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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: ritwik08 on November 08, 2008, 01:45:41 PM

Title: Naphthalene(bond lengths)
Post by: ritwik08 on November 08, 2008, 01:45:41 PM

I was asked to compare the bond strengths of various bond strengths in naphthalene. What I did was to make different resonating structure of naphthalene = 3 when only the uncharged were drawn. I counted the frequency if double bonds at various places. I came up with 2 types of bonds. Do you guys agree??
Do I need to consider the charged resonance structures as well???

I shall b very grateful if someone could provide me with the data of bond lengths in naphthalene. I googled it but with no successful results.

Title: Re: Naphthalene(bond lengths)
Post by: ritwik08 on November 09, 2008, 06:31:45 AM

Quote from: ritwik08 on November 08, 2008, 01:45:41 PM

I was asked to compare the bond strengths of various bond strengths in naphthalene. What I did was to make different resonating structure of naphthalene = 3 when only the uncharged were drawn. I counted the frequency if double bonds at various places. I came up with 2 types of bonds. Do you guys agree??
Do I need to consider the charged resonance structures as well???

I shall b very grateful if someone could provide me with the data of bond lengths in naphthalene. I googled it but with no successful results.

I also drew the charge structures now. And the order of strength of bonds changes. Wikipedia doesnt support this:
http://en.wikipedia.org/wiki/Naphthalene

Title: Re: Naphthalene(bond lengths)
Post by: spirochete on November 09, 2008, 12:41:00 PM

You are on the right track in terms of drawing the three best (non charged) resonance structures and using those to deduce bond length.  I remember solving this exact problem a few months ago in my organic class.

 At the moment I'm having trouble explaining exactly how to use these structures to precisely define relative bond lengths.  Hopefully somebody else can chime in and explain.

If you were to draw EVERY possible charged resonance structure this method technically might work as well, but there are a near infinite number of those so this is obviously not the best method.

Title: Re: Naphthalene(bond lengths)
Post by: nj_bartel on November 09, 2008, 09:00:40 PM

When you include multiply charged structures, they also tend to cancel each other out (you'll end up with a resonance form that has a (-) on a carbon and a resonance form that has a (+) on the same carbon).  Or so I've noticed.

Title: Re: Naphthalene(bond lengths)
Post by: ritwik08 on November 11, 2008, 09:05:42 AM

Will they play a part in determining stability or not???

Title: Re: Naphthalene(bond lengths)
Post by: nj_bartel on November 11, 2008, 09:25:21 AM

Not directly, no.  Was more in addition to what spirochete was saying.

Title: Re: Naphthalene(bond lengths)
Post by: ritwik08 on November 12, 2008, 06:20:07 AM

Thanks a lot for the help guys!!

Title: Re: Naphthalene(bond lengths)
Post by: AWK on November 12, 2008, 09:48:04 AM

The link to wikipedia containd bond lengths id the text.