conjugation part 2

conjugation part 2 - Conjugated Dienes Conjugated dienes...

Info iconThis preview shows pages 1–13. Sign up to view the full content.

View Full Document Right Arrow Icon
Conjugated Dienes Conjugated dienes are compounds having two double bonds joined by one σ bond. Conjugated dienes are also called 1,3-dienes. 1,3-Butadiene (CH 2 =CH-CH=CH 2 ) is the simplest conjugated diene. Three stereoisomers are possible for 1,3-dienes with alkyl groups bonded to each end carbon of the diene.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Two possible conformations result from rotation around the C—C bond that joins the two double bonds. Note that stereoisomers are discrete molecules, whereas conformations interconvert.
Background image of page 2
Draw the three possible stereoisomers of 2,4-octadiene. Pick which one is (2 E ,4 E ) 2,4-octadiene.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Draw the s-cis and s-trans conformations of (3Z,5Z)-4,5-dimethyl -3,5-octadiene s-trans s-cis
Background image of page 4
The Carbon—Carbon σ Bond Length in 1,3-Butadiene Four features distinguish conjugated dienes from isolated dienes. 1. The C—C single bond joining the two double bonds is unusually short. 2. Conjugated dienes are more stable than similar isolated dienes. 3. Some reactions of conjugated dienes are different than reactions of isolated double bonds. 4. Conjugated dienes absorb longer wavelengths of ultraviolet light.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
The Carbon—Carbon σ Bond Length in 1,3- Butadiene The observed bond distances can be explained by looking at hybridization.
Background image of page 6
A resonance argument can also be used to explain the shorter C—C σ bond length in 1,3-butadiene. Based on resonance, the central C—C bond in 1,3- butadiene is shorter because it has partial double bond character.
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms. Consequently, the π electrons are not localized between the carbon atoms of the double bonds, but rather delocalized over four atoms. This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present. This shortens the bond.
Background image of page 8
Using hybridization, compare the C-C bonds of the following three compounds. H 3 C CH 3 H 2 C CH 2 HC CH sp 3 25% s character sp 2 33% s character sp 50% s character H 3 C O O - Using resonance, why are the two C—O bonds the same length? H 3 C O O - H 3 C O O - The two resonance structures show how the electron density is delocalized over 3 atoms.
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Stability of Conjugated Dienes When hydrogenation gives the same alkane from two dienes, the more stable diene has the smaller heat of hydrogenation.
Background image of page 10
A conjugated diene has a smaller heat of hydrogenation and is more stable than a similar isolated diene. Figure 16.5 Relative energies of an isolated and conjugated diene
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
A conjugated diene is more stable than an isolated diene because a conjugated diene has overlapping p orbitals on four adjacent atoms. Thus, its π electrons are delocalized over four atoms. This
Background image of page 12
Image of page 13
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 04/01/2012 for the course CHEM 307 taught by Professor Dr.owenmcdougal during the Fall '11 term at Boise State.

Page1 / 54

conjugation part 2 - Conjugated Dienes Conjugated dienes...

This preview shows document pages 1 - 13. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online