Organic Chemistry Questions and Answers – Nuclear Magnetic Resonance – 1

This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Nuclear Magnetic Resonance – 1”.

1. Which of the following organic compound with molecular formula C3H C12 exhibits only one signal in the IH NMR spectrum?
a) 2, 2-dichloropropane
b) 1, 2-dichloropropane
c) 1, 3-dichloropropane
d) 1, 1-dichloropropane
View Answer

Answer: a
Explanation: Only (a) compound has all chemically equivalent hydrogen. So, give only one peak.Organic compound with exhibits only one signal in IH NMR spectrum is 2, 2-dichloropropane

2. The 1H NMR spectrum of CH3OCHCICH2Cl will exhibit ________
a) A three proton doublet. One proton singlet and a two proton doublet
b) A three proton singlet. One proton singlet and a two proton doublet
c) A three proton singlet. One proton triplet and a two proton doublet
d) A three proton triplet. One proton triplet and a two proton triplet
View Answer

Answer: c
Explanation: The 1H NMR spectrum of CH3OCHCICH2Cl will exhibit three proton singlet, one proton triplet and two proton doublet.
No. of peaks = n + 1
(n = no. of neighbouring chemically equivalent Hydrogen nuclii)1H NMR spectrum of CH3OCHCICH2Cl will exhibit one proton triplet & a two proton doublet

3. Which of the following 1H-NMR spectrum of compound with molecular formula C4H9NO2 shows delta 5.30 (broad, IH), 4.10(q, 2H), 2.80 (d, 3H), 1.20 (t, 3H) ppm?
a) CH3NHCOOCH2CH3
b) CH3CH2NHCOOCH3
c) CH3OCH2CONHCH3
d) CH3CH2OCH2CONH2
View Answer

Answer: a
Explanation: C4H9NO2;
chemical shift = 5.30 (broad, 1H)
chemical shift = 4.10 (q, 2H)
chemical shift = 2.80 (d, 3H)
chemical shift = 1.20 (t, 3H)
Neighbouring 1H attached to N & one will be broad as because of hydrogen attached to N
As shown in the above compound, doublet, because of neighbouring 1H attached to N and one will be broad as because of hydrogen attached to N, quatret becauseof neighbouring 3H and finally triplet, because
of neighbouring 2H.
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4. Which of the following has three types of hydrogens in the following compounds?
a) Br-CH = CH2
b) CH3 – CH2 – CH3
c) C6H5CH2
d) CH3– CH2 – CH(CH3) – N02
View Answer

Answer: a
Explanation:
The following has three types of hydrogens in the following compounds is Br-CH = CH2
We will replace them by D and check the structure if they are different then they are different.
Three different type of hydrogen is present in compound a
both are different
so, total 3 type of hydrogen
So, three different type of hydrogen is present in compound a.

5. Which of the compound show only one signal is present in the PMR spectra?
a) C3H4, C3H6
b) C4H6, C5H12
c) C8H18, C2H6O
d) All of the mentioned
View Answer

Answer: d
Explanation: All of the below compounds have only type of proton, so only one signal.Signal is present in PMR spectra is C3H4, C3H6 & C4H6, C5H12 & C8H18, C2H6O
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6. How many Hertz does 1 ppm correspond to for an PMR spectrometer operating at a radio frequency of 60 MHz and 100 MHz?
a) 6 Hz, 10 Hz
b) 60 Hz, 100 Hz
c) 100 Hz, 60 Hz
d) 10Hz, 100Hz
View Answer

Answer: b
Explanation:
Hertz does 1 ppm to operating at radio frequency of 60 MHz & 100 MHz is 60 Hz, 100 Hz

7. Compound C4H10O gave PMR spectrum consisting of two groups of lines (multiplets) with relative intensities in the ratio 3 : 2. Other compound of the same formula exhibited two lines with relative area of 9 : 1. What are these compounds.
a) Diethyl ether and n-butanol
b) t-Butyl alcohol and 1-Methoxypropane
c) diethyl ether and t-Butyl alcohol
d) Diethyl ether and 2-Methoxypropane
View Answer

Answer: c
Explanation: NMR data: Two groups of lines in 3 : 2 and two groups of lines in 9 : 1 for same molecular formula
a) CH3–CH2–O–CH2–CH3
Find compounds with relative intensities in the ratio 3 : 2 in given diagram
6Ha -> 1 group of line;
4Hb -> 1 group of line

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Therefore, Intensity => 6 : 4 => 3 : 2.
b) Diethyl ether & t-Butyl alcohol consisting of two groups of lines in the ratio 3 : 2
9H -> 1 group of line and 1H -> 1 group of line / single peak.
Therefore, Intensity => 9 : 1.

8. The distance between the centers of the peaks of doublet is called as?
a) Coupling constant
b) Spin constant
c) Spin-spin coupling
d) Chemical shift
View Answer

Answer: a
Explanation: When the proton under observation is split into several peaks by neighboring protons, the distance between these peaks is a constant called the coupling constant (J). Any two sets of protons that exhibit the same coupling constant are most likely splitting eachother signals and are said to be coupled. The distance between the centers of the peaks of doublet is called as Coupling constant
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9. H2, CH4, C2H6 and C6H6 exhibit which PMR spectra?
a) Singlet
b) Doublet
c) Triplet
d) Quintet
View Answer

Answer: a
Explanation: All structures have only one type of hydrogen. So, all give single peak.
H2, CH4, C2H6 & C6H6 exhibit Singlet PMR spectra in given figure

10. A proton Hb is coupled to four equivalent protons Ha. The multiplicity and the relative intensity of lines in the signal Hb is?
a) Doublet, I : 4
b) Triplet, I : 4 : 6
c) Quintet, 1 : 4 : 6 : 4 : 1
d) Quartet, 1 : 4 : 6 : 4
View Answer

Answer: c
Explanation: Each signal in a proton NMR spectrum may or may not be split into one or more peaks, this is called as multiplicity. The most common concept associated with signal multiplicity is the n+1 rule. According to this rule, the signal for the proton under observation will be split into n+1 peaks by protons attached to adjacent carbons, where n is the number of such protons.
No. of peaks = n + 1
(n = no. of neighbouring chemically equivalent Hydrogen nuclii)
So, Hb —-> 4Ha

Multiplicity —->n + 1 = 5 (Quintet)

And, according to Pascal triangle(shown below): Intensity–> 1 : 4 : 6 : 4 : 1
            1
         1 1
      1 2 1
   1 3 3 1
1 4 6 4 1

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