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9. STRUCTURE DETERMINATION

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.

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. e.g., C H O Mass% 55.7 7.0 37.1 Mole% = Ratio

EMPIRICAL AND MOLECULAR FORMULAE; DETERMINATION OF “SODAR” Molecular formula is an integral number of times the empirical formula. Sum of double bonds or rings (”SODAR”) For C,H,O: (2#C + 2 – #H) / 2 For C,H,O,N,Hal: (2#C + 2– #H – #Hal + #N ) / 2 If SODAR calculated from empirical formula is not an integral, this cannot be the molecular formula.

MASS SPECTROMETRY Theory and Experiment M + e– (70 eV) M+ + 2e–

E.g., mass spectrum of ethane, CH3CH3 Data Available E.g., mass spectrum of ethane, CH3CH3 Knowledge of molecular weight (from mass spec) and empirical formula (from combustion analysis) allows determination of molecular formula. abundance 0 10 20 30 m/e

SPECTROSCOPY: THE INTERACTION OF LIGHT AND MATTER

INFRARED SPECTROSCOPY See Chapter 2 COOH 3100 sp2C–H 1600 C=C 3500br, O-H 1000-1200 C–O 2900 sp3C–H 1700 C=O 4000 600 Wavenumber / cm-1

Problem. Compound A has the empirical formula C2H4O, the mass spectrum has a peak at m/e=88. The IR does not have a peak at ca. 1700 cm-1. Problem. Compound B has the empirical formula C5H10O, the mass spectrum has a peak at m/e=86. The IR does have a peak at ca. 1700 cm-1.

PROTON NUCLEAR MAGNETIC RESONANCE (1H NMR) SPECTROSCOPY Nuclear Spin (1H, 13C, 19F, 31P, …)

The Energy of Protons in a Magnetic Field

A Nuclear Magnetic Spectrometer

An early 1H NMR spectrum ...not much chemical information A modern 1H NMR spectrum …lots of chemical information

Preview: 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

(a) The number of signals in the spectrum is the number of types of proton. 1H NMR spectrum of methane one peak  one type of proton 1H NMR spectrum of ethane

1H nuclear magnetic resonance spectrum of methanol… two peaks  two types of proton

(b) The relative area of each peak (the integration) corresponds to the relative number of each type of proton 1H NMR spectrum of methanol integral ratio :  ratio of types of proton 1H NMR spectrum of p-xylene

Shielding and Deshielding The magnetic field experienced by a nucleus depends on the electron density around the nucleus The energy at which a nucleus resonates depends on the electron density

Chemical Shift 10 5 d / ppm

CH3CH3 CH3N(CH3)2 CH3OCH3 CH3F d0.9 d2.2 d3.2 d4.3 CH3Cl D3.1 CH3Br d#.# CH3l CH3Cl CH2Cl2 CHCl3 d3.1 d5.3 d7.3

d7.3 d5.3 d0.9

(c) The chemical shift indicates the environment of the proton 1H NMR spectrum of #####

Spin-Spin Coupling The magnetic field experienced by protons is influenced by the spin of adjacent protons

Some Common Sets of Multiplets Et–X i-Pr–X t-Bu–X

(d) The multiplicity of a signal indicates the number of adjacent protons 1H NMR spectrum of diisopropyl ether

Geminal protons Rings Vinyl groups

Summary: Types of information available from 1H NMR spectrum Number of signals Number of types of proton Integral of signals Relative number of each type of proton Chemical Shift Electronic environment Multiplicity Number of adjacent protons

Problem. Predict the 1H NMR spectrum of CH3CH2Cl Problem. Predict the 1H NMR spectrum of CH3COCH2CH2Cl

13C NMR SPECTROMETRY 13C (not 12C) has nuclear spin However, 13C is only present at 1.1% abundance - Signals are weak - Spectra usually acquired without multiplicity information - Larger range of chemical shifts (0 to <200 ppm)

200 150 100 50 d / ppm

USING DATA TO DETERMINE STRUCTURE 1. Write down conclusions from each individual technique - Use combustion analysis to determine empirical formula, and mass spectrometry to give molecular weight (and molecular formula) - Calculate number of rings and double bonds from molecular formula (SODAR) - Determine presence of functional groups present from infrared spectroscopy. - Use 1H and 13C NMR spectroscopy to identify other structural features. 2. Use these conclusions to determine structure 3. Check your answer (predict the spectra; does your prediction match the data provided?)

Working through a set of data… Combustion Analysis: mass%: C, ##; H, ##. Mass spec: M+ m/e = ##

Infrared spectrum

13C NMR spectrum

Infrared spectrum

1H NMR spectrum

…finishing up Use this process when determining the structure of organic compounds from spectral data.

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CHAPTER 9 ON EXAM 5 Types of Questions - Use data from elemental analysis, infrared spectroscopy and nuclear magnetic resonance spectrometry (1H and 13C) to determine the structure of organic compounds. - In class assignment - Take home assignment (more complex problems) Preparing for Exam 4: - Work as many problems as possible. - Work in groups. - Do the “Learning Group Problem” at the end of the chapter. - Work through the practice exam