Terpenes-UV-ques

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					                                                                   Terpenes
                                                   •Terpenes are natural products that
                                                   form from a pyrophosphate and are
                  Terpenes                         structurally related to isoprene.
                                                            CH3
          Compounds with multiples of a 5
              carbon structural unit              H 2C      C     CH          CH2
                    (Isoprene)
                                                    Isoprene
                                             (2-methyl-1,3-butadiene)                  O       O
                                                                                       P       P
                                                                                   O       O        O
                                                                                       O       O
                                                                        Isopentenyl pyrophosphate       Isoprene




         The Isoprene Unit                                          Question
• An isoprene unit is the carbon skeleton   • How many isoprene units does geraniol
  of isoprene (ignoring the double bonds)     contain?
                                            •    A) 2
                                            •    B) 3
                                            •    C) 4
Two isoprene units:                         •    D) 5




                                                              Isoprene Links
                                                                   Heads or Tails?
         The Isoprene Unit
• The isoprene units are joined "head-to-
  tail."
             head                 tail


                      tail head




                                                                                                                   1
        Common Carbon Backbone




                    Terpenes                                                      Question
               Classification of Terpenes

                                                                  • Farnesol is classified as a
   •Class         Number of carbon atoms
   •Monoterpene             10
   •Sesquiterpene           15
   •Diterpene               20
   •Sesterpene              25                                    •     A)    monoterpene.
                                                                  •     B)    sesquiterpene.
   •Triterpene              30
                                                                  •     C)    diterpene.
   •Tetraterpene            40
                                                                  •     D)    triterpene.




                    Terpenes                                                     Terpenes
            Representative Monoterpenes                                  Representative Monoterpenes



                              OH                     O                                     OH                     O

                                                         H                                                            H



α-Phellandrene   Menthol                 Citral              α-Phellandrene   Menthol                 Citral
 (eucalyptus)  (peppermint)          (lemon grass)            (eucalyptus)  (peppermint)          (lemon grass)




                                                                                                                          2
                    Terpenes                               Terpenes
            Representative Monoterpenes             Representative Sesquiterpenes




                                                                     H


α-Phellandrene   Menthol                Citral                α-Selinene
 (eucalyptus)  (peppermint)         (lemon grass)              (celery)




                    Terpenes                               Terpenes
            Representative Sesquiterpenes           Representative Sesquiterpenes




                              H


                       α-Selinene                             α-Selinene
                        (celery)                               (celery)




                    Terpenes                               Terpenes
              Representative Diterpenes               Representative Diterpenes




                                          OH                                      OH




                        Vitamin A                             Vitamin A




                                                                                       3
     Terpenes
                                         Common Terpenes
Representative Diterpenes
                                           OH                                     OH


                                                                  Nerol C10H18O
                               Linalool C1 0H1 8O


                                                     OH

          Vitamin A                                                      OH

                             Geraniol C1 0H1 8O
                                                                 Terpineol   C10H18O




     Limonene                                     Limonene
    CH2




                                            CH2




                       CH2




                                        Ultraviolet-Visible (UV-VIS)
                                                  Spectroscopy

                               Gives information about conjugated π electron
                                                  systems




                                                                                       4
       Transitions between electron energy states
                                                                                  Conventions in UV-VIS


                                                                X-axis is wavelength in nm (high energy at left,
                                                                low energy at right)
                      gaps between electron energy
                      levels are greater than those             λmax is the wavelength of maximum absorption
                      between vibrational levels                                                       molecule—
                                                                and is related to electronic makeup of molecule—
  Δ E = hν                                                      especially π electron system
                      gap corresponds to wavelengths
                      between 200 and 800 nm                    Y axis is a measure of absorption of electromagnetic
                                                                radiation expressed as
                                                                 Absorbance or molar absorptivity (ε)




UV and Visible light cause only two kinds of electronic
                                                                                      cis,trans-1,3-cyclooctadiene
                                                                   π→π* Transition in cis,trans-1,3-cyclooctadiene
                       transitions

                                                                        ψ4∗                                        ψ4∗

                                                                LUMO    ψ3∗                                        ψ3∗
                                                                                            Δ E = hν
                                                                HOMO ψ2                                            ψ2

                                                                        ψ1                                         ψ1
• Only organic compounds with π electrons can absorb
                                                                            Most stable                    π-Electron
energy in the UV/Visible region                                              π-electron                  configuration of
• A visible spectrum is obtained if visible light is absorbed               configuration                 excited state
• A UV spectrum is obtained if UV light is absorbed




               π→π* Transition in Alkenes                                             Substituent Effects


     HOMO-LUMO energy gap is affected by                                Methyl groups on double bond cause λmax
     substituents on double bond                                        to shift to longer wavelengths

                                                                    H             H                H              CH3
     as HOMO-LUMO energy difference decreases
                                                                        C     C                         C     C
     (smaller ΔE), λmax shifts to longer wavelengths
                                                                    H             H                H              CH3

                                                                     λmax 170 nm                       λmax 188 nm




                                                                                                                            5
A chromophore is the part of a molecule which absorbs                                      Effect of Conjugation on λmax
                  UV or visible light




                                                                                                     Substituent Effects


                                                                                           Extending conjugation has a larger effect
                                                                                           on λmax; shift is again to longer
                                                                                           wavelengths

                                                                                   H            H             H            H
                                                                                       C    C                     C   C            H
                                                                                   H            H             H            C   C
                                                                                                                      H            H

                                                                                    λmax 170 nm                   λmax 217 nm




             O
                                                                                                The Beer–Lambert Law
                 CH3

 H2C

       CH3
                                 UV   Spectrum:      (+)-carvone
                                                                                                       A = clε
                                                    236 nm
                       3.5

                        3

                       2.5
                                                                                                A = log(I / I0)
                        2                                                                       c = concentration of substance
                                                                         Series1
                       1.5                                                                      in solution
                        1
                                                                                                l = length of the cell in cm
                       0.5

                        0
                                                                                                ε = molar absorptivity
                         200   220          240           260      280
                                      Wavelength   (nm)

                                                                                   The molar absorptivity of a compound is a constant that
                                                                                      is characteristic of the compound at a particular
                                                      carvone’
                                              What is carvone’s λmax?
                                                                                                          wavelength




                                                                                                                                             6
      UV Spectrum of cis,trans-1,3-cyclooctadiene
                     cis,trans-1,3-cyclooctadiene                 H                                                             Substituent Effects
                                                                                   H
                                                                       C   C                              H
                                                                  H                                                                        λmax 217 nm
                                                                                   C             C
                                                                                                                                                    diene)
                                                                                                                                        (conjugated diene)
              2000                                                         H                               H
    Molar                                         λmax 230 nm   H 3C              H
absorptivity (ε)
                                                   εmax 2630
              1000                                                     C   C                             H                                 λmax 263 nm
                                                                 H                                                                  H conjugated triene plus
                                                                                  C              C
                                                                                                                                        two methyl groups
                   200    220   240   260   280                            H                              C            C
                         Wavelength, nm                                                          H                                  CH3




       Both the λmax and ε increase as the number of              An auxochrome is a substituent in a chromphore that
       conjugated double bonds increases                          alters the λmax and the intensity of the absorption




       Uses of UV/Vis Spectroscopy                                         Spectroscopic Analysis:
                                                                                   Infrared:

                                                                                     IR Peak(s) (cm-1)                          Chemical Function(s)




       •Identifying molecular structures
                                                                                  Ultraviolet:


       •Measuring rates of a reaction                                           Structural feature(s):

                                                                                aromatic ring    ; oxygen atom   ; diene (conjugated)     Unknown UV max: _______

                                                                                none of the above


       • Determining the pKa of a weak acid                                            Features of conjugated system (if present)                 Value (nm)

                                                                                        Base value                     210 – 215 nm



       •(Quantitative analysis: Beer’s Law)
                                                                                        Homoannular diene                 +39-69 nm

                                                                                        Extended conjugation                  +30 nm

                                                                                        Alkyl group(s)                      +5-12 nm

                                                                                        Exocyclic double bond                  +5 nm

                                                                                        TOTAL (Calculated)




                                                                                                                                                                    7
 Rate of Reaction    Rate of Reaction




pKa of a Weak Acid




                                        8

				
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