4.1.8Unit 4. Rates, Equilibria and Further Organic Chemistry
Reaction Mechanisms — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 17 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 6Nucleophilic Substitution
The mechanism that operates depends on the structure of the halogenoalkane.
Reaction
- Reaction A nucleophile such as OH⁻ replaces the halogen in a halogenoalkane, as in CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
Curly arrows
- Curly arrows A curly arrow shows the movement of a pair of electrons, starting from a lone pair or bond and finishing at an atom or bond.
SN2
- SN2 This is a one-step mechanism in which the nucleophile attacks as the leaving group departs.
SN1
- SN1 This is a two-step mechanism: the halide leaves first to give a carbocation, and then the nucleophile attacks it.
Section 2 of 6SN1 and SN2 Compared
SN1 and SN2 Compared
Primary halogenoalkane
- Primary halogenoalkane These usually react by SN2, with rate = k[halogenoalkane][OH⁻], because the carbon is open to attack.
Tertiary halogenoalkane
- Tertiary halogenoalkane These react by SN1, with rate = k[halogenoalkane], since the tertiary carbocation is stabilised by alkyl groups.
Rate-determining step
- Rate-determining step In SN1 it is the loss of the halide ion, but in SN2 it is the single step involving both reactants.
Stereochemistry
- Stereochemistry SN2 inverts the configuration at a chiral centre,
- while SN1 gives a mixture because the planar carbocation can be attacked from either side.
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two | One |
| Rate equation | k[RX] | k[RX][Nu] |
| Intermediate | Carbocation | None, transition state only |
| Favoured by | Tertiary RX | Primary RX |
Section 3 of 6Writing a Mechanism
1
Draw the halogenoalkane and show the polar C–X bond with a partial positive charge on carbon.
2
For SN2 draw an arrow from the lone pair on the nucleophile to the carbon, and an arrow from the C–X bond to the halogen.
3
For SN1 draw an arrow from the C–X bond to the halogen first, giving a carbocation and a halide ion.
4
In SN1 add a second arrow from the nucleophile's lone pair to the positive carbon.
5
Show the charges on the ions and make sure the product and the leaving group are correct.
Section 4 of 6Energy Profiles
Transition state
- Transition state A maximum on the energy profile that cannot be isolated and has partial bonds, shown in square brackets with a dagger.
Intermediate
- Intermediate A minimum between two peaks, which can sometimes be detected, and is a real species with full bonds.
SN2 profile
- SN2 profile One peak with no dip, since there is one step and no intermediate.
SN1 profile
- SN1 profile Two peaks with a dip for the carbocation between them, where the higher peak is the slow step.
Section 5 of 6Worked Example
Question
- Rate data show that the rate for 2-bromo-2-methylpropane with aqueous hydroxide is k[(CH₃)₃CBr] and for 1-bromobutane is k[C₄H₉Br][OH⁻].
- Name the mechanism for each.
- The tertiary halogenoalkane has a rate equation that includes only the halogenoalkane, so the slow step involves one species: SN1.
- The primary halogenoalkane has a rate equation that includes both reactants, so the single step involves both: SN2.
- In SN1 the first step is (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻.
Answer
- 2-bromo-2-methylpropane: SN1;
- 1-bromobutane: SN2
Section 6 of 6Exam Checklist & Tips
4.1.8 – Key Points for the Exam
- Nucleophile A species that donates a pair of electrons to form a new covalent bond.
- Carbocation An ion with a positively charged carbon atom having only three bonds.
- Transition state The highest-energy arrangement of atoms in a step, which cannot be isolated.
- Intermediate A species formed in one step and used up in another, found at an energy minimum.
- SN2 Nucleophilic substitution in one step with both species in the rate-determining step.
Common Mistakes
- Drawing a curly arrow that starts at an atom rather than at a lone pair or bond.
- Calling a transition state an intermediate.
- Saying that SN1 is favoured for primary halogenoalkanes.
- Leaving out the charge on the carbocation or the halide ion.
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