Organic Chemistry

Question 1

ALKANES

Answer 1

saturated with carbon-carbon single bonds

Question 2

ALKENES

Answer 2

unsaturated with carbon-carbon double bonds

Question 3

General formula of alkanes

Answer 3

CnH2n+2

Question 4

General formula of alkenes

Answer 4

CnH2n

Question 5

Alkane examples

Answer 5

Methane (CH4 when n=1)
Ethane (C2H6 when n=2)
Propane (C3H8 when n=3)

Question 6

Alkene examples

Answer 6

Ethene (C2H4 when n=2)
Propene (C3H6 when n=3)

Question 7

Chemical Properties of Alkanes

Answer 7

1) Combustion

(i) Complete combustion: presence of excess air to form
carbon dioxide and water
(ii) Incomplete combustion: presence of limited air to form
carbon monoxide & water are formed.

2) Substitution reaction
CH4 + Cl2 → CH3Cl ( chloromethane) + HCl

• only take place in the presence of ultraviolet light

3) Cracking
Large alkane molecules are cracked to form smaller alkenes and alkanes / hydrogen

Conditions: 600 °C & Al2O3 catalyst

C10H22 (alkane) → C4H8 (alkene) + C6H14 (alkane)

C10H22 (alkane) → 2C2H4 (ethene) + 2C3H6 (propene) + H2

Question 8

Chemical properties of alkenes

Answer 8

1) Combustion

(i) Complete combustion: presence of excess air to form
carbon dioxide and water
(ii) Incomplete combustion: presence of limited air to form
carbon monoxide & water are formed.

2) Addition reaction
(i) with hydrogen (200 °C with nickel catalyst)

C2H4 (ethene) + H2 → C2H6 (ethane)

(ii) with aqueous bromine (test for unsaturated compounds)

C2H4 (ethene) + Br-Br C2H4Br2 (dibromoethane)

Aqueous bromine (reddish-brown) becomes colourless

3) Addition Polymerisation

Conditions: heat, high pressure & catalyst

C2H4 + C2H4 + … → ( C2H4) n (polyethene)

Question 9

What is the functional group of alcohols?

Answer 9

-OH

Question 10

What is the functional group of carboxylic acids?

Answer 10

-COOH

Question 11

General formula of alcohols

Answer 11

CnH2n+1OH

Question 12

General formula of carboxylic acids

Answer 12

CnH2n+1COOH

Question 13

Examples of alcohols

Answer 13

Methanol (CH3OH when n=1)

Ethanol(C2H5OH when n=2)

Propanol (C3H7OH when n=3)

Question 14

Examples of carboxylic acid

Answer 14

Ethanoic acid (CH3COOH when n=1)

Question 15

How is alcohol made?

Answer 15

Fermentation of sugar/glucose to form ethanol

Sugar / glucose → (yeast) ethanol + carbon dioxide

C6H12O6 → 2C2H5OH + 2CO2

Conditions:
1. Yeast must be present as catalyst
2. Suitable warmth: Temp of 37ºC
(If too hot: yeast will be destroyed; if too cold: enzymes will be
inactive)
3. no O2 present in mixture
(If O2 is present: it will oxidised the ethanol to ethanoic acid)

Question 16

How is carboxylic acid made?

Answer 16

Oxidation of ethanol to form ethanoic acid.

Oxidation by
(i) oxygen in air or
(ii) by acidified potassium manganate(VII)
(KMnO4 will change from purple to colourless. This is
a test for alcohols)

Question 17

Chemical properties of alcohol

Answer 17

1) Combustion
2) Oxidation to form carboxylic acid

C2H5OH (ethanol) → [O] CH3COOH (ethanoic acid)

Oxidation by
(i) oxygen in air or
(ii) by acidified potassium manganate(VII)
(KMnO4 will change from purple to colourless. This is a test for
alcohols)

Question 18

Chemical properties of carboxylic acid

Answer 18

1) Combustion
2) Reacts like (weak) acid
- reacts with alkali to form salt & water.
- reacts with carbonates to form salt, water & CO2
- reacts with reactive metals to from salt & H2

Question 19

Using a named reagent, describe how you would be able to distinguish
unsaturated hydrocarbons from
saturated hydrocarbons.

Answer 19

Reagent used is aqueous bromine.

If saturated hydrocarbon is present,
aqueous bromine remained brown /
no visible change.

If unsaturated hydrocarbon is
present, brown aqueous bromine
will turn colourless / brown aqueous bromine will decolourise.

Question 20

Briefly describe the
manufacture of alkenes from long-chain alkanes

Answer 20

Alkenes can be obtained from longchain alkanes by cracking.

Using heat, high pressure and
Al2O3 catalyst, large alkane molecules are cracked to form
smaller alkenes and alkanes and
hydrogen

For example:
C10H22 (alkane) → C4H8 (alkene) +
C6H14 (alkane)

Question 21

Describe the importance
of cracking.

Answer 21

Cracking helps to meet the demands for smaller, more useful
alkene molecules like the petrol.

Cracking is also important to
produce hydrogen.