Ch 1 · Organic synthesis · AS & A Level Chemistry 9701

Topic 1 of 1 in Organic synthesis — AS & A Level Chemistry 9701 — 57 lessons.

Introduction

AS organic synthesis draws together every functional-group reaction of the module: recognising what is present in a molecule, predicting how it will react, and linking single-step reactions into a route from one compound to another.

Key ideas — Identifying functional groups

A molecule may contain more than one functional group, each reacting independently according to its own chemistry; 4-hydroxybutanoic acid, HOCH_2CH_2CH_2COOH, for example, contains both a hydroxyl group and a carboxylic acid group. Systematically scanning a structure for mathrmC=C, -OH, -X (halogen), mathrmC=O (aldehyde or ketone), -COOH, -COOC- (ester), -NH_2 and -mathrmC≡N identifies every group present.

🧑‍🏫 Teacher's explanation

Think of an organic molecule like a Swiss Army knife. Each tool on the knife—the blade, the scissors, the screwdriver—is a functional group. Even though they are all part of one handle, each tool performs a specific, different job. If you want to know what the knife can do, you don't look at the handle; you look at the tools.

📖 Definitions

🔍 Worked example — Easy

Identify all functional groups present in the molecule CH_3CH(OH)CH_2CHO.

🔍 Worked example — Standard

A molecule has the formula CH_3COOCH_2CH_3. Identify the functional group and predict its primary reaction type.

🔍 Worked example — Exam-style

A student is given a molecule with the following features: a C≡ N group, a C=C double bond, and a -Cl atom. Predict the products if the molecule is reacted with H_2 (catalytic Ni) followed by HCl.

⚠️ Common errors

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🎯 Exam question

(a) Draw a possible structural formula for this compound that contains both an aldehyde group and an alcohol group. [1] (b) Predict the type of reaction that would occur if this compound is reacted with H_2O in the presence of a dilute acid catalyst. [1] [2 marks]

Key ideas — Predicting reactions from functional groups

Once the functional groups are identified, the reactions available to the molecule follow directly: a mathrmC=C bond undergoes electrophilic addition; a -OH group can be oxidised (if primary or secondary), substituted or dehydrated; a -COOH group reacts with metals, alkalis and carbonates and can be esterified; an -NH_2 group acts as a base and as a nucleophile. A molecule with several groups can undergo several of these reactions, and a large excess of one reagent can drive reaction at more than one group at once.

🧑‍🏫 Teacher's explanation

Think of an organic molecule like a Swiss Army knife. The carbon skeleton is the handle, providing the structure, but the functional groups are the specific tools—the blade, the scissors, or the screwdriver—that actually do the work. If you see a 'blade' (a C=C double bond), you know it's designed to cut through things (undergo addition). If you see a 'screwdriver' (an -OH group), you know it can be used to turn or change the molecule.

📖 Definitions

🔍 Worked example — Easy

Identify the functional groups in but-3-en-2-ol and predict the reaction type when treated with K_2 extCr_2 extO_7/H^+.

🔍 Worked example — Standard

A molecule has the formula CH_3 extCH_2 extCOOH. Predict the products when it reacts with CH_3 extOH in the presence of H_2 extSO_4.

🔍 Worked example — Exam-style

Predict the products of the reaction between prop-1-ene (CH_3 extCH=CH_2) and HBr.

⚠️ Common errors

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🎯 Exam question

Key ideas — Devising a synthetic route

A multi-step route is built by identifying the functional group of the starting material and of the target, then selecting the shortest sequence of syllabus reactions that converts one into the other, choosing reagents and conditions that avoid unwanted side reactions. Converting ethene into ethanoic acid, for example, requires addition of steam (H_3PO_4 catalyst) to give ethanol, then oxidation under reflux with excess acidified potassium dichromate(VI) to give ethanoic acid.

🧑‍🏫 Teacher's explanation

Think of organic synthesis like a recipe in a professional kitchen. If you want to make a complex cake (your target molecule) but you only have flour and eggs (your starting materials), you cannot just throw them in a bowl. You need a series of intermediate steps—making a batter, then baking, then frosting—where each step transforms the substance into something slightly different until you reach the final goal.

📖 Definitions

🔍 Worked example — Easy

Identify the functional groups in the molecule CH_3CH_2OH and predict its oxidation product using acidified K_2Cr_2O_7.

🔍 Worked example — Standard

A student wants to convert ethene (C_2H_4) into ethanol (C_2H_5OH). Write the reagents and conditions required.

🔍 Worked example — Exam-style

Calculate the number of moles in 46.0 g of ethanol (C_2H_5OH). (M_r of C=12, H=1, O=16)

⚠️ Common errors

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🎯 Exam question

(a) Propose a two-step synthetic route, including reagents and conditions for each step. [3] (b) Identify the functional group present in the intermediate produced in step 1. [1] [4 marks]

Key ideas — Analysing reaction type and reagents at each step

Every step in a route can be classified by reaction type, such as addition, substitution, oxidation, reduction, elimination or condensation, and matched to the specific reagents and conditions it needs, since marks are awarded separately for the correct reagent, the correct condition and the correct reaction type. Converting a halogenoalkane to an alcohol, for instance, requires aqueous sodium hydroxide (reagent) heated under reflux (condition), an example of nucleophilic substitution (reaction type).

🧑‍🏫 Teacher's explanation

Think of organic synthesis like a complex LEGO instruction manual. Each step is a specific instruction to change one part of the structure—like swapping a blue brick for a red one. To build the final model, you can't just say 'change the brick'; you have to specify exactly which piece you are using (the reagent), how much heat or pressure you apply (the conditions), and the specific mechanism of how that piece snaps into place (the reaction type).

📖 Definitions

🔍 Worked example — Easy

Identify the reaction type, reagent, and condition required to convert chloroethane (ceC_2H_5Cl) into ethanol (ceC_2H_5OH).

🔍 Worked example — Standard

A chemist wants to convert propene (ceC_3H_6) into 2-bromopropane (ceC_3H_7Br). State the reaction type and the reagent required.

🔍 Worked example — Exam-style

A synthetic route starts with ethanol (ceCH_3CH_2OH) and aims to produce ethanoic acid (ceCH_3COOH). Describe the reagents and conditions for this two-step process, including the intermediate.

⚠️ Common errors

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🎯 Exam question

Key ideas — By-products and practical considerations

Each step can also produce by-products that must be separated from the desired organic product: nucleophilic substitution of a halogenoalkane with hydroxide gives a halide salt, esterification produces water alongside the ester, and reduction of a carbonyl compound leaves excess reducing agent to remove. Recognising these by-products, and whether a step is reversible and so requires conditions to shift its position of equilibrium, is needed to evaluate the practicality, not just the chemistry, of a route.

🧑‍🏫 Teacher's explanation

Think of organic synthesis like cooking a complex meal. If you are making a sauce (your desired product), you might end up with leftover scraps of vegetables or extra water in the pan (by-products). To get a pure sauce, you have to find ways to filter out those scraps or boil off the extra water. In chemistry, we don't just care about making the right molecule; we care about how easy it is to get it out of the mixture.

All chapters in Organic synthesis

Organic synthesis