In simple terms
A friendly intro before the formal notes — no formulas yet.
Organic Reaction Toolkit
Organic chemistry is like building with molecular LEGO®, where each functional group has its own set of connection rules. Understanding these rules allows you to predict how molecules will react and transform.
Imagine you have different types of connectors for your building blocks. One type (a nucleophile) is attracted to positive plugs, another (an electrophile) seeks out negative sockets. The shape of your block (the functional group) determines which connector you can use and what you can build.
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Nucleophilic substitution: A nucleophile attacks the partially positive carbon in a halogenoalkane, kicking out the halogen. Primary halogenoalkanes prefer a one-step (SN2) process, while tertiary ones favour a two-step (SN1) route via a stable carbocation.
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Electrophilic addition: The electron-rich C=C double bond in an alkene attacks an electrophile (like H⁺). This forms a carbocation intermediate, which is then attacked by a nucleophile. The initial attack follows Markovnikov's rule, forming the most stable carbocation.
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Nucleophilic addition: The partially positive carbon of a carbonyl group (C=O) is attacked by a nucleophile (like CN⁻). The π-bond of the C=O breaks, and a new single bond is formed. Aldehydes are generally more reactive than ketones due to electronic and steric factors.
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Condensation: Two molecules combine to form a larger molecule, with the elimination of a small molecule like water or HCl. This is key for forming esters (from a carboxylic acid and alcohol) and amides (from a carboxylic acid and amine).
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Full topic notes
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1. Reactions of Halogenoalkanes: Nucleophilic Substitution
Halogenoalkanes are characterised by the polar C-X bond (where X is a halogen). The carbon atom is electron-deficient () and is susceptible to attack by nucleophiles. This leads to a substitution reaction where the halogen atom, the 'leaving group', is replaced by the nucleophile.
Reagents: Aqueous sodium hydroxide (for OH⁻), potassium cyanide in ethanol (for CN⁻), ammonia in ethanol (for NH₂).
Mechanism: The mechanism depends on the structure of the halogenoalkane.
Primary (1°): Undergo SN2 mechanism. A single step where the nucleophile attacks the carbon at the same time as the C-X bond breaks. The rate depends on the concentration of both the halogenoalkane and the nucleophile: Rate = k[RX][Nu⁻].
Tertiary (3°): Undergo SN1 mechanism. A two-step process. First, the C-X bond breaks heterolytically to form a stable carbocation intermediate (rate-determining step). Then, the nucleophile rapidly attacks the carbocation. The rate depends only on the concentration of the halogenoalkane: Rate = k[RX].
2. Reactions of Alkenes: Electrophilic Addition
The defining feature of an alkene is the C=C double bond. This bond consists of one strong bond and one weaker, exposed bond. The high electron density of the bond makes alkenes susceptible to attack by electrophiles, leading to an addition reaction where the double bond is broken.
General Reaction: C=C + E-Nu → E-C-C-Nu
Mechanism: A two-step process. First, the bond attacks an electrophile (E⁺), forming a C-E bond and a carbocation intermediate. Second, a nucleophile (Nu⁻) attacks the carbocation.
Markovnikov's Rule: For unsymmetrical alkenes, the addition of H-X proceeds via the most stable carbocation intermediate. This means the H adds to the carbon with more hydrogens already attached.
Carbocation Stability: Tertiary (3°) > Secondary (2°) > Primary (1°). Alkyl groups are electron-donating and help to stabilise the positive charge.
Common Reagents: HBr (electrophilic addition of HBr), Br₂ (electrophilic addition of bromine, used as a test for unsaturation), H₂O/H⁺ (hydration to form an alcohol), H₂ (hydrogenation to form an alkane, a reduction reaction not addition).
3. Reactions of Carbonyls: Nucleophilic Addition
Aldehydes and ketones contain the carbonyl functional group, C=O. The large difference in electronegativity between carbon and oxygen creates a polar bond, with a carbon and a oxygen. The electron-deficient carbon is attacked by nucleophiles, leading to an addition reaction across the C=O double bond.
Reaction: Addition of HCN to form a hydroxynitrile (cyanohydrin).
Reagents: Potassium cyanide (KCN) followed by dilute acid, or a mixture of KCN and HCN. Using KCN provides a source of the CN⁻ nucleophile, but HCN is a weak acid and is required to protonate the intermediate.
Mechanism: Step 1: The CN⁻ nucleophile attacks the carbonyl carbon, and the bond of the C=O breaks, with electrons moving to the oxygen. Step 2: The resulting negative intermediate (an alkoxide) is protonated by an H⁺ source (e.g., from HCN or water) to form the hydroxynitrile product.
Importance: This reaction is useful in synthesis as it increases the carbon chain length by one carbon atom.
4. Condensation and Hydrolysis
Condensation reactions are fundamental to building larger, more complex molecules like esters, amides, and polymers. In these reactions, two functional groups react to form a new linkage, eliminating a small, stable molecule like water. Hydrolysis is the reverse process, where water is used to break these linkages, often catalysed by acid or alkali.
Esterification: Carboxylic acid + Alcohol ⇌ Ester + Water. This is a reversible reaction, catalysed by concentrated sulfuric acid. The acid acts as both a catalyst and a dehydrating agent, shifting the equilibrium to the right.
Amide Formation: Carboxylic acid + Amine → Amide + Water. This reaction is typically slow. A more vigorous method is to use an acyl chloride instead of a carboxylic acid, which reacts readily with an amine to form an amide and HCl.
Hydrolysis of Esters: Can be acid-catalysed (reversible, gives carboxylic acid and alcohol) or base-catalysed (saponification, irreversible, gives carboxylate salt and alcohol).
Hydrolysis of Amides: Requires more vigorous conditions than esters (e.g., boiling with strong acid or alkali) to break the strong C-N bond.
For mechanism questions, always include all relevant dipoles (e.g., C-Cl), charges, and curly arrows. A curly arrow must start from a lone pair or a bond and point to the atom it is forming a bond with. Examiners are very strict about the origin and destination of curly arrows.
Worked examples
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2-chloro-2-methylpropane is heated under reflux with aqueous sodium hydroxide. Name the organic product and the type of mechanism. Draw the mechanism for this reaction.
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1. Identify the reactants and structure: Reactant is 2-chloro-2-methylpropane, which is a tertiary halogenoalkane. The nucleophile is the hydroxide ion, OH⁻, from NaOH(aq).
Propene reacts with hydrogen bromide. Draw the mechanism for the major product formed and name this product.
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1. Identify reactants and reaction type: Propene (CH₃CH=CH₂) is an unsymmetrical alkene. Hydrogen bromide (HBr) is a hydrogen halide. The reaction is electrophilic addition.
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Glossary
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What is a nucleophile?
An electron pair donor. It is a species with a lone pair of electrons and/or a negative charge, attracted to electron-deficient regions (e.g., a carbon atom). Examples: OH⁻, CN⁻, NH₃, H₂O.
Key takeaways
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Reagents: Aqueous sodium hydroxide (for OH⁻), potassium cyanide in ethanol (for CN⁻), ammonia in ethanol (for NH₂).
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Mechanism: The mechanism depends on the structure of the halogenoalkane.
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Primary (1°): Undergo SN2 mechanism. A single step where the nucleophile attacks the carbon at the same time as the C-X bond breaks. The rate depends on the concentration of both the halogenoalkane and the nucleophile: Rate = k[RX][Nu⁻].
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Tertiary (3°): Undergo SN1 mechanism. A two-step process. First, the C-X bond breaks heterolytically to form a stable carbocation intermediate (rate-determining step). Then, the nucleophile rapidly attacks the carbocation. The rate depends only on the concentration of the halogenoalkane: Rate = k[RX].
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Test Your Knowledge on Organic Reactions
Test Your Knowledge on Organic Reactions
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