IR Spectroscopy Some Simple Practice Problems — Master Organic Chemistry PDF

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3/1/2018

IR Spectroscopy: Some Simple Practice Problems — Master Organic Chemistry

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IR Spectroscopy: Some Simple Practice Problems by James in Spectroscopy By itself, Infrared (IR) spectroscopy isn’t a great technique for solving the structure of an unknown molecule. However, we’ve seen that IR spectroscopy can a great technique for identifying certain functional groups in an unknown molecule – especially functional groups containing OH or C=O. For instance, in an earlier post on the structure determination of deer tarsal gland pheromone, we saw how the authors of the study used IR spectroscopy to identify the presence of a lactone functional group (i.e. a cyclic ester) by its characteristic absorbance at 1775 cm-1. Additionally, if you’ve narrowed down a structure to several possibilities, it can be very helpful in ruling various possibilities out. In this post we’re going to go through four (simple) practice problems where you’ll be provided with an IR spectrum and the molecular formula, and are then charged with the task of figuring out which molecule best fits the spectrum. Everything you need to know about IR in order to solve the problems below was presented in the previous post on how to do quick analyses of IR spectra, so go back and read that if you haven’t done so already. Let’s begin. https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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IR Spectroscopy: Some Simple Practice Problems — Master Organic Chemistry

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Problem #1: Unknown molecule with molecular formula C5H10O. Which of these five molecules is it most likely to be?

Problem #2: Unknown molecule with molecular formula C6H12O.

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Problem #3: Unknown molecule with molecular formula C6H14O .

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IR Spectroscopy: Some Simple Practice Problems — Master Organic Chemistry

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Problem #4: Unknown molecule with formula C4H8O2 (Also, smells like vomit)

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IR Spectroscopy: Some Simple Practice Problems — Master Organic Chemistry

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Answers Problem 1: You’re given the molecular formula, which is C5H10O. This corresponds to an index of hydrogen deficiency (IHD) of 1, so either a double bond or ring is present in the molecule. This immediately rules out d) whose IHD is zero and thus has a molecular formula of C5H12O. Looking at the spectrum we see a broad peak at 3300 cm-1 and no dominant peak around 1700 cm-1 (That peak halfway down around 1700 cm-1? It’s too weak to be a C=O. ) That broad peak at 3300 tells us that we have an alcohol (OH group). The only option that makes sense is e) (cyclopentanol) since it has both an OH group and an IHD of 1. It can’t be b) since that molecule lacks OH. a) and c) are further ruled out by the absence of C=O ; B is ruled out by the presence of the OH at 33oo Problem 2: A molecular formula of C6H12O corresponds to an IHD of 1 so either a double bond or ring is present in the molecule. There is no strong OH peak around 3200-3400 cm-1 (that little blip around 3400 cm-1 is too weak to be an OH). We can immediately rule out a) and e) . However, we do see a peak a little above 1700 cm-1 that is one of the strongest peaks in the spectrum. This is a textbook C=O peak. We can safely rule out b) which lacks a carbonyl. The only option that makes sense is d) (2-hexanone) since c) doesn’t match the molecular formula (two oxygens, five carbons). Note also that the C-H region shows all peaks below 3000 cm-1 which is what we would expect for a saturated (“aliphatic”) ketone. https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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IR Spectroscopy: Some Simple Practice Problems — Master Organic Chemistry

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A molecular formula of C6H14O corresponds to an IHD of zero. No double bonds or rings are present in the molecule. Using this we can immediately rule out d) and e) since their structures cannot correspond to molecular formula (they are both C6H12O) There is no OH peak visible around 3200-3400 cm-1. We can rule out a) and b) . This leaves us with c) . It’s an ether. Useful tip: ethers are “silent” in the prominent parts of the IR spectrum; this functional group is best identified through a process of deduction. Seeing an O in the formula but no OH or C=O peaks, the only logical selection is c) . Final note: e) is a cyclic ether called an “epoxide”. The important clue to distinguish c) and e) was the fact that we were given the molecular formula. In the absence of that information it would have been difficult to tell the difference without a close consultation of an IR peak table. Problem 4: The immediate giveaway is the smell of puke. That’s butyric acid for sure! More seriously: the formula of C4H8O2 corresponds to an IHD of 1. We can immediately rule out c) . Looking at the IR spectrum we see a huge peak in the 3300-2600 cm-1 region that blots out everything else. This seems like a textbook “hairy beard” typical of a carboxylic acid, but let’s look for more information before confirming it. We can at least rule out a) , which has no OH peaks. We also see a strong peak a little above 1700 cm-1 which is typical of a C=O. We can safely rule out e) which lacks carbonyl groups entirely. This leaves us with two reasonable choices: b) (the carboxylic acid) and d) the ketone / alcohol. How to choose between the two? The “hairy beard” is diagnostic. Alcohol OH peaks don’t fill up 600 wavenumber units the way that carboxylic acid peaks do. [Go back and look at a few examples from the previous post if you’d like confirmation] A more subtle way to distinguish the two might be the position of the carbonyl peak, but carboxylic acids (1700-1725 cm-1) show up largely in the same range as do ketones (1705-1725 cm-1). You might recognize that in each of these four examples we followed a simple procedure: 1. Since we were given the molecular formula, we calculated the index of hydrogen deficiency. This is a quick calculation and gives us useful information. We were able to use it to “rule out” a few answers which you might classify as “trick questions”. 2. Next, we examined the hydroxyl region around 3200-3400 cm for broad, rounded peaks (“tongues”) typical of OH groups . The most important question we want to answer is: “is there an OH present”? 3. Then, we looked at the carbonyl region from about 1650 – 1830 cm for sharp, strong peaks (“swords”) typical of C=O groups. Here we want to quickly know if there are any C=O groups present. 4. Using these three pieces of information we could then rule out various options that were given to us, narrowing down the possible options. Granted, these were relatively simple examples (only C,H, and O), but the thought process is what’s important. Two final notes in conclusion: It’s nice to be able to get a positive ID on a functional group, but ruling things out can be valuable too. The absence of an OH or C=O peak (or both) is still helpful information! We used this in Problem 3 to infer the existence of an ether by the absence of an OH or C=O. A related point: information you get about a molecule from various sources (e.g. molecular formula, UVVis, IR, mass spec, 13-C and 1H NMR) is self-consistent and should not contradict. I like to think of structure determination as being a bit like trying to infer the structure of a three-dimensional object by analyzing the shadows it casts from lights at various angles (or different wavelengths, to use a slightly https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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IR Spectroscopy: Some Simple Practice Problems — Master Organic Chemistry

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first place. It’s crucial to be able integrate each of these sources of information together. In this post we saw examples of using both index of unsaturation and IR together to draw conclusions about the functional groups present in the molecule. As we move towards increasingly complex spectral techniques, this skill of “integration” will become increasingly more important! As we’ll see, solving the structure of an unknown is a bit like filling out one of those “logic squares” you’ve likely encountered in grade school.

There’s likely room for a third post on IR spectroscopy covering some more rarely encountered functional groups and other minutiae. But for now we’re going to move on to the next technique, mass spectrometry (MS) in the next post in this series.

Related Posts: Degrees of Unsaturation (Index of Hydrogen Deficiency) Infrared Spectroscopy: A Quick Primer On Interpreting Spectra Bond Vibrations, IR Spectroscopy, and the “Ball and Spring” Model Now Available – The Spectroscopy Pack (PDF) Tagged as: degrees of unsaturation, IHD, index of hydrogen deficiency, infrared spectroscopy, IR, spectroscopy problems { 0 comments… add one now } Leave a Comment Name * E-mail * Website

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After doing a PhD in organic synthesis at McGill and a postdoc at MIT, I applied for faculty positions at universities and it didn’t work out, yada yada yada. So I decided to teach organic chemistry anyway! Master Organic Chemistry is the resource I wish I had when I was learning the subject. Copyright © 2018 MasterOrganicChemistry.com. All rights reserved Organic Chemistry Is Awesome About James Account Advanced Substitution Topics – Sun Nov 10 Alkene Mini Course – Thanks! Alkene Mini-Course Alkenes: Common Exam Problems (With Solutions) Alkyne Webinar Sun Dec 15 at 9pm EST Alkynes Are A Blank Canvas Blog https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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Common Mistakes In Organic Chemistry: Pentavalent Carbon Confirming your email address… Crash Course On Alkenes Crash Course On Alkenes – Checkout Dashboard Don’t Be Futyl, Learn The Butyls Feedback Form Styling Sandbox Page From Gen Chem To Organic Chem Getting Started Home Home-2 How To Draw A Cyclohexane Chair How To Succeed In Organic Chemistry How To Succeed In Organic Chemistry Initiation, Propagation, Termination Introduction To Spectroscopy Seminar (Including NMR) Introduction To Substitution Webinar Tuesday Nov 5 Leah Nomenclature Guest Post Manage Subscription Members Login Mission MOC Elite MOC Private Facebook Group My Organic Chemistry Story – What’s Yours? New Acid-Base Webinar NEW! Reagents App October 22 Acid-Base Webinar Online Organic Chemistry Tutoring Org 2 Post Index Organic 1 Organic 2 Organic Chemistry for the MCAT Organic Chemistry Study Advice Post Index Post Index 2 Post Index Draft post index draft 2 Products Organic Chemistry Reagent Guide Reaction Guide 1,4-addition of enolates to enones (“The Michael Reaction”) 1,4-addition of nucleophiles to enones 1,4-addition of organocuprates (Gilman reagents) to enones Acidic cleavage of ethers (SN2) Addition Of Alcohols To Alkenes With Acid Addition of aqueous acid to alkenes to give alcohols Addition of Dichlorocarbene to alkenes to give dichlorocyclopropanes Addition of dichloromethylene carbene to alkenes Addition of Grignard reagents to aldehydes to give secondary alcohols Addition of Grignard reagents to esters to give tertiary alcohols Addition of Grignard reagents to formaldehyde to give primary alcohols https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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Addition of HBr once to alkynes to give alkenyl bromides Addition of HBr to Alkenes Addition of HBr twice to alkynes to give geminal dibromides Addition of HCl once to alkynes to give alkenyl chlorides Addition of HCl to Alkenes to Give Alkyl Chlorides Addition of HCl to alkynes twice to give geminal dichlorides Addition of HI once to alkynes to give alkenyl iodides Addition of HI twice to alkynes to give geminal diiodides Addition of Hydroiodic Acid to Alkenes to Give Alkyl Iodides Addition of LiAlH4 to aldehydes to give primary alcohols Addition of LiAlH4 to ketones to give secondary alcohols Addition of NaBH4 to aldehydes to give primary alcohols Addition of NaBH4 to ketones to give secondary alcohols Addition of organocuprates (Gilman reagents) to acid chlorides to give ketones Addition to alkenes accompanied by 1,2-alkyl shift Additions to alkenes accompanied by 1,2-hydride shifts Aldol addition reaction of aldehydes and ketones Aldol Condensation Alkylation of enamines with alkyl halides Alkylation of enolates Allylic bromination of alkanes using NBS Baeyer-Villiger Reaction Base-promoted formation of enolates from ketones Basic hydrolysis of esters (saponification) Beckmann Rearrangement Bromination of alkenes with Br2 to give dibromides Bromination of aromatic alkanes to give alkyl bromides Bromination of Aromatics to give Bromoarenes Cannizarro Reaction Chlorination of alkenes with Cl2 to give vicinal dichlorides Chlorination of Arenes to give Chloroarenes Claisen Condensation of esters Cleavage of ethers using acid (SN1 reaction) Clemmensen Reduction of Ketones/Aldehydes to Alkanes Conversion of acid chlorides to aldehydes using LiAlH(O-tBu)3 Conversion of acid chlorides to esters through addition of an alcohol Conversion of alcohols to alkyl bromides using PBr3 Conversion of alcohols to alkyl chlorides using SOCl2 Conversion of alcohols to alkyl halides using HCl Conversion of Alkyl halides to ethers (SN1) Conversion of carboxylic acids into acid chlorides with SOCl2 Conversion of carboxylic acids to carboxylates using base Conversion of carboxylic acids to esters using acid and alcohols (Fischer Esterification) Conversion of tertiary alcohols to alkyl bromides using HBr Conversion of tertiary alcohols to alkyl iodides with HI Conversion of thioacetals to alkanes using Raney Nickel Curtius Rearrangement of Acyl Azides to Isocyanates Decarboxylation of beta-keto carboxylic acids Dehydration of amides to give nitriles Deprotonation of alcohols to give alkoxides Deprotonation of alkynes with base to give acetylide ions Diels Alder Reaction of dienes and dienophiles https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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Elimination (E1) of alkyl halides to form alkenes Elimination (E1) with 1,2-alkyl shift Elimination (E1) with hydride shift Elimination (E2) of alkyl halides to give alkenes Elimination of alcohols to give alkenes using POCl3 Elimination of water from alcohols to form alkenes using acid Enamine Hydrolysis Formation of Acetals from Aldehydes and Ketones Formation of alkynes through double elimination of vicinal dibromides Formation of amides from acid chlorides and amines Formation of Amides Using DCC Formation of anhydrides from acid halides and carboxylates Formation of Bromohydrins from alkenes using water and Br2 Formation of bromohydrins from alkenes using water and NBS Formation of Carboxylic Acids from Acyl Chlorides Formation of carboxylic acids from Grignard reagents and CO2 Formation of chlorohydrins from alkenes using water and Cl2 Formation of Cyanohydrins from ketones and aldehydes Formation of cyclopropanes from alkenes using methylene carbene (:CH2) Formation of Diazonium Salts from Aromatic Amines Formation of enamines from ketones/aldehydes and secondary amines Formation of epoxides from alkenes using m-CPBA Formation of epoxides from bromohydrins Formation of Gilman reagents (organocuprates) from alkyl halides Formation of Grignard Reagents from Alkenyl Halides Formation of Grignard Reagents from Alkyl Halides Formation of hydrates from aldehydes/ketones and H2O Formation of imines from primary amines and ketones Formation of organolithium reagents from alkyl halides Formation of thioacetals from aldehydes and ketones Formation of tosylates from alcohols Free Radical Addition of HBr To Alkenes Free Radical Bromination of Alkanes Free Radical Chlorination of Alkanes Friedel Crafts alkylation of arenes Friedel-Crafts acylation of aromatic groups to give ketones Halogenation of Alkynes Hell-Vollhard-Zelinsky Reaction Hofmann elimination of alkylammonium salts to give alkenes Hofmann Rearrangement of Amides to Amines Hydroboration of Alkenes Hydroboration of alkynes using BH3 to give aldehydes Hydrogenation of Alkenes to give Alkanes Hydrogenation of Alkynes to Alkanes using Pd/C Hydrolysis of acetals to give aldehydes and ketones Hydrolysis of esters to carboxylic acids with aqueous acid Hydrolysis of imines to give ketones (or aldehydes) Hydrolysis of nitriles with aqueous acid to give carboxylic acids Iodination of alkenes to give vicinal diiodides (1,2-diiodides) Iodination of Aromatics with I2 Keto-enol tautomerism Kiliani-Fischer Synthesis https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

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Nucleophilic Aromatic Substitution Via Arynes Opening of epoxides with acid and water to give trans diols Opening of epoxides with nucleophiles under acidic conditions Oxidation of aldehydes to carboxylic acids using Cr(VI) Oxidation of aldehydes to carboxylic acids with Ag2O Oxidation of aromatic alkanes with KMnO4 to give carboxylic acids Oxidation of primary alcohols to aldehydes Oxidation of Primary Alcohols to Aldehydes using PCC Oxidation of primary alcohols to carboxylic...