Download Further Physical and Organic Chemistry

Further Physical and Organic
Chemistry
Contents
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Kinetics
Equilibria
Acids and Bases
Nomenclature and Isomerism in Organic
Chemistry
Compounds containing the Carbonyl Group
Aromatic Chemistry
Amines
Amino Acids
Polymers
Structure Determination
Kinetics
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The rate for the following reaction:
A+BC+D
Is given by the following equation
Rate =k[A]n[B]m
The larger the value of k the faster the rate of reaction.
Increasing the temperature increases the value of k
m and n are the orders of reaction with respect to A and B
If the order of reaction is zero the reactant does not effect the rate of
reaction.
• If the order of reaction is 1 doubling the concentration of that
reactant doubles the rate of reaction.
• If the order of reaction is 2 doubling the concentration of that
reactant quadruples the rate of reaction.
• The units of a rate constant vary according to the actual rate
equation for the reaction.
Equilibria
• You still need to know the information you learnt
for AS.
• The equilibrium law states for the following
reaction:
– aA + bB  cC + dD
c [D]d
[C]
Kc =
[A]a[B]b
• If the reaction involves gases pressures are
used instead of concentration and the
equilibrium constant is called Kp
Equilibria
c [D]d
[C]
Kc = a b
[A] [B]
• Kc can only be changed by changing
temperature.
• In an exothermic reaction Kc decreases with
increasing temperature, this means less
products are formed.
• Increasing the concentration of one of the
reactants does not change the value of Kc but
does mean that the concentration of products
must also increase.
Acids and Bases
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A Bronsted-Lowry acid is a proton donor
A Bronsted-Lowry base is a proton acceptor
pH is a measure of the strength of acid
pH = -log10[H+], where [ ] represents the
concentration in mol dm-3.
• A related concept is the ionic product of water,
Kw
• Kw = [H+][OH-] = 10-14 mol2 dm-6 at 25°C
• Note that in water [H+]=[OH-] therefore pH of
water is 7
Strong and Weak Acids and Bases
• In a strong acid or base the ions are fully dissociated.
• This means that pH can be calculated using the
equations on the previous page.
• Weak acids and bases are partially dissociated the
degree of dissociation is measured using the acid
dissociation constant, Ka.
HA  H+ + A+ ] [A-]
[H
Ka =
[HA]
• A large Ka value shows a large degree of dissociation.
• pKa is the negative logarithm of the Ka
Nomenclature and Isomerism
• You still need to know how to name the
compounds that you learnt in AS
• You need to understand the difference and
between structural isomerism and
stereoisomerism.
• In structural isomerism the order that the atoms
are joined together are different.
• In stereoisomerism the atoms are joined
together in the same order but are arranged
differently in space.
Geometric Isomerism
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Geometric isomerism is a specific form of stereoisomerism.
Cl
H
H
C=C
H
H
C=C
Cl
Trans isomer
Trans isomer
Cl
Cl
Cis isomer
Cis isomer
This form of isomerism arises because there is no rotation
about the double bond
Optical Isomerism
H
H
C
OH
Cl
Br
C
Br
Cl
OH
These two molecules cannot be superimposed. If you don’t believe it
build a model.
This form of isomerism is called optical isomerism because the different
isomers can rotate polarised light in different directions
The central carbon is called a chiral carbon.
The two isomers are called enantiomers.
A mixture of isomers is called a racemic mixture
Carbonyl Compounds
• The carbon oxygen double bond is a polar bond.
This leaves the carbon atom electron deficient
and attractive to nucleophiles.
• Cyanide is a nucleophile can be added across
the carbonyl bond, this is a useful reaction for
increasing the length of the carbon chain.
• The resulting nitriles are hydrolysed by water in
hot acid to form carboxylic acid.
• Nitriles are reduced by sodium in ethanol to form
an amine.
Tests for Carbonyl Compounds
• Brady’s reagent produces bright orange crystals
with carbonyl compounds.
• The melting point of these crystals have very
sharp melting points which can be looked up in
data books to identify the original compound.
• There are three tests for aldehyde:
– Tollen’s reagent gives a silver mirror
– Benedict’s solution gives a red precipitate
– Acidified dichromate turns from orange to green
• These rests are specific to aldehydes and not
ketones!
Carboxylic Acids and Esters
• Carboxylic acids are weak acids but will liberate CO2
from carbonates.
• Carboxylic acids and alcohols react together, in the
presence of a
strong acid catalyst, to give esters.
• Esters often have characteristic fruity smells ( think of
fruit flavoured sweets).
• Esters are used as solvents, plasticisers and
food flavourings.
• Esters can be hydrolysed into their component acids
and alcohols. This is important in the production of soap,
glycerol and higher fatty acids from naturally-occurring
esters.
Aromatic Chemistry
• An arene is a compound which contains a
benzene ring.
• Benzene has the formula C6H6. It is represented
using the following symbol.
This is represents the
delocalised bonding that exists in
benzene. Each carbon –carbon bond is
approximately half way between the
length of a single bond and double bond. Having
the six electrons delocalised over the whole ring
confers extra stability.
Reactions of Arenes
Benzene reacts with only very reactive electrophiles.
+ HNO3
+ Br2
+Cl2
H2SO4
FeBr3/heat
FeCl3/heat
NO2 + H2O
Br
+
C2H5 +
HBr
HCl
Amines
• Amines can be categorised into primary, secondary and tertiary in a
similar manner to alcohols.
• Amines act as Bronsted –Lowry bases, that is they accept hydrogen
ions.
• Electron donating groups such as alkyl groups make the lone pair of
electrons on the nitrogen atom more negative and attractive to
hydrogen ions and increase the basic strength of the amines
compared to ammonia.
• Electron withdrawing groups such as arenes have the opposite
effect.
• Amines can be prepared by the reduction of nitriles and nitro
compounds.
• Amines and ammonia act as nucleophile with haloalkanes to form
primary and secondary amines.
• Amines have a characteristic smell of rotting fish.
Amino acids
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Amino acids have the following structure:
R
H2N –C –COOH
R
+H3N –C –COO-
H
H
As shown the molecule can have both negative and positive components, this is
called a zwitterion. In acidic solutions the ion has an overall positive charge and
in alkaline solutions the ion has an overall negative charge.
Proteins are sequences of amino acids joined by peptide links. These links can
be hydrolysed to produce the constituent amino acids.
C=O
A peptide Link
H –N
Polymers
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Addition polymers may be formed directly from compounds containing C=C
bonds.
Polyalkenes are chemically inert and therefore non biodegradable.
Condensation polymers may be formed by reactions between dibasic acids
and diols to form polyesters, and between dicarboxylic acids and diamines
or between amino acids to form polyamides.
Polyesters have the following linkage between the repeating units:
O
–C
O–
and polyamides nave the following linkage between the repeat units:
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Polyesters and polyamides can be broken down by hydrolysis and are,
therefore, biodegradable
H –N
C=O
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Structure Determination
• Mass spectrometry
• Infra-red spectroscopy
• Nuclear magnetic resonance spectroscopy
Mass Spectroscopy
• Mass spectrometry can be used to determine
the molecular formula of a compound from the
mass of the molecular ion.
• The fragmentation of a molecular ion:
• M+. → X+ + Y.
• gives rise to a characteristic relative abundance
spectrum.
• The more stable X+ species give higher peaks.
Infra Red Spectroscopy
• Certain groups in a molecule absorb infra-red radiation at
characteristic frequencies.
• Fingerprinting allows identification of a molecule by comparison of
spectra in a database.
• Looking at a spectrum it is possible to identify particular functional
groups and to identify impurities with reference to data books.
Characteristic OH absorption at 3300cm-1
Absorption
Fingerprint region
characteristic of
each substance
Wave numbers cm-1
Nuclear Magnetic Spectroscopy
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Nuclear magnetic resonance gives information about the relative number
and position of hydrogen atoms in a molecule.
Proton n.m.r. spectra are obtained using samples dissolved in proton-free
solvents (e.g. deuterated solvents and CCl4).
Chemical shift, δ , is measured next to a standard tetramethylsilane (TMS).
The chemical shift of an individual proton depends on the molecular
environment.
The area under the individual peaks on a proton n.m.r spectrum are
proportional to the number of protons with that chemical shift.
N.m.r can tell us the number of different types of protons and how many of
each type of proton there are.
In addition there is an interaction between protons on adjacent carbons
called spin-spin coupling, which leads to a splitting pattern.
If a peak is split into two there is one proton on the adjacent carbon, if it is
spit into three there are two protons on the adjacent carbon.
This is called the n+1 of spin-spin splitting.
Summary
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Kinetics
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Equilibria
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The rate of reaction is defined as k[A]n[B]m where n and m represent the orders of the
reaction
Equilibria are defined by the following equation
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Kc = [C] a[D] b
c
d
[A] [B]
Acids and Bases
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An acid is a proton donor and a base is a proton acceptor
Nomenclature and Isomerism in Organic Chemistry
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In A2 isomerism includes stereoisomerism which is concerned with the arrangement of
atoms in space.
Compounds containing the Carbonyl Group
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The carbonyl group is a polar group which leaves the carbon susceptible to nucleophilic
attack
Aromatic Chemistry
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The delocalised electrons in the arene ring confer stability on these molecules, although
they do undergo electrophilic substitution under certain conditions
Amines
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Amines act as bases and as nucleophiles
Amino Acids
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Amino acids form zwitterions and make up the back bone of poly peptides
Polymers
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Polymers are useful long chain molecules made up from repeating units of much smaller
monomers.
Structure Determination
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The structure of molecules can be achieved by a variety of spectroscopic techniques either in
isolation or together.