Organic synthesis

Question 1

What is the formation of nitriles from haloalkanes?

Answer 1

Reacting haloalkanes with sodium cyanide or potassium cynanide in ethanol increases the length of the carbon chain.
Nucleophilic substitution.
CH3CH2Cl + KCN –> CH3CH2CN + KCl

Question 2

What is the formation of nitriles from aldehydes and ketones?

Answer 2

Aldehydes and ketones react with hydrogen cyanide in a nucleophilic addition reaction to increase the number of carbon atoms, forming a hydroxynitrile.
Hydrogen cyanide is poisonous so a mixture of NaCN and H2SO4 is used to improve safety and reaction rate.

Question 3

What is the reduction of nitriles?

Answer 3

Nitriles are reduced to amines by reacting hydrogen in the presence of a nickel catalyst.
CH3CH2CN + 2H2 –Ni–> CH3CH2CH2NH2

Question 4

What is the hydrolysis of nitriles?

Answer 4

Nitriles are hydrolysed to carboxylic acids by heating with dilute aqueous acid.
CH3CH2CN + 2H2O + HCl –> CH3CH2COOH + NH4Cl

Question 5

What is alkylation of benzene rings?

Answer 5

An alkyl group from a haloalkane is transferred to a benzene ring using a Friedel Crafts catalyst like AlCl3.

Question 6

What is acylation of benzene rings?

Answer 6

Benzene reacts with an acyl chloride in the presence of AlCl3 to form a ketone.

Question 7

What is filtration under reduced pressure?

Answer 7

Used to separate a solid product from a solvent or liquid reaction mixture.
Diagram:
Buchner flask
Buchner funnel
Pressure tubing
Filter paper
Filter / vacuum pump

Question 8

What is the method of filtration under reduced pressure?

Answer 8

Slowly pour the reaction mixture from the beaker into the filter paper in the funnel.
Rinse the beaker with cold solvent to collect all the crystals.
Rinse the crystals in the funnel with more solvent and let dry for a few minutes.

Question 9

What is recrystallisation?

Answer 9

The solid product formed from filtration contains impurities which can be removed by recrystallisation.
Purification depends upon the desired product and the impurities having different solubilities in the chosen solvent.

Question 10

What is the method of recrystallisation?

Answer 10

If flammable, heat the solvent in a water bath. If water, place the conical flask on a tripod over a bunsen burner to warm.
Slowly add the minimum volume of hot solvent to the sample until it dissolves.
Once dissolved, allow to cool. Crystals should form (scratch with glass rod if not), then when no more form, filter under reduced pressure to obtain dry crystalline solid.

Question 11

What is melting point used for?

Answer 11

The melting point can determine if a sample is pure.
A pure organic substance has a very sharp range of 1 or 2 degrees.
An impure sample has a lower melting point than a pure sample.

Question 12

What are the 2-, 4-, directing groups?

Answer 12

NH2
OH
OR
R
C6H5
Halogens

Question 13

What are the 3- directing groups?

Answer 13

RCOR
COOR
COOH
SO3H
CHO
CN
NO2
NR3+

Question 14

What is chromatography?

Answer 14

It is used to separate individual components from a mixture of substances.
The stationary phase does not move and is normally a solid or liquid supported on a solid.
The mobile phase does move, and is usually a gas or liquid.
Used for analysis of drugs, plastics, flavourings, air samples and forensic science.

Question 15

What is Thin Layer Chromatography?

Answer 15

TLC is a quick and inexpensive analytical technique that indicates how many components are in a mixture.
It uses a TLC plate which is usually plastic or glass sheet, coated with a thin layer of a solid adsorbent substance, usually silica.

Question 16

What is the adsorbent in TLC?

Answer 16

It is the stationary phase.
The different components in the mixture have different affinities for the absorbent and bind with differing strengths to its surface.
Strong adsorption = less distance travelled.

Question 17

What is Rf?

Answer 17

TLC are analysed by calculating the value for the Retention Factor for each component, and comparing to known values.
Rf = distance moved by the component / distance moved by the solvent front.

Question 18

How else can TLC be interpreted?

Answer 18

Run the sample alongside pure samples of compounds that may be present.
It can then be identified visually by seeing which dots appear in the impure sample.

Question 19

What are the limitations of TLC?

Answer 19

Similar molecules have similar RF values/ similar adsorption.
Component spots can merge.
Some compounds might not have known RF values.

Question 20

Outline how TLC could be used to monitor the course of the reaction

Answer 20

Take samples from reaction mixture at regular intervals.
Run on a TLC plate, alongside cyclohexanol (and cyclohexanone) controls.

Question 21

What is gas chromatography?

Answer 21

Used to separate and identify volatile organic compounds present in a mixture.
The stationary phase is a high boiling point liquid adsored onto an inert solid support.
The mobile phase is an inert carrier gas such as helium or neon.

Question 22

What is the retention time?

Answer 22

The retention time is the time taken for each component to travel through the column.
The compound retained in the column for the shortest time has the lowest retention time and is detected first.
Compounds with stronger adsorption move slower.

Question 23

How is a gas chromatogram interpreted?

Answer 23

Retention times identify the components present by comparing to retention times of known components.
Peak integrations determine the concentrations of components in the sample.

Question 24

The student runs a second chromatogram on the sample using a more polar solvent.
Predict the effect, if any, on the Rf values of the amino acids.

Answer 24

Rf values would be larger.
Amino acids are more soluble in more polar solvent so would travel further up the plate.

Question 25

What are the limitations of GC?

Answer 25

Many chemicals have similar retention times. Similar substances have similar retention times and hide under one another. New substances don't have known retention time values. So it is often used alongside mass spectrometry.

Question 26

What is the isoelectric point?

Answer 26

The pH at which the amino acid exists as a zwitterion

Question 27

What is TMS?

Answer 27

Tetramethylsilane, (CH3)4Si is used as the standard reference chemical against which all the chemical shifts are measured. TMS has a chemical shift of 0 ppm (parts per million).

Question 28

What are deuterated solvents?

Answer 28

A solvent where the 1H atoms have been replaced by 2H atoms (D). Deuterium produces no NMR signal in the frequency ranges used in 1H and 13C spectroscopy. Deuterated trichloromethane CDCl3 is a common solvent in NMR spectroscopy.

Question 29

What is carbon-13 NMR spectroscopy?

Answer 29

It determines: the number of different carbon environments from the number of peaks. The types of carbon environments, from the chemical shift.

Question 30

How is the chemical environment of a carbon atom determined?

Answer 30

Carbon atoms that are bonded to different atoms or groups of atoms have different environments and will absorb at different chemical shifts. If two carbon atoms are positioned symmetrically, they have the same chemical shift and contribute to the same peak.

Question 31

How do electron withdrawing groups affect chemical shift?

Answer 31

Carbon atoms nearer to an oxygen atom will be shifted more to the right than other carbon atoms. E.g. in propanal, the CH2 group is also bonded to a C, like the CH3 group is, but is closer to the O so has a higher ppm.

Question 32

What is proton NMR spectroscopy?

Answer 32

Determines: the number of proton environments and the type of proton environments. The relative numbers of each type of proton - from intergration traces or ratio numbers of relative peak areas. The number of non-equivalent protons adjacent to a proton, from splitting pattern.

Question 33

What is the intergration trace?

Answer 33

The area under the peak, either an extra line or a number of relative peak areas. The ratio shows how many hydrogens there are for that peak.

Question 34

What is the n+1 rule?

Answer 34

The number of sub-peaks is one greater than the number of adjacent protons causing the splitting. n+1, where n is the number of adjacent hydrogens.

Question 35

What are the spin-spin splitting patterns?

Answer 35

n 0 singlet no adjacent H 1 doublet adjacent CH 2 triplet adjacent CH2 3 quartet adjacent CH3 4+ multiplet likely (CH3)2CH

Question 36

What are common spin-spin splitting pairs?

Answer 36

CH2 - CH3 quartet - triplet CH - CH2 triplet - doublet CH2 - CH2 triplet - triplet CH - CH doublet - doublet CH - CH3 quartet - doublet CH - (CH3)2 septet - doublet

Question 37

How are hydroxyl and amine groups shown?

Answer 37

OH and NH2 peaks can occur at any chemical shift, and are not involved in spin-spin coupling.

Question 38

What is proton exchange?

Answer 38

The proton NMR is run as normal, then a small volume of D2O (deuterium oxide) is added, and another spectrum run. Deuterium exchanges the OH and NH2 protons with D atoms. D doesn't absorb chemical shift, so the OH or NH2 peak disappears.

Question 39

How do you answer a NMR question?

Answer 39

Draw a table: Chemical shift | Splitting pattern | intergration | environment Underline the hydrogens to show which H causes the peak.

Question 40

State the region of the electromagnetic spectrum used in 1 H NMR spectroscopy. Explain why CDCl3 is used as a solvent in 1 H NMR spectroscopy.

Answer 40

Radio waves The solvent does not have any hydrogen / H / protons CDCl3 has an even number of nucleons so doesn't possess spin.

Question 41

Explain the use of two deuterated compounds in NMR spectroscopy.

Answer 41

CDCl3 used as a solvent D2O used to identify OH or NH protons