Chpt. 23: Types of Reactions In Organic Chemistry

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Chpt. 23 Types of Reactions In Organic Chemistry
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So far in organic chemistry we have studied fuels
and the various families of organic compounds. In
this final organic chapter we will study some of
the reactions that these organic families
undergo!!!!!
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This chapter involves a study of the 1)
various types of chemical reactions
that organic compounds undergo 2) synthesis of
organic compounds 3) methods used to analyse for
the presence of various organic
substances
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Part1 Types of chemical reactions that organic
compounds undergo
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Organic reactions can be classified as
follows - Substitution Reactions - Addition
Reactions - Polymerisation Reactions -
Elimination Reactions - Redox Reactions -
Reactions as Acids
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Substitution Reactions
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Apart from combustion alkanes are rather
unreactive. However, in addition to combustion
reactions ALKANES also undergo SUBSTITUTION
reactions
Definition 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.
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Types of Substitution Reactions - Halogenation
of Alkanes (Free Radical Substitution) -
Esterification (previously discussed) -
Saponification (previously discussed)
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• Halogenation of Alkanes (Halogen replaces H)
• Ordinary Level
• This involves the reaction of the alkanes with
  halogens in the presence of light e.g.
• Methane reacts with chlorine in the presence of
  
• ultraviolet light to yield
  (mono)chloromethane and HCl
• CH4 Cl2 ? CH3Cl HCl
• Space for diagram

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• Ethane reacts with chlorine in the presence of
• ultraviolet light to form
  (mono)chloroethane
• C2H6 Cl2 ? C2H5Cl HCl
• Space for diagram

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Higher Level Must understand the mechanism of
this reaction i.e. the detailed step by step
description of how the overall reaction
occurs. As discussed preciously ( chemical
bonding, thermochemistry) chemical reactions
involve the breaking and forming of bonds. When
such bonds are broken and formed it involves the
rearrangement of electrons and this is what needs
to be considered when studying reaction
mechanisms.
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Reaction Mechanism Halogenation STAGE 1
INITATION (Getting it started) - UV light
breaks down the chlorine molecule into two
chlorine atoms HOMOLYTIC FISSION Cl Cl
? Cl Cl - this is called a
photochemical reaction i.e. a rxn that is
brought about by light. This is proven to be a
light dependent reaction as it does not
occur in the dark at room temperature
uv
Covalent Bond
Unpaired electron
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STAGE 2 PROPAGATION 1 (Keep it going) - a
free chlorine atom attacks a methane molecule to
form HCl and a methyl free radical (Free
radical is any atom or group of atoms with an
unpaired electron) Cl CH4 ?
HCl CH3 - due to an incomplete
outer shell the methyl free radical is very
reactive
Unstable
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STAGE 3 PROPAGATION 2 (Keep it going) - this
methyl free radical attacks a chlorine molecule
to form chloromethane and a free Cl
atom CH3 Cl2 ? CH3Cl Cl
- the chlorine atom produced during propagation 2
can then react with another molecule of
methane as in propagation 1 - thus a
chain reaction is started!!!!!!
Cl CH4 ? HCl CH3
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- Evidence for this chain reaction - chemists
compared the amount of light falling on this
reaction with the amount of light emitted. They
were then able to determine the number of
photons that were absorbed by the reactants and
found that thousands of molecules of
chloromethane were produced for every photon
that was absorbed.
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STEP 4 TERMINATION (Grinding to a Halt) - The
chain reaction comes to an end when the
various free radicals combine with each other to
form un-reactive molecules i.e. (not free
radicals) - Some possible combinations (Chlorine
Methane) CH3 Cl ? CH3Cl
Formation of chloromethane Cl Cl ?
Cl2 Formation of chlorine molecule CH3
CH3 ? C2H6 Formation of ethane
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Evidence for this Free Radical Mechanism

• Reaction requires UV light of energy high enough
  to
• homolyse chlorine to initiate.
• For every photon absorbed very many molecules
  of a
• product are formed.
• Therefore when a mixture of an alkane and
  chlorine
• are irradiated wIth UV light for even a short
  period , a
• chain reaction occurs RXN STOPS IN THE DARK

• Formation of products such as ethane,
• chloroethane, butane etc. is evidence for the
• termination steps.

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• If radical promoters such as tetramethyl lead,
• Pb(CH3)4, or tetraethyl lead are added to the
  reaction
• mixture, there is a marked increase in the rate
  of the
• reaction.
• Pb(CH3)4 ? Pb 4CH3

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• Syllabus requires that you know the mechanism of
  free radical substitution for both methane and
  ethane.
• Free Radical Substitution of Ethane
• Propagation 1 Radical formed will be ethyl
• instead of methyl
• C2H6 Cl ? HCl C2H5
• Tetraethyl lead, Pb(C2H5)4, is needed to
  increase
• the rate of the reaction

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• Termination stage butane formed instead of
  ethane
• and some other possible combinations (Chlorine
  
• Ethane)

Cl Cl ? Cl2 Formation of
chlorine molecule
C2H5 Cl ? C2H5Cl Formation of
chloroethane C2H5 C2H5 ? C4H10 Formation
of butane
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Points to Note - almost all organic
compounds burn to form carbon dioxide and
water - exceptions to this rule are the fully
halogenated alkanes, CBrClF2 - these
type of compounds do not support
combustion and are denser than air, hence, they
are added to fabrics to reduce their tendency
to catch fire. - known as FLAME
RETARDANTS
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Esterfication ( H atom of carboxylic acid
replaced by alkyl group)
Esterification (ester formation) is an example of
a substitution reaction, with the carboxyl
hydrogen in an acid replaced by the alkyl group
of an alcohol This is also know as a
CONDENSATION REACTION Definition
A condensation reaction is one in which two
small molecules react to form a larger one with
the elimination of a smaller one like water.
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Ethanoic acid Ethanol Ethyl
Ethanoate Water
H
H
H
H
O
C
C
C
C

H
H-O
H
O-H
H
H
H
H
ESTERFICSATION
HYDROLYSIS
H
O
H
H
O

C
C
C
C
H
O
H
H
H
H
H
H
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Formation of an ester is also a reversible
reaction i.e. a rxn in which the products react
to form back the product and vice versa. In
this case the reverse reaction is called
HYDROLYSIS Definition Hydrolysis is the
chemical decomposition of a substance by water,
the water itself also being decomposed.
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Base Hydrolysis of Esters (Na/K replaces alkyl
group)
Esters can also be hydrolysed very effectively in
the presence of a base like sodium hydroxide or
potassium hydroxide. The base hydrolysis of
esters using sodium hydroxide forms the sodium
salt of the carboxylic acid (SOAP) rather than
the carboxylic acid itself and alcohol. It is
called BASE HYDROLYSIS OF AN ESTER or
SAPONIFICATION reaction
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Basic Saponification of an Ester Important
Saponification of an Ester
Ethyl Sodium Sodium
Ethanol Ethanoate Hydroxide
Ethanoate
Sodium salt of carboxylic acid
Glyceryl 3NaOH
3Sodium Glycerol Tristearate
Stearate (Soap)
Sodium salt of carboxylic acid
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Syllabus requires that students are able to draw
the structures of the reactants and products for
soap manufacture!!!
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O
H
C17H35
C
NaOH
C
O
H
O
C17H35
C
C
NaOH
H
O
O
C17H35
C
C
NaOH
H
O
H
Triester Glyceryl Tristearate (fat)
Sodium Hydroxide
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O
H
C17H35
C
C
H
H-O
O
Na
O

C17H35
C
C
H
H-O
O
Na
O
C17H35
C
C
H
H-O
O
Na
H
Sodium Stearate (Soap)
Glycerol
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Elimination Reactions
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• Definition
• An elimination reaction is one in which a small
  molecule is removed from a larger molecule to
  leave a double bond in the larger molecule.
• Alkenes can be formed from their corresponding
• alcohols using elimination reactions.
• Since water is removed this particular type of
• elimination reaction is known as a DEHYDRATION
  
• REACTION
• The change in structure is from tetrahedral
  carbon
• (alcohol) to planar carbon (alkene)

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Elimination Reaction Preparation of Ethene
Gas!!! Ethanol ? Ethene Water
Saturated Unsaturated
-H2O
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Elimination Reaction Preparation of Ethene
Gas!!!!
H
H
Ethanol is a primary alcohol
The functional group of an alcohol is O-H the
hydroxyl group. A water molecule is eliminated
C
C
H
H
H
OH
H
H
C
C
H
H
H
OH
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H
H
O

C
C
H
H
H
H
A double bond is formed between the
carbons Ethene an unsaturated compound is formed
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The laboratory preparation of ethene involves
elimination XEthanol. YHot aluminum oxide
(Catalyst)
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Question Name two features of elimination
reactions Answer - remove small molecule -
make a double bond
Note the removal of water from an alcohol to
form an alkene is the only type of elimination
reaction on the course!!!
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ADDITION REACTIONS
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Alkenes are much more reactive than alkanes and
their characteristic reactions are ADDITION
REACTIONS where the double bond opens up allowing
various substances to add on and produce
saturated compounds
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The addition of bromine to a sample of ethene
causes bromine to add across the CC double bond
to form 1,2-dibromoethane. This is an example of
an addition reaction
Br
Br
H
H
C C Br-Br C-C
H
H
H
H
H
H
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• If double bonds are stronger than single bonds
  why do alkenes react so readily with bromine???
• Double bond consists of a sigma and a pi bond.
• Pi bonds are weaker than sigma bonds (less
• overlapping of orbitals)
• When bromine adds across the double bond in
  ethene
• the energy required to break the pi bond is
  released
• when two single bonds to the bromine atoms are
  
• formed.
• Products are more stable than reactants

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Definition An addition reaction is one in which
two substances react together to form a single
substance In general, an addition reaction
involves a change in structure from planar to
tetrahedral
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• Addition Reactions involving Ethene
• (Must be aware of the industrial importance of
  the addition reactions of ethene)
• a) Addition of Hydrogen (H2)
• The addition of hydrogen to alkenes is known
  as
• HYDROGENATION
• Sunflower oil, palm oil etc. are said to be
  polyunsaturated (CC) and are thought to be less
  damaging to our health than the saturated fats
  found in dairy products.
• Adding hydrogen to some of the CC bonds in
  these oils changes them into soft solids. Thus
  hydrogenation is used in industry to convert
  vegetable oils into solid saturated materials
  used in margarine and dairy spreads

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• By controlling the degree of hydrogenation, the
  margarine can be made as hard or as soft as
  needed
• Animal fats saturated
• Vegetable Fats unsaturated

H
H
H
H
Catalyst
H
C
C
H
C
C
H2
Heat
H
H
H
H
Ethene Ethane
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• B) Addition of Chlorine (Cl2)
• The reaction between chlorine and ethene results
  in the product, 1,2-dichloroethane
• 1,2-dichloroethane, is used in industry to make
  chloroethene, the raw material for the
  manufacture of the plastic PVC - polyvinylchloride

H
H
H
H
H
C
C
H
Cl2
C
C
H
Cl
Cl
H
1,2-dichloroethane
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• C) Addition of Bromine (Br2)
• This reaction is used to test for unsaturation
  (preparation of ethene)
• Used as an additive in leaded petrol

H
H
H
H
H
C
C
H
Br2
C
C
H
Br
Br
H
1,2-dibromoethane
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• D) Addition of water (H2O)
• Addition of water is known as a HYDRATION
  reaction
• Reaction used in the manufacture of ethanol (a
  widely used solvent in industry)

H
H
H
H
H
C
C
H
HOH
C
C
H
OH
H
H
Ethanol
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• E) Addition of Hydrogen Chloride (HCl)
• Main modern use of chloroethane is the
  manufacture of ethylcellulose, a thickening agent
  and binder in paints and cosmetics.

H
H
H
H
H
C
C
H
HCl
C
C
H
Cl
H
H
Chloroethane
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In each of the reactions (a) (e) the structure
changes from planar to tetrahedral
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Ionic Addition Reaction Mechanism (Higher Level
Only)

• The double bond is made up of 2 bond pairs.

H
H
C C

• 4 electrons in total.

H
H

• One pi bond and one sigma
• bond.

• It is an electron rich region
• with a slightly negative charge.

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STAGE 1 Polarisation

• A bromine molecule is a non-
• polar molecule
• However, as it approaches
• the double bond the high
• concentration of negative
• charge in the CC bond
• causes the approaching
• Br-Br molecule to become
• polarised.

H
H
C
???
???
Br - Br.
C
H
H
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• STAGE 2 Heterolytic Fission
• Br Br
• The induced polarisation becomes so great the
  Br2
• molecule splits into Br and Br- species. This
  is
• known as heterolytic fission.
• Heterolytic Fission
• The breaking of a bond so that the bonding
• electrons(two) end up on one atom is known as
• heterolytic fission

Br Br_
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• STAGE 3 Carbonium Ion Formation
• The Br species in order to gain the electrons
  
• needed for a full outer shell attacks the C2H4
  
• molecule.
• The Br ion forms a covalent bond with one of
  the
• carbon atoms.
• The other carbon atom has lost an electron and
• so becomes positively charged CARBONIUM ION

?
H
H
H
C
Br
C
H
Br
C
C
H
H
H
H
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N.B. Note As there is a shortage of electrons
in this intermediate species, C2H4Br, it has an
overall positive charge. Modern evidence shows
that rather than bonding to one of the carbons,
the Br is attached to both in a bridged
structure. This cyclic structure is known as
cyclic bromonium ion The relatively large
size of the bromine atom allows the formation
of this three-membered ring structure
Br
H
H
?
C C
H
H
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• STAGE 4 Bromide ion attack on carbonium ion
• The presence of the carbonium ion makes the
• substance unstable and it quickly combines
  with
• the bromide ion, Br-, to form 1,2-dibromoethane

H
H
H
C
Br
H
C
Br
Br-
C
C
H
Br
H
H
H
1,2-dibromoethane
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• Evidence for Ionic Addition
• When ethene reacts with bromine in water in the
  
• presence of sodium chloride a number of
  compounds
• are formed
• - 1,2-dibromoethane
• - 1-bromo-2-chloroethane formed when the
  carbonium ion is attacked by the Cl- ion.

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- 2-bromoethanol formed when the carbonium ion
is attacked by the water molecule
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The syllabus requires you also know the mechanism
of ionic addition of Cl2 and HCl to ethene(Bk. Pg
369) Note a cyclic intermediate (stage 3) is
not formed in the case of addition of Cl2 or HCl
to ethene. This is because the Cl atom and the H
atom are to small to form a ring compound.
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Acidic Nature of the Carboxylic Acid
Group Ethanoic Acid is a far stronger acid than
ethanol. WHY??? - Inductive Effect -
Stability of carboxylate ion
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• Inductive Effect
• Upon closer analysis of the carboxyl functional
  group
• H atom is attached to O atom in C-O-H chain
• Carbon atom of carbonyl group is slightly
  positive and tends to attract electrons from the
  oxygen atom of the OH group INDUCTIVE EFFECT
• This effect facilitates the ionisation of the H
  atom in the O-H bond.
• Thus carboxylic acids lose the proton of the COOH
  group quite readily showing acidic properties

?-
O
?
C
?-
?
O
H
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• Stability of Carboxylate Ion
• When a carboxylic acid loses a proton from the
  carbonyl group it forms a negative ion called the
  carboxylate ion. Two structures of this ion are
  possible
• Each has a C-O single and C-O double bond.
• Experiment has shown that the two C-O bonds in
  the carboxylate ion are of equal length
• Suggests that the structure is in between the two
  structures shown
• Negative charge is not localised on one oxygen
  atom but is spread over the three atoms
  delocalised charge

O
-
O
C
R
R
C
-
O
O
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• New structure is a resonance hybrid of two
  possible structures and this gives it extra
  stability
• Fact that the carboxylate ion is so stable is the
  driving force for the carboxylic acid to lose a
  proton.

O
R
C
-
O