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STANDARD ANSWERS AND DEFINITIONS
Evidence for Kekule’s model to be wrong:
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.
Discuss the structure and bonding in benzene / (comparing to kekule - structure):
Label p orbitals and state that they overlap Label ‘delocalised orbitals’ State the bond lengths are the same That it is a planar molecule
Discuss the relative low reactivity of benzene / (problems with Kekule – reactivity):
Draw the enthalpy diagram This shows that benzene is not 3 c=c as it is lower in energy This means it is more stable Therefore is less reactive
Discuss the reactivity of benzene compared to alkenes
Between the C-C: 2e from a bond and 2e from the localised bond = 4e Higher electron density Polarises electrophiles more. Alkenes do not need halogen carrier. Electrophilic addition reactions
Between the C-C: 2e from bond and 1e pre C-C from delocalised bond = 3e Lower electron density Polarises electrophiles less. Benzene needs a halogen carrier. Electrophilic substitution reactions
Discuss the reactivity of phenol compared to benzene
Lone pair electrons on the O Delocalise with the electrons in the benzene ring Makes it more electron rich Ring becomes activated Polarises electrophiles more. Phenols do not need halogen carrier. Are multiply substituted.
Between the C-C: 2e from bond and 1e from C-C from delocalised bond = 3e Lower electron density Polarises electrophiles less. Benzene needs a halogen carrier. Only monosubstituted
Summary:
Benzene VS. Cyclohexene
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
Benzene VS. Phenol
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
Carbonyl Test
Test for Carbonyl Group
2,4,DNPH (Brady’s Reagent) If present, orange precipitate formed FILTER, RECRYSTALISE, FILTER, MELTING POINT DETERMINATIO0N / COMPARE TO KNOW DATA
Test to distinguish between Aldehyde and Ketone
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
Reduction of Aldehydes / ketonesMechanism of reducing an Aldehyde
NaBH 4 : Source of hydride ions, H-
Azo Dyes
1) Make Nitrous Acid NaNO 2 + HCl NaCl + HNO2 (below 10oC)
2) Make Diazonium Salt
Below 10oC because N 2 is unstable – decomposes releasing nitrogen gas Benzenediazonium salts are stabilized as the benzene ring allows the electrons from the diazonium functional group to be delocalised over the benzene ring
3) Coupling
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
Amines 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
Inductive Effect
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
Preparation of Primary/Secondary aliphatic amines
CH 3 CH 2 NH 2 + CH 3 CH 2 Cl (CH 3 CH 2 ) 2 NH (CH 3 CH 2 ) 2 NH + CH 3 CH 2 Cl (CH 3 CH 2 ) 3 N
Isoelectric Point
Usually PH6 as COOH is slightly more acidic that NH2 is basic Depends on side groups, hence the different points
Acid Hydrolysis
Heat under reflux with 6mHCl for 24 hours Always gives COOH and NH 3 +
Alkali Hydrolysis
Solution of NaOH, reflux Always gives COO-Na+^ and NH 2
Hydrolysis of Polyesters/Polyamides
Hot aq Acid/ aq Alkali As above for acid / alkali hydrolysis products
Photodegradable polymers
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 CO 2 and H 2 O
Chromatography
Stationary phase is in a fixed place (paper in paper chromatography) molecules interact with stationary phase slowing down their movement – ADSORPTION
Mobile phase moved in a definite direction (water rises up in paper chromatography) molecules interact with mobile phase speeding up their movement – SOLUBILITY
Thin Layer Chromatography – TLC Is used to check purity / separate amino acids/ monitor the extent of a reaction. Solid stationary phase- Silica Gel Liquid mobile phase- Solvent
Producing the chromatogram in TLC: 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.
Each separated component is a spot, if colourless use ninhydrin and a UV lamp
Limitations of TLC 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!!
Rf =
Distance moved by component Distance moved by solvent front
Gas Chromatography - GC
Is used to separate volatile compounds (gases) in a mixture with low boiling points The stationary phase:
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
The mobile phase:
Inert carrier gas e.g helium or nitrogen.
Separation
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
Limitations of gas chromatography:
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.
Uses for GC-MS
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
D 2 O
D replaces H in OH and NH protons Peak for OH / NH protons disappears This is due to D having 2 nucleons – no signal
TMS Reference Signal at 0
DEFINITIONS
Retention time- Is the time taken for a component to pass from inlet to detector.
Alpha Amino Acid- NH2 and COOH joined at the same C
Stereoisomers- Same structural formula different spatial arrangement of atoms
Amphoteric - Amino acids will react with both acids due to NH2 and alkalis due to COOH
Optical isomers - Mirror images cannot be superimposed upon each other
Achiral compounds - do not have 4 different groups around a carbon atom
Chiral compounds - have 4 different groups around a carbon atom
Enantiomers- the two different optical isomers
Racemic mixture - An equal mixture of the 2 isomers will not rotate plane polarised light as each isomer cancels the other out.
Stereospecific - Optical activity is important in biological systems as only one of the isomers will interact with enzymes.
Delocalised electrons – are shared between more than 2 atoms
Addition reaction – where a reactant is added to an unsaturated molecule
Substitution reaction – where an atom or group of atoms is replaced with a different atom/group of atoms
Electrophile – is an atom/group of atoms that is attracted to an electron rich centre where it accepts a pair
of electrons to form a dative covalent bond
Substitution – is where one group is replaced by another group
Curly arrow – used in mechanisms to show the movement of an electron pair / forming, breaking bonds.
