Topic 17: Further Organic

Question 1

What is a Chiral molecule?

Answer 1

A molecule with a chiral carbon (it is asymmetric)

Question 2

What is a Chiral Carbon?

Answer 2

A carbon atom with four different groups attached to it (asymetric)

Question 3

How does optical isomerism occur?

Answer 3

Results from chiral centres in a molecule with asymmetric carbon atoms

Question 4

What are optical isomers (properties)?

Answer 4

• non-superimposable mirror images

- rotate the plane of polarisation of plane polarised monochromatic light

Question 5

Define optical activity?

Answer 5

The ability of a single optical isomer to rotate the plane of polarisation of plane- polarised monochromatic light in molecules containing a single chiral centre

Question 6

What is a racemic mixture?

Answer 6

No optical activity due to equal concentrations of two opposite enantiomers

Question 7

SN1 reaction

Answer 7

• S=Substitution, N=nucleophilic , 1=unimolecular
• Forms two products
• It can form a racemic mixture as equal concentrations of opposite enantiomers are formed

Question 8

SN2 reaction

Answer 8

• S=Substituiton, N=Nucleophillic, 2=bimolecular

- If the molecule is an enantiomer it will reverse the way in which it rotates the polarised light

Question 9

Define unimolecular (SN1)

Answer 9

-1 species involved in the rate-determining step

Question 10

Define bimolecular

Answer 10

-2 species are involved in the rate-determining step

Question 11

Can Aldehydes and Ketones form Hydrogen bonds with eachother?

Answer 11

-No because they do not thave a hydrogen attached to O,N or fluorine

Question 12

Can Aldehydes and ketones form hydrogen bonds with water?

Answer 12

yes

Question 13

What reagent is used to identify both aldehydes and ketones?

Answer 13

• Brady’s reagent, 2,4-DNP (DiNitroPhenylhydrazine)

- Positive result: Bright Orange Percipitate

Question 14

How can we identify the carbonyl compound?

Answer 14

• Filter and dry the derivative
• Measure the melting point of the derivative
• Compare the value to a data booklet and check which carbonyl compound it belongs too

Question 15

Acidified Potassium dichromate (VI) test

Answer 15

• Used to distinguish between aldehydes and ketones
• Colour change from orange to green due to the reduction of the chromium ions
• Oxidises aldehydes to carboxylic acids

Question 16

Fehling’s/Benidict’s solution

Answer 16

• Colour change is due to the change from Copper (II) oxide complex to a Copper (I)
• Positive result Blue -> red precipitate

Question 17

Tollens reagent

Answer 17

-Positive result : colourless to a silver mirror

Question 18

How is the Triiodomethane reaction used?

Answer 18

• Carbonyl compound is added to an alkaline solution with iodine
• To identify the CH3-CO group in aldehydes and ketones and some alcohols
• Positive result : yellow precipitate
• Examples : Ethanol, Ethanal, propanone
• Only one aldehyde gives a positive result (ethanal) and all methyl ketones give a positive result

Question 19

Explain how you would use the triiodomethane test

Answer 19

• Add the carbonyl compound to an alkaline solution of iodine
• Warm the mixture then cool it
• If a yellow precipitate is formed there is a CH3CO group present
• The yellow precipitate is the CHI3 (yellow insoluble solid)

Question 20

Reduction of aldehydes and ketones to alcohols

Answer 20

• Lithium alluminium hydride (LiAlH4)
• This reagent is dissolved in dry ether along with the carbonyl compound
• Ketone reduced to a secondary alcohol
• Aldehyde reduced to primary alcohol [H]
• Carboxylic acid reduced to primary alcohol 2[H]

Question 21

What is the optical activity when carbonyl compounds react with KCN&HCN and why?

Answer 21

• Most of the time no optical activity (there need to be 2 different groups attached to the carbon)
• Because the RHC=O is planar and the (: CN) can attack from either side forming two enantiomers
• This forms a racemic mixture that results in no optical activity

Question 22

What type of reaction is it when you react carbonyl compounds with HCN&KCN?

Answer 22

Nucleophilic addition reaction

Question 23

Acidic Hydrolysis of Nitriles

Answer 23

R-CN + [H+] + 2H2O -> R-COOH + NH4+

Question 23

Alkaline Hydrolysis of Nitriles

Answer 23

1. R-CN + [OH]- +H2O -> R-COO- + NH3

2. use dilute acid to convert R-COO- into R-COOH

Question 24

Acidic Hydrolysis of esters

Answer 24

R1-COO-R2 + H2O ⇌ R1-COOH + R2OH

Question 25

Alkaline Hydrolysis of Esters

Answer 25

R1-COO-R2 + NaOH