Organic Chemistry 5 (mechanisms) Flashcards
Mechanism for Ionic addition
stage a
c=c bond in ethene is an area of negativity (4 electrons being shared) causes non-polar bromine to become polarised as it approaches the double bond, double bond repels electrons to bromine atom furthest away
Mechanism for Ionic addition
stage b
polarisation causes bromine to spilt up into Br+ and Br- by heterolytic fission
heterolytic fission
molecule splits to produce 2 different ions
Mechanism for Ionic addition
stage c
Br+ attacks double bond to get 2 electrons it needs, dative covalent bond, double bond becomes single and other carbon has a positive charge (carbonium ion)
Mechanism for Ionic addition
stage d
the positive carbonium ion is now attack by the Br - to form another dative covalent bond
Mechanism for Ionic addition
how do you get the evidence
carry it out in the presence of sodium chloride solution
Mechanism for Ionic addition
when carried out in sodium chloride solution, what proves the existence of the carbonium ion 1
trace amounts of 1-bromo-2-chloroethane formed when Cl- attacks the carbonium ion
Mechanism for Ionic addition
when carried out in sodium chloride solution, what proves the existence of the carbonium ion 2
2-bromoethaol is formed when the water attacks the carbonium ion
Mechanism for Ionic addition
with Cl2 instead of Br2
the Cl+ attacks the double bond first and then the Cl- attacks the carbonium ion to for 1,2-dichloroethane
Mechanism for Ionic addition
with HCl instead of Br2
the H+ attacks the double bond first and then the Cl- attacks the carbonium ion to form chloroethane
polymer
a very long chained molecule that is made when many thousands of small-chained molecules, called monomers, join together
what is polythene used for 2
plastic bags, cling film
what is polypropene used for
plastic jugs, beakers
definition of a free radical substitution reaction
a free radical substitution reaction occurs when an atom or group of atoms are replaced by another group of atoms. there are two products
free radical substitution reaction, monochlorination of methane
stage 1
initiation
free radical substitution reaction, monochlorination of methane
stage 2
propagation
free radical substitution reaction, monochlorination of methane
stage 3
termination
free radical substitution reaction, monochlorination of methane
initiation stage
ultraviolet light causes chlorine to break down into Cl• and Cl• (homolytic fission)
free radical substitution reaction, monochlorination of methane
why does it need UV light
it is a photochemical reaction
free radical substitution reaction, monochlorination of methane
4 pieces of evidence
- the reaction will not happen in the absence of u.v light
- for every photon absorbed thousands of molecules of product are formed (chain reaction)
- the formation of products such as ethane/chloroethane
- radical promoters like tetramethyl lead speed up the reaction (existence of methyl radicals)
free radical substitution reaction, monochlorination of methane stage 2 propagation, what does the Cl• do?
it attacks an unreactive methyl molecule to form HCl and a methyl radical
free radical substitution reaction, monochlorination of methane stage 2 propagation is the methyl radical reactive
yes as it has an unpaired electron
free radical substitution reaction, monochlorination of methane stage 2 propagation first equation
CH4 + Cl• -> CH3• + HCl
free radical substitution reaction, monochlorination of methane stage 2, what does the methyl radical do
attacks a chlorine molecule to produce chloromethane and a reactive chlorine atom
free radical substitution reaction, monochlorination of methane stage 2 why is the chlorine atom reactive
has an unpaired electron
free radical substitution reaction, monochlorination of methane stage 2 propagation second equation
CH3• + Cl2 -> CH3Cl + Cl•
free radical substitution reaction, monochlorination of methane stage 2 propagation
what does it mean that you produce another chlorine atom
a chain reaction ocurs
free radical substitution reaction, monochlorination of methane stage 3 termination, 3 equations
Cl• + Cl• -> Cl2
CH3• + Cl• -> CH3Cl
CH3• + CH3• -> C2H6
one promoter and formula
tetramethyl lead Pb(CH3)4
free radical substitution reaction, monochlorination of ethane
does the initiation stage change?
no
free radical substitution reaction, monochlorination of ethane main products
HCl and C2H5Cl
free radical substitution reaction, monochlorination of ethane
3 termination equations
Cl• + Cl• -> Cl2
CH3• + C2H5• -> C2H5Cl
CH3• + CH3• -> C4H10
C4H10
butane
free radical substitution reaction, monochlorination of ethane
what does the product of butane prove
the existence of ethyl radicals
free radical substitution reaction, monochlorination of ethane
one promoter
tetraethyl lead, Pb(C2H5)4
