Topic 17 Organic II Flashcards
IR is absorbed by polar bonds.
The molecule itself doesn’t have to be polar. When polar bonds absorb IR, the bonds stretch & bend.
IR can be used to monitor an organic reaction.
E.g., when an aldehyde is reduced to a primary alcohol, the C=O bond peak disappears and the O-H stretch appears. State the wavenumbers.
Peaks in the IR spectrum at wavelengths less than 1000 cm^-1
The fingerprint region attributed to the unique vibration of the whole molecule.
TMS
Tetramethylsilane is the internal reference standard in NMR with 1 proton environment containing 12 protons, so produces an intense peak.
The molecular formula contains C:H in the ratio ~1:1
Indicates a benzene ring.
The molecular formula contains C:H in the ratio CnH2n+2
The molecule is saturated.
The molecular formula contains C:H in the ratio CnH2n
The molecule is unsaturated.
What is 13C-NMR used for?
To differentiate structural isomers.
How would you form a Grignard reagent?
Halogenoalkane + Mg, heat under reflux in dry ether to form RMgBr.
Bromoalkanes & iodoalkanes are typically used to make Grignard reagents. Why are chloroalkanes less commonly used?
They are less reactive and require stronger and longer heating with Mg.
Carbonyl –> alcohol
Reduce with LiAlH4 in dry ether.
Carbonyl –> hydroxynitrile
React with HCN in KCN. HCN is a weak acid, so the KCN provides the :CN- ion and the HCN provides the H+ needed to form the OH.
Aldehyde (not methanal) and asymmetric ketones react with HCN in KCN to produce…
…a racemic mixture as the :CN- ion can attack equally from above or below the plane of the reaction centre.
RMgBr + CO2 + H+ –>
RCOOH
RMgBr + methanal –>
Primary alcohol
RMgBr + aldehyde (not methanal) –>
Secondary alcohol
RMgBr + ketone –>
Tertiary alcohol
Grignard reagent + water –>
Alkane
Test for halogenoalkanes
Add NaOH & heat. Add nitric acid then AgNO3 and note the colour of the precipitate.
Test for acyl chlorides
Add water. Steamy fumes of HCl(g).
Upon what 2 factors does retention time in gas chromatography depend?
Polarity/affinity of the component for the stationary phase.
Boiling temperature of the compound.
How do higher boiling temperatures affect the retention time in GC?
Compounds with higher boiling temperatures spend less time in the gas phase/mobile phase, so have longer retention times.
How is esterification different with acyl chlorides compared to carboxylic acids?
- No acid catalyst is needed as it occurs quickly at RTP.
- HCl by-product instead of water.
- Irreversible instead of reversible.
Why is ethanoic acid a weak acid but ethanol is not acidic?
The carbonyl group pulls e- away from the OH- group, making the H atom more electropositive, so easier to remove as an H+ ion. The carboxylate ion has the negative charge shared between 2 oxygen atoms. The electrons in the p-orbitals in the O-C-O bonds delocalise to form a delocalised pi bond.
Butanoic acid + PCl5 –>
Butanoyl chloride + HCl + POCl3
Dry conditions. Room temperature.
What reagents & conditions are required to convert a nitrile to a carboxylic acid?
Any dilute acid. Reflux. Heat. Acid hydrolysis.
What reacts with a carboxylic acid to form an ester?
Alcohol. Concentrated acid catalyst. Reflux.
What can be used instead of PCl5 (dry) to form an acyl chloride? What is the advantage of using this other reagent?
SOCl2. Inorganic byproducts are gases, so only the acyl chloride is left in the flask.
How to purify an impure sample of an organic solid:
Dissolve in the minimum volume of hot solvent.
Filter through a warmed funnel into a conical flask.
Allow the solution to cool.
Filter the mixture under reduced pressure using a Büchner funnel.
Wash the solid on the filter paper with ice cold solvent.
Dry.
Compare acid & alkali hydrolysis of an ester.
The acid is a catalyst & the alkali is a reactant. Acid hydrolysis is reversible, so lower-yielding than alkaline hydrolysis. Both require distillation to separate the products. Alkali hydrolysis requires the addition of dilute acid to convert the carboxylate salt into a carboxylic acid.
What is key about the carbocation formed in the nucleophilic addition of carbonyls?
It is planar around the reaction site, so the nucleophile attacks equally from above & below the plane.
Describe the Iodoform test.
Add I2(aq) + NaOH(aq) and warm. Methyl carbonyls react to form yellow precipitates.
