Chapter 14

How many peaks are expected in the 1H NMR spectrum of cyclohexane at room temperature?

1

What is the index of hydrogen deficiency for a compound with molecular formula C4H9N?

1

The mass spectrum of methane (CH4) shows a small peak at m/z = 17. What is the origin of this peak?

13CH4

What is the index of hydrogen deficiency for a compound with molecular formula C6H10?

2

The 1H NMR signal for a proton that couples to two other groups of nonequivalent protons will show ______ set(s) of coupling patterns. The original signal will be split into ______ peaks by n protons in the first group, and each of these peaks will split into a further ______ peaks due to m protons in the second group. Peaks often ______ so that there are fewer peaks than expected in the resulting signal. Multiple choice question.

2; n + 1; m + 1; overlap

Which structural fragment would give rise to a characteristic triplet-quartet pattern in the 1H NMR spectrum?

A

Select all the statements that correctly describe spin-spin splitting for two groups of coupled protons.

A 1H NMR signal for a given group of H's will be split into (n+1) peaks by a group consisting of n equivalent H's on an adjacent C atom. Protons that are equivalent to each other do not cause observable spin-spin splitting even when they are on adjacent atoms. spin-spin splitting caused by a group consisting of equivalent protons gives equal separation between the peaks in the signal.

Which of the following correctly describes the characteristic splitting pattern seen on the 1H NMR spectrum for an isopropyl group (shown)?

A doublet-septet pattern

Select all the statements that correctly relate the degree of shielding of a nucleus to its chemical shift.

A shielded nucleus will have a lower chemical shift than a deshielded nucleus. The signal for a shielded nucleus will be closer to the signal for TMS than the signal for a deshielded nucleus.

Select all the statements that correctly describe the 1H NMR characteristics of protons bonded to unsaturated C atoms.

Alkyne protons have lower chemical shift values than alkene protons. Aromatic protons are deshielded compared to alkyl protons. Alkene protons have chemical shift values in the 4.5-6 ppm range.

Select all the statements that correctly describe how electrons influence the chemical environment of the nuclei in a molecule and its interaction with an external magnetic field.

An external magnetic field induces an opposing local magnetic field in the electrons. A deshielded nucleus has less electron density around it than a shielded nucleus. Electrons shield the nucleus from the external field.

The halogens Br and Cl each have two naturally occurring isotopes of high abundance. The mass spectrum of a compound containing one Br or one Cl atom will show TWO molecular ion peaks that are _____ mass units apart, at M and M+ _______.

Blank 1: 2 or two Blank 2: 2 or two

Each index of hydrogen deficiency in a compound represents a decrease of ________ hydrogens from the maximum possible for an alkane. To calculate the index of hydrogen deficiency for a hydrocarbon, subtract the number of ________ atoms in the formula from the number expected in a fully saturated hydrocarbon and divide by _______.

Blank 1: 2 or two Blank 2: hydrogen or H Blank 3: 2 or two

It is very common for the 1H NMR signals for two groups of protons that are ____ to each other to "lean" toward each other in the spectrum. This distorts the normal symmetry and expected relative _______ of the peaks. The closer the chemical shifts of the signals, the _______ the effect, which may cause a pair of doublets to be mistaken for a(n) _______.

Blank 1: coupled Blank 2: intensities, intensity, height, or strength Blank 3: greater, larger, bigger, or stronger Blank 4: quartet

Two protons or groups of protons that cause spin-spin splitting in each other's NMR signals are said to be ____ . The stronger this interaction, the greater the separation between the peaks within the splitting patterns. This separation is usually measured as a frequency difference in Hz and is referred to as the _____ constant, denoted by the symbol J.

Blank 1: coupled, spin coupled, or spin-coupled Blank 2: coupling

In both the structures shown, replacement of Ha and Hb, in turn, with an atom Z will produce two different compounds that are _____ of each other. Ha and Hb are therefore _____ -topic and will give _____ NMR signals.

Blank 1: diastereomers Blank 2: diastereo Blank 3: different or distinct

In a mass spectrometer, a sample of organic molecules is vaporized and bombarded by a stream of high- ______ electrons. This causes a(n) _______ to be dislodged from the molecule, producing a cation called the _______ ion. This cation ______ further, forming smaller radicals and cations.

Blank 1: energy Blank 2: electron Blank 3: molecular or parent Blank 4: fragments, breaks, or dissociates

The region of the IR spectrum below 1500 cm-1 is called the _____ region, and this portion of the spectrum is unique for every compound. The region 4000-1600 cm-1 is called the _____ group region, and this gives specific information about the type of bonds found in the compound.

Blank 1: fingerprint Blank 2: functional

Molecular vibrations are quantized, which means that discrete vibrational energy states exist within a molecule. Each type of bond has its own specific vibrational modes, so that each type of bond vibrates at a different _____ . This leads to specific absorption peaks in the IR spectrum for each different _____ group present in the molecule.

Blank 1: frequency, energy, wavelength, or wavenumber Blank 2: functional

IR spectroscopy is primarily used by organic chemists to identify the possible ______ _______ in a compound.

Blank 1: functional Blank 2: groups

A nucleus that is deshielded will experience a relatively ______ magnetic field compared to a shielded nucleus and will therefore absorb radiation at a _____ field strength or _____-field.

Blank 1: higher, greater, stronger, bigger, high, strong, or larger Blank 2: lower, smaller, lesser, or low Blank 3: down

The 1H chemical shifts of O-H and N-H protons vary a great deal because these groups can be involved in intermolecular _____ bonding. Generally, an increase in this type of interaction _______ the shielding and causes the chemical shift of the H's to _______.

Blank 1: hydrogen or H Blank 2: decreases, reduces, or lowers Blank 3: increase

An NMR spectrum plots signal ______ on the y-axis against chemical ______ on the x-axis, which is determined by measuring the frequency of the signal relative to a reference signal. The reference standard most commonly used is ______, and it is assigned the δ value of 0 ppm.

Blank 1: intensity or strength Blank 2: shift Blank 3: TMS or tetramethylsilane

Molecules, like all other matter, have specific energy _____ of different types. For a molecule to absorb radiation, the energy of the radiation must match the energy _______ between the two energy _______ in the molecule.

Blank 1: levels or states Blank 2: difference or gap Blank 3: levels or states

The mass spectrometer detects and analyzes the cations produced by fragmentation of an organic molecule. The analyzer determines the m/z ratio for each fragment, where m represents the ______ and z represents the _____ of the cation. The value of z is almost always +1, so the m/z ratio is usually equal to the actual ______ of each individual ion.

Blank 1: mass Blank 2: charge Blank 3: mass

Protons that are separated by fewer bonds have _____ effective spin-spin coupling than protons farther away. Vicinal protons are protons on ____ atoms and are separated by _____ bonds. Vicinal coupling is therefore also called-bond coupling.

Blank 1: more, greater, larger, or stronger Blank 2: adjacent, neighboring, or next door Blank 3: 3 or three Blank 4: three or 3

The _____ rule reflects the fact that an odd number of nitrogen atoms in a compound causes the compound to have an ______ molecular mass.

Blank 1: nitrogen or N Blank 2: odd

NMR spectroscopy, or _____ magnetic resonance spectroscopy, is a very important tool in the determination of organic structures. This technique relies on the interaction of a particular nucleus with a _____ field followed by absorption of energy of a specific ________, depending on the chemical environment of the nucleus.

Blank 1: nuclear Blank 2: magnetic Blank 3: frequency or wavelength

A proton is a charged particle that spins on its own axis and therefore has two _____ states, which are equal in energy. The energies of these two states will be different in the presence of an external _____ field. It is this property that is exploited in NMR spectroscopy.

Blank 1: spin Blank 2: magnetic

NMR signals may consist of a single peak, or they may be ____ into several peaks. The number of peaks for a particular signal is called its _______.

Blank 1: split or divided Blank 2: multiplicity or splitting

Each different signal in a 1H NMR spectrum represents a different ____ of hydrogen atom. Hydrogen atoms that are ______ give the same signal.

Blank 1: type, environment, kind, or set Blank 2: equivalent, identical, equal, homotopic, the same, or chemically equivalent

Absorption of IR radiation causes changes in the ______ modes of a molecule, causing its _______ to bend, stretch, and twist. This behavior is characteristic of the type of _____ groups present in the molecule.

Blank 1: vibrational or vibration Blank 2: bonds Blank 3: functional

Select all the compounds from the following list that would give rise to two signals in the 1H NMR spectrum with an integration ratio of 2:3.

CH3CH2Br CH3CH2CH2CH3

If a sample is subjected to an external magnetic field of fixed strength, which of the following determines the specific frequency at which a particular nucleus will absorb?

Chemical environment of the nucleus

Select all the statements that correctly describe chemical shift as reported on an NMR spectrum.

Chemical shift is determined relative to a reference signal. Chemical shift increases from right to left on an NMR spectrum. Chemical shift is measured in ppm.

Select all the features of a 1H NMR spectrum that give useful information about the structure of a compound.

Chemical shift of the signals Total number of signals Spin-spin splitting of the signals

Select all the statements that correctly describe enantiotopic protons.

Enantiotopic protons are chemically equivalent. If replacement of two H's, in turn, by a test group generates a pair of enantiomers, those two H's are enantiotopic. Enantiotopic protons will give the same 1H NMR signal.

True or false: A mass spectrometer measures the masses of the cation fragments produced when an organic molecule breaks apart.

False

How many signals would be expected to appear in the 1H NMR spectrum of the compound shown?

Four signals

Select all the statements that correctly describe the influence of structural features on chemical shift of protons.

H atoms attached to double bonds or aromatic rings are especially deshielded. The chemical shift of an H will increase with increasing alkyl substitution of its attached C atom. Any electronegative atom will deshield nearby protons, resulting in a higher chemical shift.

Select all the statements that correctly describe why an IR spectrum can be used to determine functional group information.

IR radiation affects the vibrational modes of a molecule, and different types of bonds vibrate at different frequencies. Molecules that contain the same type of bonds will have similar IR spectra.

The group of signals at δ 6.5-6.65 in the spectrum shown is not a quartet. Which of the following observations allow this conclusion to be made? (Select all that apply.)

In a quartet, the two outer peaks would be smaller than the two inner peaks. The separation between the peaks in this group is not consistent.

Select all the statements that correctly describe the characteristics of an IR spectrum.

Peaks in an IR spectrum point downward. The spectrum is a plot of wavenumber versus percent transmittance. Wavenumber (and frequency) decrease from left to right.

What type of information is provided by the IR spectrum of an organic compound?

Possible identity of functional groups present

Which of the following can readily be inferred simply from the molecular formula of an organic compound? (Select all that apply.)

Possible number of multiple bonds and/or rings in the compound Possibility that the compound contains an odd number of N atoms

Select all the statements that correctly describe diastereotopic protons.

Protons attached to the C atoms of a C=C double bond can be diastereotopic. The chemical shifts for diastereotopic protons are often similar. Diastereotopic protons are chemically nonequivalent.

Which of the following parameters on a 1H NMR spectrum gives information about the relative number of protons in the compound?

Ratio of the integrated areas under the peaks

For the molecular formula C6H12, select all elements that could be present.

Ring Double bond

The mass spectrum of an organic compound has a base peak at m/z = 91 and a pair of peaks of equal height at m/z = 170/172. Which of the following is the most likely structure for this compound?

Structure B

When are two hydrogen atoms in a compound chemically equivalent?

Substitution of either one with another atom yields the same compound.

Select all the statements that correctly describe the features of the 1H NMR spectrum of an alcohol.

The 1H NMR signal for an OH proton usually does not show spin-spin splitting. The concentration of the sample can affect the chemical shift of an OH proton. The chemical shift of an OH proton varies with temperature.

Select the statement that correctly describes the interaction of a molecule with IR radiation.

The bonds of the molecule vibrate in various ways.

The mass spectrum of a given organic compound shows three clusters of peaks at m/z values of 155/157, 183/185, and 198/200. In each cluster, the lower mass peak is about three times more intense than the higher mass peak. Which of the following structural features would account for this observation?

The compound contains a Cl atom.

Select all the statements that correctly describe spin-spin coupling and the coupling constant.

The coupling constant depends on the extent of the coupling between the two groups of protons. The splitting patterns for two nuclei that couple will have the same coupling constant. Coupling results in spin-spin splitting of the NMR signals for both groups of coupled protons.

Which of the following correctly describes how energy is absorbed by a molecule?

The energy of the absorbed photon must match the difference between the two energy states in the molecule.

Select all the statements that correctly describe the chemical shifts for O-H and N-H protons in 1H NMR spectroscopy.

The greater the involvement of O-H or N-H groups in hydrogen bonding, the higher the chemical shift of the protons. The chemical shift for a given O-H or N-H group can change if a different solvent is used for the experiment. The chemical shifts for OH and NH protons can vary with temperature and concentration.

A particular NMR signal appears as a quartet. Which of the following is a reasonable interpretation of this signal?

The group of protons associated with the signal is coupled to three protons on the adjacent atom(s).

Select all the statements that are correct about the H atoms in the compound shown.

The groups marked "a" and "b" are interchangeable and therefore equivalent. All three H's in the CH3 group marked "a" are equivalent because they are interchangeable.

Which of the following correctly describes the appearance of the 1H NMR signal for a group of protons that is coupled to more than one group of nonequivalent protons? (Select all that apply.)

The individual peaks within the splitting pattern will not be evenly spaced. Peaks in the signal will often overlap so that there will be fewer peaks than are theoretically possible. The signal will be split into (n+1) peaks by one of the coupled groups and then again by the other to give a complex pattern.

Select all the statements that correctly describe the general features of mass spectrometry.

The mass spectrometer records the mass-to-charge ratio of the cation fragments produced from the original molecule. Mass spectrometry involves bombarding an organic molecule with high-energy electrons, causing it to ionize. Ionization of an organic molecule produces a cation radical that fragments further.

Select the statement that correctly describes the information recorded in a mass spectrum.

The mass spectrum is a graph of the abundance of each cation plotted against its m/z value, which typically equals its mass.

Select all the statements that correctly describe the observed splitting patterns for an isopropyl group (shown) in a 1H NMR spectrum.

The signal for the CH group will be at a higher chemical shift than the CH3 group signal. The signal for the CH3 groups will be a doublet. The two CH3 groups will give the same signal.

What does NMR spectroscopy actually measure?

The specific frequencies of energy absorbed by nuclei under the influence of a magnetic field.

Most mass spectra show a small peak at one mass unit above the molecular mass of the compound. Select the statement that correctly explains the origin of this M+1 peak.

This peak arises due to the presence of carbon-13 isotopes in the molecule.

True or false: The 1H NMR signal for an OH proton does not usually exhibit spin-spin splitting.

True

True or false: Vicinal coupling is also called three-bond coupling, since the adjacent protons are separated by three bonds.

True

An NMR active nucleus has a nonzero nuclear spin with different spin states. In the presence of an external magnetic field, nuclei that are aligned _______ the external field are at a higher energy than nuclei aligned ______ the external field.

against; with

Hydrogen atoms directly attached to double bonds or aromatic rings are especially ______ with ______ chemical shift values. Compared to these hydrogen atoms, a hydrogen atom attached to a CΞC triple bond is relatively ______.

deshielded; high; shielded

In the structure shown, substitution of Ha and Hb, in turn, by an atom Z will produce two different compounds that are ______ of each other. These protons are therefore _______ and will give ______ NMR signal(s).

enantiomers; enantiotopic; the same

Various spectroscopic techniques are used to determine the structures of organic molecules. Although each technique gives different information, all of them rely on the interaction of a(n) _____ source with a molecule to produce a change from which molecular information can be inferred.

energy

n NMR spectroscopy, the resonance frequency of the nuclei being observed varies depending on their chemical ______. This structure-dependent variation in resonance position is referred to as the chemical ______ for a particular nucleus.

environment shift

The 1H NMR spectrum of a particular compound contains 5 signals. This means that the compound _____.

has 5 different types of hydrogen atoms

The index of ______ deficiency is calculated from the molecular formula of a compound. This gives information about the possible number of ______ bonds and/or ______ in the compound.

hydrogen; multiple; rings

The presence of an electronegative atom will deshield nearby protons, causing the chemical shift value to ______. As the electronegativity of the atom increases, or the number of electronegative atoms increases, the ______ the chemical shift value of the proton will be.

increase; higher

The IR spectrum is a plot of wavenumber versus percent transmittance. The greater the amount of radiation absorbed by the sample, the ______ the percent transmittance so that peaks in the spectrum point ______ on the spectrum.

lower; downward

Select the variables that are plotted on the x and y axes in a mass spectrum.

m/z Abundance

The integrated area under an NMR signal is proportional to the ______ of protons that give rise to that signal. The areas are integrated automatically to give an integral for each peak. The integral values give the ______ of protons in the compound.

number; ratio

When an NMR active nucleus is placed in a magnetic field, this external field induces a small local field in the electrons surrounding that nucleus. This local field is in the ______ direction to the external field, causing the nucleus to experience a ______ external field. The electrons are said to ______ the nucleus from the external field.

opposite; lower; shield

Particles of electromagnetic radiation are called ______, and each has a discrete amount of energy called a ______. Since electromagnetic radiation also has wave properties, each particle is also characterized by a specific ______ and frequency.

photons; quantum; wavelength

Rapid interconversion between chair conformations gives rise to a _____ sharp peak in the 1H NMR spectrum of cyclohexane.

single

A shielded nucleus will absorb ______ from a deshielded nucleus and will have a ______ chemical shift.

upfield; lower