topic 6: organic 1 mechanisms Flashcards
radical
species with an unpaired electron
2 limitations of free radical substitution
- over substitution
- forms a mixture of products
oversubstitution
the hydrogens can KEEP getting replaced by chlorine atoms
initiation
diatomic molecule forms 2 radicals via homolytic fission using UV light
how is a radical formed
homolytic fission of a covalent bond
homolytic fission
bond splits evenly; each atom gets 1 of the 2 electrons in a covalent bond
propagation
1 radical x 1 non-radical react
termination
2 radical react to form a non-radical
reagent in FSR
halide
condition in FSR
UV
how to ensure full over substitution (tetrahaloalkane)
add excess halogen
definition of electrophilic addition
addition of an electrophile to a double bond
alkene + hydrogen
HYDROGENATION
- nickel catalyst
- alkane formed
manufacture of margarine
- uses hydrogenation
- naturally occurring veg oils are unsaturated (c=c)
- converts into a solid margarine
oxidising agent
acidified poatssium mangante
(KMnO4)
alkene + KMnO4
- oxidisation
- pale purple goes colourless. diol product formed
heterolytic fission
most electronegative atom takes BOTH e- in the covalent bond
alkene + steam
- acid catalyst
- produces alcohol
alkene + halogen
dihalogenoalkane
alkene + hydrogen halide
halogenoalkane
heterolytic bond fission results in…
formation of IONS
which atom is the electrophile
the LESS electronegative one
describe formation of Br- with HBr reaction
- H is electrophile
- accepts PAIR of electrons from c=c
- h-br breaks heterolytically forming a Br- ion
when can you have 2 diff products
Hydrogen halide + ASYMMETRIC alkene
do u have equal amounts of the products?
no
primary carbocation
C+ only bonded to ONE alkyl group (eg if its at the end of the molecule)
secondary carbonation
C+ bonded to TWO alkyl groups
which is more stable of the carbocations
secondary more than primary
major product formed from
secondary
markovnikoffs rule
the H bonds to the C attached to a greater number of H atoms
EXPLAIN markovnikovs rule
- secondary carbocations are more stable
- electrons in alkyl groups can shift towards the positive charge to stabilise it
- ‘positive inductive effect’
- in secondary, positive charge is stabilised by 2 alkyl groups whereas only 1
conditions for alkene + steam
- temp of 300 c
- pressure 65 atm
- phosphoric acid catalyst
why are pi bonds weaker than sigma
- sigma are formed in the direct overlap of orbitals
- pi from sideways overlap of orbitals
- sideways overlap is left efficient as there is less e- density in the intermolecular space
alkene + x solvent
must be non-polar organic
state of hydrogen halide
gaseous
what happens if u use a polar solvent (water)
- reacts with alkenes
- mixture of products is made
- one with the halide added
- one with OH added
why can lots of product be made in free radical substitution with only a bit of UV
A chlorine radical is regenrated for every molecule of product formed
