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CHEMISTRY, M8 2025 HSC 36

Use the data sheet provided and the information in the table to answer this question.
 

 

Consider the molecule shown.

For each of the following instrumental techniques, predict the expected features of the spectra produced.

Refer to the structural features of the molecule in your answer.

  • Infrared (IR)    (Ignore any absorptions due to \(\ce{C - C}\) or \(\ce{C - H}\) )
  • Carbon-13 NMR
  • Proton NMR
  • Mass spectrometry   (7 marks)

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Infrared (IR)

  • Strong broad \(\ce{OH}\) signal at approximately 2750 cm\(^{-1}\)
  • Strong sharp \(\ce{CO}\) signal at approximately 1700 cm\(^{-1}\)

\(^{13}\text{C NMR}\)

  • 3 distinct carbon environments 
  • 1 signal at 5−40 ppm \(\ce{(CH3)}\)
  • 1 signal at 20−50 ppm \(\ce{(CH2)}\)
  • 1 signal at 160−185 ppm \(\ce{(acid CO)}\)

\(^{1}\text{H NMR (Proton NMR)}\)

  • 3 distinct hydrogen environments
  • 1 signal at 0.7−2.1 ppm \(\ce{(CH3)}\), triplet splitting pattern, integration of 3
  • 1 signal at 2.1−4.5 ppm \(\ce{(CH2)}\), quartet splitting pattern, integration of 2
  • 1 signal at 9.0−13.0 ppm \(\ce{(COOH)}\), singlet, integration of 1

Mass Spectrometry

  • Molecular ion at 74 m/z (molar mass of propanoic acid ~74 g/mol)
Show Worked Solution

Infrared (IR)

  • Strong broad \(\ce{OH}\) signal at approximately 2750 cm\(^{-1}\)
  • Strong sharp \(\ce{CO}\) signal at approximately 1700 cm\(^{-1}\)

\(^{13}\text{C NMR}\)

  • 3 distinct carbon environments 
  • 1 signal at 5−40 ppm \(\ce{(CH3)}\)
  • 1 signal at 20−50 ppm \(\ce{(CH2)}\)
  • 1 signal at 160−185 ppm \(\ce{(acid CO)}\)

\(^{1}\text{H NMR (Proton NMR)}\)

  • 3 distinct hydrogen environments
  • 1 signal at 0.7−2.1 ppm \(\ce{(CH3)}\), triplet splitting pattern, integration of 3
  • 1 signal at 2.1−4.5 ppm \(\ce{(CH2)}\), quartet splitting pattern, integration of 2
  • 1 signal at 9.0−13.0 ppm \(\ce{(COOH)}\), singlet, integration of 1

Mass Spectrometry

  • Molecular ion at 74 m/z (molar mass of propanoic acid ~74 g/mol)

CHEMISTRY, M8 2025 HSC 17 MC

The chemical environment of an atom depends on the species surrounding that atom within a molecule.

In which of the following compounds does the number of carbon chemical environments equal the number of proton chemical environments?
 

 

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\(A\)

Show Worked Solution
  • In the diagrams below, each colour represents a different carbon or proton environment (ignoring the oxygen atoms).

  • Only \(A\) has equal number of carbon and proton environments with two of each.

\(\Rightarrow A\)

CHEMISTRY, M8 2021 VCE 30 MC

The \(^1\)H NMR spectrum of an organic compound has three unique sets of peaks: a single peak, seven peaks (septet) and two peaks (doublet).

The compound is

  1. 3-methyl butanoic acid.
  2. 2-methyl propanoic acid.
  3. 2-chloro-2-methylpropane.
  4. 1,2-dichloro-2-methylpropane.
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\(B\)

Show Worked Solution
  • On a \(^1\)H NMR spectrum:
  •    A single peak → no hydrogens on the adjacent carbon.
  •    A double peak → 1 hydrogen on the adjacent carbon.
  •    A septet peak → 6 hydrogens with the same environment on adjacent carbons.
  • Therefore, there are 3 unique hydrogen environments in this compound.
  • 2-methyl propanoic acid is the only compound with 3 different hydrogen environments.

\(\Rightarrow B\)

♦ Mean mark 47%.

CHEMISTRY, M8 2024 HSC 19 MC

Which of the following compounds produces TWO doublets in the \({ }^1 \text{H NMR}\) spectrum?
 


 

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\(A\)

Show Worked Solution
  • For two doublets to occur, the substance must have two chemically different hydrogen environments that both have one neighbouring hydrogen on an adjacent carbon atom.
     

  • This is displayed by option \(\text{A}\) where the hydrogen atoms in environments 1 and 3 both have only one neighbouring hydrogen atom each. 

\(\Rightarrow A\)

♦ Mean mark 48%.

CHEMISTRY, M8 2023 HSC 36

An organic reaction pathway involving compounds \(\text{A, B,}\) and \(\text{C}\) is shown in the flow chart.
 

The molar mass of \(\text{A}\) is 84.156 g mol\(^{-1}\).

A chemist obtained some spectral data for the compounds as shown.
 

\( \text{Data from} \ ^{1} \text{H NMR spectrum of compound C} \)
 \( Chemical \ Shift \ \text{(ppm)} \) \( Relative \ peak \ area \) \( Splitting \ pattern \)
\(1.01\) \(3\) \(\text{Triplet}\)
\(1.05\) \(3\) \(\text{Triplet}\)
\(1.65\) \(2\) \(\text{Multiplet}\)
\(2.42\) \(2\) \(\text{Triplet}\)
\(2.46\) \(2\) \(\text{Quartet}\)
\( ^{1} \text{H NMR chemical shift data}\)
\( Type \ of \ proton \)  \(  \text{δ/ppm} \)
\( \ce{R - C\textbf{H}3,R - C\textbf{H}2 - R}\) \(0.7-1.7\)
\( \left.\begin{array}{l}\ce{\textbf{H}3C - CO - \\-C\textbf{H}2 - CO -}\end{array}\right\} \begin{aligned} & \text { (aldehydes, ketones,} \\ &\text{carboxylic acids or esters) }\end{aligned}\) \(2.0-2.6\) 
\( \ce{R - C\textbf{H}O} \) \(9.4-10.00\)
\( \ce{R - COO\textbf{H}} \) \(9.0-13.0\)
 

Identify the functional group present in each of compounds \(\text{A}\) to \(\text{C}\) and draw the structure of each compound. Justify your answer with reference to the information provided.  (9 marks) 

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Compound \(\text{A}\): Alkene
 

   

Compound \(\text{B}\): Secondary alcohol

Compound \(\text{C}\): Ketone
 

   

Reasoning as follows:

  • Compound \(\text{A}\) is able to undergo an addition reaction to add water across a \(\ce{C=C}\) bond  \(\Rightarrow \)  Alkene
  • Compound \(\text{B}\) is the product of the above hydration reaction and is therefore an alcohol.
  • The \(\ce{^{13}C\ NMR}\) spectrum of Compound \(\text{A}\) confirms it is an alkene (132 ppm peak corresponding to the \(\ce{C=C}\) atoms). 3 spectrum peaks indicate 3 carbon environments. The molar mass of compound \(\text{A}\) is 84.156 g mol\(^{-1}\) which suggests symmetry within the molecule.
  • The Infrared Spectrum of Compound \(\text{B}\) has a broad peak at approximately 3400 cm\(^{-1}\). This indicates the presence of an hydroxyl group and confirms \(\text{B}\) is an alcohol.
  • Compound \(\text{C}\) is produced by the oxidation of Compound \(\text{B}\) with acidified potassium permanganate.
  • Compound \(\text{C}\) is a carboxylic acid if \(\text{B}\) is a primary alcohol or a ketone if \(\text{B}\) is a secondary alcohol.
  • Since the \(\ce{^{1}H NMR}\) spectrum of \(\text{C}\) does not show any peaks between 9.0 − 13.0 ppm, it cannot be a carboxylic acid. Compound \(\text{C}\) is therefore a ketone and Compound \(\text{B}\) is a secondary alcohol.
  • The \(\ce{^{1}H NMR}\) spectrum shows 5 peaks \(\Rightarrow \) 5 hydrogen environments.
  • Chemical shift and splitting patterns information indicate:
  • 1.01 ppm – 1.05 ppm: \(\ce{CH3}\) (next to a \(\ce{CH2}\))
  •   1.65 ppm: \(\ce{CH2}\) (with multiple neighbouring hydrogens)
  •   2.42 ppm: \(\ce{CH2}\) (next to the ketone \(\ce{C=O}\) and a \(\ce{CH2}\))
  •   2.46 ppm: \(\ce{CH2}\) (next to the ketone \(\ce{C=O}\) and a \(\ce{CH3}\))
Show Worked Solution

Compound \(\text{A}\): Alkene
 

   

Compound \(\text{B}\): Secondary alcohol

Compound \(\text{C}\): Ketone
 

   

Reasoning as follows:

  • Compound \(\text{A}\) is able to undergo an addition reaction to add water across a \(\ce{C=C}\) bond  \(\Rightarrow \)  Alkene
  • Compound \(\text{B}\) is the product of the above hydration reaction and is therefore an alcohol.
  • The \(\ce{^{13}C\ NMR}\) spectrum of Compound \(\text{A}\) confirms it is an alkene (132 ppm peak corresponding to the \(\ce{C=C}\) atoms). 3 spectrum peaks indicate 3 carbon environments. The molar mass of compound \(\text{A}\) is 84.156 g mol\(^{-1}\) which suggests symmetry within the molecule.
  • The Infrared Spectrum of Compound \(\text{B}\) has a broad peak at approximately 3400 cm\(^{-1}\). This indicates the presence of an hydroxyl group and confirms \(\text{B}\) is an alcohol.
  • Compound \(\text{C}\) is produced by the oxidation of Compound \(\text{B}\) with acidified potassium permanganate.
  • Compound \(\text{C}\) is a carboxylic acid if \(\text{B}\) is a primary alcohol or a ketone if \(\text{B}\) is a secondary alcohol.
  • Since the \(\ce{^{1}H NMR}\) spectrum of \(\text{C}\) does not show any peaks between 9.0 − 13.0 ppm, it cannot be a carboxylic acid. Compound \(\text{C}\) is therefore a ketone and Compound \(\text{B}\) is a secondary alcohol.
  • The \(\ce{^{1}H NMR}\) spectrum shows 5 peaks \(\Rightarrow \) 5 hydrogen environments.
  • Chemical shift and splitting patterns information indicate:
  • 1.01 ppm – 1.05 ppm: \(\ce{CH3}\) (next to a \(\ce{CH2}\))
  •   1.65 ppm: \(\ce{CH2}\) (with multiple neighbouring hydrogens)
  •   2.42 ppm: \(\ce{CH2}\) (next to the ketone \(\ce{C=O}\) and a \(\ce{CH2}\))
  •   2.46 ppm: \(\ce{CH2}\) (next to the ketone \(\ce{C=O}\) and a \(\ce{CH3}\))

CHEMISTRY, M8 2019 HSC 14 MC

A molecule, `text{C}_(4) text{H}_(9) text{Cl}`, is analysed. The \(\ce{^1H NMR} \) spectrum of this molecule is shown.
 

What is the structural formula of this molecule?
 


 


 


 

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`A`

Show Worked Solution
  • The \(\ce{^1H NMR} \) spectrum exhibits a single signal, which suggests that all the hydrogens in the sample are present in the same chemical environment.
  • Therefore, the answer A, where all the hydrogens are chemically equivalent is correct.

`=>A`

CHEMISTRY, M8 2021 HSC 18 MC

The table lists the information from a proton NMR spectrum.
 

Which compound could have produced this spectrum?

  1. `text{1,2,2-trichlorobutane}`
  2. `text{1,3-dichloro-2-methylpropane}`
  3. `text{2-chloro-2-methylbutane}`
  4. `text{2,2-dichlorobutane}`
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`D`

Show Worked Solution

By elimination:

There are 8 hydrogen atoms in total (3 + 3 + 2 = 8)

  • Eliminate A and C

Compound has a 3H singlet

  • Eliminate B

`=> D`


♦♦ Mean mark 39%.