Organic Chemistry

1
Organic Chemistry
2
Introduction
Important definitions HOMOLOGOUS SERIES a
family of organic compounds which all fit the
same general formula, neighbouring members differ
from each other by CH2, have similar chemical
properties and show trends in physical
properties. e.g. the alkanes all fit the general
formula CnH2n2, members of the family include
methane (CH4), ethane (C2H6) and propane (C3H8).
3
EMPIRICAL FORMULA the simplest ratio of moles
of atoms of each element in a compound. MOLECULAR
FORMULA the actual number of moles of atoms of
each element in a compound. STRUCTURAL FORMULA
(aka displayed formula or graphical formula)
shows all of the atoms and the bonds between
them. e.g. pentane (C5H12).
4
A condensed structural formula can also be used
which omits the bonds. So for pentane we can
use CH3CH2CH2CH2CH3 or CH3(CH2)3CH3 The formulas
in the data book are called SKELETAL formulas.
These must NOT be used in the exam. STRUCTURAL
ISOMERS compounds with the same molecular
formula but a different arrangement of atoms.
e.g. C4H10 represents
5
butane and
6
2-methylpropane
7
FUNCTIONAL GROUP atom or group of atoms which
gives an organic compound its characteristic
chemical properties.
8
Alkanes
suffix ane
For example
CH3CH3
ethane
9
Alkenes
suffix ene
For example
ethene
CH2CH2
10
Halogenoalkanes
prefix halo -
X Cl, Br or I
For example
chloroethane
CH3CH2Cl
11
Alcohols
suffix ol or prefix hydroxy-
For example
CH3CH2OH
ethanol
2-hydroxypropanoic acid
CH3CH(OH)COOH
12
Aldehydes
suffix al
For example
ethanal
CH3CHO
13
Ketones
suffix one or prefix oxo-
For example
propanone
CH3COCH3
3 oxobutanoic acid
CH3COCH2COOH
14
Carboxylic acid
suffix oic acid
For example
ethanoic acid
CH3COOH
15
Amines
suffix amine or prefix amino -
For example
CH3NH2
methylamine
aminoethanoic acid
H2NCH2COOH
16
Esters
suffix oate
For example
CH3COOC2H5
ethyl ethanoate
17
Aromatic compounds
Contain the BENZENE ring
formula C6H6
18
Some examples
(1-methylpropyl) benzene
nitrobenzene
4-chloro-3-methylbenzenecarboxylic acid
1-bromo-2-chlorobenzene
19
Sometimes the benzene ring is not regarded as the
key part of the molecule.
In these cases it is referred to as the PHENYL
group.
For example
phenylamine
20
aldehyde
butanal
ketone
pentan-3-one
carboxylic acid
3-methylbutanoic acid
21
alkene
alcohol
ketone
ester
carboxylic acid
22
carboxylic acid
ester
aldehyde
ether
amine
nitrile
23
Physical Properties of Organic Molecules

• Alkanes worksheet
• What type of intermolecular force would you
  expect to find between alkanes, halogenoalkanes,
  aldehydes, ketones, alcohols and carboxylic
  acids?
• Use this information to deduce the relative
  boiling points of these homologous series and
  their solubility in water.

24
The Alkanes

• The alkanes is an homologous series where all
  members fit the general formula CnH2n2.
• They have trends in physical properties e.g.
  density and m.p. and b.p. all increase with Mr.
• They all undergo similar chemical reactions.
• Alkanes are SATURATED HYDROCARBONS. i.e. they
  contain only single C to C bonds and are made up
  of C and H atoms only.

25
Alkanes are obtained from crude oil by fractional
distillation. They are mainly used as fuels. The
large Mr alkanes do not ignite easily so there is
little demand for them as fuels so they are
CRACKED to make smaller more useful alkanes and
alkenes. Apart from combustion alkanes undergo
few chemical reactions. This is for two main
reasons
26

• The bonds in alkanes are relatively strong.
• The bonds have a relatively low polarity as the
  electronegativity of C and H is similar.
• As a consequence alkanes can be used as
  lubricating oils, although they do degrade over
  time.

27
Reactions of Alkanes

• Combustion
• Alkanes burn exothermically to produce carbon
  dioxide and water if there is a plentiful supply
  of oxygen. This is known as complete combustion.
• e.g. CH4 2O2 ? CO2 2H2O
• Write equations for the complete combustion of
  butane and octane.

28
If there is a limited supply of oxygen incomplete
combustion occurs and carbon monoxide or carbon
are formed instead of carbon dioxide. e.g. CH4
1½O2 ? CO 2H2O CH4 O2 ? C 2H2O What
problems do the gases released on combustion of
alkanes cause?
29
Chlorination Methane does not react with chlorine
in the dark but in the presence of ultraviolet
light reacts to form hydrogen chloride. CH4 Cl2
? CH3Cl HCl The mechanism for this reaction is
known as free radical substitution. Substitution
replacement of an atom or group of atoms by a
different atom or group of atoms.
30
Free radical species with an unpaired
electron. Free radicals are formed by homolytic
fission of bonds. In homolytic fission one
electron from the shared pair goes to each
atom. So
31
or Cl2 ? 2Cl.
Heterolytic fission of Cl Cl would result in
the formation of Cl and Cl-. There are three
steps in the mechanism initiation, propagation
and termination
32
Free radical substitution
chlorination of methane
i.e. homolytic breaking of covalent bonds
Overall reaction equation
Conditions
ultra violet light (breaks weakest bond)
excess methane to reduce further substitution
33
Free radical substitution mechanism
UV Light
initiation step
two propagation steps
termination step
minor termination step
Also get reverse of initiation step occurring as
a termination step.
34
Further free radical substitutions
Overall reaction equations
Conditions
35
Write down two propagation steps to explain the
formation of dichloromethane. Methane reacts in
exactly the same way with bromine to form
hydrogen bromide together with bromomethane,
dibromomethane, tribromomethane and
tetrabromomethane. Write down the mechanism for
the reaction between chlorine and ethane to form
chloroethane. Use the mechanism to explain why
small amounts of butane are formed. How could
the formation of further substitution products be
minimised?
36
The Alkenes

• All fit the general formula CnH2n.
• Are unsaturated hydrocarbons as they contain a C
  C.
• Much more reactive than alkanes.
• Industrial importance of alkenes
• Making polymers (plastics)
• Hydrogenation of vegetable oils to make margarine
• Hydration of ethene to make ethanol.

37
When naming alkenes have to include position of
double bond, for example CH3CHCHCH3 is but - 2
- ene and CH3CH2CHCH2 is but -1- ene Draw out
and name all of the alkenes with the molecular
formula C6H12. Alkenes undergo ADDITION
reactions. Two substances combine to form one
new substance. Unsaturated molecules are
converted to saturated molecules.
38
Reactions of Alkenes

1. Addition of hydrogen (hydrogenation) Alkenes
   react with hydrogen in the presence of a nickel
   catalyst at 180 C to form an alkane. e.g.

C2H4 H2 ? C2H6
ethene
ethane
39
Most oils are esters of propane-1,2,3-triol (aka
glycerol) with 3 long chain carboxylic acids (aka
fatty acids). The esters are sometimes called
triglycerides. Hydrogenation of these oils
produces margarine.

• The common fatty acids include
• octadeca-9-enoic (oleic) acid unsaturated acid
  found in most fats and olive oil
• octadeca-9,12-dienoic (linoleic) acid
  unsaturated acid found in many vegetable oils
  such as soyabean and corn oil

40
Above is the triglyceride formed between
propan-1,2,3-triol and oleic acid. Hydrogenation
using a nickel catalyst and slight pressure
removes some of the CC. This enables the chains
to pack together more closely which increases the
van der Waals forces and hence m.p. so the oils
are solidified forming margarine.
41

• Addition of halogens (halogenation)
• Halogens react with alkenes at room temperature
  and pressure in a non-polar solvent to form a
  dihalogenoalkane.
• e.g.

C2H4 Br2 ? C2H4Br2
ethene
1,2-dibromoethane
42
Bromine water is used as a test for
unsaturation. In the presence of an alkene,
bromine water turns from red brown to colourless.
Alkanes do not react with bromine water.
43
Bromine Water Test For Alkenes
colorless
amber
colorless
44
3. Reaction with hydrogen halides
(hydrohalogenation) Alkenes react with hydrogen
halides (HCl, HBr etc.) to form halogenoalkanes.
The reaction occurs at room temperature and
pressure. e.g.
H
H
HBr
H C C H
H
Br
C2H4 HBr ? CH3CH2Br
ethene
bromoethane
45

• Hydration (reaction with water)
• This can be done in two ways
• a) Addition of concentrated sulphuric acid to
  form an alkyl hydrogensulphate. Water is then
  added to hydrolyse the product and produce an
  alcohol. The sulphuric acid is regenerated.

46
C2H4 H2SO4 ? CH3CH2OSO3H
CH3CH2OSO3H H2O ?C2H5OH H2SO4
ethanol
47
b) Alkenes can also undergo direct hydration to
form an alcohol. Ethene can be converted to
ethanol by reaction with steam in the presence of
a phosphoric(V) acid (H3PO4) catalyst at a
pressure of 60 70 atm and a temperature of 300
C.
What advantages and disadvantages does this
method have over production of ethanol by
fermentation?
48
5. Addition Polymerisation The formation of
polymers involves alkenes reacting with
themselves to form a long chain molecule called a
polymer. The individual molecules used to make
the polymer are called monomers. Ethene is
polymerised to form poly(ethene)
n about 100 to 10 000
49
monomer repeating unit polymer typical uses
CH2CH2 - CH2 CH2 - poly(ethene) polythene film, bags
poly(propene) polypropylene Moulded plastic, fibres
poly(phenylethene) polystyrene packaging, insulation
CH2CHCH3
- CH2 CH -
CH3
- CH2 CH -
CH2CHC6H5
C6H5
50
monomer repeating unit polymer typical uses
pipes, flooring

poly(chloroethene) polyvinylchloride PVC
CH2CHCl
- CH2 CH -
Cl
Poly (tetrafluoroethene) PTFE
CF2CF2
- CF2 CF2 -
Non-stick coating (Teflon)
51
Alcohols

• Alcohols are the homologous series with the
  general formula CnH2n1OH.
• They all contain the functional group, OH, which
  is called the hydroxyl group.
• Alcohols can be classified as primary, secondary
  or tertiary, depending on the carbon skeleton to
  which the hydroxyl group is attached.

52
RCH2OH 1 alkyl group on C next to OH so primary
alcohol, 1
R2CHOH 2 alkyl groups on C next to OH so
secondary alcohol, 2
R3COH 3 alkyl groups on C next to OH so tertiary
alcohol, 3
Draw out the structure, name and classify all the
alcohols with the formula C4H9OH.
53
Butan-1-ol primary
Butan-2-ol secondary
54
2-methylpropan-1-ol primary
2-methylpropan-2-ol tertiary
55
Reactions of Alcohols

• Combustion
• In countries such as Brazil, ethanol is mixed
  with petrol and used to power cars. Ethanol is
  less efficient as a fuel than petrol as it is
  already partially oxidised but does make the
  country less reliant on supply of petrol. As it
  can be produced by fermentation of sugar beet,
  many consider ethanol a carbon neutral fuel.

56

• Oxidation of Alcohols
• Primary alcohols are oxidised first to
  aldehydes, such as ethanal. A suitable oxidising
  agent is acidified potassium dichromate(VI)

Cr2O7/H

H2O
ethanol
ethanal
57
An aldehyde still has one hydrogen atom attached
to the carbonyl carbon, so it can be oxidised one
step further to a carboxylic acid.
Cr2O7/H
ethanal
ethanoic acid
58
In practice, a primary alcohol such as ethanol is
dripped into a warm solution of acidified
potassium dichromate(VI). The aldehyde, ethanal,
is formed and immediately distils off, thereby
preventing further oxidation to ethanoic acid,
because the boiling point of ethanal (23 C) is
much lower than that of either the original
alcohol ethanol (78 C) or of ethanoic acid (118
C). Both the alcohol and the acid have higher
boiling points because of hydrogen bonding. If
oxidation of ethanol to ethanoic acid is
required, the reagents must be heated together
under reflux to prevent escape of the aldehyde
before it can be oxidised further.
59
Secondary alcohols are oxidised to ketones. These
have no hydrogen atoms attached to the carbonyl
carbon and so cannot easily be oxidised further.
Cr2O7/H
propan-2-ol
propanone
60
Distinguishing between 1, 2 and 3 alcohols
When orange acidified potassium dichromate(VI)
acts as an oxidising agent, it is reduced to
green chromium(III) ions. Primary and secondary
alcohols both turn acidified dichromate(VI)
solution from orange to green when they are
oxidised, and this colour change can be used to
distinguish them from tertiary alcohols.
Tertiary alcohols are not oxidised by acidified
dichromate(VI) ions, so they have no effect on
its colour, which remains orange.
61
Halogenoalkanes
Named by using the name of the alkane from which
they are derived with the prefix chloro-, bromo-
or iodo-.
For example CH3CH2Br is bromoethane (CH3)2CHCH2C
l is 1-chloro-2-methylpropane
62
Remember the position of the halogen atom must be
indicated using the appropriate number so
CH3CH2CH2Cl is 1-chloropropane and CH3CHClCH3 is
2-chloropropane Halogenoalkanes can be classified
in the same way as alcohols. Draw out, name and
classify all the isomers with the formula C5H11Br.
63
Key feature of halogenoalkanes is
C X
where X Cl, Br or I
What is notable about this bond compared with
say, C C and C H?
The halogen atom is more electronegative than C
so the bond is polarised
?
?-
C X
64
ORDER OF BOND POLARITIES
?
?-
?
?-
?
?-
C Cl
C I
C Br
gt
gt
So is order of reactivity chloroalkane gt
bromoalkanes gt iodoalkanes?
Is there another factor that ought to be
considered before reaching a conclusion?
BOND ENERGIES
65
Bond energies
Bond Bond energy in kJmol-1
C - Cl
C - Br
C - I
346
290
234
This suggests that the order of reactivity
is iodoalkane gt bromoalkanes gt chloroalkanes
66
Theres only one way to find out which is
best! Do an experiment, not fight! But what do
halogenoalkanes react with?
The ? carbon atom is susceptible to attack by
NUCLEOPHILES. A nucleophile is a species with a
lone pair of electrons. e.g. OH-, NH3, CN-. When
attack by a nucleophile occurs, the carbon
halogen bond breaks releasing a halide ion. A
suitable nucleophile for experimentation is OH-
from an aqueous solution of an alkali such as
sodium hydroxide.
67
CH3CH2X OH- ? CH3CH2OH X-
X Cl, Br or I.
OH has replaced the X so overall we have
NUCLEOPHILIC SUBSTITUTION
How can we follow the rate of this reaction so
that we can determine the order of reactivity?
Halide ions ? coloured precipitates when silver
nitrate is added. So we can measure how long it
takes for a precipitate to form. See EXPERIMENT
SHEET.
68
Mechanisms for nucleophilic substitution

• SN1 unimolecular nucleophilic substitution
  (only one species in the slow step of the
  mechanism, rate determining step)
• SN2 bimolecular nucleophilic substitution (two
  species in the slow step of the mechanism, rate
  determining step)
• Use of curly arrows
• Curly arrows are used in reaction mechanisms to
  show the movement of electron pairs.

69
TAILS come from
eithera bond pairof electrons
or a lone pairof electrons
HEADS point
eithernext to an atom
orat an atom
to form a bond pair of electrons
to forma lone pairof electrons
X
70
Heterolytic fission of C Br bond
slow

Intermediate carbocation
fast
SN1
71
Transition state

SN2
72
Which is best? SN1 or SN2? For primary
halogenoalkanes SN2 For tertiary
halogenoalkanes SN1 3 halogenoalkanes cannot
undergo the SN2 mechanism as 5 bulky groups would
not fit around the C in the transition state -
steric hindrance. 1 halogenoalknes are less
likely to undergo SN1 as this would involve the
formation of a primary carbocation as an
intermediate. Alkyl groups push electron density
to the C atom they are attached to (positive
inductive effect) which stabilises the positive
charge. More alkyl groups mean a more stable
carbocation.
73
2 halogenoalkanes react via a mixture of SN1 and
SN2. The mechanism predominating depends upon
the nature of the alkyl groups and the nature of
the solvent.