Colourful Solutions > Electron sharing reactions

Introduction

Electron pairs hold atoms together in covalent compounds. In order for reactions to occur, the electrons must be involved in reaction processes. In this section we look at how the electron pairs promote and provoke chemical reaction.

Syllabus reference R3.4.1
Syllabus reference R3.4.2
Syllabus reference R3.4.3
Syllabus reference R3.4.4
Syllabus reference R3.4.5
Syllabus reference R3.4.6
Syllabus reference R3.4.7
Syllabus reference R3.4.8
Syllabus reference R3.4.9
Syllabus reference R3.4.10
Syllabus reference R3.4.11
Syllabus reference R3.4.12
Syllabus reference R3.4.13

Reactivity 3.4.1 - A nucleophile is a reactant that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons.

  • Recognize nucleophiles in chemical reactions.

Guidance

  • Both neutral and negatively charged species should be included.

Tools and links

Reactivity 3.4.2 - In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond, as another bond breaks producing a leaving group.

  • Deduce equations with descriptions and explanations of the movement of electron pairs in nucleophilic substitution reactions.

Guidance

  • Further details of the mechanisms are not required at SL.

Tools and links

Reactivity 3.4.3 - Heterolytic fission is the breakage of a covalent bond when both bonding electrons remain with one of the two fragments formed.

  • Explain, with equations, the formation of ions by heterolytic fission.

Guidance

  • Curly arrows should be used to show the movement of electron pairs during reactions.

Tools and links

  • Reactivity 3.3 - What is the difference between the bond-breaking that forms a radical and the bond-breaking that occurs in nucleophilic substitution reactions?

Reactivity 3.4.4 - An electrophile is a reactant that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from that reaction partner.

  • Recognize electrophiles in chemical reactions.

Guidance

  • Both neutral and positively-charged species should be included.

Tools and links

Reactivity 3.4.5 - Alkenes are susceptible to electrophilic attack because of the high electron density of the carbon–carbon double bond. These reactions lead to electrophilic addition.

  • Deduce equations for the reactions of alkenes with water, halogens, and hydrogen halides.

Guidance

  • The mechanisms of these reactions will not be assessed at SL.

Tools and links

  • Reactivity 3.3 - Why is bromine water decolourized in the dark by alkenes but not by alkanes?
  • Structure 2.4 - Why are alkenes sometimes known as “starting molecules” in industry?

Reactivity 3.4.6 - A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. (HL)

  • Apply Lewis acid–base theory to inorganic and organic chemistry to identify the role of the reacting species.

Guidance

Tools and links

  • Reactivity 3.1 - What is the relationship between Brønsted–Lowry acids and bases and Lewis acids and bases?

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. (HL)

  • Draw and interpret Lewis formulas of reactants and products to show coordination bond formation in Lewis acid–base reactions.

Guidance

Tools and links

  • Structure 2.2 - Do coordination bonds have any different properties from other covalent bonds?

Reactivity 3.4.8 - Coordination bonds are formed when ligands donate an electron pair to transition element cations, forming complex ions. (HL)

  • Deduce the charge on a complex ion, given the formula of the ion and ligands present.

Guidance

Tools and links

Reactivity 3.4.9 - Nucleophilic substitution reactions include the reactions between halogenoalkanes and nucleophiles. (HL)

  • Describe and explain the mechanisms of the reactions of primary and tertiary halogenoalkanes with nucleophiles.

Guidance

  • Distinguish between the concerted one-step SN2 reaction of primary halogenoalkanes and the two-step SN1 reaction of tertiary halogenoalkanes.
  • Both mechanisms occur for secondary halogenoalkanes.
  • The stereospecific nature of SN2 reactions should be covered.

Tools and links

  • Reactivity 2.2 - What differences would be expected between the energy profiles for SN1 and SN2 reactions?
  • Reactivity 2.2 - What are the rate equations for these SN1 and SN2 reactions?
  • Nature of science, Reactivity 2.2 - How useful are mechanistic models such as SN1 and SN2?

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

  • Predict and explain the relative rates of the substitution reactions for different halogenoalkanes. Different halogenoalkanes should include RCl, RBr, RI.

Guidance

  • The roles of the solvent and the reaction mechanism on the rate will not be assessed.

Tools and links

  • Structure 3.1 - Why is the iodide ion a better leaving group than the chloride ion?

Reactivity 3.4.11 - Alkenes readily undergo electrophilic addition reactions. (HL)

  • Describe and explain the mechanisms of the reactions between symmetrical alkenes and halogens, water and hydrogen halides.

Guidance

Tools and links

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. (HL)

  • Predict and explain the major product of a reaction between an unsymmetrical alkene and a hydrogen halide or water.

Guidance

Tools and links

Reactivity 3.4.13 - Electrophilic substitution reactions include the reactions of benzene with electrophiles. (HL)

  • Describe and explain the mechanism of the reaction between benzene and a charged electrophile, E+.

Guidance

  • The formation of the electrophile will not be assessed.

Tools and links

  • Structure 2.2 - What are the features of benzene, C6H6, that make it not prone to undergo addition nreactions, despite being highly unsaturated?
  • Reactivity 3.1 - Nitration of benzene uses a mixture of concentrated nitric and sulfuric acids to generate a strong electrophile, NO2+. How can the acid/base behaviour of HNO3 in this mixture be described?