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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%.