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Chemical Shift – magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11) Anisotropy of Aromatic compounds: in plane and above CH3 CH3 H H H H H H H H H H δδδδring���� 8.14-8.64 ppm δδδδMe���� -4.25 ppm H H δδδδring���� 7.27-6.95 ppm δδδδCH2���� -0.51 ppm δδδδOUTSIDE���� 9.28 ppm δδδδINSIDE���� -2.99 ppm H H H H H H H H H H H H H HH H H H Anisotropy: Aromatic Electronic effects CH2 O CH3 CH2 + O- CH3 Deshielded O O O H H 7.10 ppm COOEt COOEt H H COOEt H H EtOOC 6.83 ppm6.28 ppm O H H7.71 ppm 6.10 ppm O H H H H7.07 ppm 6.38 ppm 6.28 ppm 5.93 ppm 8 7 6 5 4 3 O 12 3 4 5 6 O 7 Electronic effects: conjugation with carbonyl 8 7 6 5 4 3 2 O 1 2 34 56 7.75 6.20 Electronic effects: conjugation with carbonyl deshielded Electronic effects: conjugation with heteroatom OH H O+ C- H H SH H 6.06 ppm 5.48 ppm S H H 5.81 ppm O H H5.78 ppm OH H4.82 ppm 6.22 ppm shielded 6.5 6.0 5.5 5.0 4.5 4.0 3.5 12 34 O 5 Electronic effects: no conjugation with heteroatom 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 12 34 O 5 2.65 2.60 2.55 6.35 6.30 4.95 4.90 shielded Electronic effects: conjugation with heteroatom O CH3 8 7 6 5 4 3 2 8.0 7.5 Electronic effects: conjugation with carbonyl deshielded deshielded o p m 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 O CH3 7.3 7.2 7.1 7.0 6.9 6.8 Electronic effects: conjugation with heteroatom Shielded shielded o p m NH CH3 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 7.0 6.5 Electronic effects: conjugation with heteroatom Shielded shielded o p m F 7.5 7.4 7.3 7.2 7.1 7.0 6.9 Cl 7.4 7.3 7.2 7.1 Br 7.6 7.5 7.4 7.3 7.2 7.1 Aromatic: inductive effect and resonance effect Calculating Shifts for aromatic compounds NMR � Common Aromatic Patterns 6/9/2011 15 ortho 6-10 Hz para 1-4 Hz meta 0-2 Hz8-10 Hz H H H H H H H H NMR � “Activating” and “Deactivating” groups and the impact of the changing electron density in the Benzene ring on Chemical Shift of ortho, meta, para protons 6/9/2011 17 Ha Ha’ Hb Hb’ Ha&Ha’Hb&Hb’ 2 Amino Protons The Molecular Ion peak from Mass Spectrometry would have indicated the presence of the single Chlorine atom and Nitrogen. � Para Di-Substituted Benzene ring � Ha & Ha’ have same Chemical Shift � Hb & Hb’ have same Chemical Shift � Ha is split into doublet by Hb � Hb is split into doublet by Ha � Two sets of peaks produced by relative electronegativity of Amino & Cl groups P-Chloroaniline (C6H6ClN) Hydrogen bond Aromatic substitution pattern: ortho 8.00 7.90 7.80 7.70 7.60 7.50 7.40 CH3 CH3 O O AA’ XX’ Typical spectra for ortho (symmetrical) 8.5 8.0 7.5 Br N+ O- O H A H B H C H D Calculated shifts δHA=8.44 δHB=7.82 δHC=7.31 δHD=8.19 HA HB HCHD meta bromo nitro benzene NMR � “Activating” and “Deactivating” groups and the impact of the changing electron density in the Benzene ring on Chemical Shift of ortho, meta, para protons 6/9/2011 21 c b a � The Methoxy group is moderately activating, while the Nitro groups are strongly deactivating (electron withdrawing) � Net effect is to Decrease the electron density about the ring protons � The a & b protons are Ortho to the strongly deactivating Nitro groups, thus, they have reduced electron density and their Chemical Shift is down field relative to the “c” proton � All protons interact to produce Spin-Spin Coupling. a b c 3H 2,4-Dinitroanisole (C7H6N2O5) AB-Spectra AMX C6 H4 O5 N2 I = 6 - 4/2 + 2/2 +1 I = 6 Phenyl = 4 I NO2 = 1 I AMX AFMX δ J C5 H4 N Br I = 5 – 4/2 – 1/2 +1/2 +1 I = 4 (aromatic ring)
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