Ch 2 · Characteristic organic reactions · AS & A Level Chemistry 9701
Topic 2 of 4 in An introduction to AS Level organic chemistry — AS & A Level Chemistry 9701 — 67 lessons.
Introduction
Every organic mechanism you meet at AS and A Level is built from a small set of reusable ideas: how a bond can break, what kind of species attacks a molecule, and what overall change occurs. Learning this vocabulary precisely now means that later mechanisms for alkanes, alkenes and halogenoalkanes can be described in exact examiner-approved language rather than vague description.
Key ideas — Saturated and unsaturated compounds
A compound is saturated if it contains only single covalent bonds between carbon atoms, as in alkanes. A compound is unsaturated if it contains at least one carbon-to-carbon multiple bond (C=C or C ≡ C), as in alkenes. Unsaturation is detected by the decolourisation of aqueous bromine, since the exposed π electrons of the double bond react readily with electrophiles.
🧑🏫 Teacher's explanation
Think of a saturated molecule like a fully packed suitcase where every compartment is filled; there is no room to add anything new without removing something else first. In organic chemistry, 'saturated' means the carbon atoms are bonded to the maximum possible number of hydrogen atoms using only single bonds.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass (M_r) of ethene, C_2H_4. (Given A_r: C=12, H=1)
🔍 Worked example — Standard
A sample of 11.2 g of propene (C_3H_6) reacts completely with bromine (Br_2). Calculate the moles of propene used. (M_r of C_3H_6 = 42)
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🎯 Exam question
(a) Classify compound X as saturated or unsaturated. [1] (b) Write the balanced chemical equation for the reaction between ethene (C_2H_4) and bromine (Br_2) to form 1,2-dibromoethane. [2] [3 marks]
Key ideas — Bond fission: homolytic and heterolytic
A covalent bond can break in two ways:
🧑🏫 Teacher's explanation
Imagine a covalent bond as a pair of friends holding hands. In homolytic fission, they decide to let go and walk away in opposite directions, each taking one hand. They are now 'lonely' and very reactive because they have an unpaired hand (an electron) looking for a partner. These are free radicals.
📖 Definitions
🔍 Worked example — Easy
Identify the type of fission occurring in the reaction: ceCl-Cl xrightarrowUV 2ceCl^bullet
🔍 Worked example — Standard
A molecule ceR-X undergoes fission to produce ceR^+ and ceX^-. What type of fission has occurred and what are the products called?
🔍 Worked example — Exam-style
In a free-radical substitution reaction of methane (ceCH_4) with chlorine (ceCl_2), a radical ce· CH_3 reacts with ceCl_2 to produce ceCH_3Cl and ceCl^bullet. Identify this stage of the mechanism.
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🎯 Exam question
(a) Name this stage of the free-radical mechanism. [1] (b) Explain why the reaction in part (a) is considered a propagation step. [2] [3 marks]
Key ideas — Nucleophiles and electrophiles
A nucleophile is an electron pair donor: a species with a lone pair (or occasionally a π bond) that is attracted to a region of positive or partial positive charge, e.g. OH^-, CN^-, NH_3. A electrophile is an electron pair acceptor: a species that is attracted to a region of negative charge or high electron density and can accept a lone pair, e.g. H^+, Br^+ (from polarised Br_2), or a carbocation. The adjectives nucleophilic and electrophilic describe reactions or reagents that behave in these ways, for example nucleophilic substitution and electrophilic addition.
🧑🏫 Teacher's explanation
Think of organic reactions as a game of 'hot and cold' played with electrons. In chemistry, electrons are like a crowd of people; they naturally want to move from where there are many people (high electron density) to where there are few people (low electron density).
📖 Definitions
🔍 Worked example — Easy
Identify the nucleophile and the electrophile in the reaction between ammonia and hydrogen chloride: NH_3 + extHCl arrow NH_4 extCl
🔍 Worked example — Standard
In the reaction C_2H_4 + Br_2 arrow C_2H_4Br_2, explain why Br_2 acts as an electrophile.
🔍 Worked example — Exam-style
A reaction involves a species X^- attacking a carbocation R^+. Classify the reaction type and identify the roles of X^- and R^+.
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🎯 Exam question
Key ideas — Classes of organic reaction
🧑🏫 Teacher's explanation
Organic chemistry is essentially the study of how molecular 'Lego' sets can be rearranged. Instead of just breaking things apart, we are looking at how specific pieces are swapped, added, or removed to create new structures. Think of it like a dance floor: in an 'addition' reaction, two people join hands to form one pair; in a 'substitution' reaction, one person leaves the dance floor so their partner can be replaced by someone else; and in 'elimination', two people might step away from the group, leaving a gap (a double bond) behind.
📖 Definitions
🔍 Worked example — Easy
Identify the type of reaction for the following: C_2H_4 + Br_2 arrow C_2H_4Br_2
🔍 Worked example — Standard
Predict the product of the substitution reaction: CH_4 + Cl_2 arrow ? + HCl
🔍 Worked example — Exam-style
A halogenoalkane reacts with aqueous NaOH to form an alcohol and NaBr. Identify the reaction type and write the balanced equation for CH_3CH_2Cl + NaOH arrow ? (assume aqueous conditions).
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🎯 Exam question
(a) Identify the type of reaction that occurs when X reacts with Br_2. [1] (b) Write a balanced equation for the reaction between X and Br_2 to show the product formed. [2] [3 marks]
Key ideas — Oxidation and reduction: [O] and [H]
Because organic redox reactions rarely involve simple electron transfer, Cambridge equations represent oxidation using the symbol [O] to denote oxygen supplied by an oxidising agent, and reduction using [H] to denote hydrogen supplied by a reducing agent. For example, the oxidation of ethanol to ethanal is written CH_3CH_2OH + [O] arrow CH_3CHO + H_2O, and the reduction of an aldehyde to a primary alcohol is written CH_3CHO + 2[H] arrow CH_3CH_2OH.
🧑🏫 Teacher's explanation
In inorganic chemistry, we often think of redox as the movement of electrons between ions. However, in organic chemistry, molecules are large and complex. Instead of tracking every single electron, we focus on the 'functional groups'—the parts of the molecule that actually react.
📖 Definitions
🔍 Worked example — Easy
Write the balanced equation for the oxidation of methanol (CH_3 extOH) to methanal (CH_2 extO) using the symbol [O].
🔍 Worked example — Standard
A sample of propan-1-ol (CH_3CH_2CH_2OH) is reacted with an oxidizing agent. It is converted into propanoic acid (CH_3CH_2COOH). Write the equation using [O].
🔍 Worked example — Exam-style
Complete the reduction of ethene (C_2H_4) to ethane (C_2H_6) using the symbol [H].
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🎯 Exam question
Key ideas — The four core mechanism types
The mechanisms studied at AS Level fall into four categories, distinguished by the species that attacks and the overall change: free-radical substitution (alkanes with halogens in UV light), electrophilic addition (alkenes with H_2, HX, X_2 or steam), nucleophilic substitution (halogenoalkanes with OH^-, CN^- or NH_3), and nucleophilic addition (carbonyl compounds attacked by, for example, CN^- or NaBH_4 acting as a source of H^-).
🧑🏫 Teacher's explanation
Organic chemistry is essentially the study of how molecules break apart and reform. Instead of looking at whole molecules as solid blocks, we look at the 'movement' of electrons. Think of a reaction like a dance floor: sometimes a dancer leaves a partner to join another group (substitution), sometimes two dancers join together to form a new couple (addition), and sometimes a couple breaks up to create two single dancers (elimination).
📖 Definitions
🔍 Worked example — Easy
Identify the type of reaction occurring when propene reacts with HBr to form 2-bromopropane.
🔍 Worked example — Standard
A reaction involves the attack of CN^- on CH_3CH_2Cl to produce CH_3CH_2CN and Cl^-. Classify this mechanism.
🔍 Worked example — Exam-style
In the free-radical substitution of methane with chlorine, the first step (initiation) involves the UV light breaking Cl-Cl. If the concentration of Cl· radicals is 2.0 × 10^-4 mol dm^-3, calculate the concentration of Cl_2 required to produce this amount if the initiation rate is proportional to [Cl_2]. (Assume a simplified rate law Rate = k[Cl_2] where k = 0.05 s^-1)
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🎯 Exam question
(a) State the type of reaction mechanism involved. [1] (b) Explain why the C-Cl bond is susceptible to attack by OH^-. [2] [3 marks]
Worked example
Question. Chlorine reacts with methane in the presence of ultraviolet light to form chloromethane and hydrogen chloride.
🔍 Worked example — Easy
A chlorine molecule (Cl_2) undergoes homolytic fission under UV light. Calculate the total number of electrons shared in the bond before fission and how many electrons each chlorine atom possesses after fission.
🔍 Worked example — Standard
In the reaction CH_4 + Cl_2 xrightarrowUV CH_3Cl + HCl, identify the type of reaction and the role of the Cl· radical in the first propagation step (CH_4 + Cl· arrow · CH_3 + HCl).
🔍 Worked example — Exam-style
Compare the electron movement in heterolytic fission of H-Cl versus homolytic fission of Cl-Cl. Which process results in ions?
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