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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: NewtoAtoms on May 24, 2008, 11:37:48 AM

Title: 1,3-butadiene
Post by: NewtoAtoms on May 24, 2008, 11:37:48 AM

Hello,

C-C has a bond length of 154 pm
C=C has a bond length of 133 pm

Can anyone please explain why 1,3-butadiene has a C triple bond C with a bond length of 146 pm. 

I thought that 3 bonds have shorter bond lenghts than double bonds, and double bonds have shorter lengths than single bonds.

EEhh....

Title: Re: 1,3-butadiene
Post by: macman104 on May 24, 2008, 12:57:11 PM

Resonance structures.  Are any of the bonds really single or double bonds, or do they resonate so they are something inbetween (like maybe 146?)

Title: Re: 1,3-butadiene
Post by: DrCMS on May 24, 2008, 01:04:17 PM

1,3-butadiene does not have a CC triple bond it has 2 C=C double bonds of typical bond length and a central C-C that is shorter than usual for C-C single bonds.  This is due to sp2-sp2 overlap for  C2-C3 sigma bond and also overlap between the C2-C3 p orbitals

Title: Re: 1,3-butadiene
Post by: NewtoAtoms on May 24, 2008, 03:35:22 PM

but according to my text the equation is:

H2C = CH-CH = CH2

If I draw this out the 2 central C atoms are triple bonded together, then bonded to each H (which are both terminal) 

My textbook continues to say that that central triple bond has a length of 146 pm... but then just assumes I should know why.

Could you explain to me further....

please :-(


Thank you!

Title: Re: 1,3-butadiene
Post by: DrCMS on May 24, 2008, 05:44:26 PM

I do not know why you think there is a triple bond but there is NOT a triple bond in 1,3-butadiene.

see this link

http://en.wikipedia.org/wiki/Butadiene

Title: Re: 1,3-butadiene
Post by: Yggdrasil on May 24, 2008, 06:11:58 PM

Quote from: NewtoAtoms on May 24, 2008, 11:37:48 AM

I thought that 3 bonds have shorter bond lenghts than double bonds, and double bonds have shorter lengths than single bonds.

This is correct.  Bond length decreases with increasing bond order.  The fact that the central bond of 1,3-butadiene has a bond length between the bond length of a single and double bond suggests that the bond order is between 1 and 2 (see macman's post).

Title: Re: 1,3-butadiene
Post by: NewtoAtoms on May 24, 2008, 11:02:36 PM

Sorry Dr. CMS...

I am just new to Chemistry so it was my mistake about the triple bond.  I do appologize. I went on the site you referenced and read and studied the material and now I realize there is no triple bond... thank you for the reference and sorry for my mistake.

Thank you everyone for the great explanations and helping me understand the bonding.

I deeply appreciate it!

New to Atoms....