What is the defining feature of an aromatic compound?
The benzene ring, C6H6
What is the kekulé model of benzene?
A ring of six carbons bonded by 3 alternating single and double bonds.
What is the delocalised model of benzene?
The model suggesting that each carbon donates an electron from its p-subshell and they combine to form a ring of delocalised electrons.
What evidence is there for delocalisation in the benzene ring?
The enthalphies of hydration for cyclohexene and the kekulé model - the kekulé model's is less than expected, suggesting that benzene's structure is more stable than the kekulé model would infer. This stability is granted to benzene by its ring of delocalised electrons.
Why is the benzene ring prone to electrophilic substitution reaction mechanisms?
Because it is a region of high electron density - this attracts electrophiles. Substitution is favoured because it preserves the benzene ring's ring of delocalised electrons and thereby its stability.
What is Friedel-Crafts acylation?
The reflux of benzene and an acyl chloride in the presence of AlCl4 in a non-aqueous environment to form a phenylketone.
What does AlCl3 act as in Friedel-Crafts acylation? How does it do this?
A halogen carrier - it accepts a lone pair from the acyl chloride (forming AlCl4-), polarising the acyl group far enough to form a carbocation.
Why are Friedel-Crafts acylation reactions useful?
Because they make benzene reactive via. the addition of an acyl group.
Define the term 'Halogen carrier'.
A catalyst which increases the positivity of an electrophile.
What is the mechanism for Friedel-Crafts acylation?
What is a by-product in the formation of phenylketones from benzene?
What is nitration? What are the conditions and reagents necessary for it?
The addition of a nitrate group to the benzene ring. It requires benzene, concentrated H2SO4 and HNO3, and warming (for mononitration, temperatures below 55°C).
How is the electrophile for the nitration of the benzene ring formed?
HNO3 + H2SO4 ----> H2NO3+ + HSO4-
H2NO3+ ----> H2O + NO2+
Why are nitrobenzene compounds useful?
Because they can be reduced to aromatic amines, which are used to manufacture dyes and pharmaceuticals.
What does the amine functional group look like?
N --- H
What is the name of this compound?
H3C -- N -- H
What does a tetramethylamine ion (quaternary ammonium ion) look like?
CH3 -- N+ -- CH3
What is the name for a benzene ring with an amine group attached?
What is a use of quaternary ammonium salts?
What is base strength dependent upon?
Lone pair availability.
Which is the stronger base, a phenylamine, ammonia, or an aliphatic amine?
The aliphatic amine - its lone pair is the most available because of inductive effects from its attached alkyl groups. The phenylamine is a weak base because the nitrogen's lone pair is delocalised with the benzene ring's.
What are the two possible methods for producing an aliphatic amine?
Reacting a haloalkane with excess ammonia whilst heating, and reduction of a nitrile.
How can a mixture of primary, secondary, tertiary amines and ammonium salts be seperated?
By fractional distillation.
What is the laboratory method for reducing a nitrile?
Using a LiAlH4 reducing agent with dilute acid; this should be done under reflux with sodium metal and ethanol.
What is the industry method for reducing a nitrile to an aliphatic amine?
Catalytic hydrogenation - high temperature, pressure, H2 gas and a nickel catalyst.
What is the equation for the reduction of ethanenitrile to ethylamine? What are the conditions for this reaction?
CH3CN + 2H2 ----> CH3CH2NH2
High temperature, pressure and a nickel catalyst.
How is it possible to form an aromatic amine?
By first producing nitrobenzene (via. nitration), then heating that nitrobenzene with tin and conc. HCl acid under reflux and finally adding an alkali.
What is the equation for the formation of a phenylamine from nitrobenzene?
C6H5(NO2) + 6[H] ----> C6H5(NH2) + 2H2O
What does the amide group look like?
R -- C = O
N -- H
What does the N - substituted amide group look like?
R -- C = O
N -- H
What are amino acids?
Compounds which contain both the amine group and the carboxyl group.
Why are amino acids amphoteric?
The amine group is basic, the carboxyl group is acidic.
What does an amino acid look like?
H -- N -- C -- C = O
¦ ¦ ¦
H H OH
What is the system for naming amino acids?
- Longest carbon chain which includes the carboxyl group
- Number the amino group w/ the carboxy carbon being 1
e.g. 2-aminobutanoic acid
What is a zwitterion?
A dipolar ion.
Where do zwitterions exist?
Near an amino acid's isoelectric point.
What is an amino acid's isoelectric point?
The pH where the overall charge on the amino acid is zero.
Describe paper chromatography with a mixture of amino acids.
- Place a concentrated of the mixture of amino acids on chromatography paper - mark the spot position with a pencil.
- Dip the paper into solvent - as the solvent spreads up the paper, the amino acids move with it at different rates.
- Spray the paper w/ ninhydrin solution to make the amino acids purple, then identify the amino acids by measuring how far they move relative to the solvent.
What is a protein?
A condensation polymer of amino acids.
When proteins form, what is the byproduct?
How is it possible to return a protein to its parent amino acids?
By hydrolysis; refluxing the condensation polmer (protein) with hot aqueous 6M HCl for 24hrs, then neutralising the product with an alkali.
What does a peptide/amide linkage look like?
R -- C = O
N -- R
What gives a protein its structure? Why is protein shape important?
Hydrogen bonds between amide groups in the protein. Protein shape is vital their function - they have a specific shape in order to work with enzymes.
What conditions affect protein shape?
pH and heat affect hydrogen bonding and therefore protein shape.
How are addition polymers formed?
Via. the reaction addition reactions of alkenes.
How are condensation polymers formed?
Through the reaction of monomers which contain two functional groups - each time a polymer linkage is formed, a small molecule is eliminated, usually water.
How can polyamides be formed?
Either by the reaction of amino acids, or by the reaction of dicarboxylic acids and diamines.
How is it possible to form a polyester?
By reacting dicarboxylic acids with diols in a condensation polymerisation reaction.
Why are addition polymers non-biodegradable?
Because they are inert and do not contain regions of varying electron density (i.e. they aren't polar).
Are condensation polymers biodegradable?
Yes - they are susceptible to hydrolysis because they have regions of varying electron density.
What are the advantages and disadvantages of placing waste plastics in landfill sites?
Landfill is cheap and easy, but it requires land, releases methane and can contaminate water supplies.
What are the advantages and disadvantages of burning waste plastics?
Burning plastics generates heat which can be used to generate electricity, but it needs to be monitored to reduce toxic gases and it produces gases which contribute to the greenhouse effect.
What is a scrubber?
A piece of machinery which neutralises the waste gases in plastic burning via. a base.
What are the advantages and disadvantages of recycling plastics?
Whilst waste plastics need to be sorted, which is expensive, recycling preserves raw material and allows the plastics to be cracked into useful monomers or to be melted and remoulded.
In what ways could the modern would optimise its use of plastics?
By reducing packaging, encouraging the re-use of plastics and developing plastics which biodegrade more quickly.