!! www.clutchprep.com
CONCEPT: PURPOSE OF ANALYTICAL TECHNIQUES Classical Methods (Wet Chemistry): Chemists needed to run dozens of chemical reactions to determine the type of molecules in a compound. EXAMPLE: Tollen s Test Instrumental Methods (Dry Chemistry): Expensive scientific instruments investigate the properties of molecules. EXAMPLE: 1 H NMR Page 2
CONCEPT: IR SPECTROSCOPY- GENERAL FEATURES IR Spectroscopy is a chemical analytical method that uses differing frequencies of infrared light to detect predictable types of chemical bonds in molecules. The frequencies will cause certain bonds to Stretching, Twisting, Wagging, Scissoring, etc. If the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum. Major regions of absorption Common IR Ranges 3200 3600 -OH Strong, Broad 3300 -NH Peaks = H s SP3 = 2900 3000 SP2 = 3000 3150 SP = 3150-3300 -CH 2200-2300 C C C N Choppy Medium, Sharp 1700 C=O Very Strong, Sharp 1650 C=C Medium, Sharp Page 3
CONCEPT: IR SPECTROSCOPY- FREQUENCIES There are specific absorption frequencies in the functional group region that we should be familiar with EXAMPLE: What are the major IR absorptions for each compounds? Page 4
PRACTICE: Answer each of the following questions based on the images below. O OH O O O H F 3 C O CF 3 A B C D E a) Which compounds show an intense peak ~ 1700 cm -1? b) Which compound shows an intense, broad peak at ~ 3400 cm -1? c) Which compound has a peak at ~1700 cm -1, but no peaks at 2700 cm -1? d) Which compound has no signal beyond the fingerprint region? Page 5
PRACTICE: The following compound contains two carbonyl groups. Identify which carbonyl group will exhibit a signal at a lower wavenumber. O O Page 6
CONCEPT: IR SPECTROSCOPY- DRAWING HYDROCARBONS Alkanes: Alkenes: Terminal Alkynes: Page 7
CONCEPT: IR SPECTROSCOPY- DRAWING ALCOHOLS AND AMINES Alcohols: 1 Amines: 2 Amines: Page 8
CONCEPT: IR SPECTROSCOPY- DRAWING SIMPLE CARBONYLS Ketones: Esters: Page 9
CONCEPT: IR SPECTROSCOPY- DRAWING COMPLEX CARBONYLS Aldehydes: Carboxylic Acids: Page 10
CONCEPT: IR SPECTROSCOPY- DRAWING CONCEALED FUNCTIONAL GROUPS Alkyl Haldies: Ethers: 3 Amines: Page 11
PRACTICE: Based on IR data given determine the structure of the unknown. Unknown compound A has molecular formula C4H11N. It shows a peak at 2900 cm -1 and peaks in the fingerprint region. PRACTICE: Based on IR data given determine the structure of the unknown. Unknown compound B has molecular formula C4H11N. It shows a single peak at approximately 3400 cm -1 as well as peaks at 2900 cm -1 and in the fingerprint region. Compound B also possesses a branched alkyl group. PRACTICE: Based on IR data given determine the structure of the unknown. Unknown compound C has molecular formula C6H10O3. It shows peaks at 2900, 1850, 1740 cm -1 and in the fingerprint region. Page 12
PRACTICE: Match the following functional group choices with the supplied infrared spectra data A) Ether B) Ketone C) Alcohol D) Alkene E) Nitrile Page 13
PRACTICE: Match the following functional group choices with the supplied infrared spectra data. A) Alkyl Halide B) Alkyne C) Carboxylic Acid D) Alkene E) Ketone Page 14
PRACTICE: Match the following functional group choices with the supplied infrared spectra data. A) Aldehyde B) Alkane C) Carboxylic Acid D) Ester E) Ether PRACTICE: Match the following functional group choices with the supplied infrared spectra data. A) Ketone B) Alkyne C) Alkene D) Alkyl Halide E) Amine Page 15
CONCEPT: 1 H NUCLEAR MAGNETIC RESONANCE- GENERAL FEATURES 1 H (Proton) NMR is a powerful instrumental method that identifies protons in slightly different electronic environments by inducing tiny magnetic fields in the electrons around the nucleus. General Spectrum: is the standard reference point for NMR Electrons protons from the effects of NMR The further downfield, the more the proton There are 4 types of information we can gain from NMR spectra. Four Types of Information 1. Total Number of Signals Describes how many different types of hydrogens are present 2. Chemical Shift Describes how shielded or deshieldied the hydrogens are 3. Height of Signals (Integration) Describes the relative ratios of each type of hydrogen 4. Spin-Splitting (Multiplicity) Describes how close or far the different hydrogens are to each other Page 16
CONCEPT: 1 H NMR TOTAL NUMBER OF SIGNALS There are as many signals on each spectrum as there are unique, non-equivalent protons. Equivalent protons are defined as protons that have the same prospective on the molecule For now, let s assume that hydrogens bound to the are equivalent Symmetry will reduce the total number of signals EXAMPLE: How many different types of protons (signals) are there on each molecule? Page 17
PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? Page 18
PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? Page 19
PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? Page 20
CONCEPT: 1 H NMR PROTON RELATIONSHIPS Hydrogens attached to the same carbon actually do have different relationships based on their chirality. The Q-Test is used to determine the specific type of chirality of each hydrogen. a. Homotopic Protons Q-Test DOES NOT yield new chiral center Protons are always homotopic and are considered (They share a signal) In general, the three hydrogens on -CH3 groups will always be homotopic b. Enantiotopic Protons Q-Test DOES yield new chiral center. No original chiral centers = protons are still (They share a signal) c. Diastereotopic Protons Q-Test DOES yield new chiral center. 1+ original chiral centers = protons are now (Each proton gets its own signal) Page 21
EXAMPLE: How many signals will each molecule possess in 1 H NMR? Page 22
PRACTICE: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. PRACTICE: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. PRACTICE: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. Page 23
CONCEPT: 1 H NMR PROTON RELATIONSHIPS d. E / Z Diastereoisomerism Q-Test DOES yield new trigonal center on terminal double bonds Protons are always diastereotopic and are (Each proton gets its own signal) EXAMPLE: How many peaks will each molecule possess in 1 H NMR? Page 24
CONCEPT: 1 H NMR CHEMICAL SHIFTS The chemical shift indicates the exact electrochemical environment that each proton is experiencing. In general, electronegative groups will pull electrons away from nuclei, deshielding them Shifts increase (move downfield) as protons become more deshielded C H 1 2 C = C 4.5 6 C C 2.5 Benzene 6 8 Z C H 2 4 Aldehyde, -CHO 9-10 OH, NH 1 5 Carboxylic Acid, -COOH 10-13 Your professor will determine how many chemical shifts you should memorize. We ll go over them just in case. EXAMPLE: Order the following five protons from most deshielded to most shielded Page 25
PRACTICE: Which of the labeled protons absorbs energy most upfield in the 1 H NMR? D O A H C E B PRACTICE: Which of the labeled hydrogens will be most de-shielded? O O O O O O O A B C D E PRACTICE: Which compound possesses a hydrogen with the highest chemical shift for its 1 H NMR signal? F F F F F F A B C D Page 26
CONCEPT: 1 H NMR SPIN-SPLITTING WITHOUT J-VALUES Also known as spin-spin coupling, or J-coupling, this describes the distances between different protons. Note: This topic can be taught with or without J-values. Check with your professor to determine how much detail you should learn. For now, we will start with the simplest explanation, (should suffice for 90% of professors), which is without J-values. Adjacent, - protons will split each other s magnetic response to the NMR We use the rule to determine how many splits we will achieve Pascal s Triangle predicts the shape of the splits we will get EXAMPLE: How will the following protons be split? Page 27
PRACTICE: Predict the splitting pattern (multiplicity) for the following molecule: PRACTICE: Predict the splitting pattern (multiplicity) for the following molecule: Page 28
PRACTICE: Which of the following compounds gives a 1 H NMR spectrum consisting of only a singlet, a triplet, and a pentet? a) CH3OCH2CH2CH2CH2OH b) CH3OCH2CH2OCH2CH3 c) CH3OCH2CH2CH2OCH3 d) CH3OCH2CH2OCH3 Page 29
CONCEPT: 1 H NMR SPIN-SPLITTING WITH J-VALUES AND TREE DIAGRAMS Coupling-Constants, also known as J-values, describe the amount of interaction that a proton will have on another. Here are some examples of common coupling-constants (measured in Hz): Pascal s Triangle only helps to predict the shapes of splits when all of the J-values are assumed to be the same. When multiple J-values are involved, tree diagrams are needed to predict the shapes of the splits. Drawing Simple Tree Diagrams: First, let s use tree diagrams to help us understand why Pascal s Triangle and the n + 1 Rule make sense. Each split represents the J-value in Hz of a single proton. What does n + 1 predict here? ANSWER Page 30
CONCEPT: 1 H NMR SPIN-SPLITTING WITH J-VALUES AND TREE DIAGRAMS Drawing Complex Tree Diagrams: Now let s use an example where multiple J-values are involved. Always split in order of highest to lowest values. Before starting, what does the n + 1 Rule predict here? ANSWER EXAMPLE: Use a tree diagram to predict the splitting pattern of the bolded proton. Page 31
PRACTICE: Draw a tree diagram for H * in the structure below. F2CH * CH(CH3)2 JH*-F = 50 Hz JH*-H = 7 Hz Page 32
CONCEPT: 1 H NMR SPIN-SPLITTING COMMON PATTERNS Some splitting patterns are highly indicative of certain structures. We can get ahead by memorizing them. EXAMPLE: Which common 1 H NMR splitting pattern seen below could help us determine the molecular structure? Page 33
CONCEPT: 1 H NMR INTEGRATION Integration describes how many of each type of hydrogen are present, expressing this information as relative ratios. Uses the Area Under the Curve (AUC) to visually demonstrate which hydrogens are most prevalent. EXAMPLE: Draw the complete NMR spectrum: Page 34
PRACTICE: Which of the following molecules gives a 1 H NMR spectrum consisting of three peaks with integral ratio of 3:1:6? Page 35
PRACTICE: Draw the approximate positions that the following compound might show in its 1 H NMR absorptions? Page 36
PRACTICE: Draw the approximate positions that the following compound might show in its 1 H NMR absorptions? Page 37
CONCEPT: 13 C NMR GENERAL FEATURES 13 C NMR is a more limited type of nuclear magnetic resonance that identifies 13 C instead of 1 H. Due to low natural incidence of the 13 C isotope, is NOT observed. ( ---------) (---------) = All of the other principles from 1 H NMR apply, except that we must learn new shift values: C H 5-45 C = C 100-140 C C 65-100 Benzene 120-150 Z C H 30-80 Carbonyl 160-210 EXAMPLE: How many 13 C signals would ethylbenzene give? EXAMPLE: Which compound(s) will give only one peak in both its 1 H and 13 C spectra? Page 38
CONCEPT: STRUCTURE DETERMINATION MOLECULAR SENTENCES The holy grail of this section is structure determination. You may be asked to produce a structure from scratch given only a MF, NMR Spectrum and IR Spectrum. Our goal is to build a strong molecular sentence by gathering clues, then propose drawings. How to build a molecular sentence: 1. Determine IHD. 2. Analyze NMR, IR and splitting patterns, integrations for major clues (i.e.). NMR = 9.1 ppm IR = 1710 cm -1 Triplet/Quartet 9.1 ppm (2H) 3. Calculate 1 H NMR Signal : Carbon Ratio. Ratio < ½ suggests symmetrical, whereas ratio > ½ suggests asymmetrical Never rule out a structure based on symmetry (you may not be able to visualize it) 4. State the number of 1 H NMR signals needed. --- DRAW POSSIBLE STRUCTURES --- 5. Use a combination of Shifts, Integrations, and Splitting to confirm which structure is correct. EXAMPLE: Build a strong molecular sentence using the following data. MF: C4H6O2 IR: peak at 2950 cm -1 1 H NMR peak at 2700 cm -1-2.2 (doublet, 4H) peak at 1720 cm -1-9.4 (triplet, 2H) Page 39
PRACTICE: Propose a structure for the following compound that fits the following 1 H NMR data: Formula: C3H8O2 1 H NMR: 3.36 δ (6H, singlet) 4.57 δ (2H, singlet) Page 40
PRACTICE: Propose a structure for the following compound that fits the following 1 H NMR data: Formula: C2H4O2 1 H NMR: 2.1 δ (singlet, 1.2 cm) 11.5 δ (0.5 cm, D 2 O exchange) Page 41
PRACTICE: Propose a structure for the following compound that fits the following 1 H NMR data: Formula: C10H14 1 H NMR: 1.2 ppm (6H, doublet) 2.3 ppm (3H, singlet) 2.9 ppm (1H, septet) 7.0 ppm (4H, doublet) Page 42
PRACTICE: Propose a structure for the following compound, C7H12O2 with the given 13 C NMR spectral data: Broadband decoupled 13 C NMR: 19.1, 28.0, 70.5, 129.0, 129.8, 165.78 δ DEPT-90: 28.0, 129.8 δ DEPT-135: 19.1 δ ( ), 28.0 ( ), 129.8 δ ( ), 70.5 δ ( ) & 129.0 δ ( ) Page 43
PRACTICE: Propose a structure for the following compound, C5H10O with the given 13 C NMR spectral data: Fully Broadband decoupled 13 C NMR and DEPT: 206.0 δ ( ); 55.0 δ ( ); 21.0 δ ( ) & 11.0 δ ( ). Page 44
PRACTICE: Provide the structure of the unknown compound from the given information. Formula: C4H10O IR: 3200-3600 cm -1 1 H NMR: 0.9 ppm (6H, doublet) 1.8 ppm (1H, nonatet) 2.4 ppm (1H, singlet) 3.3 ppm (2H, doublet) Page 45
PRACTICE: Provide the structure of the unknown compound from the given information. Formula: C4H9N IR: 2950 cm -1, 3400 cm -1 1 H NMR: 1.0 ppm (4H, triplet) 2.1 ppm (4H, triplet) 3.2 ppm (1H, singlet) Page 46
CONCEPT: MASS SPECT- INTRODUCTION Mass Spectrometry is usually accomplished through a technique called electron impact ionization (EI) Electrons are beamed at molecules, generating high energy intermediates called radical cations This is known as the molecular ion or as the parent ion. Only fragment cations are deflected by the magnetic field, the smaller ones more than the bigger ones. Detects the mass-to-charge ratio, which means that it detects the MW of cationic fragments Page 47
CONCEPT: MASS SPECT- FRAGMENTATION Ionization Potentials: Some electrons require less energy to ionize than others. Simple Fragmentation Mechanisms: The molecular ion will often fragment into smaller, sometimes more stable ion fragments. The stability of the cation fragment usually determines the relative amounts of fragments observed Radicals tend to form on the less stable side of the fragment Common Splitting Fragments: EXAMPLE: Fragmentation of Butane Page 48
PRACTICE: Draw the most likely ion fragment for the following molecules a. b. PRACTICE: Propose the molecular ion and likely fragmentation mechanism for the following molecule. What would be the value of the base peak? Page 49
CONCEPT: MASS SPECT- COMMON ISOTOPES Isotopes are often visible on a mass spectrum, due to their differing weights. They can be used for structure determination. Understanding the (M + 1) Peak 1.1% of all carbon is found as 13 C, adding a small but distinctive (M + 1) peak proportional in size to the number of carbons. This proportion is fairly consistent, so it gives rise to two helpful equations Understanding the (M + 2) Peak The halogens Cl and Br give distinctive (M + 2) peaks due to their unusual patterns of isotopic abundance 35 Cl = 75.8% and 37 Cl = 24.2%, yielding an approximate 3:1 ratio at (M + 2) 79 Br = 50.7% and 81 Br = 49.3%, yielding an approximate 1:1 ratio at (M + 2) The Nitrogen Rule Unlike carbon, nitrogen forms 3 bonds. We can use this information to determine the number of nitrogens in a molecule. Even or odd molecular weight of parent ions usually indicates and even or odd number of nitrogens present Page 50
PRACTICE: Propose the number of carbons for a compound that exhibits the following peak in its mass spectrum: a. (M) + at m/z = 72, relative height = 38.3% of base peak (M+1) + at m/z = 73, relative height = 1.7% of base peak b. Predict the approximate height of the (M + 1) peak for the molecule icosane, molecular formula C20H42. c. Draw the expected isotope pattern that would be observed in the mass spectrum of CH2Br2. In other words, predict the relative heights of the peaks at M, (M + 2), and (M + 4) peaks. Page 51