Download TYPES OF REACTIONS IN ORGANIC CHEMISTRY

TYPES OF REACTIONS IN ORGANIC CHEMISTRY
SUBSTITUTION REACTIONS
A substitution reaction is a chemical reaction in which an atom or group of atoms in a
molecule is replaced by another atom or group of atoms.
(a) HALOGENATION OF ALKANES
Initiation
UV light breaks down the Chlorine molecule into two Chlorine atoms (homolytic
fission).
/
Propagation
A chlorine atom attacks the methane molecule to form hydrogen chloride and a methyl
free radical.
A methyl free radical attacks a chlorine molecule to form chloromethane and a chlorine
atom.
Termination
The combining together of free radicals and atoms with each other stops the reaction.
EVIDENCE FOR THE MECHANISM
~ the reaction will not occur in the dark at room temperature
~ the energy supplied is not enough to break the C - H bond
~ No molecular hydrogen produced - hence no hydrogen free radicals formed
~ Ethane is produced in small amounts, can only be explained by CH3 + CH3
~ The presence of tetramethyl - lead greatly speeds up the reaction as it is a source
of methyl free radicals.
(b )ESTERIFICATION
~ Esters are formed when an alcohol and a carboxylic acid react together,
(condensation reaction)
~ The H attached to the -OH group in the carboxylic acid molecule has been
replaced by the alkyl group, it is a type of substitution reaction.
~ The reverse reaction of esterification is called hydrolysis
ADDITION REACTIONS
An addition reaction is one in which two substances react together to form a single
substance.
Polarisation
The high concentration of negative charge in the C = C double bond causes the Br - Br
bond to be polarised
HETROLYTIC FISSION
The induced polarisation becomes so great, the Br 2 molecule splits into Br + and Br(hetrolytic fission)
CARBONIUM ION FORMATION
The carbonium ion is formed by the Br + attacking the double bond
BROMIDE ION ATTACK ON THE CARBONIUM ION
EVIDENCE FOR THE MECHANISM
);- When bromine water containing sodium chloride reacts with ethane, two other
compounds are formed in addition to 1,2 - dibromoethane. These are 1 - bromo2 - chloroethane and 2 - dibromoethanol.
);- When ethane reacts with bromine in water, 2 - bromoethanol is formed, evidence
that the carbonium ion exists
);- Chlorine reacts by the same mechanism.
);- In the case of HCI, the H+ ion attacks the C = C first. Cl then reacts with the
carbonium ion to form chloroethane
POL YMERISATION REACTOINS
~ Polymers are long chain molecules made by joining together many small
molecules
~ Polymers consist of a repeating structure
~ Monomers are the small molecules from which the polymer is made.
~ Molecules of ethene link with each other to form polythene.
~ Molecules of propene link with each other polypropene
~ Crude oil, a source of alkenes, is the raw material for the manufacture of
polymers.
ELIMINATION REACTION
An elimination reaction is one in which a small molecule is removed from a larger one
to leave a double bond in the larger molecule.
~ The removal of water from an alcohol to form an alkene is an elimination reaction
(Dehydration reaction)
REDOX REACTIONS
>- When a primary alcohol reacts with an oxidising agent, it is converted to an
aldehyde
>- Oxidising agents are sodium dichromate, acidified potassium permanganate
>- If the oxidising agent is in excess, the aldehyde is further oxidised to a carboxylic
acid.
>- Ethanol in alcoholic drinks is oxidised to ethanal in the liver
REACTIONS OF ALCOHOLS AND CARBOXYLIC ACIDS AS ACIDS
REACTION OF ALCOHOL
Sodium reacts with ethanol to form a salt called sodium ethoxide and hydrogen gas
ACIDIC NATURE OF THE CARBOXYLIC ACID GROUP
~ The carbonyl group has the effect of drawing electrons away from the - OH
group and this facilitates the ionisation of the H atom in the -OH bond.
~ The fact that the carboxylate ion is stabilised by resonance is the driving force for
the carboxylic acid to lose a proton.
ORGANIC SYNTHESIS
Petroleum '\ ______________________ ---..- Alkanes
Alkenes
~-------------~
Alcohols
Alkynes
Esters
Aldehydes
Ketones
Carboxylic acid
Soaps