TOPIC 9. STRUCTURE DETERMINATION (chapter 9 and ww2. collard/CHEM2311/notes/ 9. STRUCTURE...

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Transcript of TOPIC 9. STRUCTURE DETERMINATION (chapter 9 and ww2. collard/CHEM2311/notes/ 9. STRUCTURE...

  • D.M. Collard 2007

    9-2

    TOPIC 9. STRUCTURE DETERMINATION(chapter 9 and part of chapter 2)

    9-6

    OBJECTIVES

    1. Use combustion analysis to determine empirical formula.2. Determine molecular weight (and molecular formula) from mass

    spectrometry.2. Calculate number of rings and double bonds from molecular formula.3. Determine functional groups present from infrared spectroscopy.4. Use 1H and 13C NMR spectroscopy to identify other structural features. 5. Combine conclusions from individual techniques to determine the structure

    of organic compounds.

    Problems in this section will be restricted to the following classes of aliphatic and aromatic compounds:

    Alkanes, alkenes, alkynes, alkyl halides, alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, acyl chlorides, anhydrides, amides, amines, nitriles

  • D.M. Collard 2007

    9-7

    ???

    COMBUSTION ANALYSIS

    CxHyOz + O2 xCO2 + (y/2)H2O

    Measure mass of CO2 and H2O formed from a known mass of compound;data cited as mass% of each element present. Generally do not measure mass%O.e.g., C H OMass% 54.51 9.09

    Mole Ratio

    =

    =

    Empirical formula

    Be careful when rounding, elemental analyses provided 0.4 mass% - do not round by more than 0.05 to 0.1. Multiply ratios before rounding.

    9-8EMPIRICAL AND MOLECULAR FORMULAS;

    DETERMINATION OF SODARMolecular formula is an integral number of times the empirical formula.

    C2H4OSum of double bonds and rings (SODAR)

    For C,H,O: (2#C + 2 #H) / 2

    For C,H,O,N,Hal: (2#C + 2 #H #Hal + #N) / 2Each Hal replaces a HEach N adds an extra HS,O: no effect on calculation

    If SODAR calculated from empirical formula is not a positive integral (or 0), this cannot be the molecular formula.

    C2H4O MOLECULAR FORMULA COULD BE: ???

  • D.M. Collard 2007

    9-12If SODAR 4: consider the possible presence of a benzene ring

    OO

    9-13

    MASS SPECTROMETRY

    Theory and Experiment

    M + e (70 eV) M+ + 2eM+ lower mass ions

    S:9.12-9.18

  • D.M. Collard 2007

    9-14Data Availablee.g., mass spectrum of ethane, CH3CH3

    Ions also fragment, which can give further clues about the structure (beyond scope of this course).

    Knowledge of molecular weight (from mass spec) and empirical formula (from combustion analysis) allows determination of molecular formula.

    0 10 20 30m/e

    abun

    danc

    e

    9-15

    ???

    Empirical formula: C2H4O

    Mass spectrum: M+ m/e = 88

    MOLECULAR FORMULA IS:

  • D.M. Collard 2007

    9-16SPECTROSCOPY:

    THE INTERACTION OF LIGHT AND MATTER

    HighestMost dangerous

    Very dangerous

    Microwaveoven, rotationof molecules

    IR - bondsvibrating

    Tanning raysskin cancer

    What wesee

    Lowest energyradio/TVNMR

    High energy - kills life

    S:9.1-9.2

    9-17

    INFRARED SPECTROSCOPY

    Molecular Vibrations: Stretching and Bending

    Analogy to masses and springs:

    Spring constant (strength) , Frequency,

    Masses ,

    Prob:2.29,43,45,

    46

    S:2.16

  • D.M. Collard 2007

    9-18Electromagnetic RadiationE = h c =

    For molecular vibrations, Frequency, = 6 x 1012 to 1.25 x 1014 HzWavelength, = 50 to 2.5 mDefine wavenumber,

    = 200 to 4000 cm-1

    Instrumentation SpectrumA simple scheme

    IR source

    sample

    referencedetector

    =1

    (in cm)

    wavenumber / cm-14000 600%

    tran

    smitt

    ance

    9-20Selected Infrared AbsorptionsFunctional Group Range cm-1 Intensity and shapesp3CH 2850-2960 medium to strong; sharpsp2CH 3010-3190 medium to strong; sharpspCH about 3300 medium to strong; sharpCC 1620-1660 weak to medium; sharpCC 2100-2260 weak to medium; sharpNH 3300-3500 medium; broadOH (very dilute) about 3600 medium; sharpOH (H-bonded) 3200-3550 strong; broadOH (carboxylic acid) 2500-3000 medium; very broadCO 1050-1150 medium to strong; sharpCO (aldehyde,ketone) 1690-1740 very strong; sharpCO (ester) 1735-1750 very strong; sharpCO (carboxylic acid) 1710-1780 strong; sharpCO (amide) 1630-1690 strong; sharpCN 2200-2260 medium; sharp

  • D.M. Collard 2007

    9-21Validating resonance theory

    CO (ester) 1735-1750 cm-1CO (amide) 1630-1690 cm-1

    Which is the stronger bond?

    Can we understand this using resonance theory?

    N

    O

    O

    O

    N

    O

    O

    O

    9-22Infrared spectroscopy indicates the presence of particular bonds in a sample. The combination of bonds indicates which functional groups are present.You do not need to memorize the data in Table 2.7 (or the previous slide) for this class. However, you must develop experience at interpreting IR spectra to be able to determine the presence of particular functional groups.

    Wavenumber / cm-14000 600

    Ether

    Carboxylic acid

    Ester

    Aldehyde or ketone

    Alcohol

    C-OC=OO-H

  • D.M. Collard 2007

    9-23Hexane

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    9-241-Hexene

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    CH

    CH2

  • D.M. Collard 2007

    9-25

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    1-HexyneC

    CH

    9-26

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    Toluene

    CH3

  • D.M. Collard 2007

    9-27Butyl methyl ether

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    O

    9-28

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    1-Hexanol

    OH

  • D.M. Collard 2007

    9-29

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    2-HexanoneO

    9-30

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    Hexanoic Acid

    OH

    O

  • D.M. Collard 2007

    9-31

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    Methyl Propionate

    9-34

    ???

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    C4H8O2

    Structure contains:

    Draw eight possible structuresconsistent with this data.

  • D.M. Collard 2007

    9-36

    4000 3000 2000 1500 1000 500Wavenumber (cm-1)

    Tran

    smitt

    ance

    (%) 100

    50

    0

    Problem: C8H8O2. What functional group(s) is(are) present? What other structural features?

    9-37Problem: Determine the structure of the compound with the following empirical formula and IR spectrum.Empirical formula: C4H8OIR; peak at 1720 cm-1; no broad peak at 3300, no strong peaks at 1000 to 1200 cm-1, no strong peaks at 1600-1650 cm-1

  • D.M. Collard 2007

    9-39

    Important infrared adsorptions

    Wavenumber / cm-14000 600

    1700

    1000-1200 CO

    2900 sp3CH

    3100 sp2CH

    3500, O-H

    COOH

    1600 C=C

    9-40HYDROGEN (PROTON) NUCLEAR MAGNETIC

    RESONANCE (1H NMR) SPECTROSCOPYNuclear Spin (1H, 13C, 19F, 31P)

    S:9.3-9.4

    N

    N

    S

    S

    S

    N

  • D.M. Collard 2007

    9-41The Energy of the Spin States of Hydrogen Nuclei in a Magnetic Field

    Bo

    EBo=0

    The frequency required to flip the spin state from to is called resonance frequency

    9-42An early 1H NMR spectrum

    ...not much chemical information

    A modern 1H NMR spectrum

    lots of chemical informationE=h

    E=h

    abso

    rptio

    n

  • D.M. Collard 2007

    9-44

    Ha

    Hb

    If the applied magnetic field is held constant, the protons Ha and Hbabsorb irradiation with different frequencies.

    B

    E

    hab

    s orp

    tion

    The spectrum

    9-45Preview: Types of Information available from a 1H NMR spectrum

    A 1H nuclear magnetic resonance spectrum contains information about the:

    (a) number of different types of proton

    (b) relative number of each type of proton

    (c) proximity to functional groups

    (d) the number of adjacent protons

    HCH

    Cl CH

    HH

  • D.M. Collard 2007

    9-46(a) The number of signals in the spectrum is the number of types of proton.

    1H NMR spectrum of methane CH4

    one peak one type of proton

    1H NMR spectrum of ethane CH3CH3

    one peak one type of proton

    abso

    rptio

    nab

    s orp

    tion

    h

    h

    9-47Protons that are related to each other by a rotation or plane of symmetry are identical (chemical shift equivalent)

    HCH

    H CH

    HH

    H

    HH

    H H

    H

    CH3CH2CH2CHCH3CH3

    CH3

    HH

    H H

    H

    Cl

    ClH

    H H

    H

    CH3

    HCl

    H H

    H

    Hax

    Hax

    HeqHeq

    Hax

    HaxHax

    HaxHeq

    Heq

    Heq

    Heqlow temperature: 2 signals

    high temperature (room T): 1 averaged signal

    H3C C CH3

    OH

    H

  • D.M. Collard 2007

    9-481H nuclear magnetic resonance spectrum of methanol

    CH3OH

    two peaks two types of proton

    abso

    rptio

    n

    h

    9-51

    CH3H3C

    HH

    H H

    H3C OH

    (b) The relati