Organic reaction mechanisms flashcards

Organic reaction mechanisms describe the step-by-step movement of electrons during chemical reactions. Students must understand how bonds break and form, distinguishing between homolytic fission, which produces radicals, and heterolytic fission, which produces ions. The four main mechanism types are free radical substitution, electrophilic addition, nucleophilic substitution, and electrophilic substitution. Students need to identify nucleophiles and electrophiles, draw curly arrows correctly to show electron pair movement, and understand why certain reagents react with particular functional groups. The concept of carbocation stability and the difference between SN1 and SN2 pathways in nucleophilic substitution are also important. Understanding mechanisms allows students to predict products, explain selectivity, and appreciate how reaction conditions influence which pathway is followed.

1. Card 1

   What does a curly arrow represent in a reaction mechanism?

   A curly arrow shows the movement of a pair of electrons, always drawn from a bond or lone pair towards where the electrons move to.

2. Card 2

   What is the difference between homolytic and heterolytic bond fission?

   Homolytic fission splits a bond equally, giving one electron to each fragment, forming radicals. Heterolytic fission gives both electrons to one fragment, forming ions.

3. Card 3

   Define a nucleophile and give one example.

   A nucleophile is an electron-pair donor that attacks electron-deficient centres. Examples include OH-, NH3, and CN-, all of which have lone pairs available.

4. Card 4

   Define an electrophile and give one example.

   An electrophile is an electron-pair acceptor that attacks electron-rich centres. Examples include Br2, H+, and NO2+, which are electron-deficient species.

5. Card 5

   What type of mechanism do alkenes undergo with bromine water?

   Alkenes undergo electrophilic addition. The pi bond acts as the electron donor, attacking the electrophilic bromine molecule to form a cyclic bromonium ion intermediate.

6. Card 6

   What is the mechanism for the reaction of methane with chlorine in UV light?

   This is free radical substitution, proceeding through initiation, propagation, and termination steps, with homolytic fission of Cl2 producing chlorine radicals.

7. Card 7

   What is the difference between SN1 and SN2 nucleophilic substitution?

   SN1 is a two-step mechanism via a carbocation intermediate; SN2 is a one-step concerted mechanism where the nucleophile attacks as the leaving group departs.

8. Card 8

   Why do tertiary halogenoalkanes favour SN1 over SN2?

   Tertiary halogenoalkanes form stable tertiary carbocations in SN1. Steric hindrance from three alkyl groups also blocks the back-side attack required for SN2.

9. Card 9

   What is the role of the benzene ring in electrophilic aromatic substitution?

   The delocalised pi electron system of benzene acts as the nucleophile, donating electrons to the electrophile. Addition is avoided to preserve aromaticity.

10. Card 10

    What is a half-headed curly arrow and when is it used?

    A half-headed curly arrow (fishhook arrow) represents the movement of a single electron, used exclusively when drawing free radical mechanisms.

11. Card 11

    What is a carbocation and how does its stability vary with substitution?

    A carbocation is a carbon bearing a positive charge. Stability increases from primary to secondary to tertiary because alkyl groups donate electron density, dispersing the charge.

12. Card 12

    In nucleophilic addition to carbonyl compounds, what acts as the nucleophile when HCN is used?

    The cyanide ion (CN-) acts as the nucleophile, attacking the electrophilic carbonyl carbon. The reaction produces a hydroxynitrile, also called a cyanohydrin.

• Students often draw curly arrows starting from an atom rather than from a bond or lone pair, which misrepresents where the electron pair originates.
• A common error is confusing SN1 and SN2 by assuming that all halogenoalkanes follow the same pathway, ignoring how the degree of substitution and steric effects determine the mechanism.
• Many students incorrectly use full curly arrows in free radical mechanisms instead of half-headed fishhook arrows, failing to show that single electrons are moving.

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