Why are alkenes and alkynes called unsaturated hydrocarbons?

Why alkenes and alkynes are called unsaturated hydrocarbons is actually due to the double anfd triple bonds they possess.

Alkenes and alkynes are called unsaturated compounds basically for two reasons 

1. Both do not have enough hydrogen atoms due to the double and multiple 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 consequently they burn with a more smoky flame due to their unsaturation. 

What is an unsaturated compound?

An unsaturated compound simply put is a compound that has double or multiple bonds.

Alkenes and alkynes are called unsaturated and they participate in some basic reactions. 

why alkenes are considered unsaturated is just simply because of their double bond.

What is the difference between saturated and unsaturated compounds?

A saturated compound has a single carbon-to-carbon bond while an unsaturated compound has a double or triple bond.

There are chemical reactions specific to unsaturated compounds.

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

Alkanes	Alkenes	Alkynes
Combustion Substitution Cracking	Combustion Addition Polymerization	Combustion Addition Substitution (terminal alkynes) polymerization

I’m going to talk about these reactions briefly now.

Just to help understand this concept,a brief recap of the reactions of saturated and unsaturated hydrocarbons.

Summary of Reactions of Alkanes  Alkenes and Alkynes

Reactions of 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 or incompletely to produce carbon monoxide and water.

For example, let’s look at the combustion of ethane.

Complete combustion 

2C₂H₆ +7O₂ === 4CO₂ + 6H₂0

Incomplete combustion

 2C₂H₆ +5O₂====4CO + 6H₂0

2. Substitution reactions 

Alkanes can undergo substitution reactions when they react mainly with halogens.  This type of reaction is a photochemical reaction.

A photochemical reaction is a reaction that takes place in the presence uv light.

Example

C₂H₆ + Cl₂ ===C₂H₅Cl +HCl

Other examples of photochemical reaction include photosynthesis in plants, depletion of ozone, the photographic reaction in film (decomposition of silver bromide)

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 because the heavy hydrocarbon is broken into smaller hydrocarbons.

Example;

Cracking of Decane

C₁₀H₂₂===C₆H₁₄+ C₄H₈

Reactions of Alkenes and Alkynes

Alkenes and alkynes are called unsaturated compounds because they have double and triple bonds respectively. 

The reactions they participate in are so different from the reactions alkanes undergo. 

Alkenes 

Alkenes undergo combustion, addition, and polymerization. 

1. Combustion reactions

The combustion reactions are just the same as the ones I gave in alkanes.

It is either complete combustion( excess air) or incomplete combustion ( insufficient air)

Complete combustion 

C₂H₄ + 3O₂ === 2CO₂ + 2H₂O

Incomplete combustion 

C₂H₄ + O₂ === 2CO + 2H₂O

2.Addition reactions 

An addition reaction is a reaction in which a single product Is formed.

a. Hydrogenation 

C₂H₄+ H₂===C₂H₆

b. Chlorination 

   C₂H₄+Cl₂ ===C₂H₆Cl₂

c. Hydrohalogenation

  C₂H₄+ HBr ===C₂H₅B_r

c. Hydration 

C₂H₄+ H₂O==C₂H₅OH

d.Hydrolysis

in hydrolysis, an acid or base is used with water to form ethanol. The hydrolysis and hydration of alkenes yield alkanols.

C₂H₄+ H₂SO₄ === C₂H₅HSO₄

In the hydrolysis of ethene, an intermediate is first formed ehylhydrogentetraoxosulphate VI.

The intermediate ethylhydrogentetraoxosulphate VI reacts with water to form ethanol.

C₂H₅HSO₄ + H₂O === C₂H₅OH + H₂SO₄

3. Polymerization reactions 

Polymerization of alkenes yields polymers.

Examples Polymerization of ethene yields polythene. 

n(C₂H₄)====(C₂H₄)n

Alkynes 

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

1. 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

2. Addition reactions 

Hydrogenation 

C₂H₂ +2H₂===C₂H₆

Chlorination

C₂H₂ +2Cl₂===C₂H₂Cl₄

3..Polymerization reactions 

Polymerization of ethyne yields benzene 

3C₂H₂ ===C₆H₆

4. 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 bonds they possess.  

The properties of alkenes and alkynes show that unsaturated compounds differ from saturated compounds.

Why are alkenes and alkynes said to be unsaturated compounds?

Alkynes and aslkynes are unsaturated compounds due to the presence of the double and triple bond they possess. the presence of the double or multiple bonds shows that the hydrogen atoms is not enough.