3.0 Organic Chemistry 1

3.4 Recommended practice of topic summary

For better retention of what is learnt, the learner is encouraged to prepare, for every topic read, a condensed one-page summary notes and a reaction scheme (flow chart, or map). Begin with a simple scheme, then build and enrich it gradually as you revise and understand the topic more. Refer to "Answers to Questions" sections, where you find it necessary, to enable you write the summary. Here is an example for alkenes.

Alkenes

Unsaturated hydrocarbons with carbon-carbon double bonds

Laboratory preparation of ethene (an alkene)

Dehydration of ethanol using concentrated sulphuric (VI) acid. CH₃CH₂OH H₂SO₄(l) H₂O + CH₂CH₂

Industrial production

From cracking of higher alkanes

General formula

Belong to a homologous series with the general formula, C_nH_2n

Nomenclature of alkenes

General name: Alk-x-ene where x is the position of the double bond.

Example: CH₃CH₂CH₂CHCHCH₃ is Hex-2-ene

Physical properties of alkenes:

Largely insoluble in water; soluble in organic (oily) solvents e.g. tetrachloromethane, colourless, low density, low boiling and melting points.

Chemical properties of alkenes

Ethene extinguishes a glowing splint, is neutral, decolourizes chlorine water, bromine water, potassium manganate (VII) solution, and potassium chromate (VI) even in the dark.

With halogens (Cl₂, Br₂, I₂, F₂), e.g. C₂H₂(g) + Cl₂(g) CHClCHCl(g) (halogenation/chlorination, etc)

With hydrogen e.g. CH₂CH₂(g) + H₂(g) Ni/140 OC CH₃CH₃(g) (hydrogenation),

With hydrogen halides, e.g. CH₂CH₂ + HCl(g) CH₃CH₂Cl (hydro-halogenetation)

With chlorine water, CH₂CH₂(g) + HOCl(aq) CH₂ClCH₂OH(l)

Self-addition reaction: nCH₂=CH₂ (-CH₂CH₂-)n (addition polymerization)

Test for alkenes

Alkenes decolourize chlorine water, bromine water, and potassium manganate (VII) even in the dark.

Uses of alkenes

Welding fuel (ethyne), manufacture of polythene and plastics, and alcohol

NB: The reaction scheme does not show all the products and equations for each step. The learner should practice writing them on a separate workbook.

Questions 3.4(a)

1. Name the specific addition reactions and processes labelled I, II, III, IV, V, VI in Figure 3.4.
2. Prepare a condensed summary together with a reaction scheme for alkanes, using ethane as an example.

Answers to Questions 3.4(a)

Questions 3.4(b)

1. From what we have learnt in this topic, explain how you would test for each of the following groups of substances.
   1. Alkanes (e.g. ethane)
   2. Alkenes (e.g. ethene)
   3. Alkynes (e.g. ethyne)
2. Tests for the substances named in Question 1 assume that they have already been confirmed as hydrocarbons. That is, their products of combustion in oxygen are carbon (IV) oxide (CO₂) and water (H₂O) only. From your prior knowledge of chemical behavior of carbonates, acids, hydrated salts, oxygen and water, describe how each of the following substances can be confirmed.
   1. Carbon (IV) oxide (CO₂)
   2. Water (H₂O)
   3. Hydrogen gas (H₂)
   4. Oxygen gas (O₂)

Answers to Questions 3.4(b)

3.5 Project 3

Design, preferably in a group, an experiment to determine factors that make the burning of oil fuels more efficient, to reduce energy losses. In the design, explain clearly the procedure you would follow, the materials and apparatus you require and how you would use them, the measurements you would take, and how you would use them to identify the factors.