IR-2 Group Frequencies

IR-2 Group Frequencies - Group Frequencies Functional Group...

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Group Frequencies Functional Group Analysis
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IR Peaks are Associated with Separate Functional Groups Although all the bonds in a molecule vibrate, to a first approximation, they can be treated as if they vibrate independently. O-H C-O OH
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IR Group Frequencies
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Hydrocarbons (overview) (a) C -H stretch R -C ˹$ -H 3300 cm -1 , more s-character R 2 C=C-H 3000 ~ 3100 cm -1 C-C-H(alkane) 2800 ~ 3000 cm -1 (b) C-C Too week to observe C=C medium 1620 ~ 1680 cm -1 C ˹$ medium 2100 ~ 2600 cm -1 aromatics 2 sharp peaks 1500 ~ 1600 cm -1 (c) out-of-plane bendings Diagnostic to identify substitution patterns
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Cyclopropane H H 3100 ~ 3070 cm -1
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Epoxides O H H 3060 ~ 3040 cm -1
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1-Octene
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Alkene Substitution Patterns
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1-Hexene and 3-Hexene !
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Other Alkenes
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Methyl Pentenes CH CH 3 CH 3 1370 & 1385 cm -1 C CH 3 CH 3 1370 & 1395 (weaker) cm -1 CH 3
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Exocyclic Alkenes ! (C=C) 1780 1678 1657 1651 1655 cm -1 ! ring ! (bond angle) sp 180 sp2 120 sp3 109 α p-character then, s-character stronger C=C bond ν
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Exocyclic Alkene Examples 1649 1667 1676
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1-Hexyne
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1-Hexyne and 2-Hexyne
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Aromatic Substitutions Mono-substituted 2 peaks ~ 700 cm -1 730 ~ 770 cm -1 Disubstituted ortho- 1 peak 735 ~ 770 cm -1 meta- 2 peaks 680 ~ 725 cm -1 750 ~ 810 cm -1 para- 1 peak 790 ~ 840 cm -1
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Benzene Substitution Patterns
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Toluene and p-Xylene CH 3 CH 3 CH 3
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Propyl Benzene and Isopropyl Benzene
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• O-H Stretch – 3650 ~ 3600 cm-1, sharp (monomeric) – 3500 ~ 3200 cm-1, broad (hydrogen bonded) Usually concentration dependent Some exceptions O O H O H Always H-bonded Always monomeric
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Tert-Butanol, Neat and solution Neat CCl 4 solution H-bonded monomeric
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• C-O Stretch – Phenol 1220 cm-1 – Tertiary alcohol 1150 cm-1 – Secondary alcohol 1100 cm-1 – Primary alcohol 1050 cm-1
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