AQA A-LEVEL CHEMISTRY 7405/3 Paper 3
Alkene definition - ANSWER Unsaturated hydrocarbons.
General formula of alkenes - ANSWER CnH2n
What is required somewhere in the structure of alkenes? - ANSWER A carbon-
carbon double bond.
What is the arrangement of bonds around the C=C? (Shape and bond angle) -
ANSWER Planar, 120 degrees.
When do numbers needed to be added to the name? - ANSWER When positional
isomers can occur.
What is stereoisomerism? - ANSWER Stereoisomers have the same structural
formulae but have a different spatial arrangement of atoms.
What type of isomerism can alkenes exhibit? - ANSWER E-Z stereoisomerism.
Why do E-Z isomers exist? - ANSWER Due to restricted rotation about the C=C
bond.
When do E/Z stereoisomers arise? - ANSWER (a) There is restricted rotation
around the C=C double bond.
(b) There are two different groups/atoms attached both ends of the double bond.
What type of bonds easily rotate? - ANSWER Single carbon-carbon covalent bonds.
Z-but-2-ene - ANSWER
E-but-2-ene - ANSWER
How do you name E-Z stereoisomers? - ANSWER 1) Determine the priority groups
on both sides of the double bond.
2) If the priority atom is on the same side of the double bond it is labelled Z from the
german zusammen (The Zame Zide!).
3) If the priority atom is on the opposite side of the double bond it is labelled E from
the german entgegen (The Epposite side!).
What is the priority group? - ANSWER The atom with the bigger Atomic number is
classed as the priority atom.
Electrophile definition - ANSWER An electron pair acceptor.
Addition reaction definition - ANSWER A reaction where two molecules react
together to produce one.
, What type of reaction do alkenes undergo? - ANSWER Addition reactions.
Why do alkenes experience addition reactions? - ANSWER The double bonds in
alkenes are areas with high electron density. This attracts electrophiles and the
alkenes undergo addition reactions.
Reaction of bromine with alkenes
What is the change in functional group? - ANSWER Alkene → dihalogenoalkane
Reaction of bromine with alkenes
What is the reagent? - ANSWER Bromine.
Reaction of bromine with alkenes
What is the conditions? - ANSWER Room temperature (not in UV light).
Reaction of bromine with alkenes
What is the mechanism? - ANSWER Electrophilic addition.
Reaction of bromine with alkenes
What is the type of reagent? - ANSWER Electrophile Br+.
Reaction of bromine with alkenes
mechanism example with methene - ANSWER
Reaction of bromine with alkenes
What happens to the alkene in terms of polarity when this reaction takes place? -
ANSWER As the Br2 molecule approaches the alkene, the pi bond electrons repel
the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar
and ELECTROPHILIC (Brδ+ ).
Reaction of bromine with alkenes
What is the intermediate formed called? - ANSWER The INTERMEDIATE formed,
which has a positive charge on a carbon atom is called a CARBOCATION.
Reaction of hydrogen bromide with alkenes
What is the change in functional group? - ANSWER Alkene → halogenoalkane
Reaction of hydrogen bromide with alkenes
What is the reagent? - ANSWER HCl or HBr.
Reaction of hydrogen bromide with alkenes
What is the conditions? - ANSWER Room temperature.
Reaction of hydrogen bromide with alkenes
What is the mechanism? - ANSWER Electrophilic addition
Reaction of hydrogen bromide with alkenes
What is the type of reagent? - ANSWER Electrophile Hδ+
Reaction of hydrogen bromide with alkenes
Alkene definition - ANSWER Unsaturated hydrocarbons.
General formula of alkenes - ANSWER CnH2n
What is required somewhere in the structure of alkenes? - ANSWER A carbon-
carbon double bond.
What is the arrangement of bonds around the C=C? (Shape and bond angle) -
ANSWER Planar, 120 degrees.
When do numbers needed to be added to the name? - ANSWER When positional
isomers can occur.
What is stereoisomerism? - ANSWER Stereoisomers have the same structural
formulae but have a different spatial arrangement of atoms.
What type of isomerism can alkenes exhibit? - ANSWER E-Z stereoisomerism.
Why do E-Z isomers exist? - ANSWER Due to restricted rotation about the C=C
bond.
When do E/Z stereoisomers arise? - ANSWER (a) There is restricted rotation
around the C=C double bond.
(b) There are two different groups/atoms attached both ends of the double bond.
What type of bonds easily rotate? - ANSWER Single carbon-carbon covalent bonds.
Z-but-2-ene - ANSWER
E-but-2-ene - ANSWER
How do you name E-Z stereoisomers? - ANSWER 1) Determine the priority groups
on both sides of the double bond.
2) If the priority atom is on the same side of the double bond it is labelled Z from the
german zusammen (The Zame Zide!).
3) If the priority atom is on the opposite side of the double bond it is labelled E from
the german entgegen (The Epposite side!).
What is the priority group? - ANSWER The atom with the bigger Atomic number is
classed as the priority atom.
Electrophile definition - ANSWER An electron pair acceptor.
Addition reaction definition - ANSWER A reaction where two molecules react
together to produce one.
, What type of reaction do alkenes undergo? - ANSWER Addition reactions.
Why do alkenes experience addition reactions? - ANSWER The double bonds in
alkenes are areas with high electron density. This attracts electrophiles and the
alkenes undergo addition reactions.
Reaction of bromine with alkenes
What is the change in functional group? - ANSWER Alkene → dihalogenoalkane
Reaction of bromine with alkenes
What is the reagent? - ANSWER Bromine.
Reaction of bromine with alkenes
What is the conditions? - ANSWER Room temperature (not in UV light).
Reaction of bromine with alkenes
What is the mechanism? - ANSWER Electrophilic addition.
Reaction of bromine with alkenes
What is the type of reagent? - ANSWER Electrophile Br+.
Reaction of bromine with alkenes
mechanism example with methene - ANSWER
Reaction of bromine with alkenes
What happens to the alkene in terms of polarity when this reaction takes place? -
ANSWER As the Br2 molecule approaches the alkene, the pi bond electrons repel
the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar
and ELECTROPHILIC (Brδ+ ).
Reaction of bromine with alkenes
What is the intermediate formed called? - ANSWER The INTERMEDIATE formed,
which has a positive charge on a carbon atom is called a CARBOCATION.
Reaction of hydrogen bromide with alkenes
What is the change in functional group? - ANSWER Alkene → halogenoalkane
Reaction of hydrogen bromide with alkenes
What is the reagent? - ANSWER HCl or HBr.
Reaction of hydrogen bromide with alkenes
What is the conditions? - ANSWER Room temperature.
Reaction of hydrogen bromide with alkenes
What is the mechanism? - ANSWER Electrophilic addition
Reaction of hydrogen bromide with alkenes
What is the type of reagent? - ANSWER Electrophile Hδ+
Reaction of hydrogen bromide with alkenes
