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Addition Reactions

In this type of reaction, atoms are ADDED to an UNSATURATED organic molecule (alkene or alkyne). The total number of atoms in the original molecule will increase.
The are essentially 4 types of addition reactions :

Hydrogenation (reacting with H₂)
Hydrohalogenation (reacting with, example HBr)
Halogenation (reacting with halogens, example Br₂)
Hydration (reacting with water H₂O)

Functional groups completely change.

HYDROGENATION

Hydrogen H₂ is added to an alkene.
Reaction condition: catalyst Pt / Ni / Pd

Diagram 1

When the double breaks, to become single, each "half" hangs on to each adjacent carbon.
So the 2 individual H atoms can join in.
The alkene (ethene) becomes the alkane (ethane)

HYDROHALOGENATION

A hydrohalogen (such as HCl) is added to an alkene.
Reaction condition: No water present.

Diagram 2

The H and Cl simply "hooks" onto each adjacent carbon where the double bond was.
The alkene (ethene) changes into the haloalkane (chloroethane).

Another example of HYDROHALOGENATION
This example shows what happens if the alkene is much longer.

Diagram 3

C₁ has 2 H atoms.
C₂ has 1 only H atom.
When this molecule reacts, C₁ attracts the H from the HCl to itself.
The chlorine atom Cl then has no option but to attach itself to C₂.
The alkene (but-1-ene) becomes the haloalkane (2-chlorobutane).

This is called Markovnikov's Rule.
When a compound HX reacts with a alkene, the hydrogen atom H from HX will attach itself to the C that has the greater number of hydrogen atoms, and the X will attach itself to the carbon with the fewer number of hydrogen atoms.

HALOGENATION

A halogen such as Br₂ reacts with an alkene.
Reaction condition: Room temperature.

Diagram 4

The 2 Br atoms just hooks on to each adjacent carbon.
The alkene (ethene) changes to the haloalkane (1,2-dibromoethane)

HYDRATION

This is a reaction with water and an alkene.
Reaction condition: reaction must take place in a dilute, strong acid which must act as a catalyst. Example sulphuric acid H₂SO₄ or phosphoric acid H₃PO₄

Diagram 5

The H-OH of the water (H₂O) hooks to each adjacent carbon.
The alkene (ethene) is converted into the alcohol (ethanol).

Another example of HYDRATION
This example shows what happens if the alkene is much longer.

Diagram 6

Notice that the H from the water has moved to the first carbon, since this carbon originally had more H atoms already.
The OH thus had to attach itself to the second carbon.
The ethene (but-1-ene) becomes the alcohol (butan-2-ol)

This is another example of Markovnikov's Rule.