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 =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.

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= [C]c [D]d [A]a[B]b • If the reaction involves gases pressures are used instead of concentration and the equilibrium constant is called Kp

5. Equilibria Kc= [C]c [D]d [A]a[B]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.

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 = -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

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. Cl H C=C H Cl Geometric Isomerism • Geometric isomerism is a specific form of stereoisomerism. H H C=C 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. C C Optical Isomerism H H OH Br Cl Cl Br 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

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 • 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!

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 astrong acid catalyst, to give esters. • Esters often have characteristic fruity smells ( think of fruit flavoured sweets). • Esters are used as solvents, plasticisers andfood 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. R R H2N –C –COOH +H3N –C –COO- H H 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. C=O A peptide Link H –N

18. –C O O– 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 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 C=O 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, 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.

23. Summary • Kinetics • The rate of reaction is defined as k[A]n[B]m where n and m represent the orders of the reaction • 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 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 • 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 in isolation or together.