AC 1-4 Flashcards
Define aromatic.
A compound that contains a benzene ring.
Define aliphatic.
Any compound that doesn’t contain a benzene ring.
Define alicyclic.
A compound that contains only non-aromatic ring(s).
Molecular formula of benzene.
C6H6.
How many bonds does each carbon atom form in benzene?
3 sigma bonds. 2 are with neighbouring carbons, and 1 is with a hydrogen.
Each carbon in benzene has 1 electron in a p-orbital, which extends above and below the plane of the molecule. What is the result of this?
The p-orbitals overlap to form a delocalised pi-electron system.
State 3 unexpected properties of benzene.
- Lower reactivity. It doesn’t undergo typical alkene addition reactions.
- All C-C bonds are the same length.
- Hydrogenation is less exothermic than expected.
Define rate of reaction.
The rate of change of the concentration of a reactant or product with respect to time.
In a rate equation, what is k?
K is the rate constant. It is a measure of the probability that a reaction will happen when particles collide.
What does k depend on?
K is temperature dependent only. If temperature increases, the value of k also increases.
How can you calculate the units of k?
mol^(1-x)dm^(3x-3)s^-1, where x is the overall order of reaction.
If successive half lives decrease, this means…
The reaction is zero order, and rate is independent of concentration.
If successive half lives are constant, this means…
The reaction is first order, and the rate is directly proportional to the concentration.
For a first order reaction, k is equal to…
The natural long of 2 divided by the half life.
If successive half lives increase, this means…
The reaction is second order.
The rate determining step is…
The slowest step in the reaction pathway.
If asked to propose a reaction mechanism…
Propose the rate determining step as the first step.
The number of particles of a particular reactant in the rate determining step…
Is equal to the order of reaction for that particular reactant.
What must be done to deduce the rate equation from a reaction mechanism that doesn’t have the rate determining step first?
Add up all the previous steps up to and including the slow step.
In the Arrhenius equation, what unit of temperature must be used?
Kelvin.
Why are aromatic compounds less reactive than alkenes?
The delocalised pi-electron system in aromatic compounds means the overall electron density is lower, so they are less attractive to electrophiles.
By what mechanism do aromatic compounds react?
Electrophilic substitution.
Why do aromatic compounds tend to react by substitution instead of addition?
An addition reaction would mean losing the favourable delocalised pi-electron system, whereas a substitution reaction means it is still present in the product.
What are the reaction conditions for the nitration of benzene?
React at 50 degrees Celsius with nitric acid and a sulphuric acid catalyst.
What is required to halogenate benzene?
A halogen molecule, and a halogen carrier catalyst. These two things react together to form the electrophile.
What would you observe when benzene is brominated?
The decolorisation of bromine, and steamy fumes of HBr.
What’s an alkylation reaction?
The substitution of an alkyl group into a benzene ring. The alkyl electrophile is formed by reacting the alkyl with a halogen carrier catalyst, often AlCl3.
What’s an acylation reaction?
The reaction of benzene with an acyl chloride, to form an aromatic ketone. The acyl chloride must react with a halogen carrier catalyst to form an electrophile.
In thin layer chromatography, what are the stationary and mobile phases?
The stationary phase is the thin layer of solid mounted onto a piece of glass. The mobile phase is the organic solvent.
What separates different substances in TLC?
Their adsorption to the stationary phase. Stronger adsorption means slower migration.
What causes the solvent to move up the plate?
Capillary action.
What does the strength of adsorption depend on?
- The attractive forces between the component and the stationary phase.
- The solubility of the component in the mobile phase.
State 2 advantages of TLC.
Fast and cheap.
State the primary limitation of TLC.
It has very limited resolution, so compounds with similar Rf values will not be clearly separated.
In gas chromatography, what are the stationary and mobile phases?
The stationary phase is a liquid adsorbed onto an inert solid support. The mobile phase is an inert gas, often helium.
How does GC work?
- A small sample is injected into the machine and vaporised.
- It is carried into the column by the stream of inert gas, where it passes through the stationary phase.
- Compounds are separated by their retention times.
What are components separated by in GC?
Their solubility in the liquid stationary phase. Greater solubility means greater retention time.
What do the areas under the peaks of a gas chromatogram represent?
The relative quantity of each component.
What are the steps to calibrating a GC machine?
- Inject a series of samples containing different, known amounts of a substance.
- Measure the peak area for each of the samples.
- Plot a calibration curve, showing peak area as a function of the concentration of the substance.
- The unknown sample can then be injected, and its peak area can be compared with the calibration curve.
State 2 advantages of GC-MS.
- Very good resolution.
2. Can operate with very tiny amounts of sample mixture.
State 3 limitations of GC-MS.
- Similar compounds often have similar retention times, so they may be difficult to separate.
- Previously unknown compounds have no reference retention times or mass spectra for matching.
- The equipment is sophisticated and expensive.
