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 (tertiary/branched haloalkanes)
- 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 (primary haloalkanes)
- 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 ( add HCN and KCN)
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
E.g of reaction :
CH3CO + HCN —> CH3CH(OH)CN
INCREASE CHAIN LENGTH
How does 2,4- DNPH detect presence of carbonyl compounds?
Carbonyl group undergoes condensation reaction with 2,4-dinitrophenylhydrazine - 2 molecules join tgt/release H2O or HCl
If aldehyde/ketone present = DEEP ORANGE PRECIPITATE which is purified by recrystallisation
MP of precipitate used/compared to data booklet to identify specific aldehyde/ketone
What is the iodoform test?
Tri-iodomethane forms YELLOW precipitate with METHYL KETONES (have CH3CO- group) and ethanal (also has CH3CO- group) - methyl group attached to C=O
Sample heated with alkaline solution (NaOH) of IODINE
1. Halogenation: all 3 H atoms in CH3 group replaced with iodine atoms, to form -CI3 group
2. Intermediate compound HYDROLYSED by alkaline solution (NaOH) to from SODIUM SALT (RCO2-Na+) + yellow precipitate of CHI3 (tri-iodomethane)
CH3COCH3 + 3I2 + 4NaOH —> CHI3 + CH3COONa + 3NaI + 3H2O
What alcohols will also be detected in iodoform test and why?
If alcohol contains a R-CH(OH)-CH3 - produce yellow precipitate too
- includes methyl secondary alcohols (like hexan-2-ol) + the primary alcohol, ethanol
Iodine in NaOH reacts to from iodate ions which oxidise SECONDARY ALCOHOL —> KETONE
General formula for carboxylic acids?
CnH2n+1COOH
Physical properties of carboxylic acids?
Contain 2 polarised groups : C=O and O-H
So ALOT OF INTERMOLECULAR FORCES - HIGH BP/MP
O-H bond : hydrogen bonding : high mp/bp + soluble in water/polar solvents of shorter chained carboxylic acids
How to form carboxylic acids through OXIDATION?
Oxidise PRIMARY ALCOHOLS and ALDEHYDES
- use acidified potassium dichromate or acidified potassium manganate
-reflux
The ox agents (K2Cr2O7 / KMnO4 are REDUCED )
orange Cr2O72- reduced —> green Cr3+ ions
purple MnO4- reduced —> colourless Mn2+ ions
Hydrolysis of nitriles?
Produce carboxylic acids
- use dilute acid/alkali followed by acidification - heat under reflux
Using:
DILUTE ACID : forms carboxylic acid + ammonium salt
DILUTE ALKALI: forms sodium carboxylate salt + ammonia
Acidification changes carboxylate ion —> carboxylic acid
-CN group turns into -COOH group
E.g reaction :
CH3CH(OH)CN + 2H2O + H+ —> CH3CH(OH)COOH + NH4+
Reduction of carboxylic acids?
Use reducing agent - lithium tetrahydridoaluminate , LiAlH4 in dry ether at room temp
Reduced to PRIMARY ALCOHOL
- adding water at end will destroy excess LiAlH4
Carboxylic acids with bases?
Reaction with :
Metal oxide - metal salt and water produced
2CH3COOH (aq) + MgO (s) → (CH3COO)2Mg (aq) + H2O (l)
Magnesium ethanoate formed
ALKALIS: salt and water formed (neutralisation)
CH3COOH (aq) + KOH (aq) → CH3COOK (aq) + H2O (l)
Reaction with potassium hydroxide —> potassium ethanoate formed
CARBONATE : metal salt, water,CO2
2CH3COOH (aq) + K2CO3 (s) → 2CH3COOK (aq) + H2O (l) + CO2 (g)
Reaction with potassium carbonate —> potassium ethanoate formed
Carboxylic acid with phosphorus (V) chloride?
Forms ACYL CHLORIDE
CH3CH2COOH (l) + PCl5 (s) → CH3CH2COCl (l) + POCl3 (l) + HCl (g)
Propanone acid + PCl5 —> propanoyl chloride , phosphorus trichloride oxide and hydrogen chloride (STEAMY FUMES)
- liquid products separated by fractional distillation
Carboxylic acid with alcohols?
ESTER FORMED : -COOR group / sweet,fruity smell
Reaction : CONDENSATION REACTION
Conc H2SO4 catalyst
- water is lost!!!
How are esters named?
1st part of name from ALCOHOL
2nd part of name from carboxylic acid
What makes carboxylic acids weak acids and what does this tell us about the position of equilibrium?
In aqueous solutions they are PARTIALLY IONISED:
CH3COOH —> CH3COO- + H+
Equilibrium lies to LEFT + conc of H+ is smaller than conc of carboxylic acid
Rate equation for SN2 mechanism?
As its a 1step mechanism - RDS depends on conc of nucleophile + halogenoalkane
Rate = k[halogenoalkane][nucleophile]
OH- (nucleophile) is 1st order unlike in Sn1, where’s it’s 0 order
Hydrolysis of esters - acid? Conditions and products?
Reforms CARBOXYLIC ACID + ALCOHOL
CONDITIONS: heat under reflux with strong acid (dilute HCl) (equilibrium mixture reached - hydrolysis not complete)
Hydrolysis of esters - alkaline? Products and conditions? Example reaction?
Carboxylic acid produced reacts with excess alkali to form CARBOXYLATE SALT + ALCOHOL
Conditions: heat under reflux with dilute alkali (NaOH) - reaction goes to completion , so IRREVERSIBLE
- ester fully hydrolysed
Ethyl propanoate + NaOH —> sodium propanoate (CH3CH2COO-Na+) + ethanol
How is the sodium carboxylate from hydrolysis of ester in alkaline conditions CONVERTED INTO CARBOXYLIC ACID?
ACIDIFICATION
Sodium carboxylate (-COO-) ion needs to be PROTONATED by an acid (HCl) —> form carboxylic acid (-COOH)
What are acyl chlorides and acid anhydrides?
Derivatives of carboxylic acids
Acyl chloride —> substitution of -OH group by chlorine atom (ethanoyl chloride)
Acid anhydride —> substitution of -OH group by alkanoate (eg ethanoic anhydride)
Nucleophilic addition- elimination reaction?
Nucleophilic addition of small molecule across C=O bond, followed by elimination of small molecule
ACYL CHLORIDES:
-Hydrolysis
-Reaction with alcohol TO FROM ESTER
-Reaction with ammonia + primary amines to form AMIDES
Hydrolysis of acyl chlorides?
Forms CARBOXYLIC ACID + HCl
- water molecule added across C=O bond + HCl elimated
Propanoyl chloride + H2O —> CH3CH2COOH + HCl
- OH- is nucleophile + removes chlorine
Acyl chlorides forming esters?
React with alcohols -
alcohol added across C=O bond
HCl molecule eliminated
Propanoyl chloride + ethanol (CH3CH2OH) —> ethyl propanoate + HCl
- (CH2CH3O-) replaces chlorine
Formation of amides from acyl chlorides?
ADD AMINE OR AMMONIA
Amine or ammonia molecule added across C=O bond
HCl eliminated
Propanoyl chloride + ammonia (NH3)—> propanamide (CH3CH2CONH2) + HCl
- NH2 replaces chlorine
Propanoyl chloride + methylamine (CH3NH2)—> methyl propanamide (CH3CH2CONHCH3) + HCl
- (NHCH3) replace chlorine
How to identify condensation polymers?
Monomers linked by ester or amide bonds
- all formed by ELIMINATION OF WATER
How is polyester formed?
SERIES OF CONDESATION REACTIONS - lose water
DIcarboxylic acid monomers + DIOL monomers
- monomer linked with ester bonds
OH removed from carboxylic acid and H removed from alcohol (FOR EVERY ESTERIFICATION REACTION)- expelled as WATER
Formation of polyesters from HYROXYCARBOXYLIC ACIDS?
Single monomer contains alcohol group at one end, carboxylic group at other end
- can react 2 of these monomer to FORM POLYESTER + releases water
Example of polyester - what is it formed from and uses?
Terylene (PET)
From benzene-1,4-dicarboxylic acid + ethane-1,2-diol
- used for clothes - make them crease free/longer lasting
- treated tyrelene used for fizzy drink bottles/food containers
Polyester properties?
Fibres are :
Strong
Flexible
Abrasion resistant
Differences in esterification reactions when ethanoyl chloride is used instead of ethanoic acid?
When ethanoyl chloride used:
- HCl is byproduct (instead of water)
- doesn’t need catalyst bc reaction very fast at room temperature
- reaction is irreversible
Which alcohol produces both but-1-ene and but-2-ene in an elimination reaction?
Butan-2-ol
When carbonyls react with 2,4 DNPH:
Why is the filter paper and funnel warmed i an oven before filtering?
- Make sure the solution doesn’t cool down
- prevent crystallisation taking places on filter paper /in funnel which would reduce yield
When carbonyls react with 2,4 DNPH:
How does ‘allowing the filtrate to cool and crystallise’ and ‘filtering crystals under reduced pressure’ remove impurities from crystalline product?
Cool/crystallise : soluble impurities present stay in solution/remain dissolved
Filter under reduced pressure: removes more soluble impurities
How to make gignard reagent ?
Reflux magnesium with halogenoalkane in dry ether
How to produce primary ,secondary, tertiary alcohols and carboxylic acids from gignard reagent?
React Gignard with :
METHANAL —> primary alcohol
ALDEHYDES —> secondary alcohol
KETONES —> tertiary alcohol
CO2 —> carboxylic acid
2 step reaction:
1. Add gignard in dry ether
2. Add dilute acid (HCl) to hydrolyse/protonate
