lectures 19-20 preview

lectures 19-20 preview - Lecture 19 Chemistry 310N Organic...

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

View Full Document Right Arrow Icon
6/15/09 Lecture 19 Chemistry 310N    Organic Chemistry II for Prehealth Professionals Unique number: 53185 Tu,Th 12:30-2:00 pm, Welch 2.224   Professor:  Jonathan Sessler, Ph.D. [email protected]
Background image of page 1

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

View Full DocumentRight Arrow Icon
6/15/09 Chapter 20 Conjugated Systems
Background image of page 2
6/15/09 Conjugated Dienes From heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes. H 0 -237 (-56.5) 1,3-Butadiene -126 (-30.1) -127 (-30.3) kJ (kcal)/mol Name 1-Pentene 1-Butene trans- 1,3-Pentadiene 1,4-Pentadiene trans- 2-Butene -115 (-27.6) cis- 2-Butene -120 (-28.6) -226 (-54.1) -254 (-60.8) Structural Formula
Background image of page 3

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

View Full DocumentRight Arrow Icon
6/15/09 Conjugated Dienes Butadiene Conjugation of the double bonds in 1,3- butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol . 2H 2 + catalyst H 0 = 2(-127 kJ/mol)           = -254 kJ/mol) 2 2 2H 2 + H 0  = -237 kJ/mol catalyst
Background image of page 4
6/15/09 Conjugated Dienes Butadiene Conjugation of double bonds in butadiene gives the molecule an additional stability of approximately 17 kJ/mol.
Background image of page 5

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

View Full DocumentRight Arrow Icon
6/15/09 Structure of Butadiene MOs Combination of four parallel 2p atomic orbitals gives two π -bonding MOs (this screen) and two π -antibonding MOs (the next screen).
Background image of page 6
6/15/09 Structure of Butadiene MOs the two π -antibonding MOs of butadiene.
Background image of page 7

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

View Full DocumentRight Arrow Icon
6/15/09 Conjugated Systems Systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds. 3-Cyclohexenone (less stable) 2-Cyclohexenone (more stable) O O
Background image of page 8
6/15/09 1,2- and 1,4-Addition Addition of one mol of HBr to butadiene at -78°C gives a mixture of two constitutional isomers. We account for these products by the following 1-Bromo-2-butene 10% (1,4-addition) -78°C + + 3-Bromo-1-butene 90% (1,2-addition) CH 2 = CH- CH= CH 2 HBr CH 2 = CH- CH- CH 2 CH 2 - CH= CH- CH 2 1,3-Butadiene H H Br Br
Background image of page 9

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

View Full DocumentRight Arrow Icon
6/15/09 1,2- and 1,4-Addition The key intermediate is a resonance-stabilized allylic carbocation. CH 2 = CH- CH= CH 2 H- Br CH 2 = CH- CH-CH 2 CH 2 -CH= CH-CH 2 + + + CH 2 = CH- CH-CH 2 Br H Br CH 2 -CH= CH-CH 2 Br Br (1,4-Addition) (1,2-Addition) _ _ H H H
Background image of page 10
6/15/09 1,2- and 1,4-Addition Addition of one mole of Br2 to butadiene at -15°C also gives a mixture of two constitutional isomers. We account for the formation of these 1,2- and -15°C 3,4-Dibromo-1-butene (54%) (1,2-addition) 1,4-Dibromo-2-butene (46%) (1,4-addition) + + 1,3-Butadiene CH 2 = CH- CH= CH 2 Br 2 CH 2 - CH= CH- CH 2 CH 2 - CH- CH= CH 2 Br Br Br Br
Background image of page 11

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

View Full DocumentRight Arrow Icon
6/15/09 Experimental Information For addition of HBr at -78°C and Br2 at - 15°C, the 1,2-addition products predominate; at higher temperatures (40° to 60°C), the 1,4-addition products predominate. If the products of the low temperature
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 06/15/2009 for the course CH 53185 taught by Professor Sessler during the Spring '09 term at University of Texas.

Page1 / 90

lectures 19-20 preview - Lecture 19 Chemistry 310N Organic...

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