Chapter 14 Organic Chemistry Solutions Manual PDF

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Chapter 14 Infrared Spectroscopy and Mass Spectrometry Review of Concepts Fill in the blanks below. To verify that your answers are correct, look in your textbook at the end of Chapter 14. Each of the sentences below appears verbatim in the section entitled Review of Concepts and Vocabulary.  

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Spectroscopy is the study of the interaction between _______ and ________. The difference in energy (ΔE) between vibrational energy levels is determined by the nature of the bond. If a photon of light possesses exactly this amount of energy, the bond can absorb the photon to promote a __________________ excitation. IR spectroscopy can be used to identify which _____________________ are present in a compound. The location of each signal in an IR spectrum is reported in terms of a frequency-related unit called _________________. The wavenumber of each signal is determined primarily by bond __________ and the __________ of the atoms sharing the bond. The intensity of a signal is dependent on the ______________ of the bond giving rise to the signal. _________________ C=C bonds do not produce signals. Primary amines exhibit two signals resulting from ___________ stretching and _____________ stretching. Mass spectrometry is used to determine the ___________________ and ________________________ of a compound. Electron impact ionization (EI) involves bombarding the compound with high energy _______________, generating a radical cation that is symbolized by (M)+• and is called the molecular ion, or the __________ ion. Only the molecular ion and the cationic fragments are deflected, and they are then separated by their ____________________ (m/z). The tallest peak in a mass spectrum is assigned a relative value of 100% and is called the __________ peak. The relative heights of the (M)+• peak and the (M+1)+• peak indicates the number of ___________________. A signal at M15 indicates the loss of a _________ group; a signal at M29 indicates the loss of an _________ group. ______________ alkanes have a molecular formula of the form CnH2n+2. Each double bond and each ring represents one degree of _______________.

Review of Skills Fill in the blanks and empty boxes below. To verify that your answers are correct, look in your textbook at the end of Chapter 14. The answers appear in the section entitled SkillBuilder Review. 14.1 Analyzing an IR Spectrum

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14.2 Distinguishing Between Two Compounds Using IR Spectroscopy

14.3 Using the Relative Abundance of the (M+1)+• Peak to Propose a Molecular Formula

14.4 Calculating HDI

Mistakes to Avoid We have seen that the IR spectrum of an alkene will exhibit a signal near 1650 cm-1 (the characteristic signal for a C=C bond) if there is a dipole moment associated with the C=C bond. In such a case, the dipole moment changes as the C=C bond vibrates, creating an oscillating electric field that serves as an antenna to absorb the appropriate frequency of IR radiation. If the C=C bond does not have a dipole moment, then it cannot efficiently absorb IR radiation, and the signal near 1650 cm-1 will be absent. For example, consider the following two compounds:

Don’t be confused by the terms ‘symmetrical alkene’ and ‘unsymmetrical alkene’. We might refer to the first compound as an unsymmetrical alkene (in reference to the dipole moment), but the truth is that this alkene still does possess some symmetry (an axis of symmetry, as well as two planes of symmetry).

But these symmetry elements are not relevant for determining whether a C=C bond has a dipole moment, and therefore, they are not relevant for determining whether or not a C=C bond will produce a signal in an IR spectrum. When we refer to a symmetrical alkene, we are referring to the symmetry of the two vinylic positions:

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With this in mind, let’s consider the following alkene:

While this molecule does possess some symmetry, you should avoid falling into the trap of calling it a symmetrical alkene and erroneously deciding that the C=C double bond will not produce a signal. In fact, the dipole moment for this C=C bond is expected to be quite large (because of the combined inductive effects of the chlorine atoms. One vinylic position (the one connected to the two chlorine atoms) is more electron-deficient (+) than the other vinylic position. As a result, this C=C bond is expected to produce a rather strong signal in the IR spectrum.

Solutions 14.1. (a) The CH bond is expected to produce the signal with the largest wavenumber, because bonds to H typically produce high-energy signals (due to the low mass of the hydrogen atom). Among the remaining two bonds, the triple bond is stronger than the double bond, so we expect the double bond to produce the signal with lowest wavenumber.

(b) Each of the bonds in this case is a single bond. The CH bond is expected to produce the signal with the larger wavenumber, because bonds to H typically produce high-energy signals (due to the low mass of the hydrogen atom).

14.2. (a) This compound exhibits an sp2 hybridized carbon atom that is connected to a hydrogen atom. As such, this CH bond (highlighted) should produce a signal above 3000 cm-1 (at approximately 3100 cm-1).

(d) This compound exhibits an sp2 hybridized carbon atom that is connected to a hydrogen atom. As such, this C-H bond (highlighted) should produce a signal above 3000 cm-1 (at approximately 3100 cm-1).

(e) This compound has two sp2 hybridized carbon atoms, but neither of them are connected to hydrogen atoms. And there are no sp hybridized carbon atoms. Therefore, we do not expect a signal above 3000 cm-1. (f) This compound exhibits an sp hybridized carbon atom that is connected to a hydrogen atom. As such, this C-H bond (highlighted) should produce a signal above 3000 cm-1 (at approximately 3300 cm-1).

14.3. (a) One of the carbonyl groups (upper left) is not conjugated, so it is expected to produce a signal at approximately 1720 cm-1. The other carbonyl group (bottom right) is conjugated to a C=C  bond, so it is expected to produce a signal at approximately 1680 cm-1.

(b) This compound has three sp2 hybridized carbon atoms, but none of them are connected to hydrogen atoms. And there are no sp hybridized carbon atoms. Therefore, we do not expect a signal above 3000 cm-1. (c) This compound has two sp hybridized carbon atoms, but neither of them are connected to hydrogen atoms. And there are no sp2 hybridized carbon atoms. Therefore, we do not expect a signal above 3000 cm-1.

(b) One of the ester groups (bottom right) is not conjugated, so it is expected to produce a signal at

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approximately 1740 cm-1. The other carbonyl group (upper left) is conjugated to a C=C  bond, so it is expected to produce a signal at approximately 1710 cm-1.

(c) The carbonyl group of a ketone is expected to produce a signal at approximately 1720 cm-1, while the carbonyl group of an ester is expected to produce a signal at approximately 1740 cm-1.

(which is not directly connected to the chlorine atom). Therefore, the C=C bond in this compound has a larger dipole moment than the C=C bond in the other compound. As a result, we expect the chloroalkene to be more efficient at absorbing IR radiation (thereby producing a stronger signal). (b) The C=C bond in the compound shown below will have a larger dipole moment because one vinylic position is connected to two chlorine atoms while the other vinylic position is not directly connected to any chlorine atoms. As a result, the two vinylic positions are in very different electronic environments, giving rise to a large dipole moment. We therefore expect this C=C bond to be more efficient at absorbing IR radiation (and therefore produce a stronger signal).

14.6. If we draw all significant resonance structures of 2-cyclohexenone, we see that one of the vinylic positions is electron-deficient (highlighted in the third resonance structure):

14.4. The C=C  bond in the conjugated compound produces a signal at lower wavenumber (1600 cm-1) because it has some single bond character, as seen in the third resonance structure below. This additional single bond character renders the C=C  bond weaker (relative to the C=C  bond of the other compound, which does not exhibit any single bond character).

As a result, the two vinylic positions experience very different electronic environments, giving rise to a large dipole moment. With a large dipole moment, this C=C bond is expected to be very efficient at absorbing IR radiation, thereby producing a strong signal. 14.7. The vinylic CH bond should produce a signal at approximately 3100 cm-1.

14.5. (a) The second compound has an electronegative chlorine atom, which withdraws electron density via induction.

This causes the two vinylic positions to experience different electronic environments. The vinylic position connected directly to the chlorine atom is expected to be more electron-poor (+) than the other vinylic position

14.8. The narrow signal is produced by the OH stretching in the absence of a hydrogen bonding effect. The broad signal is produced by OH stretching when hydrogen bonding is present. Hydrogen bonding effectively lowers the bond strength of the OH bonds, because each hydrogen atom is slightly pulled away from the oxygen atom to which it is connected. A longer bond length (albeit temporary) corresponds with a weaker bond, which corresponds with a lower wavenumber.

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CHAPTER 14 14.9. (a) The broad signal between 3200 and 3600 cm-1 is characteristic of an alcohol (ROH). (b) This spectrum lacks broad signals above 3000 cm-1, so the compound is neither an alcohol nor a carboxylic acid (both of which produce broad signals that reach as high as 3600 cm-1). (c) The extremely broad signal that extends from 2200 to 3600 cm-1 is characteristic of the O-H stretching of a carboxylic acid (RCO2H). The signal just above 1700 cm-1 is also consistent with a carboxylic acid (for the C=O bond of the carboxylic acid). (d) This spectrum lacks broad signals above 3000 cm-1, so the compound is neither an alcohol nor a carboxylic acid (both of which produce broad signals that reach as high as 3600 cm-1). (e) The broad signal between 3100 and 3600 cm-1 is characteristic of an alcohol (ROH). (f) The extremely broad signal that extends from 2200 to 3600 cm-1 is characteristic of the OH stretching of a carboxylic acid (RCO2H). The signal around 1700 cm-1 is also consistent with a carboxylic acid (for the C=O bond of the carboxylic acid). 14.10. (a) The strong signal just above 1700 cm-1 is consistent with the stretching of the carbonyl group (C=O) of a ketone. (b) The extremely broad signal that extends from 2200 to 3600 cm-1 is characteristic of the OH stretching of a carboxylic acid (RCO2H). The signal just above 1700 cm-1 is also consistent with a carboxylic acid (for the C=O bond of the carboxylic acid). (c) The signal at approximately 3400 cm-1 is consistent with the stretching of the NH bond of a secondary amine. (d) The two signals at 3350 and 3450 cm-1 are consistent with the stretching of the NH bond (symmetric and asymmetric) of a primary amine. (e) The strong signal just above 1700 cm-1 is consistent with the stretching of the carbonyl group (C=O) of a ketone. (f) The broad signal between 3200 and 3600 cm-1 is characteristic of an alcohol (ROH). 14.11. The Csp3H bonds can stretch symmetrically, asymmetrically, or in a variety of ways with respect to each other. Each one of these possible stretching modes is associated with a different wavenumber of absorption, giving a large number of overlapping peaks. 14.12. (a) Begin by drawing a line at 1500 cm-1 and ignoring everything to the right (the fingerprint region). Then, look for any signals associated with double bonds (16001850 cm-1) or triple bonds (21002300 cm-1). In this case, there is a weak signal between 1600 and 1700 cm-1, consistent with an alkene. Finally, we draw a line at 3000 cm-1, and we look for signals to the left of this line. In this case, there is a signal at approximately 3100 cm-1, which is consistent with a Csp2H bond of an alkene. Among the possible structures, the alkene is the

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structure that is consistent with the signals in the spectrum.

(b) Begin by drawing a line at 1500 cm-1 and ignoring everything to the right (the fingerprint region). Then, look for any signals associated with double bonds (16001850 cm-1) or triple bonds (21002300 cm-1). In this case, there is a strong signal between 1700 and 1800 cm-1, consistent with a carbonyl group. Among the possible structures, only two of them exhibit C=O bonds. One of these structures has carboxylic acid groups, and the spectrum does not match that compound (because that compound is expected to give a broad signal from 2200-3600 cm-1, which is absent in our spectrum). The following structure (an ester) is consistent with the IR spectrum.

(c) Begin by drawing a line at 1500 cm-1 and ignoring everything to the right (the fingerprint region). Then, look for any signals associated with double bonds (16001850 cm-1) or triple bonds (21002300 cm-1). In this case, there are none. Next, we draw a line at 3000 cm-1, and we look for signals to the left of this line. In this case, there are none. With no characteristic signals for any functional groups, this spectrum is consistent with an alkane.

(d) The broad signal between 3200 and 3600 cm-1 is characteristic of an alcohol (ROH). There is only one alcohol among the possible structures given.

(e) The extremely broad signal that extends from 2200 to 3600 cm-1 is characteristic of the OH stretching of a carboxylic acid (RCO2H). The signal just above 1700 cm-1 is also consistent with a carboxylic acid (for the C=O bond of a carboxylic acid group). Notice that this signal appears to be comprised of two overlapping signals, which can likely be attributed to symmetric and asymmetric stretching of the two carboxylic acid groups.

(f) The two signals at 3350 and 3450 cm-1 are consistent with the stretching of the NH bonds (symmetric and asymmetric) of a primary amine. There is only one primary amine among the possible structures given.

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14.13. The following five signals are expected (presented in order of increasing wavenumber): 1) The C=C bond (expected to be ~ 1650 cm-1) 2) The C=O bond of the carboxylic acid group (expected to be ~ 1720 cm-1) 3) All Csp3H bonds (expected to be...