Chemistry A2 Level Ocr F324 Standard Answers

Front 1

Evidence for Kekule’s model to be wrong:

Back 1

 All C-C bond lengths are the same length, between C-C and C=C.
 Only reacts with Br2 with a halogen carrier
 Benzene is lower in energy than Kekule’s structure suggests its should be.

Front 2

Discuss the structure and bonding in benzene / (comparing to kekule - structure):

Back 2

Front 3

Discuss the relative low reactivity of benzene / (problems with Kekule – reactivity):

Back 3

Front 4

Discuss the reactivity of benzene compared to alkenes

Back 4

Front 5

Discuss the reactivity of phenol compared to benzene

Back 5

Front 6

Benzene VS. Cyclohexene

Back 6

Cyclohexene
 Electrophillic Addition
 Electrons are localised
 Between C-C 2e from  bond and 2e from localised bond = 4e
 Higher electron density, polarises electrophiles more
 Don’t need a halogen carrier
Benzene
 Electrophillic Substitution
 Electrons are delocalised
 Between C-C 2e from  bond and 1 e from the C-C from delocalised bond = 3e
 Lower electron density, polarises electrons less
 Need a halogen carrier

Front 7

Benzene VS. Phenol

Back 7

Phenol- Multiple Substitution
 Lone pair of electrons on O
 Delocalise with the  electrons in the Benzene ring
 Makes more electron rich
 Ring becomes activated, polarises electrophiles more
 Phenols do not need a halogen carrier
Benzene- Mono-substitution
 Electrophillic Substitution
 Electrons are delocalised
 Between C-C 2e from  bond and 1 e from pre C-C from delocalised  bond = 3e
 Lower electron density, polarises electrons less
 Need a halogen carrier

Front 8

Nitration of benzene:

Back 8

Front 9

Halogenation of benzene:

Back 9

Front 10

Carbonyl Test
Test for Carbonyl Group

Back 10

 2,4,DNPH (Brady’s Reagent)
 If present, orange precipitate formed
 FILTER, RECRYSTALISE, FILTER, MELTING POINT DETERMINATIO0N / COMPARE TO KNOW DATA

Front 11

Test to distinguish between Aldehyde and Ketone

Back 11

 Warm with Tollens Reagent (silver nitrate dissolved in ammonia)
 If aldehyde present, silver mirror forms as the aldehyde is oxidised
 If ketone present no change as ketone cannot be oxidised

Front 12

Reduction of Aldehydes / ketonesMechanism of reducing an Aldehyde

Back 12

Front 13

Azo Dyes

Back 13

The Azo dye is now stable as there is extensive delocalisation over both arenas via the azo group, -N=N-
This also gives rise to the colours

Front 14

Amines

Back 14

 A weak base because of lone pair of electrons on N accept protons
 proton acceptors
 lone pair electrons are donated forming a dative covalent bond

Front 15

Inductive Effect

Back 15

 Alkyl groups- positive inductive effect – stronger base
 The alkyl group gives a small push of electrons towards LP on the N
 This makes it form a dative covalent bond more readily
 Ammonia - no inductive effect as nothing attached to functional group
 Benzene Ring – Negative inductive effect
 Benzene ring has small pull of electrons away from Nitrogen atom
 The LP electrons are delocalised into the benzene ring
 Makes them less readily available to form a dative covalent bond
 Weaker base

Front 16

Fatty acid - shorthand:

Back 16

Front 17

Trans fats cholesterol:

High Density Lipoproteins

Back 17

 Carry cholesterol out of the blood and out of the body
 Good

Front 18

Low Density Lipoproteins

Back 18

 Carry about 65% of cholesterol around the body and deposit lipids onto artery walls
 This restricts blood flow
 Bad

Front 19

Making Biodiesel - Transesterification:

Back 19

 The waste oil is filtered then reacted with methanol and sodium hydroxide (catalyst) to form
biodiesel.
 This also increases the atom economy of fats.

Front 20

Preparation of Alphatic Amines

Back 20

 Warm halogenoalkens with excess ammonia
 CH3CH2Cl + NH3 >>> CH3CH2NH2 + HCl
 NH3 + HCl >>> NH4Cl

Front 21

Preparation of Primary/Secondary aliphatic amines

Back 21

 CH3CH2NH2 + CH3CH2Cl >>> (CH3CH2)2NH
 (CH3CH2)2NH + CH3CH2Cl >>> (CH3CH2)3N

Front 22

Isoelectric Point

Back 22

 Usually PH6 as COOH is slightly more acidic that NH2 is basic
 Depends on side groups, hence the different points

Front 23

Acid Hydrolysis

Back 23

 Heat under reflux with 6mHCl for 24 hours
 Always gives COOH and NH3+

Front 24

Alkali Hydrolysis

Back 24

 Solution of NaOH, reflux
 Always gives COO-Na+
and NH2

Front 25

Hydrolysis of Polyesters/Polyamides

Back 25

 Hot aq Acid/ aq Alkali
 As above for acid / alkali hydrolysis products

Front 26

Photodegradable polymers

Back 26

 Blended with light sensitive catalysts so become weak, brittle when exposed to light
 Can also have C=O which absorb UV light and break
 Photodegradable plastics break to form shorter waxy hydrocarbon molecules before bacteria
breaks them further into CO2 and H2O

Front 27

Chromatography

Back 27

Chromatography

Front 28

Stationary phase

Back 28

 is in a fixed place (paper in paper chromatography)
 molecules interact with stationary phase slowing down their movement – ADSORPTION

Front 29

Mobile phase

Back 29

 moved in a definite direction (water rises up in paper chromatography)
 molecules interact with mobile phase speeding up their movement – SOLUBILITY

Front 30

Thin Layer Chromatography – TLC

Back 30

 Dissolve sample.
 Draw a pencil line and spot sample using a capillary tube, allow to dry.
 Place plate in a tank of solvent - solvent must be below line, seal the tank.
 Separation is by adsorption - allow solvent to almost reach the top, draw a line here - solvent front.

Front 31

Limitations of TLC

Back 31

 Similar compounds often have too similar Rfvalues.
 Unknown compounds have no Rf value for comparison.
 It is hard to find a solvent that will have the correct amount of solubility - Goldilocks!!

Front 32

Rf Values

Back 32

Front 33

Gas Chromatography - GC

Back 33

Gas Chromatography - GC
 Is used to separate volatile compounds (gases) in a mixture with low boiling points

Front 34

The stationary phase:
GC

Back 34

 Depends what is separated whether you use a liquid or solid lining of the chromatography column
 e.g liquid long chain alkane (high boiling point)
 e.g solid silicone polymer

Front 35

The mobile phase:
GC

Back 35

 Inert carrier gas e.g helium or nitrogen.

Front 36

Separation
GC

Back 36

 Different components slowed by different amounts- separation – retention times
 Each component leaves the column at a different time and is detected as it leaves the column.
 Each peak represents a component
 Area under each peak is proportional to the abundance of each component

Front 37

Limitations of gas chromatography:

Back 37

 Similar retention times + peak shapes most compounds cannot be positively identified.
 Not all substances can be separated.
 Unknown compounds have no reference retention times.
Due to the limitations, gas chromatography is usually used in conjunction with spectroscopy.

Front 38

Uses for GC-MS

Back 38

1) Forensics - scenes of crime
2) Environmental analysis - air pollutants, waste water, pesticides in food.
3) Airport security - explosives in luggage / airport security
4) Space probes - planetary atmospheres

Front 39

Chiral Compound

Back 39

 Optical isomers are one type of Stereo isomers (cis / trans is the other).
 They are non-superimposable
 Chiral carbon has 4 different groups attached
 Always draw in 3D

Front 40

Properties of optical isomers:

Back 40

 They rotate plane polarised light.
 One isomer rotates it in one direction and the other in the opposite direction

Front 41

Problems with Chiral drugs

Back 41

 One optical isomer may have serious side effects
 Expensive/ difficult to separate isomers
 One optical isomer may have serious side effects
 Reduces the effectiveness
 Dose size

Front 42

Overcoming Chiral drug synthesis problems

Back 42

 Use and enzyme catalyst
 Chiral Synthesis
 Chiral Catalyst
 Chiral Pool Synthesis

Front 43

1) Using enzymes as biological catalysts:

Back 43

 Nature is steroespecific, if this can be used only one isomer will be produced.
 If a biocatalyst is used it will only catalyse the production of one isomer.

Front 44

2) Chiral pool synthesis:

Back 44

 This starts the synthesis pathway with a stereospecific enantiomer
 All of the following synthesis steps should lead to a pure optical isomeric drug

Front 45

3) Use of transition metal complexes:

Back 45

 Some act as catalysts that will produce only one optical isomer

Front 46

Cause of Stereo Isomerism
a) Optical isomerism

Back 46

C with 4 different groups attatched
Mirror images of each other

Front 47

b) Cis trans isomerism

Back 47

 C=C has restricted rotation
 Both C in C=C attached to two different groups

Front 48

NMR:
Interpretation: always gives you 2 pieces of information

Back 48

1) splitting pattern – number of adjacent H’s
2) Chemical shift – adjacent functional groups
 If the numbers of H’s are given above the peak, make sure you use these too

Front 49

D2O

Back 49

 D replaces H in OH and NH protons
 Peak for OH / NH protons disappears
 This is due to D having 2 nucleons – no signal

Front 50

TMS

Back 50

 Reference Signal at 0

Front 51

Draw the structure of an optically active carboxylic acid with molecular formular of (C6H12O2). with five peaks in its13C n.m.r. spectrum.

Back 51

(CH3)2CHCH(CH3)COOH