Chirality Flashcards
What is chiral carbon/chiral centre?
Carbon atom with 4 different atoms/groups of atoms attached to it
- will be asymmetric
What are enantiomers?
Compounds with 1 chiral centre have 2 optical isomers - enantiomers
- mirror images of each other - non-superimposable
Properties of optical isomers - chemcial and physical?
Chemcial : identical BUT optical isomers interact with biological sensors differently
- e.g an enantiomer of carvone smells on spearmint, other smells of caraway
Physical: identical BUT differ in their ability to rotate the plane of polarised light
How can rotation of plane polarised light be used to determine the identity of an optical isomer?
- pass plane polarised light through a sample
- Depending on which isomer the sample contains, the plane of polarised light will be rotated clockwise or anti-clockwise by a fixed number of degrees
Enantiomers main differences are that they rotate plane of light clockwise or anticlockwise (+ / -)
What is a racemic mixture and why are they optically inactive?
Mixture containing equal amounts of each enantiomer
- optically inactive as enantiomers cancel out each others effect (1 rotates light clockwise, other rotates light anticlockwise) - plane of polarised light will not change
What kind of mechanism can SN1 and SN2 occur in?
Nucleophilic substitution
What is the SN1 mechanism ?
2 step reaction
- the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion - RDS/SLOW (rate= k[Haloalkane] )
- leaves a trigonal planar, tertiary carbocation - the planar, tertiary carbocation attacked by the nucleophile
- nucleophile is able to attack from above/below of the planar carbocation, which results in the formation of a racemic mixture
What is the SN2 mechanism?
1 step reaction
- nucleophile donates pair of electrons to δ+ carbon atom of the haloalkane - forms new bond
At same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond
The halogen as an X- ion
E.g substitution of bromothane by OH- ions to form ethanol
Br atom on bromoethane causes STERIC HINDRANCE- OH- ion nucleophile can only attack from opposite side of C-Br bond
As C-OH bond forms, the C-Br bond breaks - the Br atom to leave as Br-
So: the molecule has undergone an inversion of configuration
If SN2 starts with enantiopure reactant , u get entiopure product
Difference between aldehyde and ketone in terms of structure?
ALDEHYDE: C=O always have H bonded to it
- carbonyl group always on position 1 on chain
KETONE: carbonyl group in middle of chain
C=O attached to 2 alkyl groups
What intermolecular forces exist in carbonyls and why are their MP’s lower than their corresponding alcohol?
- have a dipole in their structure due to electronegative oxygen in carbonyl group
- so aldehydes/ketones have PERMANENT DIPOLE-DIPOLE interactions + London forces between molecules
MP/BP lower than alcohol as no hydrogen bonds between molecules as **no polar bonds (e.g O-H, N-H) to result in δ+ hydrogen in H bonding
Why do smaller aldehydes/ketones dissolve in water?
Have a lone pair of electrons on Oxygen in C=O group
- can form H bonds with δ+ hydrogen from H2O
Why can’t larger aldehydes and ketones dissolve in water?
Have Longer hydrocarbon chains
- strength of the POTENTIAL hydrogen bonding of carbonyl + water < combined strength of intermolecular forces of carbonyl and H bonding in water
How can oxidation with ACIDIFIED POTASSIUM DICHROMATE distinguish between ketone+aldehyde?
Aldehydes OXIDISED to CARBOXYLIC ACIDS
Ketones RESISTANT TO OXIDATION - as they dont have readily available H atom like aldehydes
- need very strong [O] that will break C-C bond
Heating with acidified potassium dichromate:
Aldehyde oxidised - ORANGE —> GREEN
Ketone not oxidised - NO COOUR CHANGE
How does Tollens’ reagent distinguish between aldehydes+ ketones?
When gently warmed with Tollens’:
ALDEHYDE : oxidised / [Ag(NH3)2]+ ions in reagent reduced to solid metallic silver, Ag
Ag+ ions are oxidising agents/get reduced themselves
So + result = SILVER MIRROR FORMED
Ketone : no silver mirror seen as KETONE NOT OXIDISED
How does FEHLING’S distinguish between aldehydes and ketones?
Warmed with FEHLING’S:
Aldehyde is OXIDISED to carboxylic acid
- BLUE —> RED
FEHLING’S contains Cu2+ ions in NaOH :
- reduced from Cu2+ complex ions (BLUE) to Cu+ (BRICK RED PRECIPITATE of CuO)
Ketone REMAINS BLUE - not oxidised
What is Benedict’s solution?
Cu2+ ions dissolved in SODIUM CARBONATE
Reduction of carbonyls? Which mechanism is it?
NUCLEOPHILIC ADDITION reaction
- ALDEHYDES reduced to PRIMARY ALCOHOL
- Ketone reduced to SECONDARY ALCOHOL
- use lithium aluminium hydride in dry ether (LiAlH4)
- hydride ion reduces C=O group
addition of HCN to carbonyl compounds?
NUCLEOPHILIC ADDITION
2 step process:
1. :CN- attacks δ+ carbonyl carbon to form negatively charged intermediate (carbonyl O is δ-)
2. (-) Oyxgen atom in intermediate quickly reacts with AQUEOUS H+ (from HCN, water or dilute acid) to form 2-hyroxynitrile compounds
INCREASE CHAIN LENGTH
