Topic 17: Further Organic Flashcards
What is a Chiral molecule?
A molecule with a chiral carbon (it is asymmetric)
What is a Chiral Carbon?
A carbon atom with four different groups attached to it (asymetric)
How does optical isomerism occur?
Results from chiral centres in a molecule with asymmetric carbon atoms
What are optical isomers (properties)?
- non-superimposable mirror images
- rotate the plane of polarisation of plane polarised monochromatic light
Define optical activity?
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
What is a racemic mixture?
No optical activity due to equal concentrations of two opposite enantiomers
SN1 reaction
- S=Substitution, N=nucleophilic , 1=unimolecular
- Forms two products
- It can form a racemic mixture as equal concentrations of opposite enantiomers are formed
SN2 reaction
- S=Substituiton, N=Nucleophillic, 2=bimolecular
- If the molecule is an enantiomer it will reverse the way in which it rotates the polarised light
Define unimolecular (SN1)
-1 species involved in the rate-determining step
Define bimolecular
-2 species are involved in the rate-determining step
Can Aldehydes and Ketones form Hydrogen bonds with eachother?
-No because they do not thave a hydrogen attached to O,N or fluorine
Can Aldehydes and ketones form hydrogen bonds with water?
yes
What reagent is used to identify both aldehydes and ketones?
- Brady’s reagent, 2,4-DNP (DiNitroPhenylhydrazine)
- Positive result: Bright Orange Percipitate
How can we identify the carbonyl compound?
- 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
Acidified Potassium dichromate (VI) test
- 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
Fehling’s/Benidict’s solution
- Colour change is due to the change from Copper (II) oxide complex to a Copper (I)
- Positive result Blue -> red precipitate
Tollens reagent
-Positive result : colourless to a silver mirror
How is the Triiodomethane reaction used?
- 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
Explain how you would use the triiodomethane test
- 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)
Reduction of aldehydes and ketones to alcohols
- 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]
What is the optical activity when carbonyl compounds react with KCN&HCN and why?
- 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
What type of reaction is it when you react carbonyl compounds with HCN&KCN?
Nucleophilic addition reaction
Acidic Hydrolysis of Nitriles
R-CN + [H+] + 2H2O -> R-COOH + NH4+
Alkaline Hydrolysis of Nitriles
- R-CN + [OH]- +H2O -> R-COO- + NH3
2. use dilute acid to convert R-COO- into R-COOH
