why are alkenes and alkynes called unsaturated hydrocarbons?

Alkenes and alkynes are called unsaturated compounds basically for two major reasons

  1. Both do not have enough hydrogen atoms due to the bonds they have
  2. Both alkenes and alkynes have double and triple bonds respectively
    It is important to note that alkenes and alkynes undergo some reactions peculiar to unsaturated compounds due to the bonds they contain.

Additionally, they both burn with more smoky flame due to their unsaturation.

What is an unsaturated compound?

An unsaturated compound is a compound that has double or multiple bonds.
Alkenes and alkynes are called unsaturated, and they participate in some basic reactions.

Although I have talked about some important basic facts in revision note for organic chemistry concerning some facts and conversions in organic chemistry.

Why alkenes are considered unsaturated is just simply because of its double bond.

What is the difference between saturated and unsaturated compounds?

A saturated compound has single carbon to carbon bond while an unsaturated compound has double or triple (multiple bond) bond.
There are chemical reactions specific to unsaturated compounds.

I will like to summarize the reactions that alkanes, alkenes and alkynes undergo.

Characteristics Hydrocarbons reactionsAlkanesAlkenesAlkynes
Combustion reactionyesyesyes
substitution reaction yesNoyes
Cracking reactionyesNoNo
Polymerization reactionNoyesyes
Addition reaction Noyesyes

Also it is pertinent to note that Polymerization reactions is common to unsaturated hydrocarbons alkynes and alkenes.

I’m going to talk about these reactions briefly now.
Just a brief recap of reactions of saturated and unsaturated hydrocarbons.

Summary of Reactions of Alkanes Alkenes and Alkynes

A. Alkanes

Alkanes are saturated hydrocarbons, and they exhibit only three basic reactions.
Alkanes undergo only combustion, substitution and cracking.

1.Combustion reactions

Alkanes undergo combustion either completely to give out carbon iv oxide and water while incompletely to produce carbon monoxide and water.

Example, let’s look at the combustion of ethane.

Complete combustion

C2H4 +O2====CO2 + H20

Incomplete combustion

2.Substitution reactions

Alkanes can undergo substitution reactions when they react mainly with halogens. This type of reaction is a photochemical reaction.
It takes place in the presence of uv light.

C2H6 + Cl2 ===C2H5Cl +HCl

3.Cracking reactions

Cracking is the splitting or breaking down of long chain hydrocarbons into smaller hydrocarbons.
In cracking, the number of hydrocarbons is increased.

Cracking of Decane


Alkenes and Alkynes

Alkenes and alkynes are called unsaturated compounds because they have double and triple bond respectively.
The reactions they participate in are so different from the reactions alkanes undergo.

B. Alkenes

Alkenes undergo combustion, addition and polymerization.

1.Combustion reactions

The combustion reactions are just the same as the one I gave in alkanes.
It is either complete combustion (excess air) or incomplete combustion (insufficient air)

Complete combustion
C2H4 + O2==CO2 + H20

Incomplete combustion
C2H4 +O2==CO2 + H20
2.Addition reactions

Addition reaction is a reaction in which a single product Is formed.
a. Hydrogenation
C2H4+ H2===C2H6
b. Chlorination
C2H4+Cl2 ===C2H6Cl2
c. Hydration
C2H4+ H2O==C2H5OH

3.Polymerization reactions

Polymerization of alkenes yields polymers .

Examples Polymerization of ethene yields polythene.


Alkynes undergo four basic chemical reactions; combustion, addition, Polymerization and substitution reactions.

a. Combustion reactions

Combustion reactions will produce carbon iv oxide and water on complete combustion and carbon II oxide and water.

Complete combustion

2C2H2 +5O2 ===4CO2 +2H2O
Incomplete combustion
2C2H2 +6O2===4CO +2H2O

b. Addition reactions
C2H2 +2H2===C2H6
C2H2 +2Cl2===C2H2Cl4
Polymerization reactions
Polymerization of ethyne yields benzene.
Substitution reactions
Terminal alkynes undergo Substitution reactions just like alkanes.

They undergo electrophilic substitution reactions with ammoniacal solutions of silver nitrate or copper I chloride.
C2H2 + 2AgNO3 ==Ag2C2 + 2HNO3
C2H2 + 2CuCl ==Cu2C2 + 2HCl
It is evident that alkenes and alkynes are called unsaturated compounds because of the double and triple bond they possess.
The properties alkenes and alkynes show that unsaturated compounds differ from saturated compounds.

Table summerizing the chemical properties of hydrocarbons

Characteristic properties Reasons
Why are alkenes more reactive than alkanes?This is due to the presence of pi bond from the second bond in the double bond.
why are alkenes not used as fuels?Presence of double bond makes them unsaturated, and they do not burn cleanly.
Why are alkenes called olefins?because lower members can form oily products when treated with chlorine or bromine
Why are alkanes insoluble in water?This is because they are non-polar.
Why are alkanes relatively unreactive?This is because there is no much difference in the electronegativities of the carbon and hydrogen in alkanes
Why are alkanes good fuels??Because they burn with a non-smoky green flame (thus not polluting the environment)
Why are alkynes acidic?Alkynes are acididc because they can release hydrogen to form alkyne ions
Why are alkynes more acidic than alkenes?This is because they have more hydrogen atoms
Why are alkenes linear?This sis due to the presence of triple bond restricting their rotation
why are alkenes and alkynes called unsaturated hydrocarbons?Alkenes and alkynes are called unsaturated compounds because they have double and triple bond respectively.

In conclusion the chemistry of hydrocarbons is very interesting and it is very important to understand that alkanes are saturated while alkynes and alkynes are unsaturated hydrocarbons.

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