Edexcel IAL Chemistry A2 Topic 20 Organic Synthesis

Pearson Edexcel IAL Chemistry syllabusChemistry YCH11First assessment 2019

Topic 20 questions combine analytical data with synthesis planning: calculate formulae, identify functional groups, choose reagents and justify purification or separation steps.

Exam points

Deduce empirical, molecular and structural formulae from combustion and spectral data.
Complete synthesis routes with reagents, intermediates and functional-group changes.
Choose purification or separation methods such as distillation, reflux or chromatography.

[Maximum number: 1]

Grignard reagents react with
□ A water giving primary alcohols
□ B all aldehydes giving secondary alcohols
□ C ketones giving secondary or tertiary alcohols
□ D carbon dioxide giving carboxylic acids
(Total for Question 16 = 1 mark)

[Maximum number: 9]

Azo dyes, such as Organol Brown, can be made from benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$ , using the reaction scheme shown.

Due to the toxicity of benzene, the first step is never carried out in a school laboratory.

(a)

Draw a diagram of the apparatus used to warm under reflux in this experiment.
(3)

[ 3 ]

(b)

The excess acid is removed from the reaction mixture. The layer containing nitrobenzene is separated and dried before being purified by distillation.

Identify a suitable drying agent.

[ 1 ]

(c)

Reaction of the diazonium compound with an alkaline solution of naphthalene-1-ol produces the solid azo dye, Organol Brown. The solid is purified by recrystallisation.

Procedure
Step 1 The impure Organol Brown is dissolved in a minimum volume of hot solvent.

Step 2 The solution is filtered hot through a preheated funnel.
Step 3 The solution is cooled and filtered using a Buchner funnel.
Step 4 The solid is rinsed with a small amount of ice-cold solvent.
Step 5 The solid is dried in a desiccator.

[ 4 ]

(i)

State why a minimum volume of hot solvent is used in Step 1.
(1)

[ 1 ]

(ii)

Explain why a preheated funnel is used in Step 2.
(1)

[ 1 ]

(iii)

Give a reason for each of the two filtrations in Steps 2 and 3.

[ 2 ]

(d)

The melting temperature of the recrystallised Organol Brown is measured to check its purity.

State what you would observe if the sample was pure.
(1)

[ 1 ]

[Maximum number: 8]

This question is about the alkaline hydrolysis of an ester, X.
X is an alkyl benzoate and can be represented by the formula $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOR}$ , where R is the alkyl group.

The equation for the hydrolysis is

\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOR}+\mathrm{NaOH} \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COONa}+\mathrm{ROH}

Procedure

Step 1 Measure $5.0 \mathrm{~cm}^{3}$ of X and pour it into a pear-shaped flask. Add $25 \mathrm{~cm}^{3}$ (an excess) of aqueous sodium hydroxide solution and a few anti-bumping granules.

Step 2 Heat the flask and contents under reflux for 20 minutes.
Step 3 Allow the apparatus to cool and then rearrange it for distillation.
Distil the mixture and collect about $2 \mathrm{~cm}^{3}$ of the alcohol ROH.
Step 4 Allow the pear-shaped flask to cool, pour the contents into a beaker and add excess dilute hydrochloric acid.
Impure benzoic acid forms as crystals in the mixture.
Step 5 Recrystallise the benzoic acid using water as the solvent.
Step 6 Weigh the dry crystals and determine their melting temperature.

(a)

A student drew a diagram of the apparatus set up for distillation in Step 3. There are three errors in the diagram.

Assume the apparatus is clamped correctly and an appropriate heat source is used.

Identify the three errors and how they should be corrected.

[ 3 ]

(b)

State what should be done to separate the benzoic acid from the mixture produced in Step 4, before carrying out Step 5.

[ 1 ]

(c)

Describe the first stage in the recrystallisation process in Step 5.

[ 1 ]

(d)

The melting temperature of pure benzoic acid is $122^{\circ} \mathrm{C}$ .

State two ways in which the melting temperature changes if the benzoic acid is not pure.
(2)

[ 2 ]

(e)

The molar mass of $\mathbf{X}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOR}$ , is $178 \mathrm{~g} \mathrm{~mol}^{-1}$ .

[ 1 ]

(i)

Deduce the formula of the alkyl group, R.

[ 1 ]

[Maximum number: 9]

Ketones are useful starting compounds in organic synthesis.
This question is about butanone.

butanone

(a)

Devise a reaction scheme to prepare propan-1-ol from butanone, using no more than four steps.

Identify the reagents and essential conditions for each step and give the name or structure of each of the intermediate compounds.

[ 4 ]

(b)

Devise a reaction scheme to prepare 2-methylbut-2-ene from butanone, using no more than four steps.

Identify the reagents and essential conditions for each step and give the name or structure of each of the intermediate compounds.

[ 5 ]

[Maximum number: 10]

This question is about compounds with the molecular formula $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}$ .

(a)

Compound A is thought to be another isomer of hexanoic acid.

10 g of compound A is found to contain 6.21 g of carbon and 1.03 g of hydrogen, with the remainder being oxygen.

Use the data to calculate the empirical formula of compound A.
You must show all your working.

[ 3 ]

(b)

State how you might use your answer to (b)(i) and a mass spectrum of compound A to prove that compound A is an isomer of hexanoic acid.

[ 1 ]

(c)

*(iii) A series of tests was performed on compound A.

Deduce the structure for compound A.
Justify your answer by using all the test results.
(6)

[ 6 ]

A compound Q contains the elements carbon, hydrogen and oxygen only.

Combustion analysis of 4.91 g of Q produces 14.6 g of carbon dioxide and 3.58 g of water.

Show that the molecular formula of Q is $\mathrm{C}_{10} \mathrm{H}_{12} \mathrm{O}$ .
You must show all your working.
[ $M_{\mathrm{r}}$ of Q=148 ]

[Maximum number: 6]

Benzoic acid is a white crystalline solid with the structure shown.

It is found in many plants as it is an important building block for the biosynthesis of a variety of compounds, such as plant hormones and attractants for pollinators.

The role of benzoic acid in the chemical industry is also widespread and approximately 500000 tonnes are produced annually. It is used in the synthesis of many compounds, including medicines, dyes and insect repellents.

Such synthetic dyes are often classified as aryl azo dyes. These dyes have a range of vivid colours and a wide range of uses in many industries, including food and textiles. Their synthesis involves the formation of a diazonium ion. This ion then reacts with a phenol in a coupling reaction, to form the dye. The relative simplicity of the reactions involved and ready availability of starting materials make azo dyes cheap to produce.

Salts of benzoic acid, such as calcium benzoate and sodium benzoate, are used in the food industry as preservatives.

Devise a reaction scheme to produce benzoic acid from benzene, via bromobenzene and then a Grignard reagent.

Include the reagents and essential conditions for each step and give the name or structure of each of the intermediate compounds.

Details of practical procedures and reaction mechanisms are not required.

Exam points

Question 16

Question 3

Question 3(a)(ii)

Question 3(b)

Question 3(d)

Question 3(d)(i)

Question 3(d)(ii)

Question 3(d)(iii)

Question 3(e)

Question 4

Question 4(a)

Question 4(c)(ii)

Question 4(d)

Question 4(e)

Question 4(f)

Question 4(f)(i)

Question 20

Question 20(b)

Question 20(c)

Question 20

Question 20(b)(i)

Question 20(b)(ii)

Question 20(b)(iii)

Question 15(a)

Question 16(a)