Organic Chemistry III: Organic Synthesis (Edexcel A Level Chemistry): Flashcards

The key steps are:

1. Draw the structures of the starting molecule and target molecule

2. Determine whether they have the same number of carbon atoms (if not, chain extension is needed)

3. Work out compounds that can be made from the starting molecule and those that lead to the target

4. Match common intermediates and identify the required reagents and conditions

False.

Nitrobenzene is reduced to aminobenzene (phenylamine/aniline) using Sn / HCl, not bromine water.

A haloalkane is converted to a nitrile using aqueous ethanolic KCN with heat. The reaction proceeds by nucleophilic substitution, extending the carbon chain by one carbon atom.

To oxidise a primary alcohol to a carboxylic acid, the reagents are K₂Cr₂O₇ / H₂SO₄ and the conditions require reflux rather than distillation.

Benzene is converted to nitrobenzene using HNO₃ / H₂SO₄ (concentrated). The reaction type is nitration (electrophilic substitution).

A reaction scheme is a planned sequence of chemical steps — each with specified reagents and conditions — that converts a starting material into a target molecule via one or more intermediates.

An acyl chloride reacts with NH₃ to form a primary amide. The reaction type is nucleophilic addition-elimination.

True.

Examiners may require candidates to design a synthesis with up to four steps, including selecting appropriate reagents, conditions and control measures for hazardous substances.

A Grignard reagent is an organomagnesium halide of the form RMgX (e.g. CH₃CH₂MgI), prepared by reacting a halogenoalkane with magnesium in dry ether. The alkyl group behaves as a nucleophile and is used to increase carbon chain length.

A halogenoalkane is dissolved in dry ether and reacted with magnesium metal. For example, 1-iodoethane reacts with Mg to give ethyl magnesium iodide (CH₃CH₂MgI).

A Grignard reagent reacts with methanal and then water to produce a primary alcohol; with a longer-chain aldehyde to produce a secondary alcohol; and with a ketone to produce a tertiary alcohol.

A carboxylic acid is formed. For example, CH₃CH₂MgI reacts with CO₂ then water to give propanoic acid (CH₃CH₂COOH), extending the carbon chain by one carbon atom.

True.

Grignard reagents are highly reactive and are destroyed by water. Dry ether is used as the solvent to prevent the reagent from reacting with moisture before it can be used in synthesis.

The product is butan-2-ol, a secondary alcohol. The ethyl group from CH₃CH₂MgI adds to the carbonyl of ethanal (CH₃CHO), and water workup gives CH₃CH₂CH(OH)CH₃.

False.

The alkyl group (R group) in a Grignard reagent carries a partial negative charge and acts as a nucleophile, attacking the electrophilic carbonyl carbon.

The product is 2-methylbutan-2-ol, a tertiary alcohol (CH₃CH₂C(CH₃)(OH)CH₃). Ketones react with Grignard reagents to give tertiary alcohols after aqueous workup.

Simple distillation is a technique used to separate compounds from a mixture based on differences in boiling point. The compound with the lowest boiling point distils first and is collected in a receiving vessel.

An electric heating mantle is used because it provides controllable heating and avoids an open flame. Many organic chemicals are flammable, so a Bunsen burner would create a fire risk.

False.

Cooling water enters at the bottom of the condenser and exits at the top. This countercurrent flow maximises heat exchange and keeps the condenser walls cool.

In simple distillation, the distillate collected is that given off at ±2 °C of the boiling point of the desired product; fractions outside this range should be discarded.

The aldehyde has a lower boiling point than the alcohol because it lacks the hydrogen bonding present in alcohols. Distilling it off immediately prevents further oxidation to the carboxylic acid.

Reflux is a technique in which a reaction mixture is heated in a flask fitted with a vertical condenser, allowing vapours to condense and return to the flask. It allows a reaction to proceed at high temperature without loss of reactants, products or solvent.

Steam distillation separates an insoluble liquid from an aqueous solution. Its advantages are:

1. The insoluble liquid distils at a temperature below its normal boiling point

2. It reduces the risk of thermal decomposition of the product

True.

Anti-bumping granules provide nucleation sites for bubble formation, promoting smooth boiling and preventing sudden violent surges (bumping) in the flask.

Two examples are:

1. Oxidation of a primary alcohol to a carboxylic acid using acidified potassium dichromate

2. Esterification of an alcohol and carboxylic acid using an acid catalyst

Recrystallisation is a purification technique for impure solids. The solid and impurities are dissolved in the minimum volume of hot solvent; as the solution cools, the pure product crystallises out, leaving impurities in solution.

Add a small amount of water to the separating funnel and observe which layer increases in volume — that is the aqueous layer. The other layer is the organic layer.

When using a drying agent, if it clumps together there is still water present; more is added until some remains as a fine powder in the organic liquid.

False.

Anhydrous calcium chloride is used specifically to dry hydrocarbons. For compounds containing nitrogen or oxygen functional groups, more general-purpose drying agents such as anhydrous magnesium sulfate or calcium sulfate are used instead.

Filtration under reduced pressure (Buchner filtration) uses a Buchner funnel and flask connected to a vacuum pump. It is faster than gravity filtration and is used after recrystallisation to recover product crystals more efficiently.

Solvent extraction is a purification technique using a separating funnel to separate an organic product from an aqueous layer based on differences in solubility and density. The layers are allowed to settle and then drained separately.

Using the minimum volume of hot solvent reduces product loss: if too much solvent is used, more of the pure product remains dissolved as the solution cools and is not recovered by crystallisation, lowering the yield.

True.

Slow cooling allows larger, more uniform crystals to form. These are easier to collect by filtration and dry compared to the fine crystals produced by rapid cooling.

Sodium carbonate solution is added to neutralise acidic impurities (such as excess acid catalyst) and wash them into the aqueous layer, removing them from the organic product.

Melting point determination is a technique used to identify a substance and assess its purity by measuring the temperature at which a solid melts and comparing it to a known data book value.

The sample must be totally dry and finely powdered — crush it with the back of a spatula on filter paper or a white tile to remove moisture and ensure even heating.

False.

Impurities lower the melting point of a solid and broaden the melting point range.

Boiling point determination is a technique used to identify a liquid and assess its purity by measuring the temperature at which it boils, usually via distillation, and comparing it to a literature value.

A pure solid has a sharp melting point, whereas an impure solid melts over a broad range.

Impurities cause the boiling point to appear higher than the literature value and may cause the liquid to boil over a range of temperatures rather than at a single fixed point.

True.

A pure substance melts at a precise temperature, whereas an impure substance melts over a broad range.

A slow heating rate allows accurate determination of the melting point range; use the first run to find the approximate range, then repeat at a much slower rate for precision.