from: amberbarton on February 19, 2013, 06:57:51 PM
I don't really understand how you get from the reactants to the products.

I know that.

You are almost there, what you have suggested so far is correct, you just need to finish it.

You know you have this intermediate so far:

You know CH₂(CN)₂ (malononitrile) is one of the final products. Draw a circle around the part of the intermediate that corresponds to malononitrile. Which bond needs to break to make malononitrile?
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 20, 2013, 04:20:46 AM: The C-C bond needs to break... I'm not sure how lysis could occur without another reactant though.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: Dan on February 20, 2013, 05:50:13 AM: Hint: Why is this compound unstable?
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 20, 2013, 06:03:02 AM: I honestly don't know. If anything I would guess repulsion between the areas of high electron density surrounding each of the oxygens.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: Dan on February 20, 2013, 06:11:04 AM: Ok, google carbonyl hydration, and/or think about the instability of this one:
Title: Re: How do you hydrolyse dicyanoethene?
Post by: joyb on February 20, 2013, 07:06:00 AM: I think I'm going to have to give up on this one...
Title: Re: How do you hydrolyse dicyanoethene?
Post by: Dan on February 20, 2013, 07:51:00 AM: Hint:

:requil: acetone + HCl
Title: Re: How do you hydrolyse dicyanoethene?
Post by: orgopete on February 20, 2013, 12:03:06 PM: Quote from: amberbarton on February 19, 2013, 08:46:27 AM
The question asks to identify the products and mechanism of the following reaction:
CH₃CH=C(CN)₂ + H₂O → C₂H₄O + C₃H₂N₂
The problem is on a sheet about addition to a C=C bond. I can't work out how you get two products with those molecular formulae. If anyone could help it would be much appreciated!

If C3H2N3 is malononitrile, then the starting material can be formed if the arrow for the above reaction is reversed.
Title: Re: How do you hydrolyse dicyanoethene?
Post by: camptzak on February 24, 2013, 12:50:37 AM: water acts as a nucleophile and attacks the carbon attached to the methyl and the hydrogen. The anion on the carbon with cyano groups then grabs a proton.
A lone pair on the alcohol group comes down and kicks off the carbon with the two cyanide groups and a hydrogen attached. The anion grabs another proton forming your C₃H₂N₂.
Also you have your acetylaldehyde.

The acetylaldehyde and the hydrate would be in equilibrium with eachother.

wont the cyano groups move to carboxylic acid groups in water?