Ch 2 · Characteristic organic reactions · AS & A Level Chemistry 9701
Topic 2 of 4 in An introduction to A Level organic chemistry — AS & A Level Chemistry 9701 — 57 lessons.
Introduction
AS Level established three mechanism types: free-radical substitution, electrophilic addition and nucleophilic substitution. A2 organic chemistry adds two more, electrophilic substitution and addition-elimination, each characteristic of a specific class of compound because of the electronic structure of its functional group. Recognising which mechanism a functional group favours, and why, is essential before the detailed mechanisms of arenes and acyl chlorides are met later in this module.
Key ideas — Electrophiles, nucleophiles and mechanism families
An electrophile is an electron-pair acceptor, typically a species with a positive or partial positive charge, such as Br^+ or NO_2^+. A nucleophile is an electron-pair donor, such as OH^- or NH_3. Organic mechanisms are classified by combining the type of attacking species (electrophilic, nucleophilic or free-radical) with what happens to the substrate (substitution, addition, or, at A2, the two-step addition-elimination). Every mechanism is drawn using curly arrows, each representing the movement of one pair of electrons, starting from a lone pair or a bond and ending at the site that gains the new bond.
🧑🏫 Teacher's explanation
Think of organic reactions as a dance between two characters: the 'giver' and the 'taker'. A nucleophile is like a generous person with a surplus of cash (electrons) looking to spend it, while an electrophile is like a person with an empty wallet (a positive charge or empty orbital) looking to receive.
📖 Definitions
🔍 Worked example — Easy
Identify the nucleophile and the electrophile in the reaction between NH_3 and H^+.
🔍 Worked example — Standard
Calculate the molar mass (M_r) of CH_3 extCl where C=12, H=1, Cl=35.5.
🔍 Worked example — Exam-style
Explain why benzene undergoes electrophilic substitution rather than electrophilic addition when reacting with Br_2 in the presence of FeBr_3.
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🎯 Exam question
Key ideas — Electrophilic substitution
In electrophilic substitution, an electrophile attacks an electron-rich system and one atom of the substrate is subsequently replaced by the electrophile, with a hydrogen ion released as the leaving group. The defining feature is that the substrate's original bonding framework is regenerated once the mechanism is complete: an electrophile adds to the delocalised π system of an arene, forming a non-aromatic intermediate, and then a proton is lost from the same carbon to restore full delocalisation across the ring. This mechanism is characteristic of arenes, where the electron-rich delocalised ring attracts electrophiles but where addition would destroy the very stability that made the ring attractive in the first place.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where everyone is moving in a synchronized, rhythmic pattern (this is your delocalised π electron system). An electrophile is like an energetic newcomer trying to crash the party. In an alkene, the newcomer just pushes two people aside to join the circle (addition). But in an arene, the dancers are so stable in their pattern that they won't just let someone join; instead, they kick someone else out to make room for the newcomer so the rhythm can continue. This 'kick out' is the substitution part.
📖 Definitions
🔍 Worked example — Easy
Identify the electrophile in the nitration of benzene (C_6H_6 + HNO_3 arrow C_6H_5NO_2 + H_2O) and state its charge.
🔍 Worked example — Standard
Explain why benzene undergoes electrophilic substitution rather than electrophilic addition.
🔍 Worked example — Exam-style
In the reaction of benzene with chlorine (C_6H_6 + Cl_2 arrow C_6H_5Cl + HCl), calculate the molar ratio of benzene to hydrogen chloride produced.
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🎯 Exam question
Key ideas — Addition-elimination
In addition-elimination, a nucleophile first adds to an electron-deficient carbon, forming a tetrahedral intermediate in which the original double bond is temporarily broken; a leaving group is then eliminated from that same carbon, reforming a double bond and releasing a small, stable molecule or ion. Unlike electrophilic substitution, this mechanism is driven by nucleophilic attack, and unlike simple nucleophilic substitution at a saturated carbon, it proceeds through a genuine two-stage addition step followed by a separate elimination step, rather than a single concerted or stepwise displacement.
🧑🏫 Teacher's explanation
Imagine a crowded elevator. Two people try to squeeze in at once (the addition step), making the elevator uncomfortably packed and unstable. To restore order and make space, one person must get off (the elimination step). In organic chemistry, this is how certain molecules react when they have a double bond or a highly reactive functional group.
📖 Definitions
🔍 Worked example — Easy
Ethanoic acid chloride reacts with ammonia to form an amide. Write the balanced equation for the reaction between ethanoyl chloride and ammonia: ceCH_3COCl + NH_3 arrow ?
🔍 Worked example — Standard
Calculate the molar mass of the product formed when propanoyl chloride (ceC_2H_5COCl) reacts with water (ceH_2O) via addition-elimination.
🔍 Worked example — Exam-style
A reaction involves the attack of OH^- on CH_3CH_2COCl. The intermediate is CH_3CH_2C(O^-)(Cl)OH. What is the final organic product?
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🎯 Exam question
Key ideas — Why addition-elimination is characteristic of acyl chlorides
The carbonyl carbon of an acyl chloride, RCOCl, carries a strong partial positive charge for two reasons: the electronegative oxygen and chlorine atoms both withdraw electron density inductively, and the carbon is only sp^2 hybridised with an exposed, planar π^* system. This makes the carbonyl carbon highly electrophilic and open to attack from above or below the plane by a nucleophile such as water, an alcohol or ammonia, forming a tetrahedral intermediate. Because chloride is an excellent leaving group and the carbonyl π bond is thermodynamically favourable to reform, this intermediate rapidly collapses, ejecting Cl^- and regenerating a C=O double bond in the product. The combination of a very electrophilic carbon and a very good leaving group makes addition-elimination fast and essentially irreversible for acyl chlorides, unlike the more sluggish or reversible addition-elimination seen with other carbonyl derivatives.
🧑🏫 Teacher's explanation
Imagine a high-security building with a very attractive, glowing VIP room (the carbonyl carbon). This room is highly attractive because it is electron-deficient, like a vacuum sucking in electrons. However, the room is guarded by a very weak latch (the Cl atom). A nucleophile, like a person looking for a place to settle, rushes in to attack this carbon. This creates a temporary, unstable 'crowded' state where the carbon is bonded to too many things at once.
📖 Definitions
🔍 Worked example — Easy
Predict the products of the reaction between ethanoyl chloride and water: CH_3 extCOCl + H_2O arrow ?
🔍 Worked example — Exam-style
Explain why the reaction RCOCl + R'OH arrow RCOOR' + HCl is highly exothermic and irreversible.
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🎯 Exam question
Key ideas — Substitution in arenes compared with addition in alkenes
Alkenes and arenes both possess reactive π electron density, yet they respond to electrophiles in opposite ways. An alkene undergoes electrophilic addition: its isolated π bond is broken entirely, and both carbons form new sigma bonds, because this converts one weaker π bond into two stronger sigma bonds, releasing energy overall. An arene undergoes electrophilic substitution instead: addition across just one of its formal double bonds would remove two carbons from the delocalised ring, destroying the extensive delocalisation that stabilises the whole system and costing far more energy than is recovered by forming new sigma bonds. By substituting a hydrogen for the electrophile rather than adding across the ring, the arene loses only one proton and fully restores its aromatic delocalisation, which is energetically far more favourable.
🧑🏫 Teacher's explanation
Imagine a group of friends holding hands in a circle to form a stable, unbreakable ring. This is like an arene (like benzene). If two people in the circle suddenly decide to grab something else outside the circle, the whole ring breaks, and everyone becomes unstable. This is why benzene doesn't like 'addition' reactions; it wants to keep its ring intact.
📖 Definitions
🔍 Worked example — Easy
Identify the type of reaction occurring when ethene reacts with Br_2 to form 1,2-dibromoethane.
🔍 Worked example — Standard
Explain why benzene reacts with Cl_2 in the presence of AlCl_3 to form chlorobenzene and HCl, rather than benzene hexachloride.
🔍 Worked example — Exam-style
Calculate the mass of benzene (C_6H_6) that reacts with Cl_2 to produce C_6H_5Cl and HCl. Assume A_r values: C=12, H=1, Cl=35.5.
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🎯 Exam question
Worked example
Question. Benzene reacts with bromine in the presence of iron(III) bromide, while ethene reacts readily with bromine alone.
🔍 Worked example — Easy
Identify the mechanism for the reaction: C_6H_6 + Br_2 xrightarrowFeBr_3 C_6H_5Br + HBr.
🔍 Worked example — Standard
Predict the product and mechanism for the reaction: C_2H_4 + Br_2 arrow C_2H_4Br_2.
🔍 Worked example — Exam-style
Explain why benzene is more stable than a hypothetical 'cyclohexatriene' structure.
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Practice
Answers