Topic 17 Organic II

Question 1

IR is absorbed by polar bonds.

Answer 1

The molecule itself doesn’t have to be polar. When polar bonds absorb IR, the bonds stretch & bend.

Question 2

IR can be used to monitor an organic reaction.

Answer 2

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.

Question 3

Peaks in the IR spectrum at wavelengths less than 1000 cm^-1

Answer 3

The fingerprint region attributed to the unique vibration of the whole molecule.

Question 4

TMS

Answer 4

Tetramethylsilane is the internal reference standard in NMR with 1 proton environment containing 12 protons, so produces an intense peak.

Question 5

The molecular formula contains C:H in the ratio ~1:1

Answer 5

Indicates a benzene ring.

Question 6

The molecular formula contains C:H in the ratio CnH2n+2

Answer 6

The molecule is saturated.

Question 7

The molecular formula contains C:H in the ratio CnH2n

Answer 7

The molecule is unsaturated.

Question 8

What is 13C-NMR used for?

Answer 8

To differentiate structural isomers.

Question 9

How would you form a Grignard reagent?

Answer 9

Halogenoalkane + Mg, heat under reflux in dry ether to form RMgBr.

Question 10

Bromoalkanes & iodoalkanes are typically used to make Grignard reagents. Why are chloroalkanes less commonly used?

Answer 10

They are less reactive and require stronger and longer heating with Mg.

Question 11

Carbonyl –> alcohol

Answer 11

Reduce with LiAlH4 in dry ether.

Question 12

Carbonyl –> hydroxynitrile

Answer 12

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.

Question 13

Aldehyde (not methanal) and asymmetric ketones react with HCN in KCN to produce…

Answer 13

…a racemic mixture as the :CN- ion can attack equally from above or below the plane of the reaction centre.

Question 14

RMgBr + CO2 + H+ –>

Answer 14

RCOOH

Question 15

RMgBr + methanal –>

Answer 15

Primary alcohol

Question 16

RMgBr + aldehyde (not methanal) –>

Answer 16

Secondary alcohol

Question 17

RMgBr + ketone –>

Answer 17

Tertiary alcohol

Question 18

Grignard reagent + water –>

Answer 18

Alkane

Question 19

Test for halogenoalkanes

Answer 19

Add NaOH & heat. Add nitric acid then AgNO3 and note the colour of the precipitate.

Question 20

Test for acyl chlorides

Answer 20

Add water. Steamy fumes of HCl(g).

Question 21

Upon what 2 factors does retention time in gas chromatography depend?

Answer 21

Polarity/affinity of the component for the stationary phase.
Boiling temperature of the compound.

Question 22

How do higher boiling temperatures affect the retention time in GC?

Answer 22

Compounds with higher boiling temperatures spend less time in the gas phase/mobile phase, so have longer retention times.

Question 23

How is esterification different with acyl chlorides compared to carboxylic acids?

Answer 23

• No acid catalyst is needed as it occurs quickly at RTP.
• HCl by-product instead of water.
• Irreversible instead of reversible.

Question 24

Why is ethanoic acid a weak acid but ethanol is not acidic?

Answer 24

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.

Question 25

Butanoic acid + PCl5 –>

Answer 25

Butanoyl chloride + HCl + POCl3
Dry conditions. Room temperature.

Question 26

What reagents & conditions are required to convert a nitrile to a carboxylic acid?

Answer 26

Any dilute acid. Reflux. Heat. Acid hydrolysis.

Question 27

What reacts with a carboxylic acid to form an ester?

Answer 27

Alcohol. Concentrated acid catalyst. Reflux.

Question 28

What can be used instead of PCl5 (dry) to form an acyl chloride? What is the advantage of using this other reagent?

Answer 28

SOCl2. Inorganic byproducts are gases, so only the acyl chloride is left in the flask.

Question 29

How to purify an impure sample of an organic solid:

Answer 29

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.

Question 30

Compare acid & alkali hydrolysis of an ester.

Answer 30

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.

Question 31

What is key about the carbocation formed in the nucleophilic addition of carbonyls?

Answer 31

It is planar around the reaction site, so the nucleophile attacks equally from above & below the plane.

Question 32

Describe the Iodoform test.

Answer 32

Add I2(aq) + NaOH(aq) and warm. Methyl carbonyls react to form yellow precipitates.