AC 1-4

Question 1

Define aromatic.

Answer 1

A compound that contains a benzene ring.

Question 2

Define aliphatic.

Answer 2

Any compound that doesn’t contain a benzene ring.

Question 3

Define alicyclic.

Answer 3

A compound that contains only non-aromatic ring(s).

Question 4

Molecular formula of benzene.

Answer 4

C6H6.

Question 5

How many bonds does each carbon atom form in benzene?

Answer 5

3 sigma bonds. 2 are with neighbouring carbons, and 1 is with a hydrogen.

Question 6

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?

Answer 6

The p-orbitals overlap to form a delocalised pi-electron system.

Question 7

State 3 unexpected properties of benzene.

Answer 7

1. Lower reactivity. It doesn’t undergo typical alkene addition reactions.
2. All C-C bonds are the same length.
3. Hydrogenation is less exothermic than expected.

Question 8

Define rate of reaction.

Answer 8

The rate of change of the concentration of a reactant or product with respect to time.

Question 9

In a rate equation, what is k?

Answer 9

K is the rate constant. It is a measure of the probability that a reaction will happen when particles collide.

Question 10

What does k depend on?

Answer 10

K is temperature dependent only. If temperature increases, the value of k also increases.

Question 11

How can you calculate the units of k?

Answer 11

mol^(1-x)dm^(3x-3)s^-1, where x is the overall order of reaction.

Question 12

If successive half lives decrease, this means…

Answer 12

The reaction is zero order, and rate is independent of concentration.

Question 13

If successive half lives are constant, this means…

Answer 13

The reaction is first order, and the rate is directly proportional to the concentration.

Question 14

For a first order reaction, k is equal to…

Answer 14

The natural long of 2 divided by the half life.

Question 15

If successive half lives increase, this means…

Answer 15

The reaction is second order.

Question 16

The rate determining step is…

Answer 16

The slowest step in the reaction pathway.

Question 17

If asked to propose a reaction mechanism…

Answer 17

Propose the rate determining step as the first step.

Question 18

The number of particles of a particular reactant in the rate determining step…

Answer 18

Is equal to the order of reaction for that particular reactant.

Question 19

What must be done to deduce the rate equation from a reaction mechanism that doesn’t have the rate determining step first?

Answer 19

Add up all the previous steps up to and including the slow step.

Question 20

In the Arrhenius equation, what unit of temperature must be used?

Answer 20

Kelvin.

Question 21

Why are aromatic compounds less reactive than alkenes?

Answer 21

The delocalised pi-electron system in aromatic compounds means the overall electron density is lower, so they are less attractive to electrophiles.

Question 22

By what mechanism do aromatic compounds react?

Answer 22

Electrophilic substitution.

Question 23

Why do aromatic compounds tend to react by substitution instead of addition?

Answer 23

An addition reaction would mean losing the favourable delocalised pi-electron system, whereas a substitution reaction means it is still present in the product.

Question 24

What are the reaction conditions for the nitration of benzene?

Answer 24

React at 50 degrees Celsius with nitric acid and a sulphuric acid catalyst.

Question 25

What is required to halogenate benzene?

Answer 25

A halogen molecule, and a halogen carrier catalyst. These two things react together to form the electrophile.

Question 26

What would you observe when benzene is brominated?

Answer 26

The decolorisation of bromine, and steamy fumes of HBr.

Question 27

What’s an alkylation reaction?

Answer 27

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.

Question 28

What’s an acylation reaction?

Answer 28

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.

Question 29

In thin layer chromatography, what are the stationary and mobile phases?

Answer 29

The stationary phase is the thin layer of solid mounted onto a piece of glass. The mobile phase is the organic solvent.

Question 30

What separates different substances in TLC?

Answer 30

Their adsorption to the stationary phase. Stronger adsorption means slower migration.

Question 31

What causes the solvent to move up the plate?

Answer 31

Capillary action.

Question 32

What does the strength of adsorption depend on?

Answer 32

1. The attractive forces between the component and the stationary phase. 2. The solubility of the component in the mobile phase.

Question 33

State 2 advantages of TLC.

Answer 33

Fast and cheap.

Question 34

State the primary limitation of TLC.

Answer 34

It has very limited resolution, so compounds with similar Rf values will not be clearly separated.

Question 35

In gas chromatography, what are the stationary and mobile phases?

Answer 35

The stationary phase is a liquid adsorbed onto an inert solid support. The mobile phase is an inert gas, often helium.

Question 36

How does GC work?

Answer 36

1. A small sample is injected into the machine and vaporised. 2. It is carried into the column by the stream of inert gas, where it passes through the stationary phase. 3. Compounds are separated by their retention times.

Question 37

What are components separated by in GC?

Answer 37

Their solubility in the liquid stationary phase. Greater solubility means greater retention time.

Question 38

What do the areas under the peaks of a gas chromatogram represent?

Answer 38

The relative quantity of each component.

Question 39

What are the steps to calibrating a GC machine?

Answer 39

1. Inject a series of samples containing different, known amounts of a substance. 2. Measure the peak area for each of the samples. 3. Plot a calibration curve, showing peak area as a function of the concentration of the substance. 4. The unknown sample can then be injected, and its peak area can be compared with the calibration curve.

Question 40

State 2 advantages of GC-MS.

Answer 40

1. Very good resolution. | 2. Can operate with very tiny amounts of sample mixture.

Question 41

State 3 limitations of GC-MS.

Answer 41

1. Similar compounds often have similar retention times, so they may be difficult to separate. 2. Previously unknown compounds have no reference retention times or mass spectra for matching. 3. The equipment is sophisticated and expensive.