Sapling Problem from Chp 5 #23

Sapling Homework Problem

From Chapter 5

 #23

Introduction

Disjumbled Spectroscopy/Spectrometry Concept Cloud

Spectroscopy and Spectrometry problems are arguably some of the most difficult types of problems we’ve faced this semester because they test our knowledge of functional groups and ability to quickly think on our feet.  Similar to sight-reading a piece of music in a live-audition situation or running an unfamiliar cross-country course for the first time, these problems can appear to be almost like an impossible game of tetris if one does not have a strategy in mind ahead of time.  

Reviewing for the final exam this past weekend, I decided to devote my last blog entry to both a particular spectrometry/spectroscopy problem and my overall strategy for solving these types of problems.

The Problem

Please click on image to enlarge

Problem number 23 from chapter 5 of Sapling asks us to provide the structure of a compound based upon the given molecular formula, IR spectroscopy data, and proton NMR spectrum.                                                 

Problem Solving Strategy

Please click on image to enlarge

Before beginning to work on the problem, I've found it helpful to map out my strategy. By starting with the degrees of unsaturation, continuing onto IR and Proton NMR examinations, and ending with educated guessing, I've found that this plan of attack helps me focus on getting through the problem and ending up with a reasonable answer to the question.

Step 1: Degrees of Unsaturation

Please click on image to enlarge

Presented with a molecular formula for the compound, the first step to solving this problem involves determining the total number of pi bonds and rings present in the compound's structure.  Using the general formula for the degrees of unsaturation provided to us in class, one can accurately determine from a compound's molecular formula both whether a particular compound is acyclic or cyclic and/or an  alkane, alkene, or alkyne. For this particular problem, I determined that this compound has both one degree of unsaturation and is a type of alkene (please see above graphic for my work).

Step 2: Examination of IR Data

Please click on image to enlarge

One of several types of instrumental techniques, Infrared spectroscopy is a powerful analytical tool that allows us to differentiate between different types of functional groups. As the textbook states on page 615, an infrared spectrum (and ultimately experimental data) is obtained by passing infrared radiation through a sample of a compound  and plotting the percent transmission or radiation versus the wave number of radiation transmitted. Once completed, downward spikes on the graph indicate the absorption of energy (also referred to as absorption bands) and the specific wavenumber at which these spikes occur can identified to a specific type or functional group and/or bond.

Provided with three unique absorption bands in this problem, I compared these experimental data points to those presented in textbook tables 14.4 and 14.5 and was able to identify three types of functional groups/bonds present in the compound's structure (please see above graphic for my work).

Step 3: Examination of Proton NMR Spectrum

Please click on image to enlarge

Produced by the pulsing of proton nuclei between alpha and beta-spin states after being exposed to rf radiation (textbook page 650), Proton Nuclear Magnetic Resonance Imaging provides valuable information about the carbon-hydrogen framework of an organic compound (textbook 596). From helping us determine the total relative number of protons that produce each signal in the compound's structure (Integration) to the total number of different types of protons and the distance between each proton signal peak, proton NMR is useful in helping identify the structure of an unknown compound.

In this particular problem, I was able to determine that the provided proton NMR indicated the presence of 6 different types of protons, an integration of 2:2:2:3:3:1, and specific information related to the coupling constants (please see above graphic for my work).

Step 4: Educated Guessing

Please click on image to enlarge

Please click on image to enlarge

 Arguably the hardest part of this type of problem is coming up with possible structures, comparing possible structures with data presented, and eliminating those possible structures that do not fit the experimental data provided in this problem. 

The graphic to the left is a summary of the puzzle pieces that I worked with to come up with possible structures. The graphic to the right shows the some of these possible structures and how they relate to the experimental data provided. The differentiating concept between all four structures turned out to be the presence of 6 different types of protons.

 The Final Answer to the Question

Please click on image to enlarge