Infrared Spectroscopy · Infrared Spectroscopy Alkanes •Simple spectra with few peaks •C-H...
Transcript of Infrared Spectroscopy · Infrared Spectroscopy Alkanes •Simple spectra with few peaks •C-H...
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Infrared Spectroscopy
Correlation Chart (Adapted from Pavia Table 2.3)
etc….complete table can be found on class web site
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Infrared Spectroscopy
The IR Spectrum
(s)(m)(w)
strongmediumweak
(s)
(m)
(w)
fingerprint regionfunctional group region
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Infrared Spectroscopy
Alkanes
• Simple spectra with few peaks
• C-H stretch most prominent (around 3000 cm-1)- sp3 C-H stretch between 3000-2840 cm-1
• saturated, unstrained hydrocarbons typically to the right of 3000 cm-1
• C-H stretch on vinyl or aromatic carbon to left of 3000 cm-1
- CH2/CH3 bending vibrations between 1475-1350 cm-1
• CH2 bend around 1465 cm-1
• CH3 bend around 1375 cm-1
- long chain (four or more CH2 groups in long chain may see a band at 720 cm-1
• weak, often obscured in more complex molecules
• C-C stretch – not interpretively useful
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Infrared Spectroscopy
Alkanesdecane
cyclohexane
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Infrared Spectroscopy
Alkanes
(bromomethyl)cyclopropane
Br
cyclopropane C-Hca. 3100 cm-1
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Infrared Spectroscopy
Alkenes
• Spectra more complex than alkanes
• =C-H stretch to left of 3000 cm-1
- sp2 C-H stretch between 3095-3010 cm-1
• =C-H out of plane bending vibrations between 1000-650 cm-1
- can sometimes be used to determine degree of alkene substitution- may be obscured in complex systems
• C=C stretch occurs between 1660 - 1600 cm-1
- intensity varies with dipole - conjugation shifts absorbance to lower frequencies; increases intensity
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Infrared Spectroscopy
Alkenes
1-hexene
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Infrared Spectroscopy
Alkenes
cis-2-pentene
C=C
sp2 C-H
CH3H3C
H H
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Infrared Spectroscopy
Alkenes
trans-2-pentene
C=C
sp2 C-H
(very weak)
CH3
H3C
H
H
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Infrared Spectroscopy
Alkenes
Conjugation Effects
• Conjugation moves C=C stretch to lower frequencies (~15-20 cm-1)
• Conjugation increases amount of s-character; weakens bond, lowers force constant
• May see multiple absorbances if alkene is conjugated to another double bond
1630 cm-11640 cm-11643 cm-1
O
1617 cm-11646 cm-1
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Infrared Spectroscopy
Alkenes
trans-1,3-pentadiene
CH2
H3C
H
H
H
C=C
sp2 C-H
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Infrared Spectroscopy
Alkenes
C=C stretch: effect of ring strain in endocyclic alkenes
1646 cm-11650 cm-1 1656 cm-11566 cm-11611 cm-1
decreasing absorption frequency
1646 cm-1 ~1611 cm-1
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Infrared Spectroscopy
Alkenes
C=C stretch: substituent effects in endocyclic alkenes
1646 cm-1
1656 cm-1
1566 cm-1
1611 cm-1
1675 cm-1
1788 cm-1
1641 cm-1
1650 cm-1
1681 cm-1
1883 cm-1
1675 cm-1
1679 cm-1
R RR
R R
R
R
R
R
R
R
R
increasing absorption frequency
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Infrared Spectroscopy
Alkenes
C=C stretch: effect of ring strain in exocyclic alkenes
1651 cm-11940 cm-1 1780 cm-1 1678 cm-1 1657 cm-1
increasing absorption frequency
1655 cm-1
more s character
more p character
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Infrared Spectroscopy
Alkenes
C=C stretch in acyclic alkenes
H
HR
H
H
RR
H
H
RR
R
H
HR
R
R
HR
H
R
RR
R
~1645 cm-1
~1660 cm-1
~1675-1670 cm-1
~1655 cm-1
~1675-1670 cm-1
~1665-1660 cm-1
OR
RR
R~1680-1660 cm-1
(usually strong)
vinyl
cis
trisubstituted
vinylidene
trans
tetrasubstituted
vinyl ether
* *
*
* =C-H bending absorbance may supplement assignment