Use the Data Booklet to help you answer this question. This question is about amino acids and peptide (amide) links. 1. Draw the structure of the zwitterion formed by phenylalanine. 2. Draw the structure of serine at high pH. 3. Draw the structures of both dipeptides formed when phenylalanine reacts with serine. In each structure show all the atoms and bonds in the amide link. 4. An amide link is also formed when an acyl chloride reacts with a primary amine. Name and outline a mechanism for the reaction between CH3CH2COCl and CH3CH2NH2. Give the IUPAC name of the organic product. Name of mechanism Mechanism IUPAC name of organic product

A-Level AQA Chemistry Carboxylic Acids & Derivatives: Use the Data Booklet to help you

Use the Data Booklet to help you answer this question - AQA - A-Level Chemistry - Question 6 - 2017 - Paper 2

Question 6

Use the Data Booklet to help you answer this question. This question is about amino acids and peptide (amide) links. 1. Draw the structure of the zwitterion formed... show full transcript

Worked Solution & Example Answer:Use the Data Booklet to help you answer this question - AQA - A-Level Chemistry - Question 6 - 2017 - Paper 2

Step 1

Draw the structure of the zwitterion formed by phenylalanine.

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Answer

The zwitterion of phenylalanine shows the amino group ( $-NH_3^+$ ) protonated and the carboxyl group ( $-COO^-$ ) deprotonated. The structure can be represented as:

        NH_3^+
          |  
     H2N-C-COO^-
          |  
        C6H5

Where the carbon chain includes the phenyl group.

Step 2

Draw the structure of serine at high pH.

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Answer

At high pH, serine has its amino group protonated and the hydroxyl group ( $-OH$ ) remains. The structure can be represented as:

        NH_2
          |  
     H2N-C-COO^-
          |  
        CH2
          |  
        OH

Here, the carboxylic acid is in its deprotonated form ( $-COO^-$ ).

Step 3

Draw the structures of both dipeptides formed when phenylalanine reacts with serine.

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Answer

The two dipeptides formed can be illustrated as:

Phenylalanine-Serine (Phe-Ser):

        NH_2
          |  
     H2N-C-CO(NH)-C-COO^-
               |  
              CH2
               |  
               OH

Serine-Phenylalanine (Ser-Phe):

        NH_2
          |  
     H2N-C-CO(NH)-C6H5
          |  
          CH2
          |  
          COO^-

In both structures, the amide link is denoted by the bond between the carbonyl carbon and the amino nitrogen.

Step 4

Name and outline a mechanism for the reaction between CH3CH2COCl and CH3CH2NH2.

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Answer

The mechanism is known as nucleophilic addition-elimination. In this reaction:

The nucleophile (ethanamine, CH3CH2NH2) attacks the carbonyl carbon of the acyl chloride (CH3CH2COCl).
A tetrahedral intermediate is formed.
The chloride ion is eliminated leading to the formation of the amide:
$CH3CH2C(O)NHCH2CH3$
The IUPAC name of the organic product is N-ethylpropanamide.

Use the Data Booklet to help you answer this question - AQA - A-Level Chemistry - Question 6 - 2017 - Paper 2

Worked Solution & Example Answer:Use the Data Booklet to help you answer this question - AQA - A-Level Chemistry - Question 6 - 2017 - Paper 2

Draw the structure of the zwitterion formed by phenylalanine.

Draw the structure of serine at high pH.

Draw the structures of both dipeptides formed when phenylalanine reacts with serine.

Name and outline a mechanism for the reaction between CH3CH2COCl and CH3CH2NH2.

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