Hydrogenation, Hydration, Bromination of unsaturated Hydrocarbons. Combustuin

1. Chemical Properties of
Hydrocarbons
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2. Contents
Introduction: Learning Objectives
2
Reading Material & Conclusions
Chemical Properties
Combustion Reactions
Addition: Bromination, Hydrogenation & Hydration
Oxidation

3. Introduction: Learning Objectives
2. Predict the products of reactions of
Hydrocarbons
3. Describe each type of Chemical
Reaction
1. Understand the chemical properties of
Hydrocarbons
4. Give examples of each type of
Chemical Reaction
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4. Chemical Properties
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5. Chemical Properties: Hydrocarbons
Alkanes
Single Bonds  𝜎 𝑏𝑜𝑛𝑑𝑠
Note: Addition Reaction (synthesis): A + B  C Substitution Reaction: AB +CD  AC + BD (replacement of atoms)
Alkenes &
Alkynes
Double & 
Triple Bonds
𝜋 𝑏𝑜𝑛𝑑𝑠
Stable and Strong
Addition Reaction:
Substitution Reaction:
Unstable
Addition Reaction:
Substitution Reaction:
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6. Combustion Reactions
Note: Combustion is reaction (burning) of a compound with oxygen gas (O2) from the air and transferring
energy to surroundings as light and heat.
CxHx + O2 CO2 + H2O
General Reaction
• Burning
• O2 needed
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7. Combustion Reactions: Types
Complete
Combustion
Incomplete
Combustion
CxHx + O2 CO2 + H2O CxHx + O2 CO + H2O
CH4 + 2O2 CO2 + 2H2O 2CH4 + 3O2 2CO + 4H2O
Due to Low amount of Oxygen (for 1 mole
CH4  1.5 mole O2), CO produced
Due to High amount of Oxygen (for 1 mole
CH4  2 mole O2), CO2 produced
Note: You can think of CO2 as a complete bicycle with two tires and CO as an incomplete bicycle with only 1 tire,
because there wasn’t enough tires left to build it completely. 7

8. Combustion Reactions
A blue flame means complete combustion of the gas and more
heat is released.
Orange, yellow or red flames means incomplete combustion
of the gas. Less heat is produced
Please see the Video
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9. Addition: Bromination, Hydrogenation & Hydration
Bromination Hydrogenation Hydration
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10. Bromination
Bromination  Addition of
Bromine (Addition Reaction)
• Chemical test for an
unsaturated hydrocarbons.
Note: Bromine water is decolorized when shaken with an alkene.
b) Cyclohexene+ Br2  ?
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11. Bromination: Youtube Video
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12. Hydrogenation: Recall Preparation of Alkanes
Saturation of alkene with hydrogen atoms with the help of a catalyst (Ni, Pd or Pt)
Alkene +H2 Alkane
Ni, Pd, or Pt
+ H2 CH3-CH2-CH2-CH3
2-Butene Butane
Pd,
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13. Hydrogenation reaction can be used in food industry to convert liquid oils (like sunflower) to their solid
alternatives.
A molecule below is found in sunflower oil. Double bonds in the chains are targets of hydrogenation reaction.
Due to density change in the molecule chains, liquid oil becomes solid.
Hydrogenation
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14. Hydrogenation
Oleic Acid: found in olive oil and other unsaturated fats
(liquid)
Stearic Acid: found in saturated fats (solid)
+ H2
Pd
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Hydration
• Hydration  Addition of Water to Unsaturated Hydrocarbons : Used for Alcohol Production in Industry
• Dehydration  Removal of Water from Alcohols : Used for Alkene Production
Alkene
Symmetrical Asymmetrical
CH2=CH2, CH3-CH=CH-CH3, etc. CH3-CH=CH2, CH3-CH=CH-CH2-CH3
Draw imaginary line through the double bond and compare the sides. If they are exactly same, then it is
symmetrical hydrocarbon, if the sides are different, then asymmetrical.

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Hydration: Symmetrical Alkenes
In hydration reaction strong acid (H2SO4) is used. Sulfuric acid produces H+ ion in water. This H+ ion adds to one
of carbons adjacent to the double bond. The other carbon adds OH- group from H2O. Double bond between
carbons breaks and single bond remained as new compound is formed.

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Hydration: Symmetrical Alkenes: Examples

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Hydration: Asymmetrical Alkenes
In hydration reaction with asymmetrical alkenes, the H+ ion adds to the carbons adjacent to the double bond and
that has the greatest number of hydrogen. The other carbon with lower number of hydrogens, add OH- group
from H2O. Double bond between carbons breaks and single bond remained as new compound is formed. This is
called Markovnikov’s rule. We also can say: “The rich get richer”.
CH3-CH = CH2 + H2O CH3-CH- CH2
OH H
H2SO4
+ H2O
H2SO4
?

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Oxidation: Potassium Permanganate (KMnO4)
•KMnO4 is a salt with purple colour. It is a solid compound. Dissolves in
water.
•This reaction is used in laboratory for a chemical test of unsaturated
hydrocarbons.
•Unsaturated hydrocarbons change the colour of KMnO4 solution.
CH2 = CH2 + KMnO4 + H2O CH2OH – CH2OH + MnO2 + KOH
ethene ethandiol-1,2 or
ethylene glycol
antifreeze
CH2 = CH2 + [O] KMnO4 CH2OH – CH2OH
Ethylene glycol is a colorless viscous hygroscopic liquid used as an antifreeze.
This reaction is applied in the industry to produce an antifreeze, which is used in cars.

20. Reading Material & Conclusions
Question: Why does alkyne and alkene undergo addition reaction whereas alkane does not?
Alkanes are already bonded strongly, while alkenes and alkynes have weak π-bonds. They want to
undergo addition reactions to turn π-bonds to stronger σ's and become more stable.
Explanation:
Alkenes and alkynes are unsaturated - they have π-bonds, so don't have the full number of hydrogen
that they could have.
This means that they are more unstable than alkanes, since π-bonds aren't as strong as σ-bonds. The
alkenes and alkynes want to form more σ-bonds and have a structure more like an alkane, so they
undergo addition reactions.
Addition reactions are where more atoms are added to the molecule, not swapped or taken away. This
means that the π-bonds have to be taken away and used as σ-bonds with the new atoms, rather than
the σ-bonds already there being reattached - it's easier to break π than σ.
Alkanes do not undergo this reaction because they already only have single σ-bonds, and so they
cannot become more stable or stronger structurally - they are already at the peak, and so can only
swap things around in substitution reactions.
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Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such
an addition is the reductive removal of the double bond functional group. Regioselectivity is not an issue,
since the same group (a hydrogen atom) is bonded to each of the double bond carbons. The simplest
source of two hydrogen atoms is molecular hydrogen (H2), but mixing alkenes with hydrogen does not
result in any discernible reaction. Although the overall hydrogenation reaction is exothermic, a high
activation energy prevents it from taking place under normal conditions. This restriction may be
circumvented by the use of a catalyst, as shown in the following diagram.
An example of an alkene addition reaction is a process called hydrogenation. In a hydrogenation reaction,
two hydrogen atoms are added across the double bond of an alkene, resulting in a saturated alkane.
Hydrogenation of a double bond is a thermodynamically favorable reaction because it forms a more
stable (lower energy) product. In other words, the energy of the product is lower than the energy of the
reactant; thus it is exothermic (heat is released). The heat released is called the heat of hydrogenation,
which is an indicator of a molecule’s stability.
Reading Material & Conclusions
Link

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Reading Material & Conclusions
For the Hydration of Alkenes please go to the the Link

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