Further Physical and Organic Chemistry

7/28/2019 Further Physical and Organic Chemistry

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Further Physical and Organic

Chemistry


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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


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Kinetics

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 ofreaction.

If the order of reaction is 1 doubling the concentration of thatreactant doubles the rate of reaction.

If the order of reaction is 2 doubling the concentration of thatreactant quadruples the rate of reaction.

The units of a rate constant vary according to the actual rateequation for the reaction.


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Equilibria

You still need to know the information you learntfor AS.

The equilibrium law states for the following

reaction: aA + bB cC+dD

Kc= [C]c[D]d

[A]a[B]b

IfthereactioninvolvesgasespressuresareusedinsteadofconcentrationandtheequilibriumconstantiscalledKp


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Equilibria

Kc= [C]c[D]d

[A]a[B]b

Kc can only be changed by changingtemperature.

In an exothermic reaction Kc decreases withincreasing temperature, this means lessproducts are formed.

Increasing the concentration of one of the

reactants does not change the value of Kc butdoes mean that the concentration of productsmust also increase.


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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 = -log10[H+], where [ ] represents theconcentration in mol dm-3.

A related concept is the ionic product of water,Kw

Kw = [H+][OH-] = 10-14 mol2 dm-6 at 25C

Note that in water [H+]=[OH-] therefore pH ofwater is 7


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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 theequations on the previous page.

Weak acids and bases are partially dissociated the

degree of dissociation is measured using the aciddissociation constant, Ka.HA H++A-

Ka= [H+][A-]

[HA]

A large Ka value shows a large degree of dissociation.

pKa is the negative logarithm of the Ka


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Nomenclature and Isomerism

You still need to know how to name thecompounds that you learnt in AS

You need to understand the difference and

between structural isomerism andstereoisomerism.

In structural isomerism the order that the atomsare joined together are different.

In stereoisomerism the atoms are joinedtogether in the same order but are arrangeddifferently in space.


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Geometric Isomerism

Geometric isomerism is a specific form of stereoisomerism.

C=C

Cl

ClH

H

C=C

Cl Cl

H H

Trans isomer Cis isomer

This form of isomerism arises because there is no rotation

about the double bond

Trans isomer Cis isomer


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Optical Isomerism

C

H

Cl

Br

OHC

H

ClBr

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


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Carbonyl Compounds

The carbon oxygen double bond is a polar bond.This leaves the carbon atom electron deficientand attractive to nucleophiles.

Cyanide is a nucleophile can be added acrossthe carbonyl bond, this is a useful reaction forincreasing 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.


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Tests for Carbonyl Compounds

Bradys reagent produces bright orange crystalswith carbonyl compounds.

The melting point of these crystals have verysharp 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!


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Carboxylic Acids and Esters

Carboxylic acids are weak acids but will liberate CO2from carbonates.

Carboxylic acids and alcohols react together, in thepresence of astrong acid catalyst, to give esters.

Esters often have characteristic fruity smells ( think offruit flavoured sweets).

Esters are used as solvents, plasticisers andfood flavourings.

Esters can be hydrolysed into their component acidsand alcohols. This is important in the production of soap,glycerol and higher fatty acids from naturally-occurringesters.


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Aromatic Chemistry

An arene is a compound which contains abenzene ring.

Benzene has the formula C6H6. It is representedusing the following symbol.

This is represents thedelocalised bonding that exists inbenzene. Each carboncarbon bond isapproximately half way between the

length of a single bond and double bond. Havingthe six electrons delocalised over the whole ringconfers extra stability.


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Reactions of Arenes

Benzene reacts with only very reactive electrophiles.

+ HNO3H2SO4

NO2 + H2O

+ Br2

+Cl2

FeBr3/heat

FeCl3/heat

Br + HBr

C2H5 + HCl


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Amines

Amines can be categorised into primary, secondary and tertiary in asimilar manner to alcohols.

Amines act as BronstedLowry bases, that is they accept hydrogenions.

Electron donating groups such as alkyl groups make the lone pair ofelectrons on the nitrogen atom more negative and attractive tohydrogen ions and increase the basic strength of the aminescompared to ammonia.

Electron withdrawing groups such as arenes have the oppositeeffect.

Amines can be prepared by the reduction of nitriles and nitrocompounds.

Amines and ammonia act as nucleophile with haloalkanes to formprimary and secondary amines.

Amines have a characteristic smell of rotting fish.


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Amino acids

Amino acids have the following structure:

H2NCCOOH

R

H

+H3NCCOO-

R

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

HN

A peptide Link


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Polymers 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 acidsand diols to form polyesters, and between dicarboxylic acids and diaminesor 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

C

O

O

C=O

H

N


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Structure Determination

Mass spectrometry

Infra-red spectroscopy

Nuclear magnetic resonance spectroscopy


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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.


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Infra Red Spectroscopy

Certain groups in a molecule absorb infra-red radiation atcharacteristic frequencies.

Fingerprinting allows identification of a molecule by comparison ofspectra in a database.

Looking at a spectrum it is possible to identify particular functionalgroups and to identify impurities with reference to data books.

Wave numbers cm-1

Absorption

Characteristic OH absorption at 3300cm-1

Fingerprint region

characteristic of

each substance


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Nuclear Magnetic Spectroscopy

Nuclear magnetic resonance gives information about the relative numberand position of hydrogen atoms in a molecule.

Proton n.m.r. spectra are obtained using samples dissolved in proton-freesolvents (e.g. deuterated solvents and CCl4).

Chemical shift, , 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 ofeach 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 isspit into three there are two protons on the adjacent carbon.

This is called the n+1 of spin-spin splitting.


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Summary Kinetics

The rate of reaction is defined as k[A]n[B]m where n and m represent the orders of thereaction

Equilibria

Equilibria are defined by the following equation Kc= [C]c[D]d

[A]a[B]b 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 ofatoms in space.

Compounds containing the Carbonyl Group

The carbonyl group is a polar group which leaves the carbon susceptible to nucleophilicattack

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 Polymers are useful long chain molecules made up from repeating units of much smaller

monomers. Structure Determination

The structure of molecules can be achieved by a variety of spectroscopic techniques either inisolation or together.

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Further Physical and Organic Chemistry