Mechanisms Flashcards
nucleophilic substitution, electrophilic addition (26 cards)
nucleophilic substitution of bromoethane and aqueous alkali NaOH
forms ethanol (an alcohol) + Bromide ion
Nucleophilic substitution of bromoethane reacting with ethanolic (concentrated) potassium cyanide (KCN in ethanol) when heated under reflux.
forms PROPANENITRILE
Nucleophilic substitution reaction with the ammonia NH3 - Bromoethane (halogenoalkane) reacts with an ethanolic solution of excess (concentrated) ammonia (NH3 in ethanol) is heated under pressure.
forms ETHYLAMMONIUM
second step: an ammonia molecule may remove one of the hydrogens on the -NH3+.
forms a ETHYLAMINE (primary amine)
Nucleophiles have lone pairs.
They can form a dative bond with H+. They are bases.
Under different conditions OH- will act as a base. what will it divert from the expected and what is the mechanism called?
Different conditions result in different results. Remove H+ ions from halogenoalkanes. Elimination
Conditions of elimination
Heat
Ethanol as solvent (no water present)
concentrated potassium hydroxide
In an elimination reaction
An organic molecule lose a molecule.
What is the small molecule lost by organic molecule?
Hydrogen halide (HX)
The halogenoalkanes are HEATED with ETHANOLIC KOH causing the C–X bond to break heterolytically forming an X- ion.
Forms an ALKENE as the organic product.
Elimination Mechanism
Alkenes are susceptible to attack by electrophile (lone pair acceptors)
the C=C double bond is very rich due to the electron cloud of the pi bond.
In addition reaction, the C=C bond open up
an atom or group atoms joins onto each C of the C=C double bond.
What does electrophilic addition reaction include the addition of:
Steam
Hydrogen Halide (HX)
Halogens
Hydrogen Halide (HBr)
polar molecule due to the hydrogen and halide having different electronegativity.
HBr
The bromine atom has stronger pull on the electrons in H - Br bond.
Br atom has a partial negative (delta negative) and the H atom a partial positive
Electrophilic addition But-2-ene + H-Br
the H atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene.
The H-Br bond breaks heterolytically forming a Br- ion.
This results in the formation of a highly reactive cation intermediate which reacts with the Br-.
2 Bromobutane
Br2
Non polar molecule as both atoms have similar electronegativity and therefore share equally the electrons in the covalent bond.
Electrophilic addition: ethene + Br2
when a bromine molecule gets closer to the double bond of an alkene, the high electron density repels the electron pair in the Br-Br away from the closest Br atom.
Electrophilic addition reaction with (cold) concentrated H2SO4: The partially positive (δ+) hydrogen atom acts as an electrophile. It is attracted to the high electron density of the c=c double bond in the alkene and accepts a pair of electrons. δ
The H-O breaks heterolytically, forming a hydrogen sulfate ion, HSO4-.
A highly reactive carbocation intermediate is formed which reacts with the HSO4-.
Produces Ethyl (alkyl) hydrogen sulfate
What is the name that refers to the positively charged ion H3O+?
A hydroxonium ion
How is a hydroxonium formed?
Formed when an acid molecule dissolves in water and donates a hydrogen ion (proton) to a water molecule.
This is how H+ ions exist in aqueous solutions.
water is a weak electrophile. what is the exception to this?
Unless it undergoes (electrophilic) addition reactions with alkenes in the presence of a strong acid which acts a catalyst.
What are the strong acids that could be used?
H2SO4, H3PO4
