RPA

Question 1

compare the Kekulé and delocalised models for benzene in terms of p-orbital overlap

Answer 1

Kekule: pi electrons are very isolated
delocalised: extensive sideways overlap of p-orbitals produces a system of pi bonds which are spread out over the whole carbon ring. Because the electrons are no longer held between just two carbon atoms, but are spread over the whole ring, the electrons are said to be delocalised.

Question 2

review the evidence for a delocalised model of benzene as opposed to kekulé

Answer 2

bond lengths - x-rays show bond lengths are all the same, somewhere in between c=c and c-c (kekule would have 2 types of bonds)
enthalpy change of hydrogenation - benzene is more stable (has a less exothermic ∆Hhydrogenation), than expected, of the kekule structure due to delocalisation of pi-electrons lowering the energy
resistance to reaction - where kekulé

Question 3

describe two examples of electrophilic substitution of arenes (must be able to draw mechanisms too) - conditions/reagents

Answer 3

1. mono nitration of benzene: concentrated nitric acid in the presence of concentrated sulfuric acid catalyst, carried out at 50degrees C
2. monohalogenation of benzene: halogen in the presence of a halogen carrier catalyst; carried out in the dark at room temp.

Question 4

explain the relative resistance to bromination of benzene, compared with alkenes

Answer 4

delocalised pi-electron system has insufficient electron density to polarise Br-Br bond and react (decolourising it) so requires a catalyst to react in substitution with non-diluted bromine.
localised electron density of the c=c bond is sufficient to polarise Br-Br bond. Cyclic alkenes will react with cold dilute bromine water with no catalyst required in an addition reaction

Question 5

describe the reactions of phenol with aqueous alkalis and with sodium

Answer 5

With aqueous alkalis phenol acts as an acid, forming water and salt (phenoxide ion)
Phenol donates protons more readily than alcohols because the presence of the benzene ring weakens the O-H bond.
With sodium, phenol gives sodium phenoxide salt and hydrogen gas.

Question 6

describe the reaction of phenol with bromine

Answer 6

2,4,6-tribromophenol is formed.

No catalyst required and occurs with bromine water

Question 7

explain the relative ease of bromination of phenol compared with benzene
- at which sites does substitution occur?

Answer 7

No catalyst required and occurs with bromine water (benzene requires catalyst and pure bromine to react) showing the increased reactivity of the benzene ring in phenol. This is because there is an increased electron density on the ring due to electron-pair donation to the benzene ring from an oxygen p- orbital in phenol (lone pair electrons on oxygen delocalised on the benzene ring).
Substitution occurs at 2,4,and 6 on the ring because of the increased electron density at these sites compared to others.

Question 8

state the uses of phenols

Answer 8

production of plastics, antiseptics, disinfectants and resins for paints.

Question 9

describe the oxidation of primary alcohols

Answer 9

Oxidising agent Cr2O72–/H+ (ie K2Cr2O7/H2SO4), heat. Colour change form orange to green.
the oxidation of primary alcohols forms aldehydes and carboxylic acids;
The oxidation can be carefully controlled and the product distilled immediately so that further oxidation to a carboxylic acid does not occur
primary alcohol + [O] –> aldehyde + H2O

Question 10

describe the oxidation of secondary alcohols

Answer 10

Oxidising agent Cr2O72–/H+ (ie K2Cr2O7/H2SO4), heat under reflux. Colour change form orange to green.

primary alcohol + [O] –> ketone + H2O

Question 11

describe the reduction of carbonyl compounds using NaBH4

Answer 11

forms alcohol:
ketone + 2[H] –> secondary alcohol
aldehyde + 2[H] –> primary alcohol

Question 12

outline the mechanism for nucleophilic addition reactions of aldehydes and ketones with hydrides, such as NaBH4

Answer 12

(draw)

Question 13

describe the use of 2,4-dinitrophenylhydrazine

Answer 13

To detect the presence of a carbonyl group in an organic compound: an orange crystalline precipitate forms (2,4-dinitrophenylhydrazone)

the exact carbonyl compound can be identified from the melting point of this derivative after it has been recrystallised.

Question 14

describe the use of Tollens’ reagent (ammoniacal silver nitrate)

Answer 14

To detect the presence of an aldehyde group,

To distinguish between aldehydes and ketones: aldehydes are oxidised to carboxylic acids with the reduction of silver ions to silver so a silver mirror forms. Ketones do not react.

Question 15

explain the water solubility of carboxylic acids

Answer 15

they form hydrogen bonds with surrounding water molecules

negative dipole on lone pair of O in carboxylic acid interacts with positive dipole on H of water molecule

Question 16

describe the reactions of carboxylic acids with metals, carbonates and bases

Answer 16

.

Question 17

describe esterification of carboxylic acids

Answer 17

with alcohols, in the presence of an acid catalyst forms ester + water

Question 18

describe esterification of acid anhydrides

Answer 18

acid anhydride + alcohol –> carboxylic acid + ester
better yield of ester obtained than ester made from carboxylic acid
no catalyst required

Question 19

describe the acid hydrolysis of esters

Answer 19

ester is heated under reflux with dilute H2SO4/HCl

ester is broken down by water to form carboxylic acid + alcohol

Question 20

describe the alkaline hydrolysis of esters

Answer 20

aqueous sodium/potassium hydroxide is refluxed with the ester leading to the formation of carboxylate salts and alcohol.
Sometimes called saponification - basis of soap making/

Question 21

state the uses of esters

Answer 21

in perfumes and flavourings

Question 22

what is a triglyceride?

Answer 22

a triester of glycerol (propane-1,2,3-triol) and fatty acids
glycerol - an alcohol with three OH groups
fatty acids - long chain carboxylic acids whose hydrocarbon chains can be saturated or unsaturated

Question 23

compare the structures of saturated fats, unsaturated fats and fatty acids
cis/trans?

Answer 23

saturated - no C=C in hydrocarbon chain
unsaturated - C=C present in hydrocarbon chain
fatty acids can be saturated or unsaturated
Unsaturated fats can be cis or trans isomers of each other. In cis fat molecules cannot pack closely together so exist in liquid form. Trans fats are more linear and can pack together closely and thus have a high melting point than cis fats so are more solid.

Question 24

Explain the significance of this shorthand formula and name the fatty acids each example refers to

1. 16,0
2. 18,1(9)
3. 18,2(9,12)

Answer 24

1. 16 carbon atoms, no double bonds, hexadecanoic acid
2. 18 carbon atoms, 1 double bond positioned on carbon 9. Octadec-9-enoic acid
3. 18 carbon atoms, 2 double bonds positioned on carbons 9 and 12. Octadec-9,12-enoic acid,

Question 25

Describe the link between unsaturated and saturated fats and current concerns about heart disease and obesity.

Answer 25

trans fatty acids are associated with low-density lipoproteins (LDLs, carry cholesterol + triglycerides from liver –> tissues) - which carry a ‘bad’ form of cholesterol, proven to cause an increased risk of coronary heart disease and strokes due to deposits of atheroma (made up in part of cholesterol) which block arteries.

Question 26

describe and explain the increased use of esters of fatty acids as biodiesel

Answer 26

biodiesel is a renewable fuel produced by the transesterification of triglycerides in fats and oils as a cheaper, more environmentally friendly alternative to fossil fuels

Question 27

explain the basicity of amines

Answer 27

the lone pair on nitrogen is able to form a dative covalent bond with hydrogen and is therefore a proton acceptor

Question 28

describe the reactions of amines with acids

Answer 28

they form salts

acid + base –> salt

Question 29

describe the preparation of aliphatic amines

Answer 29

substitution of halogenoalkanes with excess ethanolic ammonia:
halogenoalkanes are warmed with an excess of ammonia, with ethanol as the solvent

Question 30

describe the preparation of aromatic amines

Answer 30

reduction of nitroarenes using tin and concentrated hydrochloric acid
nitrobenzene/other nitroarenes are reduced using a mixture of tin and conc. HCl heated under reflux

Question 31

describe the synthesis of an azo dye

- use of product?

Answer 31

Reaction of an aromatic amine with nitrous acid (

Question 32

what is the general formula for an α-amino acid?

Answer 32

RCH(NH2)COOH

Question 33

what is a zwitterion?

when do amino acids exist as zwitterions?

Answer 33

zwitterions are formed by the donation of a hydrogen ion from the carboxyl group to the amino group. They thus have no overall charge because both +ve and -ve charges are present.
at a pH value called the isoelectric point amino acids exist as zwitterions as there is no net electrical charge

Question 34

why might the isoelectric point differ for different amino acids?

Answer 34

presence of different R groups

Question 35

describe the acid–base properties of α-amino acids at different pH values

Answer 35

at a pH that is more acidic than the isoelectric point:
- the amino acid behaves as a base and accepts a proton from the acid
- the amino acid forms a positively charged ion
At a pH that is more alkaline than the isoelectric point:
- the amino acid behaves as an acid and donates a proton to the base
- the amino acid forms a negatively charged ion

Question 36

explain the formation of a peptide (amide) linkage between α-amino acids

Answer 36

condensation reaction - a dipeptide is formed with the elimination of a water molecule
-OC-NH- bond, peptide linkage

Question 37

how are polypeptides and proteins formed?

Answer 37

condensation polymerisation to form a long chain of amino acids joined together by peptide linkages
for each amino acid added, one molecule of water is formed

Question 38

describe the acid hydrolysis of proteins and peptides

Answer 38

Protein/peptide is heated under redux with aqueous 6moldm-3 HCl
amino acids formed are generally positively charged because of H+ ions from the acid.

Question 39

describe the alkaline hydrolysis of proteins and peptides

Answer 39

Polypeptide/protein is heated at just above 100degrees in a solution of aqueous sodium hydroxide and broken down into amino acids in the for of their sodium salts (-COO-Na+)

Question 40

what are optical isomers?

Answer 40

stereoisomers that are non-superimposable mirror images of each other about an organic chiral centre (four different groups attached to a carbon atom)
optical isomers rotate plane-polarised light differently

Question 41

how would you dentify chiral centres in a molecule of given structural formula?

Answer 41

the carbon atom with 4 different atoms/groups of atoms attached to it is chiral

Question 42

name two different types of stereoisomerism

Answer 42

optical isomerism and E/Z isomerism