Structured response question -SL05

1.a) This question is about four compounds A, B, C, and D, which can be made from ethene by the following reactions. All of the compounds are liquid at room temperature and each compound's molecular formula is shown. Two of the reagents needed for the reactions are shown below the arrows.

+ H2O
+ C2H6O

Magnesium was added to each compound and the only compound that produced a gas was C.

i) Use the information above to identify each of the compounds A, B, C, and D, giving the name of each one

ans (i)

Magnesium tests for the presence of a carboxylic acid. C produced a gas with magnesium, therefore C is an acid, ethanoic acid.

C is fromed from B, C2H4O, therefore B is an aldehyde. Ethanoic acid is formed from ethanal, therefore B is ethanal.

B, ethanal is formed from A, C2H6O, therefore A is a primary alcohol, ethanol. This fits well into the scheme, as A is formed by reaction of ethene, C2H4 with water.

The final product D, is formed by reaction of ethanoic acid with C2H6O. The product, D, contains all of the carbon atoms therefore the two molecules have added together (and a water molecule has been eliminated). This is an esterification reaction and D is ethyl ethanoate, an ester.

ii) State the type of reaction occurring when C is converted to D and state the catalyst that is used. What other product is formed?
ans (ii)

Esterification, the other product is water. The catalyst used for this reaction is concentrated sulfuric acid.

iii) Write the structural formulas of A and C
ans (iii)
A - Ethanol
C - Ethanoic acid

  b) State a use of the compound of the same type as D in the food industry.

ans (b)
Esters are used as flavourings and scents.

  c) Identify the gas formed when C reacts with magnesium and write an equation for the reaction occurring. Name the other product of the reaction.

ans (c)

Active metals react with acids producing hydrogen gas.

2CH3COOH + Mg (CH3COO)2Mg + H2

  d) Arrange the compounds A, B, C, in order of increasing boiling point (lowest first). Explain your choice by referring to the intermolecular forces in each case.

ans (d)

The boiling point depends on the strength of the intermolecular forces. The strongest of these is hydrogen bonding (for molecules of similar mass). Carboxylic acids have the strongest hydrogen bonding followed by alcohols. Aldehydes have dipole-dipole interactions which are the next strongest intermolecular bonding forces. All of the molecules have dispersion attractions and as they have similar masses, these forces are roughly equal

Therefore the weakest force (and therefore the lowest boiloing points) are found in esters, aldehydes, alcohols and acids in increasing order; D < B < A < C.

  d) Compound C has another isomer. Name and give the structural formula of this isomer.

ans (d)

  d) Some of the compounds A, B, C, and D, exist as optical isomers. State the structural feature that is present in a compound which exists as optical isomers.

ans (d)