Module 6: Organic Chemistry and Analysis Flashcards
Kekulé model of benzene?
Six members ring of carbon atoms joined by alternate single and double bonds
Evidence to disprove kekulés model?
- Lack of reactivity of benzene - if the model was right, it would decolourise bromine in an electrophilic addition reaction.
- Length of C-C bonds in benzene - X-ray diffraction, measure lengths in a molecule. Benzene found to have 0.139nm length but single bond would be 0.153nm
- Hydrogenation enthalpies - would expect it to have hydrogenation enthalpy which is three times that of cyclohexane. But the actual is 152kjmol-1 less than expected.
Delocalised model of benzene?
- each carbon atom uses 3/4 electrons in bonding to the two other carbons and one to the hydrogen atom.
- each carbon has on electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atom.
- sideways overlap of P-orbitals, above and below plane of carbon atoms, form electron density ring above and below.
- Overlapping creates system of pi bonds
- The six electrons in this system are delocalised
When is phenyl used instead of benzene in naming?
When benzene ring is attached to alkyl chain with a functional group or to an alkyl chain which 7 or more carbon atoms.
Benzene becomes substituent.
E.g. phenylethanone, 2-phenyloctane
Type of reaction for benzene and it’s derivatives?
Electrophilic substitution
Nitration of benzene?
Benzene + HNO3
Catalysed by H2SO4 and heated to 50*C
Produce nitrobenzene and H2O
Halogenation of benzene?
Requires a halogen carrier (AlCl3, FeCl3 etc)
Get bromination and chlorination
Alkylation of benzene?
Substitution of hydrogen atom in benzene ring by a alkyl group.
Requires reacting with haloalkane In presence of AlCl3 halogen carrier catalyst
Acylation reaction of benzene?
Benzene reacts with acts chloride in presence of AlCl3 catalyst
Aromatic Ketone formed
Oxidation of Aldehydes?
Aldehydes can be oxidised to carboxylic acids.
Reflux with acidified dichromate VI ions, Cr2O7^2- / H+
(K2Cr2O7/H2SO4)
Oxidation of Ketones?
Do not undergo oxidation reactions
Carbonyl compounds?
Aldehyde
Ketone
Presence of C double bond O
Nucleophilic Addition reaction
Why is the carbonyl compounds reaction Nucleophilic addition?
It has a C=C bond which is polar
Oxygen is more electronegative than carbon, Electron density lies closer to the oxygen atom.
Creates delta + carbon and delta - oxygen
Nucleophile will be attracted to slightly positive carbon atom.
Aldehydes can be reduced yo what with the addition of what?
Aldehydes + 2[H] —-> Primary alcohol
Requires - NaBH4 / H2O
Ketones reduces to what?
Ketone + 2[H] —> secondary alcohol
Requires - NaBH4 / H2O
Carbonyl compounds with HCN?
Aldehyde or ketone + HCN —> hydroxynitrile
Requires - H2SO4/ NaCN
( CN- / H+ )
How do you detect a carbonyl compound?
Add 2,4-dinitrophenylhydrazine
If no crystal form, add H2SO4
Yellow/orange precipitate forms which is 2,4-dinitrophenylhydrazone
Means there’s a ketone or aldehyde
Distinguishing between aldehyde and ketone?
Tollens reagent
Add silver nitrate in aqueous ammonia
If silver mirror is formed then an aldehyde is present.
Ag+ + e- —> Ag (s) The silver mirror : reduction reaction
Aldehyde + [O] —> carboxylic acid : oxidation reaction
What is a phenol?
Type of organic chemical containing a hydroxyl -OH functional group directly bonded to an aromatic ring.
Why is phenol classified as a weak acid?
Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring.
When dissolved in water, phenol partially dissociates to produce phenoxide ion and a proton.
Phenol + Sodium Hydroxide
Salt - Sodium Phenoxide (O-Na+)
Water
Neutralisation reaction.
Type of reactions Phenols undergo?
Electrophilic Substitution
Bromination of Phenol?
Phenol + Bromine 3Br2 (bromine water) –> white precipitate of 2,4,6-tribromophenol.
This decolourises bromine water, orange to colourless.
Halogen carrier not required.
+3HBr
Nitration of Phenol?
Phenol + dilute nitric acid (HNO3) –> mix of 2-nitrophenol and 4-nitrophenol
+H20
Compare reactivity of Phenol to Benzene?
Phenol has increased reactivity. This is due to the lone pair of electrons from the oxygen p-orbital of the -OH group, being donated to the pi-system of phenol. The electron density of the benzene ring in phenol is increased, this attracts electrophiles more readily than benzene.
What activates an aromatic ring?
NH2 (phenylamine) reacts more readily with electrophiles, directs second substituent to positions 2 and 4
What deactivates and aromatic ring?
NO2 (nitrobenzene) reacts less readily with electrophiles, directs second substituent to position 3
What is a carboxyl group?
Contains carbonyl group and hydroxyl group.
Carboxylic acid
Solubility of carboxylic acids?
Both the C=O and O-H bonds are polar allowing carboxylic acids to form hydrogen bonds with water molecules.
Qualitative analysis - test for Allene
Add bromine water
Decolourise from Orange to clear
Qualitative analysis- test for haloalkane
Silver nitrate and ethanol
Warm to 50•C
Chloro white ppt
Bromo cream ppt
Iodo yellow ppt
Qualitative analysis- carboxylic acid
Aqueous sodium carbonate
Effervescence
Qualitative analysis- primary secondary alcohol and aldehyde
Acidified potassium dichromate warm
Orange to green
What is the stationary phase in chromatography?
Does not move and is normally a solid or a liquid supported on a solid
What is the mobile phase in chromatography?
Does move as is normally a liquid or a gas
What is adsorption?
Process by which the solid silica holds the different substances in the mixture to its surface.
Separation is achieved by the relative adsorption of substances with the stationary phase.
Interpreting TLC
Rf = distance moved by the component / distance moved by the solvent front
