3.4 Electron-pair sharing reactions (AHL)

Understanding points

Reactivity 3.4.6—A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair

donor. (AHL)

Reactivity 3.4.7—When a Lewis base reacts with a Lewis acid, a coordination bond is formed.

Nucleophiles are Lewis bases and electrophiles are Lewis acids. (AHL)

Reactivity 3.4.8—Coordination bonds are formed when ligands donate an electron pair to

transition element cations, forming complex ions. (AHL)

Reactivity 3.4.9—Nucleophilic substitution reactions include the reactions between

halogenoalkanes and nucleophiles. (AHL)

Reactivity 3.4.10—The rate of the substitution reactions is influenced by the identity of the leaving group. (AHL)

Reactivity 3.4.11—Alkenes readily undergo electrophilic addition reactions. (AHL)

Reactivity 3.4.12—The relative stability of carbocations in the addition reactions between

hydrogen halides and unsymmetrical alkenes can be used to explain the reaction mechanism. (AHL)

Reactivity 3.4.13—Electrophilic substitution reactions include the reactions of benzene with

electrophiles. (AHL)

Lewis acids and bases

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Lewis acid (=Electrophile)

Lewis base (=Nucleophile)

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Lone pair acceptor

Lone pair donor

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e- deficient +ve charge or ẟ+ part (electron lover)

e- rich -ve charge/lone pair on O, N, Cl (nucleus (+) lover)

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BF3 BeCl2 Transition metal ions, e.g. Cu2+ (Usually has an empty orbital to accept electrons)

OH-, Cl- NH3, H2O(can have a negative charge or lone pair(s))

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Examples of Lewis Acid/Base Reactions: BF3 + NH3 → BF3NH3 Possible ˙.˙ of incomplete octet in boron and lone pair in nitrogen **note that when the Lewis base (NH3) donates e- to the Lewis acid (BF3) it forms a coordinate bond Cu2+ (aq) + 6H2O (l) → [Cu(H2O)6]2+ (aq) Ligands(Lewis Base) form coordinate bonds to vacant d orbitals in the central atom (Lewis Acid)

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https://lh7-us.googleusercontent.com/mIjQnv88SNB4DALz78Q7UXGxTI6jSJGxnvJsHmmm89DpAjmuyoKO6DrtiW6zaGf3h-NBHUK0p8A_aFfM4TtWt4q6ygSeBT2zmx25OIKZ4B6GqQ4FPFGBkJtK8wlTmpH1emi_AR5oV0XNj4dHFZ9wzyA

https://lh7-us.googleusercontent.com/RKtO_o4bUyDBx1ZPbSb-faCrFjeubTcf72vhFfyceIEEN-aA2yQMlrXycsTJrHawClWPWZ41lL6iRbtlrbLk0mLNP2SDWzbItRIXeAI5lq66Lyun3QDl6-1jlBJgHrWKgBH2MLo91I8HokXE6B1UPj0

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All bronsted lowry base are lewis ˙.˙ e- may be used to be dative covalent bonded to H+ but not all lewis base are bronsted lowry as e- donated may be used for other than H+

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No conjugates for lewis acid and base rxn ˙.˙ only 1 product formed from 2 reactants

Nucleophilic Substitution Reactions: halogenoalkanes

Mechanism

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SN1

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SN2

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https://lh7-us.googleusercontent.com/XdFXnj3k5TYVwQFKQ1um9MvCBpZdS-rjRmH0KVVdvF1ERVArmFcSfCcVbSSIRIOX8KgmH5CT_1bxQvn-iR-qECPHfGjDamKTT530zu99mDZ2G2PYKIyMgTJ49QZpx7dpXyj8QEtbWsahbcyNUqArPS0

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Type

SN1Substitution Nucleophilic Unimolecular (1)

SN2Substitution Nucleophilic Bimolecular (2)

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Bond fission

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Heterolytic fission of C-X bond → leaving group

Mechanism

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• Two step • Formation of carbocation intermediate

• One step • Unstable transition state

Alkane

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Tertiary, secondary

Primary, secondary

Energy Profile

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https://lh7-us.googleusercontent.com/gepkW_X3ZAUz_zFwmRBWtnzu6ytXUqk43HM6zQJ41EyDRzKIfpm3MlcbtGAZtR9o4IXdks3L9LExNnzDqGB4bZ97TCGuzcTU5bNbH8UIWwM_81cFCBGaDJfaYNpsQtwCTRwfkmEsjJGiaYQ-Z7YahaA

https://lh7-us.googleusercontent.com/vDg978BBT3yswKQHow43lFQ0dzEktNXQ2RIx_oKteCwRi-oeh5BKhnooXE2svzWiPWk0-eZbbxVZs1xczE0sxv0xzif0PpwtbHgmyg80AcVlinHrHmFilQ9A7ZggDLk54wc4QwwCa1aDq-PItS0W-jc

Rate Factors affecting rate

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Unimolecular Rate = k [halogenoalkane]

Biomolecular Rate = k[halogenoalkane][nucleophile]

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1. SN1 > SN2 (in alkaline/ethanolic solution) • Formation of transition state requires more Ea 2. Leaving group: F < Cl < Br < I HalogenoalkanePolarity & ease to be nucleophilic attacked (n.a.)Bond strength & ease to break C-Xfluoro-↑ (most polar and prone to n.a.)* ↓ (least polar and less prone to n.a.)↑ (strongest and hardest to break) ↓ (weakest and easiest to break)chloro-bromo-iodo- *though C-F is the most polarized bond and susceptible to n.a, ultimately the C-X bond needs to be broken in the reaction, and due to C-F being a very strong bond it reacts very slowly. 3. Solvent *Measured by colorimetry of silver halide precipitate formation upon rxn with AgNO3

Solvent

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polar, protic solvents (H-bonding) • stronger ion-dipole to solvate/stabilise carbocation intermediate e.g. Water, alcohol, carboxylic acid

polar, aprotic solvents (non H-bonding) • Polar enough to solvate the nucleophile but does not participate in H-bonding with the nucleophile (would make Nu less reactive) e.g. (CH3)2CO, CH3CN

Change in stereochemistry

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• 50/50 mix of enantiomers (racemic mixture - optically inactive)

• Inversion of configuration (optically active)

Electrophilic addition reactions: alkenes

Mechanism

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e- rich 𝛑 bond prone to electrophilic attack

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Involves any alkene with molecules such as halogens or hydrogen halides

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Symmetrical alkene

Br2 becomes polarised by nearby 𝛑 bond due to e- repulsion & becomes an electrophile Brẟ+- Brẟ- 𝛑 bond acts as a nucleophile and attacks the ẟ+ Br; 𝛑 bond breaks to form a C-Br covalent bond Heterolytic fission of Br2 and formation of carbocation intermediateUnstable carbocation intermediate attacked by Br- ion

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Unsymmetrical alkene

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Markovnikov’s Rule • Hydrogen will attach to the carbon bonded to the greater number of hydrogens. (allow +ve charge on carbon with more alkyl groups) • Formation of the more stable cation intermediate is favored 1. 𝛑 bond acts as a nucleophile and attacks the ẟ+ H; 𝛑 bond breaks to form a C-H covalent bond 2. heterolytic fission of HBr and formation of carbocation intermediate 3. Unstable carbocation intermediate attacked by Br- ion (propene onwards) 4. Theoretically, two carbocation intermediates (1o vs 2o) can be formed because of carbon’s position # 5. Hence, two products are formed (1-bromopropane & 2-bromopropane) 6. Experimentally, 2-bromopropane is formed (the major product) because it is made from a secondary carbocation intermediate which is more stable than the primary carbocation involved in making 1-bromopropane because it has a greater positive inductive effect. *greater stability of carbocation : 3o>2o>1o • due to the positive inductive effect (alkyl groups can donate electrons to stabilize positive charge)

https://lh7-us.googleusercontent.com/0tXUUYfM2NSwFNkcclcLFVgPn0nKo41rI8K9hyevlidFfHqgMnVESikTm5VPOM_qk-3mM4Jkp9ShBDnDRcPi3Su2JX9t0Gq0iUQO4P7XnfokIwC31qNjBrklAoQHktkdNVF7lDmu1Aqn5b-K2qepSlI

Electrophilic substitution reactions: benzene

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Benzene undergoes electrophilic substitution

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The e- rich 𝛑 system is prone to electrophilic attack

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Electrophilic addition reaction is not favored to preserve delocalised ring of e-s

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𝛑 system = additional bonding = energy released = lower energy state = stability

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Nitration of benzene

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Benzene reacts with concentrated nitric acid (HNO3) and sulfuric acid (H2SO4) at around 50oC to produce nitrobenzene (C6H5NO2) and water

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Concentrated sulfuric acid acts as the catalyst for the reaction

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H2SO4 + HNO3 → HSO4- + H2O + NO2+ (NO2+ is the electrophile)

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Halogenation of benzene

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Uses Cl2 (+AlCl3 in dry ether)

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Retrosynthetic Analysis

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Target molecule → precursor 1 → precursor 2 → Initial Reactant

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Butanoic acid → Butan-1-ol → 1-chlorobutane → Butane

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