Module 6: Organic and Analysis

Question 1

General formula of an alkane

Answer 1

C_nH_2n+2

Question 2

Tollen’s Reagent

Answer 2

Formed by reacting a basic solution of silver nitrate with ammonium hydroxide.

Used to identify Aldehydes.

It works because as Silver ions are reduced to elemental silver, aldehyyde is oxidised to carboxylic acid.

Ketones cannot be further oxidised, so no reaction with Tollen’s.

Question 3

2,4-Dinitrophenylhydrazine

(Brady’s reagent)

Answer 3

This reacts with the carbonyl group of aldehydes and ketones to give a yellow/orange precipitate.

This does not react with esters or with carboxylic acids.

Precipitate is collected by filtration and purified by recrystallisation. Melting point measured to identify the aldehyde or ketone

Question 4

Conditions for aromatic nitration

Answer 4

Mixture of of conc nitric acid in conc sulfuric acid

Temperature below 50 ^oC (ensures only one nitro group on ring)

Reflux condenser fitted to prevent loss of volatile substances

Question 5

Conditions for bromination

Answer 5

Use elemental bromine (Br₂) and a catalyst - either AlBr₃ or FeBr₃.

Question 6

Conditions for chlorination

Question 7

Conditions for Friedel-Crafts alkylation

Question 8

Conditions for Friedel-Crafts acylation

Question 9

Describe or draw the Kekule structure of benzene

Question 10

Discuss the shortcomings of the Kekule structure of benzene

Answer 10

• Unlike alkenes, benzene is resistant to addition reactions
• Benzene is more stable that the kekule model predicts
• X-ray diffraction techniques have shown that all 6 bonds in benzene are the same length (not three shorter C=C bonds as kekule predicted)

Question 11

What evidence is there for Benzene not having a Kekule structure?

Answer 11

Essentially there are 3 headings:

• Aromatic compounds undergo electrophilic substitution (whereas alkenes undergo electrophilic addition)
• C-C bond lengths in benzene are all the same length and are longer than C=C (as in alkenes) and shorter than C-C (as in alkanes).
• The energy released by hydrogenation of benzene is about 150kJ less than predicted for cyclohexatriene

Question 12

Draw or describe how a sigma bond forms

Question 13

Draw or describe how a pi bond is formed

Question 14

Explain why phenols neutralise sodium hydroxide but do not react with sodium carbonate.

Question 15

Compare and explain the conditions for nitrating phenols with the conditions for nitrating benzene.

Question 16

Directing effect of electron donating groups (and examples of electron donating groups)

Answer 16

2- 4- and 6- directing effect

e.g. OH and NH₂

Question 17

Directing effect of electron withdrawing groups (and examples of electron withdrawing groups).

Answer 17

3- and 5- directing effect

e.g. NO₂

Question 18

Describe the solubility of carboxylic acids

Answer 18

very soluble in polar solvents such as water.

this is because hydrogen bonds can form between the carboxylic acid functional group and water

Question 19

chemical properties of carboxylic acids

Answer 19

weak acids that partially ionise in solution releasing the H+ ion from the carboxylic acid group , forming a carboxylate ion.

Question 20

Thin Layer Chromatography (TLC)

Answer 20

stationary phase- thin piece of inert material e.g glass, covered with an absobent chemical

Mobile phase- organic solvent

Rf=distance moved by component/ distance moved by solvent

Question 21

alkenes with a strong sigma bond have a 99% chance of stealing your electrophile

Question 22

Gas Chromatography

Answer 22

Used to seperate volatile liquids

Stationary Phase- The tube is coated in a liquid ( retention time depends on solubility) or a solid ( retention time depends on adsorption).

Mobile phase- Inert carrier gas

Results shown on a gas chromatogram, where the area under each peak is proportional to the relative amount of each component.

Question 23

Retention Time

Answer 23

The time taken for the substance to pass through the coiled tube and reach the detector in a gas chromatogram

Compared with values in a reference table to identify the substance (must ensure the conditions are the same)

Question 24

NMR internal standard

Answer 24

Tetramethylsilane (CH₃)₄Si

Good internal standard because:

• Contains both carbon and hydrogen so can be used for both types of NMR
• Produces one sharp signal as all of the hydrogen atoms are in the same environment
• non-toxic
• Inert so unlikely to react with the chemicals being investigated

Question 25

Structure of an a-amino acid

Answer 25