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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: sh0ebox on January 29, 2008, 09:29:13 PM

Title: Conjugate Base of CH2=CH2?
Post by: sh0ebox on January 29, 2008, 09:29:13 PM

I know this doesn't seem very much like organic chemistry, but I am having trouble drawing the conjugate base for ethylene, CH2=CH2.

I am able to draw the conjugate acid just fine, looking something like this: (https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg255.imageshack.us%2Fimg255%2F5361%2Fconjugateacidjc5.jpg&hash=cc9f2bc88131c3b2a25b15a7760446090186347e)

but I can't seem to figure out how I would go about drawing the conjugate base form. So far everything I've been able to conjure up has not been the correct form. Can anyone give me a little insight? Thanks in advance :)

Title: Re: Conjugate Base of CH2=CH2?
Post by: AWK on January 30, 2008, 01:16:08 AM

Usually C2H5+ is written as:

Title: Re: Conjugate Base of CH2=CH2?
Post by: Rico on January 30, 2008, 08:37:56 AM

Hey sh0ebox

This is definetely also organic chemistry! The conjugate base of ethylene is what you get if ethylene acts as an acid, i.e. if you deprotonate ethylene (see jpeg attached). The protons on ethylene is not that acidic though, it has a pKa of around 44 and therefore you need a very strong base to deprotonate, something like an alkyl-lithium.

Rico

Title: Re: Conjugate Base of CH2=CH2?
Post by: sh0ebox on January 30, 2008, 10:04:50 AM

Quote from: Rico on January 30, 2008, 08:37:56 AM

Hey sh0ebox

This is definetely also organic chemistry! The conjugate base of ethylene is what you get if ethylene acts as an acid, i.e. if you deprotonate ethylene (see jpeg attached). The protons on ethylene is not that acidic though, it has a pKa of around 44 and therefore you need a very strong base to deprotonate, something like an alkyl-lithium.

Rico

Thanks for the response! I am wondering, however, why the f.c. on the C-H bond is a negative --- are there lone pairs attached to that carbon that are just not drawn in? Because I looked through my answers and I tried to draw the same figure that you drew except with lone pairs attached to the C (It indicated that I needed to draw all charges and lone pairs if there were any) but it was incorrect. I also drew the same structure without lone pairs but found the f.c. to be +. This one bit is just a little bit confusing to me. Thanks again!

Title: Re: Conjugate Base of CH2=CH2?
Post by: 0000000 on January 30, 2008, 11:22:03 AM

Quote from: sh0ebox on January 30, 2008, 10:04:50 AM

Thanks for the response! I am wondering, however, why the f.c. on the C-H bond is a negative --- are there lone pairs attached to that carbon that are just not drawn in?

the carbon atom must be negative, precisely there will be resonate stabilization but we can say that one of the C atoms will contain lonely electron pair.
Just to remind you that according to the Bronsted theory an acid is a proton donor, therefore the H atom will leave the pair of the electrons to the carbon, that so there will be negative charge and the conjugate base will be an carboanion

Title: Re: Conjugate Base of CH2=CH2?
Post by: Rico on January 30, 2008, 11:39:07 AM

Hey sh0ebox

Sorry the structure was drawn a bit sloppy, but of course the negative charge on the carbon atom implies a lone pair (see jpeg attached)!

Nice to see that somebody is really looking into things  :)

Rico

 

Title: Re: Conjugate Base of CH2=CH2?
Post by: sh0ebox on January 30, 2008, 11:45:21 AM

Awesome, thanks so much for your help guys that really cleared things up!  ;D