厦门大学化学化工学院：《有机化学》课程教学资源（课件讲稿，双语版）Chapter 13 Conjugated Unsaturated Systems（13.1-13.5）Introduction

Chapter 13 Conjugated Unsaturated Systems

Chapter 13 Conjugated Unsaturated Systems Chapter 13 Conjugated Unsaturated Systems

ORGANIC CHEMISTRY Chapter 13 13.1 Int troduction Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.1 Introduction

ORGANIC CHEMISTRY Chapter 13 Allyl radical CH2-=CH-CH2 888 Allyl cation CH=CH-CH 888 1.3-Butadiene CH2=CH-CH=CH 888 o molecules with delocalized t bonds are called conjugated unsaturated systems Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 CH2 CH CH2 Allyl radical C C C Allyl cation CH2 CH CH2 C C C 1,3-Butadiene CH2 CH CH CH2 C C C C Molecules with delocalized π bonds are called conjugated unsaturated systems

ORGANIC CHEMISTRY Chapter 13 Conjugated unsaturated systems have a p orbital on a carbon adjacent to a double bond The p orbital can come from another double or triple bond The p orbital may be the empty p orbital of a carbocation or a p orbital with a single electron in it (a radical) o Conjugation affords special stability to the molecule o Conjugated molecules can be detected using UV spectroscopy Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 Conjugated unsaturated systems have a p orbital on a carbon adjacent to a double bond The p orbital can come from another double or triple bond The p orbital may be the empty p orbital of a carbocation or a p orbital with a single electron in it (a radical) Conjugation affords special stability to the molecule Conjugated molecules can be detected using UV spectroscopy

ORGANIC CHEMISTRY Chapter 13 13.2 Allylic Substitution and the allyl Radical Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.2 Allylic Substitution and the Allyl Radical

ORGANIC CHEMISTRY Chapter 13 Reaction of propene with bromine varies depending on reaction conditions g At low temperature the halogen adds across the double bond g at high temperature or at very low concentration of halogen an allylIC substitution occurs low temperature CH,=CH—CH2+X CH,-CH—CH CCI (addition reaction) high temperature CH2=CH—CH3+X2 or low concentration of X CH2=CH—CH2X+HX Propene (substitution reaction Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 Reaction of propene with bromine varies depending on reaction conditions At low temperature the halogen adds across the double bond At high temperature or at very low concentration of halogen an allylic substitution occurs

ORGANIC CHEMISTRY Chapter 13 CH2=CH-CH3 X2 or A CH2-CH-CH2x +HX Allylic substitution Allylic hydrogen atoms H足 CH2=CH-CH2 Allylic radical Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 CH2 CH CH3 + X2 hv or CH2 CH CH2X + HX Allylic substitution C C C H H H H H CH2 CH CH2 Allylic radical Allylic hydrogen atoms

ORGANIC CHEMISTRY Chapter 13 13.2A Allylic Chlorination(High Temperature Allylic chlorination can be performed at high temperature in the gas phase 400°C CH2=CH-CH3 Cl2 CH=CH-CHoCI Hcl Relative stability of free radicals Allylic or allyl>3°>2°>1°> vinylic or vinyl Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.2A Allylic Chlorination (High Temperature) ¾ Allylic chlorination can be performed at high temperature in the gas phase CH2 CH CH3 + Cl2 400°C CH2 CH CH2Cl + HCl Allylic or allyl > 3° > 2° >1°> vinylic or vinyl Relative stability of free radicals:

ORGANIC CHEMISTRY Chapter 13 13.2B Allylic Bromination with N-Bromo succinimide low Concentration of Br2) light or roor CH=CH-CH2+ N-Br CH2=CH-CH,Br CCI N-Bromosuccinimide (NBS) N一H NBS provides a continuous low concentration of bromine for the radical reaction a low bromine concentration favors allylic substitution over alkene addition Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.2B Allylic Bromination with N-Bromo￾succinimide (Low Concentration of Br2) CH2 CH CH3 N Br O O N-Bromosuccinimide (NBS) + light or ROOR CCl4 CH2 CH CH2Br N H O O + ¾ NBS provides a continuous low concentration of bromine for the radical reaction ¾ A low bromine concentration favors allylic substitution over alkene addition

ORGANIC CHEMISTRY Chapter 13 The radical reaction is initiated by a small amount of bromine radical formed by exposure of nbs to light or peroxides CH2=CH-CH2-H+·Br—→CH2=CH-CH2+HBr CH,=CH—CH,·+BrBr CH,=CHCH,Br+·Br n-Br hBr H t br- Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 ¾ The radical reaction is initiated by a small amount of bromine radical formed by exposure of NBS to light or peroxides N H O O + Br N Br 2 O O + HBr