20.1 Types of organic reactions Flashcards
Explain nucleophiles
Nucleophiles - electron rich - attack areas with electron deficiency - act as Lewis bases - donate e pair - new covalent bond
Ex: OH-, H2O, NH3, CN-
Explain electrophiles
Electrophiles are electron deficient - accept e pairs from nucleophiles - act as Lewis bases
Ex: H+, NO2+
Explain nucleophilic substitution
Sn1/Sn2 - depends on what kind of halogenalkane (1/2/3)
Halogenalkanes - subsitution involves heterolytic fission (bond breaks, e to one of the two)
Halogen - leaving group
Reaction of primary halogenalkane
Sn2
- Nucleophile attacks as e are attracted to halogen
- Transition state
- LG leaves
Bimolecular reaction - rate depends on both the conc of halogenalkane and nucleophile:
rate = k [halogenalkane] [nucleophile]
Sn2 stereospecific because 3D arrangement of reactants influences arrangement of products
Favoured by polar aprotic solvents - leave nucleophile not solvated (ex propanone)
Explain structural inversion of molecule
When nucleophile attacks, attacks on opposite side of LG - after the reaction molecule is inversed - inversion
Reaction of tertiary halogenalkane
Sn1
- LG leaves by breaking the bond heterolytically
- Carbocation forms (stabilised by three alkyl groups)
- Nucleophile attacks
Due to steric hindrance a nucleophile cannot attack - LG leaves (slow, so rate determining step) but carbocation is symetrical - nucleophile can attack from any side - Sn1 not stereospecific
Unimolecular - rate determined only by conc of halogenalkane
rate = k [halogenlakane]
Polar protic (H bonds) solvents favour Sn1 - solvate the carbocation (ex water, alcohols, carboxylic)
Reaction of secondary halogenalkanes
Sn1/Sn2 - mixture of both
Reaction rates in halogenlakanes
Rate of LG leaving by halogenalkane halogen
Determined by two opposing:
1) polarity of halogen - C bond (the least polar break off faster)
2) strength of halogen - C bond (the weakset break off more easly)
But bond strength more important - LG according to 2)
Sn1 vs Sn2
Reaction of alkenes
Electrophilic addition
Alkenes - unsaturated - double bonds - 120 degrees - open structure for C attack - pi bonds electron dense - electrophiles attracted - electrophiles produced in heterolytic fission
Addition of halogens or hydrogen halides
Electrophilic addition reaction mechanism
- Halogen/halide molecule is polarised by an alkene - electron rich region
- Halogen/halide splits in heterolytic fission - as poles have been formed (ex: Br+ and Br- / H+ and Br-)
- Electrophilic halogen (ex Br+) atom breaks pi bond - attaches to now alkane - left carbocation
- Carbocation reacts withe the other halogen atom (ex Br-)
- Halogenalkane formed
Explain unsymmetric electrophilic addition
Unsymetrical alkene reacts with halide - two possible products - Markinkov’s rule: H will attach to that C that is already bonded to the greater number of H
Reaction of benzene
Electrophilic substitution
Delocalised pi e - attract electrophiles - high activation E - slow
- Electrophile attacks pi bond
- Non aromatic carbocation forms
- H leaves and the electrophile stays
Explan nitration of benzene
Electrophilic substitution
- Electrophile - nitronium oxide NO2+ - generated by nitrating mixture
- NO2+ attacks pi bond (temp. H2SO4)
- Carbocation forms
- H leaves
Explain production of NO2+
By nitrating mixture
Explain reduction of carbonyl compounds
Reduction (reverse of oxidation) - involves reducing agents: NaBH4, LiAlH4
Explain reduction of nitrobenzene
Nitrobenzene can be converted into phenylamine
- Nitrobenzene interacted with Sn and conc HCl - phenylammonium ion
- Phenylammonium ion reacts with NaOH - removes one H - phenylamine
Summary of all reaction mechanisms
