Structured response question -HL01
1.a) This question is about four compounds A, B, C, and D. B, C and D can be made from A by the following reaction scheme.
Sodium was added to each liquid compound. Gas bubbles formed slowly in A, but rapidly in C. The infrared spectra of the compounds showed characteristic absorptions at the following wavenumbers (cm-1)
i) Explain by reference to the data table how the infrared absorptions listed above can be used to identify the functional groups present in each of the compounds A, B, C, and D. Use this information to identify each of the compounds A, B, C, and D, giving the name and structural formula of each one.
A has an absorption that is characteristic of an OH group with H bonding. Therefore A is an alcohol or an acid.
B has a characteristic absorption of a C=O group, characteristic of aldehydes, ketones, esters and acids. B further reacts to form a compound C which has the same absorption, AND the -OH frequencies characteristic of hydrogen bonded acids and alcohols. Only aldehydes can be converted to acids, and only primary alcohols can be converted to aldehydes.
D has an absorption characteristic of C=C groups.
ans (ii)ii) State the type of reaction when A is converted to B and C and state a suitable reagent and conditions for each one. Explain why B is much harder to obtain than C in this reaction.
These are oxidation reactions. The reagent used is the same in both cases, acidified potassium (or sodium) dichromate(VI). However, in order to make the aldehyde the reaction mixture is distilled and to make the acid the reaction mixture is refluxed.
B is harder to obtain, as it itself is easily oxidised and must be removed form the reaction mixture as soon as it is formed.
iii) State the type of reaction when A is converted to D and name a catalyst that is used. What other product is formed as well as D.ans (iii)
|This is a dehydration reaction. The reagent of choice is phosphoric acid (concentrated) although sulfuric acid also can be used. The other product is water.|
b) Identify the gas formed when A and C react with sodium and write an equation for each of the reactions occurring. Name the other product of the reaction between sodium and C.ans (b)
Both A and C react with sodium forming hydrogen gas.
Compound A: 2CH3CH2CH2CH2OH + 2Na 2CH3CH2CH2CH2ONa + H2
Compound C: 2CH3CH2CH2COOH + 2Na 2CH3CH2CH2COONa + H2
The other product is sodium butanoate.
c) Arrange the compounds A, B and C in order of increasing boiling point. Explain your choice.ans (c)
Carboxylic acids have the highest boiling point due to strong hydrogen bonding between molecules. They are followed by alcohols which also have hydrogen bonding, although not so effective. Aldehydes have dipole-dipole interactions that are stronger than the forces between alkene molecules. All of the molecules have dispersion interactions.
Correct order of increasing boiling point: B < A < C
d) None of the compounds A, B, C, and D exist as optical isomers.
State the structural feature that is present in a compound that exists as optical isomers. Draw the structural formula of the isomer of A that exists as optical isomers.