Chapter 18, Problem 83IP

Interpretation Introduction

Interpretation: For the given structure of compound the expected signals and chemical shift of protons are predicted.

Concept Introduction:

• Electrophiles are electron deficient species. In Electrophilic substitution reaction a group or atom in a compound is replaced by electrophile. This kind of reaction occurs predominantly in aromatic compounds. Electrophilic substitution reactions of aromatic compounds are known as aromatic electrophilic substitution reactions.
  • Benzene is an electron rich Aromatic compound. It undergoes aromatic electrophilic substitution reaction. 

• The delocalized nature of pi electrons in benzene attributes a special property to benzene called resonance.

• If the substituent’s on benzene Carbon is Electron rich groups they are known as activating groups. They are ortho- and para- directing groups because these groups when directly bonded to benzene Carbon increases the electron density at ortho and para positions. So they direct the incoming electrophile towards ortho and para position in electrophilic substitution reactions.

• If the substituent’s on benzene Carbon is Electron withdrawing groups they are known as deactivating groups. They are meta-directing groups because these groups when directly bonded to benzene Carbon decreases the electron density at ortho and para positions and so the incoming electrophile is directed towards meta position.

• ${}^{\text{1}}\text{H}\text{NMR}$: The ${}^{\text{1}}\text{H}\text{NMR}$ spectrum gives information on the different electronic environment of protons. The number of signal (proton types) generated in ${}^{\text{1}}\text{H}\text{NMR}$ are predicted by performing symmetry operations (rotation or reflection symmetry).

• Factors affect the chemical shift values: The electronegative atoms which present nearer to the subjected protons withdraw electron density around the protons inductively makes the protons deshielded and shift the chemical shift value towards downfield.