Reaction Mechanism Flashcards
What is a substitution reaction?
The breaking of a sigma bond and replacement with a new sigma bond
What is an addition reaction?
The breaking of a pi bond and replacement via formation of two sigma bonds
What is an elimination reaction?
The breaking of 2 sigma bonds and formation of a pi bond
What are non-polar reactions?
Reactions involving free radicals - 1 valence electron
What are polar reactions?
Reactions in which a nucleophile donates a pair of electrons to an electrophile
What is a nucleophile?
Electron donor
What are the features of a nucleophile?
Electron-rich
Neutral or negatively charged
Have lone pairs of electrons or electrons in pi bonds
What is an electrophile?
Electron pair acceptor
What are the features of an electrophile?
Electron deficient
Neutral or positively charged
Contains a polar bond with partial positive charge
What is homolytic bond cleavage?
Breaking of a covalent bond where the shared pair of electrons is split evenly
Forms radicals
What is heterolytic bond cleavage?
Breaking of covalent bond where one species takes both the electrons of the shared pair
Electrons move towards the more electronegative pair
What is the order of stability with carbocations?
Most stable
Tertiary
Secondary
Primary
Least stable
What are the types of nucleophilic substitution reactions?
SN1 and SN2
What is the difference between SN1 and SN2 mechanism?
SN1 occurs stepwise (involves intermediate)
SN2 occurs in a single step
List the steps in SN1 nucleophilic substitution mechanism
- Heterolytic bond breaking to form carbocation intermediate (the electrophile)
- Bond formation as nucleophile donates pair of electrons
- Bond breaking to form a neutral product
What is the reagent and catalyst for hydrogenation of an alkene?
H2 with Platinum or Palladium
Syn conformer is formed
What is the reagent and catalyst for addition of an alkyl halide to a alkene?
Alkyl halide with ether as catalyst
What is the reagent and catalyst for addition of an alcohol to a alkene?
H2O with acid catalyst (H+)
What is the reagent and catalyst for addition of an ether (-OR) to an alkene?
Alcohol with acid catalyst
What is Markownikoff’s rule?
Addition of an unsymmetrical reagent to an unsymmetrical alkene, gives as the major product the compound in which the electropositive part of the reagent (usually H) has bonded to the carbon of the C=C bond that is directly bonded to a greater number of hydrogen atoms.
What is the reagent and catalyst for halogenation of alkenes?
Halogen / no catalyst required
Anti conformer formed
Why does the addition of halogens to alkene form trans (anti) isomer of the product?
- The pi bond (electron dense) donates a pair of electrons to the partial positive halogen - both carbons form a sigma bond to the halogen giving the halogen a positive charge (bromonium ion intermediate is formed)
- The halogen anion formed cannot attack the partial positive carbon from the positive halogen side hence donates its electrons from the other side forming trans isomer
What is the reagent and catalyst for hydrogenation of alkyne?
H2 / Pt or Pd catalyst
What is the reagent and catalyst to form (Z)-pent-2-ene from pent-2-yne?
H2 / Lindlar catalyst
What is the reagent and catalyst to form (E)-pent-2-ene from pent-2-yne?
Li with liquid ammonia and then water
What is the reagent and catalyst for formation of a ketone or aldehyde from alkynes?
Add one mole of water with aqueous sulphuric acid/HgSO4 (mercury sulphate)
Why does ketone form when you add water to alkynes with H2SO4/HgSO4 catalyst?
The -enol compound forms by Markownikoff addition where the H is bonded to the carbon already bonded to H. The presence of the double bond means further addition will occur to form the ketone -> the more stable compound. Tautomeric equilibrium formed
What is an alkynide anion and how is it formed?
Formed by a terminal alkyne losing its hydrogen as a weak acid to a strong base. The carbon nucleophile (aklynic anion) can be used for carbon chain extension.
What is benzene?
A 6-membered cyclic aromatic compound in which all the carbons are sp2 hybridised leaving 1 unhybridised p orbital.
Do benzenes undergo electrophilic addition?
No
What is a benzene ring with an amine group called?
Aniline
What is an aldehyde substituted benzene ring called?
Benzaldehyde
What are the prefixes used to indicate the relationship between two substituents of a disubstituted benzene?
Ortho 1,2
Meta 1,3
Para 1,4
