PODAR INTERNATIONAL SCHOOL (CAMBRIDGE)
Cambridge Assessment International Education
“SARASWATI” BEHIND ARCADIA, HIRANANDANI ESTATE, PATLIPADA, THANE (WEST) - 400607
Grade: _______________ Subject: Chemistry
Worksheet No: AY23-24/Sub / _No__ Date of Issue: __________ Date of Submission: ____________
Unit 19: Reactions of Organic Compounds
19.1 Characteristic Reactions of Different Homologous Series
A. Chemistry of Alkanes:
a. Alkanes are unreactive compounds because they are saturated molecules.
b. They can not take part in addition reactions.
c. They are unaffected by acids or alkalis.
d. They can take part in Substitution reactions
e. Most common Fuels are Alkanes:
A. Ethane – CH4
B. Natural Gas: Methane (75 to 90%), Ethane (5 – 10 %), Propane & Butane (3-6%)
C. CNG – Compressed Natural Gas
D. LPG (Liquefied Petroleum Gas) contains Propane and/or Butane
E. Petrol, Kerosene, and diesel – used in industry and transport.
f. Combustion of Alkanes:
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g. Substitution Reactions:
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B. Chemistry of Alkenes:
a. Fraction Supply & Demand
• Crude oils vary considerably in their composition and some need more refining than others
• Supply is how much of a particular fraction can be produced from refining the crude oil
• Demand is how much customers want to buy
• General the demand for certain fractions outstrips the supply so this is why cracking is
necessary to convert surplus unwanted fractions into more useful ones
• This is mostly larger, heavier fractions that are cracked into smaller lighter fractions
Supply & demand graph for crude oil fractions
• You can see from the chart that fuel oil and bitumen are surplus fractions so they are cracked
and modified to produce petrol, kerosene and diesel
b. Alkenes are unsaturated hydrocarbons
c. Much more reactive than alkanes
d. The most important source of alkenes is from the CATALYSTIC CRACKING of long-chain
alkanes from petroleum.
e. Catalytic Cracking:
A. Catalytic cracking takes place in a huge reactor.
B. Catalytic Cracking involves heat, pressure and a catalyst to break larger hydrocarbons in
to smaller, ligher molecules.
C. Long chain alkanes -→ alkenes + alkanes of lower relative molecular mass
D. The catalyst we use is called a zeolite. This is an ionic compound containing aluminium,
silicon, and oxygen.
E. Catalytic cracking requires a heat of 500oC
F. Examples:
A. Example 1:
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B. Example 2:
G. Molecules of long chains may be cracked in different places.
For example: Here are a number of ways that octane might be cracked:
Products from these reactions, like hexane, could be cracked themselves, producing
even smaller alkanes and alkenes.
H. Note that ALL CRACKING REACTIONS GIVE
A. An Alkane with a shorter chain than the original and a shorter alkene
OR
B. Two or more alkenes and hydrogen
I. All products of cracking of useful. Example:
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f. Combustion of Alkenes:
g. Addition Reactions
A. Bromination of Alkenes:
a. Alkenes are unsaturated molecules, atoms can add to them via the C=C double
bond, so a reaction occurs.
b. Alkenes readily react with halogens e.g. bromine, at room temperature and
pressure.
c. Bromine water is used in a simple test for unsaturated alkenes to distinguish them
from saturated alkanes.
d. This double bond makes alkenes much more reactive than alkanes, the bromine
water test for alkenes is just one example.
e. The equations illustrate what happens if gaseous alkenes are bubbled into a solution
of bromine.
f. Alkanes are saturated – no double bond – and atoms cannot add – so no reaction.
g. A few examples are set out below in various styles of formulae to
give dibromo halogenoalkanes
A. ethene + bromine ====> 1,2–dibromoethane
When you bubble an alkene gas or mix a liquid alkene with bromine solution (water
or hexane) the colour of the mixture changes from red-brown-orange to colourless.
The decolourisation clearly indicates a chemical reaction has take place and is a
simple test for unsaturation.
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B. propene + bromine ====> 1,2–dibromopropane
....
CH3CH=CH2 + Br2 ====> CH3–CHBr–CH2Br
C.
B. Hydrogenation of Alkenes:
• Alkenes also undergo addition reactions with hydrogen in which an alkane is formed
• These are hydrogenation reactions and occur at 150ºC using a nickel catalyst
• Hydrogenation reactions are used to manufacture margarine from vegetable oils
• Hydrogen reacts with the vegetable oil when it passes over nickel catalyst at
about 60oC. By hydrogenating some, not all, of the carbon-carbon double bond,
the liquid vegetable oil can be made in a solid but spreadable fat.
Hydrogen atoms add across the C=C in the hydrogenation of ethene to produce an alkane
C. Catalytic Addition of Steam:
• Alkenes also undergo addition reactions with steam in which an alcohol is formed.
• Since water is being added to the molecule it is also called a hydration reaction
• The reaction is very important industrially for the production of alcohols and it occurs
using the following conditions:
o Temperature of around 300ºC
o Pressure of 60 - 70 atm
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o Concentrated phosphoric acid catalyst
A water molecule adds across the C=C in the hydration of ethene to
produce ethanol
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