Organic Synthesis (Cambridge (CIE) A Level Chemistry): Flashcards
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What reagent is used to test for alkenes, and what is the positive result?
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What reagent is used to test for alkenes, and what is the positive result?
Bromine water is used. A positive result is the decolourisation of the bromine water from orange to colourless, indicating the presence of a C=C double bond.
Define nucleophilic substitution.
A nucleophilic substitution is an organic reaction mechanism in which a nucleophile attacks a carbon atom carrying a partial positive charge, replacing an atom or group carrying a partial negative charge.
True or False?
Tollens' reagent produces a red precipitate with aldehydes.
False.
Tollens' reagent produces a silver mirror with aldehydes. A red/orange precipitate is produced by Fehling's solution, not Tollens' reagent.
Acidified potassium dichromate oxidises primary alcohols first to .......... and then to .........., with the solution changing colour from ....... to green.
Acidified potassium dichromate oxidises primary alcohols first to aldehydes and then to carboxylic acids, with the solution changing colour from orange to green.
How do you distinguish between an aldehyde and a ketone using Fehling's solution?
Fehling's solution gives a positive result (blue to red/orange precipitate) with aldehydes only. Ketones do not react with Fehling's solution. Both give an orange precipitate with 2,4-DNPH, so this reagent cannot distinguish them.
True or False?
LiAlH4 can reduce carboxylic acids to primary alcohols, whereas NaBH4 cannot.
True.
LiAlH4 is a stronger reducing agent and reduces carboxylic acids to primary alcohols (via an aldehyde intermediate). NaBH4 cannot reduce carboxylic acids.
Define condensation reaction.
A condensation reaction is a reaction in which two organic molecules join together and in the process eliminate a small molecule such as water or hydrogen chloride.
Amines are produced by heating halogenoalkanes under pressure with ....... dissolved in .......... solvent.
Amines are produced by heating halogenoalkanes under pressure with NH3 dissolved in ethanol solvent.
What colour change is observed when acidified potassium permanganate reacts with an alkene, and what organic product is formed?
The purple KMnO4 solution turns colourless. Acidified KMnO4 causes oxidative cleavage of the C=C double bond, producing carboxylic acids (from CH2= or CH= groups) or ketones (from C= groups with no attached hydrogen).
What is a multi-step synthetic route in organic chemistry?
A multi-step synthetic route is a sequence of reactions used to convert a starting material into a target organic molecule, passing through one or more intermediate compounds.
Define intermediate.
An intermediate is a compound formed during a multi-step synthesis that is then converted into the final product in a subsequent reaction step.
True or False?
To convert propan-1-ol into propanoic acid in one step, you heat it with acidified potassium dichromate under reflux.
True.
Heating a primary alcohol under reflux with acidified K2Cr2O7 oxidises it fully to the carboxylic acid (rather than stopping at the aldehyde, which requires distillation).
To make an ester from a primary alcohol in two steps: first ....... the alcohol to a carboxylic acid, then react it with an alcohol using hot, concentrated ....... as catalyst.
To make an ester from a primary alcohol in two steps: first oxidise the alcohol to a carboxylic acid, then react it with an alcohol using hot, concentrated H2SO4 as catalyst.
How would you convert propanal into 2-hydroxybutanoic acid using two further reaction steps?
Step 1: Nucleophilic addition of HCN to propanal to form a hydroxynitrile.
Step 2: Hydrolysis of the nitrile group using dilute acid (or dilute alkali followed by acidification) to give the 2-hydroxy carboxylic acid.
True or False?
A sodium carboxylate salt can be made by reacting a carboxylic acid with sodium carbonate.
True.
Carboxylic acids react with carbonates (including Na2CO3) to produce the sodium carboxylate salt, water and CO2 gas. This is a straightforward acid-base reaction.
When devising a multi-step route, you must know which ....... group each starting material contains and which ....... each functional group can undergo.
When devising a multi-step route, you must know which functional group each starting material contains and which reactions each functional group can undergo.
Outline how ethyl propanoate can be made from propan-1-ol in two steps.
Step 1: Oxidise propan-1-ol to propanoic acid using acidified K2Cr2O7 under reflux.
Step 2: React propanoic acid with ethanol in the presence of hot, concentrated H2SO4 (condensation/esterification) to give ethyl propanoate and water.
Define synthetic route.
A synthetic route is a planned sequence of chemical reactions that converts a given starting material into a desired target molecule, specifying reagents and conditions for each step.
What reagent converts a halogenoalkane into a nitrile, and what is the mechanism?
Ethanolic KCN (potassium cyanide in ethanol) converts a halogenoalkane into a nitrile by nucleophilic substitution. The CN- ion acts as the nucleophile, replacing the halogen atom.
Define hydroxynitrile.
A hydroxynitrile is an organic compound containing both a hydroxyl (OH) group and a nitrile (CN) group. It is formed by nucleophilic addition of HCN (or NaCN) to a carbonyl compound such as an aldehyde.
True or False?
Hydrolysis of a nitrile with dilute aqueous sulfuric acid under reflux produces a carboxylic acid and an ammonium salt.
True.
Refluxing a nitrile with dilute H2SO4 (aq) causes hydrolysis of the CN group to give a carboxylic acid and an ammonium salt as products.
To synthesise hexanoic acid from 1-chloropentane: step 1 — react with ....... in ethanol to form a nitrile; step 2 — hydrolyse the nitrile with ....... under reflux.
To synthesise hexanoic acid from 1-chloropentane: step 1 — react with KCN in ethanol to form a nitrile; step 2 — hydrolyse the nitrile with dilute H2SO4 under reflux.
Why does adding KCN to a halogenoalkane increase the carbon chain length by one?
The CN- nucleophile adds one carbon atom (as part of the CN group) when it replaces the halogen. After hydrolysis of the nitrile to a carboxylic acid, the product has one more carbon atom than the original halogenoalkane.
True or False?
Nucleophilic addition of HCN to an aldehyde produces a hydroxynitrile with the same number of carbon atoms as the aldehyde.
False.
Nucleophilic addition of HCN increases the carbon chain by one. The CN group adds across the C=O bond, so the hydroxynitrile has one more carbon than the original aldehyde.
In a two-step synthesis of a 2-hydroxy carboxylic acid from an aldehyde: step 1 is nucleophilic addition of ....... to form a .........., and step 2 is ....... with dilute acid.
In a two-step synthesis of a 2-hydroxy carboxylic acid from an aldehyde: step 1 is nucleophilic addition of HCN to form a hydroxynitrile, and step 2 is hydrolysis with dilute acid.
When analysing an unfamiliar synthetic route, what is the first step you should take?
Identify the functional groups present in the starting material and product. Then match the transformation to a known reaction type (e.g. oxidation, nucleophilic substitution, hydrolysis) to select appropriate reagents and conditions.
Define nitrile hydrolysis.
Nitrile hydrolysis is the reaction of a nitrile (CN group) with dilute aqueous acid or alkali under reflux to produce a carboxylic acid (and an ammonium salt when using acid).
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