A-level Only Organic Chemistry Flashcards
What is the structure and features of benzene?
- Aromatic compound
- Ring of six carbon atoms
- Six hydrogen atoms
- Ring of delocalised electrons
- Very stable
What is the enthalpy change of hydrogenation of benzene?
-208KJmol-1
What is an Arene and its features?
- Compounds that contain benzene
- High melting points = delocalised ring
- Low boiling points = non-polar molecules, often cannot be dissolved in water.
Why can benzene undergo electrophilic substitution?
- Delocalised ring inn benzene is an area of high electron density, so is susceptible to attack from electrophiles.
- Aromatic amines and nitrobenzene can be made from benzene.
How does Nitrobenzene form?
- Electrophile is the NO2+ = reactive intermediate
- When heated with benzene these reagents lead to the substitution of NO2+ onto the benzene ring, removing a hydrogen
-This is a mono-substitution of a single NO2+, at 55oC
What is Friedel-Crafts Acylation?
The delocalised electron ring in benzene can also act as a nucleophile, which can attack on acyl chlorides.
What is needed for Friedel-Crafts Acylation to take place?
A reactive intermediate, produced from the acyl chloride and an aluminium chloride catalyst.
What happens at the end of the Friedel-Crafts Acylation reaction?
- The H+ ion removed from the ring reacts with the AlCl4- ion to reform the aluminium chloride, showing it to be a catalyst.
- Reaction produces phenylketone.
What are Amines?
- One or more of the hydrogen atoms in ammonia is replaced with an organic group
- Either primary, secondary, tertiary
What are Amines?
- One or more of the hydrogen atoms in ammonia is replaced with an organic group
- Either primary, secondary, tertiary
Amines can be produced in two ways.
What is one way amines can be produced?
- Reaction of a halogenoalkane with ammonia in a sealed tube.
- One mole of halogenoalkane reacts with two moles of ammonia producing a primary and an ammonium salt.
- Can be repeated until all the hydrogen atoms have been replaced with organic groups.
Halogenoalkane with ammonia:
Why does this reaction have a low efficiency?
- Multiple number of possible substitutions = mixture of products produced.
- So reaction requires conditions that means only a single substitution occurs,
- Excess ammonia can be added or the mixture of products can be separated using fractional distillation.
How are amines produced via the redaction of Nitriles?
And what are the necessary conditions?
- Reducing nitriles via hydrogenation = amines.
- Reduction LiAIH4, a reducing agent.
- Acidic conditions or a combination of hydrogen and Nickel.
How are aromatic amines produced?
- Reduction of nitrobenzene using concentrated hydrochloric acid (HCl) and a tin catalyst.
What is a cationic surfactants?
- Complexes with a positive and negative end.
- Good conditioners as the two ends are attracted to different substances, preventing static building up on surfaced.
What are the properties of amines as bases?
- Weak bases = lone pair of electrons on the nitrogen atom can accept protons.
- Base strength of amines depends on how available the electron pair is on the molecule.
- More available the electrons the more likely it is to accept a proton meaning its a stronger base.
Explain why butylamine is a stronger base than ammonia?
- Lone pairs more available
- Positive inductive effect
What is the inductive effect?
Different functional groups can affect how available a lone electron pair is by changing electron density around the bond.
What is the inductive effect in benzene rings?
- Draw electron density away from the nitrogen making it ‘less available’
What is the inductive effect of Alkyl groups?
- Push electron density towards the nitrogen making it ‘more available’, more alkyl groups means more ‘pushing’.
- ALIPHATIC amines are stronger bases and AROMATIC amines are weaker.
Explain Nucleophilic substitution of amines?
- Amines can act as nucleophiles because the lone pair is attracted to delta positive regions on other molecules.
- So amines can substitute halides on halogenoalkane to form 1,2, or 3 amines and 4 ammonium salts.
Explain Nucleophilic Addition-elimination in amines?
- acyl chlorides produce amides and N-substituted amides.
- Same reaction can also occur with acid anhydrides to produce an amide and a carboxylic acid.
What factors affect NMR?
- The chemical shift (position of the peak) is affected by the environment (what the atom is attached to)
- Due to the shielding or de-shielding effects of adjacent atoms
- The less electron presence (I.e more electronegative atoms around the carbon) the higher magnetic field the nucleus is exposed to hence harder to flip and the ppm value will be higher.
What is Tetramethylsilane (TMS) used for?
- Added to a sample to calibrate the spectrum
- TMS chemical shift is set to 0
- It is used because its signal is away from everything else
- Only gives one strong signal
- non toxic and inert
- low bp so can be removed from a sample easily
What is C13 NMR ?
- Analyses the different carbon environments
- Carbon environments that are near to an oxygen have delta values that are shifted to the right
- As oxygen is very electronegative and changes the bond environment and how it absorbs energy
- Molecules that have symmetry may display fewer ppm peaks than the number if carbon atoms in a molecule as they have fewer environments.
What is H1 NMR (Proton NMR)?
- Analyses the different hydrogen environments
- Also measured against TMS standard
What solvent is needed and why?
- A Non-hydrogen-containing solvent, so it doesn’t produce any ppm peaks on the spectrum.
- E.g. CCl4 is a common solvent used along with deuterated solvents containing deuterium, an isotope of hydrogen.
Explain the peaks of a H1 NMR spectra?
- Peaks show where each environment is positioned within the molecule
- Peaks split into small clusters = how many hydrogens are on the adjacent carbon atom
- Smaller peaks are a splitting pattern and follow ‘n+1’ rule where n = number of hydrogens on adjacent carbon.
What is spin-spin coupling?
- Causes splitting of the peak = due to the hydrogen bonded to adjacent carbons
- This if for non-equivalent hydrogens only
What is integration?
- The size of the peak is proportional to the number of hydrogens it represents
- It gives the values of chemically equivalent protons
Test for Alkenes?
- Add bromine water
- Shake
- If the compound reaction is unsaturated, an addition reaction will take place and the coloured solution will decolourise
Why is it better to prepare primary amines form nitriles rather than from halogenoalkanes?
Halogenoalkanes = impure products/ mix of products forms/ lower atom economy so further reactions needed
Nitriles = no further reactions needed/ one product formed/ higher atom economy
What is chromatography?
An analytical technique used to separate and identify component molecules of a mixture.
What is the mobile phase?
A substance that allows molecules to move over or through the stationary phase. Can be a liquid or gas!
- more soluble products move further with the mobile phase.
What is the stationary phase?
Substance that has affinity to molecules in the mixture being analysed.
- The greater the affinity of a molecule to the stationary phase, the shorter the distance it moves with the mobile phase
How do you calculate Rf value?
Distance moved by molecule
Rf = —————————————————
Distance moved by solvent
What is Thin-Layer chromatography?
A metal plate is coated with.
- Thin layer of silica and the solvent moves up the plate.
- The plate is then dried in a fume cupboard to reduce toxic fumes.
- The chemical traces can be viewed using a UV lamp and the distances travelled can be measured.
- A developing agent can be added, such as iodine, to allow traces to be seen by the naked eye.
What is column chromatography?
A vertical column is packed with a solid, powdered substance which acts as the stationary phase.
A solvent containing the mixture being analysed is then added moves down the column as mobile phase.
Varying affinities = drain out of column at different times = collected as separate samples. Time taken to drain out of the column = retention time.
What is gas chromatography?
Thin tube is packed with a solid, powdered substance which acts as the stationary phase.
High pressure gas is passed through this tube as the mobile phase.
Used to separate volatile liquids which are fed into the gas chromatography machine as vapours
Analysis machine records a retention time for each component molecule in the mixture, allowing them to be identified.
What is GC-MS chromatography?
A combination of analytical techniques, gas chromatography and mass spectrometry.
Molecules are separated using gas chromatography, then each molecule is fed into a mass spectrometer so it can be accurately identified.
GC-MS is a much faster analytical process that produces more accurate results for molecule analysis and identification.
