MC test: Synthetic routes

Multiple choice test on 20.2 Synthetic routes

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

What is meant by retro-synthesis in organic chemistry?

Retro-synthesis means planning a synthesis backwards, i.e. by starting at the product and taking it back a step at a time to simple, available starting materials. 

 

Butanone can be synthesised from but-2-ene in three steps.

Step 1: C4H8 → C4H9Br

Step 2: C4H9Br→ C4H9OH

Step 3: C4H9OH→ C4H8O

Which row contains the correct descriptions for all the steps in this synthesis?

RowStep 1Step 2Step 3
1Electrophilic additionElectrophilic substitutionReduction
2Free radical substitutionNucleophilic substitutionOxidation
3Electrophilic additionNucleophilic substitutionOxidation
4Free radical substitutionElectrophilic substitutionReduction

Electrophilic addition of HBr to but-2-ene gives 2-bromobutane. Nucleophilic substitution of 2-bromobutane with NaOH gives butan-2-ol. Oxidation of butan-2-ol gives butanone.

 

Starting with pent-1-ene which route will produce pentanal as one of the organic products?

Adding hydrogen chloride to pent-1-ene gives 2-chloropropane as the Markovnikov product which will ultimately give pentan-2-one when oxidized, not the aldehyde. 1-chloropentane, which can be converted into pentan-1-ol and then oxidised to pentanal, can be obtained by first converting pent-1-ene into pentane then reacting pentane with chlorine in ultraviolet light. Note that only 1-chloropentane can be used from the reaction of pentane with ultraviolet light as other isomers of chloropentane will also be formed.

 

Which reagents and conditions are required to make butanoic acid from 1-chlorobutane?

Step 1. CH3CH2CH2CH2Cl + NaOH → CH3CH2CH2CH2OH + NaCl

Step 2. 3CH3CH2CH2CH2OH + 2Cr2O72− + 16H+ → 3CH3CH2CH2COOH + 4Cr3+ + 11H2O

 

Starting with ethanoic acid as the only organic reagent, a route to make ethyl ethanoate consists of two separate steps. 

Step 1: Convert some of the ethanoic acid into ethanol.

Step 2: React the ethanol formed in Step 1 with the remainder of the ethanoic acid to make the ester.

Which row gives the correct reagents and general conditions required?

RowStep 1Step 2
1NaBH4 in ethanol followed by acidificationAdd concentrated H2SO4 and warm
2LiAlH4 in ether followed by acidificationAdd concentrated H2SO4 and warm
3Warm with acidified K2Cr2O7 solutionReflux with concentrated H2SO4
4Warm with acidified K2Cr2O7 solutionWarm with NaOH solution

Although sodium borohydride is a good reducing agent it does not reduce carboxylic acids to primary alcohols. Esters are made by warming a carboxylic acid and an alcohol in the presence of a small amount of concentrated sulfuric acid to act as a catalyst.

 

Starting with propanal as the only organic compound which route will produce propyl propanoate? 

Propanal is an aldehyde. It can be reduced to a primary alcohol (propan-1-ol) and oxidised to a carboxylic acid (propanoic acid). Propan-1-ol and propanoic acid will react when warmed in the presence of concentrated sulfuric acid to give an ester (propyl propanoate).

 

Phenylamine can be synthesised from benzene in three separate steps.
Which row contains the reagents required in the correct order?

RowStep 1Step 2Step 3
1a mixture of concentrated sulfuric acid & nitric acidsodium hydroxide solutiontin and concentrated hydrochloric acid
2tin and concentrated hydrochloric acida mixture of concentrated sulfuric acid & nitric acidsodium hydroxide solution
3sodium hydroxide solutiontin and concentrated hydrochloric acida mixture of concentrated sulfuric acid & nitric acid
4a mixture of concentrated sulfuric acid & nitric acidtin and concentrated hydrochloric acidsodium hydroxide solution

Benzene is first converted into nitrobenzene using a mixture of concentrated nitric acid and concentrated sulfuric acid. Nitrobenzene is then converted into phenylammonium chloride using tin and concentrated sulfuric acid. Phenylamine is obtained by reacting phenylammonium chloride with sodium hydroxide solution.

 

Benzene can be converted into phenylamine in three separate steps.
Which row contains the description of the processes taking place in each step in the correct order?

RowStep 1Step 2Step 3
1reductionnitrationnucleophilic substitution
2electrophilic substitutionreductionacid-base reaction
3electrophilic additionnitrationacid-base reaction
4electrophilic substitutionreductionnucleophilic substitution

Benzene undergoes electrophilic substitution (the electrophile is NO2+) to form nitrobenzene,, which is then reduced to phenylammonium chloride. Phenylammonium chloride undergoes an acid-base reaction with sodium hydroxide to form phenylamine.

 

Butanone can be synthesised from 2-bromobutane in two separate steps.
Which row contains the reagents required in the correct order?

RowStep 1Step 2
1dilute sodium hydroxide solutionlithium aluminium hydride in ether followed by acidification
2acidified potassium dichromate(VI) solutiondilute sodium hydroxide solution
3dilute sodium hydroxide solutionacidified potassium dichromate(VI) solution
4lithium aluminium hydride in ether followed by acidificationdilute sodium hydroxide solution

2-bromobutane is first converted into a secondary alcohol (butan-2-ol) by a nucleophilic substitution reaction with hydroxide ions. Butan-2-ol is then oxidised to butanone by heating with an acidified solution of potassium dichromate(VI).

 

The conversion of propane into propanal can occur by the following separate steps:

Step I. CH3CH2CH3  +  Cl2 → CH3CH2CH2Cl + HCl

Step II. CH3CH2CH2Cl + NaOH → CH3CH2CH2OH + NaCl

Step III. CH3CH2CH2OH  → CH3CH2CHO + 2H+ + 2e−

In which steps is the organic reactant being oxidised?

In Step I propane is losing a hydrogen atom and the chlorine is reduced from (0) to (−1) so propane is being oxidized. In Step II there is no change in oxidation states in any of the atoms so no redox process is taking place. In Step III propan-1-ol is losing electrons so it is being oxidized to propanal.

 

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