Organic Reaction Mechanisms
Organic reaction mechanisms trace the step-by-step movement of electron pairs as covalent bonds break and form during a chemical reaction. At A-Level Chemistry, you need to draw curly-arrow mechanisms accurately for nucleophilic substitution, nucleophilic addition, electrophilic addition, electrophilic substitution, and elimination reactions.
- Electrophile
- An electron pair acceptor, often positively charged or possessing a partial positive charge.
- Carbocation
- An organic ion containing a positively charged carbon atom.
- Nucleophilic Substitution
- A reaction where a nucleophile attacks a polar molecule and replaces a leaving group (e.g., haloalkanes with hydroxide ions).
Curly-Arrow Organic Mechanisms (Nucleophilic Substitution & Addition)
3 Golden Rules for Curly Arrows:
1. Start at electrons: Every curly arrow MUST start at a lone pair of electrons or the middle of a covalent bond (never start an arrow on a δ+ or positive charge).
2. Show dipoles: Always label δ+ and δ− on polar bonds like C–Br or H–Br.
3. Balance charges: If a neutral molecule loses a leaving ion (:Br⁻), the intermediate carbocation must carry a formal + charge.
Key Concepts & Exam Definitions (5)
- Electrophile
- An electron pair acceptor, often positively charged or possessing a partial positive charge.
- Carbocation
- An organic ion containing a positively charged carbon atom.
- Nucleophilic Substitution
- A reaction where a nucleophile attacks a polar molecule and replaces a leaving group (e.g., haloalkanes with hydroxide ions).
- Electrophilic Addition
- A reaction where an electrophile attacks the electron-rich double bond of an alkene, adding to the molecule.
- Elimination
- A reaction where atoms are removed from a molecule to form a double bond (e.g., converting a haloalkane to an alkene).
Quick-Fire Flashcards (5)
Click any card to check the answer, then mark it as Known to track your progress.
What are the reagents and conditions for nucleophilic substitution of a haloalkane to an alcohol?
What intermediate is formed during electrophilic addition to an alkene?
Which carbocation is the most stable: primary, secondary, or tertiary?
What condition dictates whether OH- acts as a nucleophile or a base with a haloalkane?
What type of mechanism is the nitration of benzene?
Exam-Style Questions & Mark Schemes (3)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Outline the mechanism for the reaction between bromoethane and cyanide ions (CN-).
Click “Check Mark Scheme” to compare your answer, This is nucleophilic substitution. A curly arrow goes from the lone pair on the carbon of the CN- ion to the partially positive carbon bonded to bromine in bromoethane. Simultaneously, a curly arrow goes from the C-Br bond to the bromine atom. This forms propanenitrile and a bromide ion.
- Q2
Explain why the reaction of propene with HBr produces 2-bromopropane as the major product and 1-bromopropane as the minor product.
Click “Check Mark Scheme” to compare your answer, The H+ from HBr adds to propene to form a carbocation intermediate. Adding H to carbon-1 forms a secondary carbocation. Adding H to carbon-2 forms a primary carbocation. The secondary carbocation is more stable due to the positive inductive effect of two alkyl groups (compared to one). Thus, the secondary carbocation forms more frequently, leading to 2-bromopropane as the major product.
- Q3
Describe the generation of the electrophile in the nitration of benzene.
Click “Check Mark Scheme” to compare your answer, Concentrated nitric acid and concentrated sulfuric acid are reacted together. Sulfuric acid is a stronger acid and donates a proton to nitric acid. HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+. The NO2+ (nitronium ion) is the electrophile.
Common Exam Questions (3)
What does a curly arrow represent in a reaction mechanism?
A curly arrow represents the movement of a pair of electrons, starting from either a lone pair or a covalent bond and pointing to the atom or position where a new bond forms.
What is a nucleophile?
A nucleophile is an electron-pair donor. It possesses a lone pair of electrons (and often a negative or partial negative charge) that is attracted to an electron-deficient (delta-positive) carbon atom.
What is Markovnikov's rule?
When a hydrogen halide (HX) adds across an unsymmetrical alkene, the hydrogen atom bonds to the carbon atom that already carries more hydrogen atoms, because the reaction proceeds via the more stable secondary or tertiary carbocation intermediate.
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