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Further Physical and Organic Chemistry

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Further Physical and Organic Chemistry Contents Kinetics Equilibria Acids and Bases Nomenclature and Isomerism in Organic Chemistry Compounds containing the Carbonyl ... – PowerPoint PPT presentation

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Title: Further Physical and Organic Chemistry


1
Further Physical and Organic Chemistry
2
Contents
  • Kinetics
  • Equilibria
  • Acids and Bases
  • Nomenclature and Isomerism in Organic Chemistry
  • Compounds containing the Carbonyl Group
  • Aromatic Chemistry
  • Amines
  • Amino Acids
  • Polymers
  • Structure Determination

3
Kinetics
  • The rate for the following reaction
  • A B ? C D
  • Is given by the following equation
  • Rate kAnBm
  • 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.

4
Equilibria
  • You still need to know the information you learnt
    for AS.
  • The equilibrium law states for the following
    reaction
  • aA bB Â cC dD
  • Kc Cc Dd
  • AaBb
  • If the reaction involves gases pressures are used
    instead of concentration and the equilibrium
    constant is called Kp

5
Equilibria
  • Kc Cc Dd
  • AaBb
  • 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.

6
Acids and Bases
  • 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 -log10H, where represents the
    concentration in mol dm-3.
  • A related concept is the ionic product of water,
    Kw
  • Kw HOH- 10-14 mol2 dm-6 at 25C
  • Note that in water HOH- therefore pH of
    water is 7

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 Â H A-
  • Ka H A-
  • HA
  • A large Ka value shows a large degree of
    dissociation.
  • pKa is the negative logarithm of the Ka

8
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.

9
Geometric Isomerism
  • Geometric isomerism is a specific form of
    stereoisomerism.

H
H
CC
Cl
Cl
Trans isomer
Cis isomer
Trans isomer
Cis isomer This form
of isomerism arises because there is no rotation
about the double bond
10
Optical Isomerism
H
H
OH
Br
Cl
Cl
Br
OH
These two molecules cannot be superimposed. If
you dont 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
11
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.

12
Tests for Carbonyl Compounds
  • Bradys 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
  • Tollens reagent gives a silver mirror
  • Benedicts solution gives a red precipitate
  • Acidified dichromate turns from orange to green
  • These rests are specific to aldehydes and not
    ketones!

13
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.

14
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.

15
Reactions of Arenes
  • Benzene reacts with only very reactive
    electrophiles.

H2SO4
NO2 H2O
HNO3
FeBr3/heat
Br HBr
Br2
FeCl3/heat
Cl2
C2H5 HCl
16
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.

17
Amino acids
  • Amino acids have the following structure

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.
CO
A peptide Link
H N
18
Polymers
  • Addition polymers may be formed directly from
    compounds containing CC 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
  • and polyamides nave the following linkage between
    the repeat units
  • Polyesters and polyamides can be broken down by
    hydrolysis and are, therefore, biodegradable

CO
H N
19
Structure Determination
  • Mass spectrometry
  • Infra-red spectroscopy
  • Nuclear magnetic resonance spectroscopy

20
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.

21
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
22
Nuclear Magnetic Spectroscopy
  • 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, d , 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 n1 of spin-spin splitting.

23
Summary
  • Kinetics
  • The rate of reaction is defined as kAnBm
    where n and m represent the orders of the
    reaction
  • Equilibria
  • Equilibria are defined by the following equation
  • Kc Cc Dd
  • AaBb
  • Acids and Bases
  • An acid is a proton donor and a base is a proton
    acceptor
  • Nomenclature and Isomerism in Organic Chemistry
  • In A2 isomerism includes stereoisomerism which is
    concerned with the arrangement of atoms in space.
  • Compounds containing the Carbonyl Group
  • The carbonyl group is a polar group which leaves
    the carbon susceptible to nucleophilic attack
  • Aromatic Chemistry
  • The delocalised electrons in the arene ring
    confer stability on these molecules, although
    they do undergo electrophilic substitution under
    certain conditions
  • Amines
  • Amines act as bases and as nucleophiles
  • Amino Acids
  • Amino acids form zwitterions and make up the back
    bone of poly peptides
  • Polymers
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