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A Reactions Review
1.
2.
3.
4.
5.
6.
FREE RADICAL SUBSTITUTION
POLYMERIZATION
ELECTROPHILIC ADDITION ( types)
A) Hydration (add water, HOH)
B) Halogenation (add H2)
C) Bromination (add Br2)
D) Hydrohalogenation (add HCl, HBr,HI (acids)
ELIMINATION (Dehydrohalogenation)
NUCLEOPHILIC SUBSTITUTION (add :OH-,:CN-,H2O, 2:NH3)
OXIDATION
A) 10 ALCOHOLS TO ALDEHYDES THEN TO CARBOXYLIC ACIDS
B) 20 ALCOHOLS TO KETONES
REACTIONS OF ORGANIC COMPOUNDS
POLYMERS
ALKANES
DIBROMOALKANES
ALKENES
KETONES
ALCOHOLS
ALDEHYDES
HALOGENOALKANES
AMINES
NITRILES
CONVERSIONS
CARBOXYLIC ACIDS
14 REACTIONS OF ORGANIC COMPOUNDS
POLYMERS
DIBROMOALKANES
KETONES
P
F
C
D
ALKANES
ALKENES
E
ALCOHOLS
M
A
B
N
L
ALDEHYDES
G
HALOGENOALKANES
O
H
AMINES
I
NITRILES
CARBOXYLIC ACIDS
A
Initiation
Free radical substitution
Cl2
Propagation
Cl• + CH4
Cl2 + CH3•
Termination
Cl• + Cl•
Cl• + CH3•
CH3• + CH3•
——> 2Cl•
radicals created
——> CH3• + HCl
——> CH3Cl + Cl•
radicals used and
then re-generated
——>
——>
——>
radicals removed
Cl2
CH3Cl
C2H6
Summary
Due to the lack of reactivity of alkanes you need a very reactive species to persuade them to
react. This is done by shining UV light on the mixture (heat could be used)
CONVERSIONS
ELECTROPHILIC ADDITION OF HBr
B
(Hydrohalogenation)
Reagent
Hydrogen bromide... it is electrophilic as the H is slightly positive
Condition
Room temperature.
Equation
C2H4(g) + HBr(g) ———> C2H5Br(l)
bromoethane
Mechanism
Step 1: As the HBr nears the alkene, the Pi bond breaks
The δ positive H joins a carbon (hydrogenation)
The :Br ion forms.
Also a carbocation (positively charged carbon species) is formed.
Step 2
The bromide ion (halogen) is a nucleophile and attacks the carbocation.
Br across the double bond.
CONVERSIONS
C
ELECTROPHILIC ADDITION OF Br2
Reagent
Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 )
Conditions
Room temperature. No catalyst or UV light required!
Equation
C2H4(g) + Br2(l)
——>
CH2BrCH2Br(l)
1,2 - dibromoethane
Mechanism
It is surprising that bromine
should act as an electrophile
as it is non-polar.
CONVERSIONS
D
ELECTROPHILIC ADDITION with Water
(or HYDRATION)
Reagent
steam
Catalyst
Sand coated with phosphoric acid (BOTH WAYS)
Product
alcohol OR Ethene (depending on direction with Chatelier's principle)
H3PO4, SiO2
Equation
C2H4(g) +
H2O(g)
C2H5OH(g)
ethanol ⍙H = -45 KJ/mo
So it is no surprise that the mechanism for hydration of alkenes is identical to that of dehydration
of alcohols, but in the reverse order of steps. D and M are the same mechanism, in reverse
CONVERSIONS
M
ELIMINATION OF WATER (DEHYDRATION)
Reagent/catalyst
phosphoric acid (H3PO4) conc. sulphuric acid (H2SO4) or conc.
Conditions
reflux at 180°C
Equation
e.g. C2H5OH(l)
CH2 = CH2(g) + H2O(l)
So it is no surprise that the mechanism for hydration of alkenes is identical (as it is a
REVERSABLE reaction) to that of dehydration of alcohols, but in the reverse order of steps. D and
M are the same mechanism, in reverse
Mechanism
CONVERSIONS
ELECTROPHILIC ADDITION OF H2
E
or HYDROGENATION
Reagent
Conditions
Product
Equation
hydrogen
nickel catalyst - finely divided
alkanes
C2H4(g) + H2(g)
———>
C2H6(g)
ethane
Ni
Use
margarine manufacture (Hydrogenated fats for spreading
on bread)
CONVERSIONS
POLYMERISATION OF ALKENES
F
ETHENE
POLY(ETHENE)
PROPENE
POLY(PROPENE)
CHLOROETHENE
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE
TETRAFLUOROETHENE
PVC
POLY(TETRAFLUOROETHENE)
PTFE
CONVERSIONS
“Teflon”
G
NUCLEOPHILIC SUBSTITUTION
AQUEOUS SODIUM HYDROXIDE
Reagent
Conditions
Product
Nucleophile
Equation
Aqueous* KOH or NaOH
Reflux in aqueous solution
Alcohol
hydroxide ion
OH¯(aq)
(SOLVENT IS IMPORTANT)
e.g. C2H5Br(l) + NaOH(aq) ———> C2H5OH(l)
+
NaBr(aq)
Mechanism
* WARNING
It is important to quote the solvent when answering questions.
Elimination takes place when ethanol is the solvent
The reaction (and the one with water) is known as HYDROLYSIS
CONVERSIONS
NUCLEOPHILIC SUBSTITUTION
H
AMMONIA
Reagent
Conditions
Product
Nucleophile
Aqueous, alcoholic ammonia (in EXCESS)
Reflux in aqueous, alcoholic solution under pressure
Amine
Ammonia (NH3)
e.g. C2H5Br + 2NH3 (aq / alc) ——> C2H5NH2 + NH4Br
Equation
(i)
(ii)
C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr
HBr + NH3 (aq / alc) ——> NH4Br
Mechanism
Notes
The equation shows two ammonia molecules.
The second one ensures that a salt is not formed.
Excess ammonia is used to prevent further substitution (SEE NEXT SLIDE)
CONVERSIONS
NUCLEOPHILIC SUBSTITUTION
I
POTASSIUM CYANIDE
Reagent
Conditions
Product
Nucleophile
Aqueous, alcoholic potassium (or sodium) cyanide
Reflux in aqueous , alcoholic solution
Nitrile (cyanide)
cyanide ion (CN¯)
Equation
e.g. C2H5Br +
KCN (aq/alc) ———> C2H5CN + KBr(aq)
Mechanism
Importance
it extends the carbon chain by one carbon atom
the CN group can then be converted to carboxylic acids or amines.
K
Hydrolysis
C2H5CN
+
CONVERSIONS
2H2O
——>
C2H5COOH +
NH3
ELIMINATION
L
Reagent
Alcoholic sodium (or potassium) hydroxide
Conditions
Reflux in alcoholic solution
Product
Alkene
Mechanism
Elimination
Equation
C3H7Br
+
NaOH(alc)
———>
C3H6
+
H2O
+
NaBr
Mechanism
the OH¯ ion acts as a base and picks up a proton
the proton comes from a C atom next to the one bonded to the halogen
the electron pair moves to form a second bond between the carbon atoms
the halogen is displaced;
overall there is ELIMINATION of HBr.
With unsymmetrical halogenoalkanes, a mixture of products may be formed.
CONVERSIONS
OXIDATION OF PRIMARY ALCOHOLS
N
Primary alcohols are easily oxidised to aldehydes
e.g.
———>
CH3CH2OH(l) + [O]
CH3CHO(l) + H2O(l)
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O]
———>
OXIDATION TO
ALDEHYDES
DISTILLATION
CH3COOH(l)
OXIDATION TO
CARBOXYLIC ACIDS
REFLUX
Aldehyde has a lower boiling point so
distils off before being oxidised further
Aldehyde condenses back into the
mixture and gets oxidised to the acid
CONVERSIONS
O
OXIDATION OF ALDEHYDES
Aldehydes are easily oxidised to carboxylic acids
e.g. CH3CHO(l) + [O]
•
•
•
•
———>
CH3COOH(l)
one way to tell an aldehyde from a ketone is to see how it reacts to mild oxidation
ALDEHYES are EASILY OXIDISED
KETONES are RESISTANT TO MILD OXIDATION
reagents include TOLLENS’ REAGENT
and FEHLING’S SOLUTION
TOLLENS’ REAGENT
Reagent
ammoniacal silver nitrate solution
Observation
a silver mirror is formed on the inside of the test tube
Products
silver + carboxylic acid
Equation
Ag+ + e- ——> Ag
FEHLING’S SOLUTION
Reagent
a solution of a copper(II) complex
Observation
a red precipitate forms in the blue solution
Products
copper(I) oxide + carboxylic acid
Equation
Cu2+ + e- ——> Cu+
CONVERSIONS
OXIDATION OF SECONDARY ALCOHOLS
P
Secondary alcohols are easily oxidised to ketones
e.g.
CH3CHOHCH3(l) + [O]
———> CH3COCH3(l) + H2O(l)
The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment
with a powerful oxidising agent they can be further oxidised to a mixture of acids
with fewer carbon atoms than the original alcohol.
CONVERSIONS