📚 A-Level Chemistry: SN1 Mechanism from Jun 18 Paper 2 | A-Level化学：2018年6月试卷2 SN1反应机理
In the June 2018 A-Level Chemistry Paper 2, one question tested students' ability to deduce and draw the mechanism for the hydrolysis of a secondary haloalkane using kinetic data. This article unpacks the SN1 mechanism step by step, linking it directly to the exam question and covering everything from rate equations to stereochemical outcomes.
在2018年6月的A-Level化学试卷2中,有一道题通过动力学数据考查了学生推导并绘制仲卤代烷水解机理的能力。本文将逐步拆解SN1机理,紧扣真题情景,涵盖从速率方程到立体化学结果的所有关键内容。
1. Exam Question Context | 考试题目背景
for two bromoalkanes. For 1-bromobutane, the rate equation was rate = k[1-bromobutane][OH- h no dependence on hydroxide ion concentration. Students were asked to deduce the mechanisms for both reactions and to draw the mechanism of 2-bromobutane, including all curly arrows and relevant species.
2018年6月试卷2的第5题向学生提供了两种溴代烷的水解数据。对于1-溴丁烷,速率方程为 rate = k[1-溴丁烷][OH-]。对于2-溴丁烷,速率方程为 rate = k[2-溴丁烷],即速率与氢氧根离子浓度无关。题目要求学生推断两个反应的机理,并画出2-溴丁烷的机理,包括所有的曲线箭头和相关物种。
2. Kinetic Data and Mechanism Deduction | 动力学数据与机理推导
The second-order kinetics observed for 1-bromobutane indicate that both the haloalkane and the nucleophile are involved in the rate-determining step. This is characteristic of an SN2 mechanism, where bond making and bond breaking occur simultaneously in a single step.
1-溴丁烷表现出的二级动力学表明,卤代烷和亲核试剂都参与了决速步。这是SN2机理的特征,即断键和成键在一步中协同发生。
In contrast, the first-order kinetics for 2-bromobutane reveal that only the haloalkane appears in the rate equation. The rate-determining step must involve only the haloalkane, which strongly suggests a unimolecular process – the SN1 mechanism, where the leaving group departs before the nucleophile attacks.
相反,2-溴丁烷的一级动力学表明,速率方程中只出现卤代烷的浓度。这意味着决速步仅涉及卤代烷分子,强烈指向单分子过程——SN1机理,即离去基团在亲核试剂进攻之前先行离去。
3. Introduction to the SN1 Mechanism | SN1机理简介
SN1 stands for Substitution Nucleophilic Unimolecular. The mechanism proceeds in two distinct steps: first, the carbon–halogen bond breaks heterolytically to form a planar carbocation intermediate; second, the nucleophile attacks the electron-deficient carbocation to form the substitution product.
SN1代表单分子亲核取代。该机理分为两个独立的步骤:第一步,C–X键异裂生成平面型碳正离子中间体;第二步,亲核试剂进攻缺电子的碳正离子,形成取代产物。
For 2-bromobutane, the reaction is: CH3CH2CHBrCH3 + OH- → CH3CH2CH(OH)CH3 + Br-. The slow step is the departure of the bromide ion, which determines the overall rate.
对于2-溴丁烷,反应为:CH3CH2CHBrCH3 + OH- → CH3CH2CH(OH)CH3 + Br-。慢速步骤是溴离子的离去,该步决定了总反应速率。
4. Step 1: Formation of the Carbocation | 第一步:碳正离子的形成
The first step is the rate-determining step. The C–Br bond breaks heterolytically, with both bonding electrons moving onto the bromine atom. A curly arrow is drawn from the middle of the C–Br bond towards the Br atom. This produces a bromide ion and a planar, sp2-hybridised secondary carbocation: (CH3CH2)(CH3)CH+.
第一步是决速步。C–Br键发生异裂,两个成键电子都移向溴原子。曲线箭头从C–Br键的中部指向溴原子。这一步生成了一个溴离子和一个平面型的sp2杂化二级碳正离子:(CH3CH2)(CH3)CH+。
(CH3CH2)(CH3)CHBr ⟶ (CH3CH2)(CH3)CH+ + :Br–
The activation energy for this step is relatively high because a secondary carbocation is less stable than a tertiary one, but the polar protic solvent (water/alcohol) helps stabilise both the carbocation and the leaving bromide ion through solvation.
该步骤的活化能相对较高,因为二级碳正离子的稳定性不如三级碳正离子,但极性质子溶剂(水/醇)通过溶剂化作用帮助稳定了碳正离子和离去的溴离子。
5. Step 2: Nucleophilic Attack on the Carbocation | 第二步:亲核试剂进攻碳正离子
In the second fast step, the hydroxide ion acts as a nucleophile. A curly arrow is drawn from a lone pair on the oxygen of OH- towards the electron-deficient carbon of the carbocation. The carbon–oxygen bond forms, yielding butan-2-ol.
在第二步快反应中,氢氧根离子作为亲核试剂。曲线箭头从OH-中氧原子的一对孤对电子出发,指向碳正离子中缺电子的碳原子。碳–氧键形成,生成丁-2-醇。
(CH3CH2)(CH3)CH+ + –OH ⟶ CH3CH2CH(OH)CH3
Because the carbocation is planar, the hydroxide ion can attack from either face with equal probability. This has profound stereochemical consequences, as the product is formed as a racemic mixture if the starting haloalkane is chiral at the reacting carbon.
由于碳正离子是平面型的,氢氧根离子可以从平面的任一侧以相等概率进攻。这会产生深刻的立体化学后果:如果起始卤代烷在反应碳上具有手性,产物将以外消旋混合物形式存在。
6. Energy Profile of the SN1 Reaction | SN1反应的能量曲线
The energy profile diagram for an SN1 reaction features two distinct humps. The first hump, corresponding to the carbocation formation, is significantly higher and represents the rate-determining step. After this peak, the energy drops to a local minimum where the unstable carbocation intermediate sits.
SN1反应的能量曲线图有两个明显的峰。第一个峰对应于碳正离子的形成,明显更高,代表决速步。经过这个峰后,能量下降到一个局部最低点,不稳定的碳正离子中间体就位于此处。
A second, much smaller hump represents the activation energy for the attack of the nucleophile on the carbocation. The overall reaction is exothermic, with the products lying at a lower energy than the reactants. The intermediate, though at a local minimum, is higher in energy than both reactants and products.
第二个小得多的峰代表亲核试剂进攻碳正离子的活化能。总反应是放热的,产物能量低于反应物。中间体处于局部能量最低,但其能量仍高于反应物和产物。
7. Stereochemistry: Racemisation in SN1 | 立体化学:SN1中的外消旋化
In the exam question, 2-bromobutane has a chiral centre at C-2 (attached to H, CH3, CH2CH3, and Br). The departure of the bromide ion generates an achiral, planar carbocation. The incoming nucleophile can then bond to either lobe of the empty p orbital.
在试题中,2-溴丁烷的C-2是一个手性中心(连接着H、CH3、CH2CH3和Br)。溴离子离去后生成一个非手性的平面碳正离子。随后靠近的亲核试剂可以进攻空p轨道的任意一叶。
This results in the formation of equal amounts of the two enantiomers of butan-2-ol – a racemic mixture. The product is optically inactive, even though the starting material was a single enantiomer. This loss of optical purity is a hallmark of the SN1 mechanism at a chiral centre.
这导致生成等量的丁-2-醇两种对映异构体
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