(a)(i) Give three characteristics of homologous series. (ii) Name two groups of compounds which form such a series. (b) A saturated organic compound A conta...
Question 1 Report
(a)(i) Give three characteristics of homologous series.
(ii) Name two groups of compounds which form such a series.
(b) A saturated organic compound A containing two carbon atoms reacted with ethanoic acid in the presence of a mineral acid to form a compound B with a sweet smell. (i) Name the functional group present in A.
(ii) Draw the structure of A.
(iii) Write a chemical equation to show the formation of B.
(iv) Name the compound B.
(c)(i) Write a balanced equation for the reaction between ethyne and excess bromine.
(ii) Give IUPAC name of the product of the reaction in (c)(i)
(iii) State two conditions under which cracking takes place
(d)(i) Outline the preparation of ethanol from starch.
(ii) Give two properties of starch.
(iii) Give a reason why starch does not reduce Fehlings solution
(iv) Describe briefly a chemical test to confirm the presence of starch.
Answer Details
(a)(i) Characteristics of a homologous series
- Members have the same general formula.
- Successive members differ by a \(\ce{-CH2-}\) unit.
- They have the same functional group and therefore similar chemical properties.
- They show a gradual change in physical properties, such as boiling point and density, as molecular mass increases.
(ii) Examples are alkanes and alkanols (alcohols).
(b) Compound A is ethanol, because a two-carbon alcohol reacts with ethanoic acid to form a sweet-smelling ester.
(i) The functional group in A is the hydroxyl group, \(\ce{-OH}\).
(ii) Structure of A, ethanol
(iii)
\[\ce{CH3COOH + CH3CH2OH <=>[conc. H2SO4][heat] CH3COOCH2CH3 + H2O}\]
(iv) B is ethyl ethanoate.
(c)(i)
\[\ce{HC#CH + 2Br2 -> CHBr2CHBr2}\]
(ii) The IUPAC name of the product is 1,1,2,2-tetrabromoethane.
(iii) Conditions for cracking
- High temperature, usually about \(500\text{ to }700\,^\circ\mathrm{C}\).
- Use of a suitable catalyst, such as silica-alumina or zeolite.
(d)(i) Preparation of ethanol from starch
- Warm starch with malt containing the enzyme diastase. The starch is hydrolysed to maltose:
\[\ce{(C6H10O5)_{n} + \tfrac{n}{2}H2O ->[diastase] \tfrac{n}{2}C12H22O11}\] - Add maltase to convert maltose to glucose:
\[\ce{C12H22O11 + H2O ->[maltase] 2C6H12O6}\] - Add yeast and keep the mixture warm, about \(30\,^{\circ}\mathrm{C}\), under anaerobic conditions. Zymase in the yeast ferments glucose to ethanol and carbon(IV) oxide:
\[\ce{C6H12O6 ->[zymase] 2C2H5OH + 2CO2}\] - Distil the fermented liquid to obtain ethanol.
(ii) Two properties of starch
- It is a white, tasteless solid and is insoluble in cold water.
- When warmed with water, it forms a colloidal paste or solution.
(iii) Starch does not reduce Fehling's solution because it has no free reducing carbonyl group.
(iv) Add iodine solution to the sample. A blue-black colour confirms the presence of starch.
Download The App On Google Playstore
Everything you need to excel in your exams
