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Chemistry > Hydrocarbons > Properties of Alkenes

Properties of Alkenes

Alkenes are an unsaturated form of hydrocarbons that are formed by double bonding between the carbon atoms. There is at least one such double bond in their structure. The simplest alkene with one double bond is ethene (C2H4). Alkenes are an important part of our everyday lives, and also have many industrial uses. Let us take a look at some properties of alkenes.

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Physical Properties of Alkenes

Physical properties of alkenes are quite similar to those of alkanes. Let us take a look at few physical properties

  • Alkenes exist naturally in all three states. The first three alkenes are gases, and the next fourteen are liquids. Alkenes higher than these are all solids.
  • All alkenes are insoluble in water, due to the weak van der Waal forces.
  • But alkenes are soluble in organic solvents like benzene or acetone because here the van der Waal forces will be replaced by new ones, making alkenes fully soluble.
  • The boiling points of alkenes depend on their molecular structure. The bigger their molecular chain the higher the boiling points. So the higher alkenes have very high boiling points
  • The polarity of alkenes will depend on their functional groups

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Chemical Properties of Alkenes

Alkenes are unsaturated compounds, which makes them highly reactive. Most of these chemical reactions occur at the Carbon-Carbon double bonds. This makes alkenes far more reactive than alkanes. Alkenes undergo three types of main reactions, which are as follows

Addition Reactions

  • Addition of Hydrogen: In the presence of nickel or platinum alkenes will react to add to its molecular chain one diatomic molecule of hydrogen (dihydrogen). And in this process, they become alkanes due to the rearrangement of atoms.
  • Addition of Halogens: Halogens will react with alkenes to form vicinal dihalides. From the halogens, iodine will not react with alkenes. But Bromine reacts with alkenes and will attach at the unsaturated site. In fact, the reaction is used to as proof of unsaturation.

C2H4(g) + Br2 (aq) → C2H4Br2 (aq)

  • Addition of Halides: These reactions follow a certain rule, the Markovnikov rule. This rule states that the negative portion of the reactant (the molecule which gets added to the chain) will attach itself to the carbon with the least number of hydrogen atoms attached. So when a hydrogen halide will react with an alkene, the hydrogen will attach at the double bond to the atom with more hydrogen atoms attached. The halide ion, on the other hand, will attach to that carbon atom that has the lesser hydrogen atoms attached.

CH3-CH=CH2+ HBr → CH3-CH(Br)-CH3

  • Addition of Water: According to the Markovnikov rule, water will react with an alkene to form alcohols. This happens in the presence of sulphuric acid.

CH2=CH2 + H2O  → CH3CH2OH

Oxidation Reactions

• Combustion Reaction: The combustion of alkenes is very exothermic, it will give out huge amounts of thermal energy. A practical example of this reaction is seen in welding of metals. It is known as oxy-ethylene welding.

CH2=CH2 + 3O2 → 2CO2 + 2H2O

• Oxidation by Pottasium Permanganate: When alkenes are reacted with cold dilute KMnO4 also known as Baeyer’s reagent, it forms vicinal glycols. It will also decolourize the pink colour of KMnO4. So it is used for testing unsaturation in compounds.

Solved Question for You

Question: What are the major organic products of the following reaction?

Properties of Alkenes

  1. I and III
  2. I and IV
  3. II and III
  4. II and IV

Solution: Option B. Treatment of alkenes with bromine Br2 gives vicinal dibromide. The bromines add to opposite faces of the double bond anti addition. Sometimes the solvent is mentioned in this reaction – a common solvent is carbon tetrachloride CCl4. In the above compound, bromine should be added in an anti manner. so major organic products will be 1,4 compounds as they are added in an anti manner. the other two are in a minor amount.

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