Organic ChemistryREACTIONS

Grade 12 Physical Science

Mrs KL Faling

SUBSTITUTION REACTIONS

• This is a reaction where an atom or group of

atoms is replaced by another atom or group of

atoms

• Substitution only occurs in saturated compounds

(single bonds)

• Substitution reactions offer require heating and/or

a catalyst

Example:

One of the H’s from methane is replaced

by a chlorine. The H and the other chlorine then form HCℓ

TYPE 1:Alkanes + HALOGENS (HALOGENATION)

• Alkanes are generally non-reactive NON-

POLAR nature

• Can react with non-polar HALOGENS

• Occurs in presence of sunlight, UV light or heat

EXAMPLE:

Practice

•Ethane + Bromine

•Butane + Iodine

•Methane + Fluorine

•Hexane + Chlorine

NOTE: The bromine test for saturation

• Bromine is used to test what homologous series an

unknown hydrocarbon belongs to:

• If Bromine is added to an alkane (saturated) a non-

spontaneous substitution reaction occurs and the

solution remains brown.

• (If the hydrocarbon is unsaturated a spontaneous

addition reaction occurs and the water goes clear)

SUBSTITUTION IN ALCOHOLS

• Alcohols react with hydrogen halides to

produce halo-alkanes and water

• (OPPOSITE TO THE HYDROLYSIS of halo-

alkanes)

• The reaction is thus reversible

NOTE:

• Reactions only occur with hydrogen and

chlorine, bromine or iodine

• HF is not reactive

Primary and secondary alcohols

• React slowly

• React best at high temperature

• An acid like H2SO4 should be added as a catalyst

PRACTICE

• Methanol + HBr

• Ethanol + HI

• Propan-1-ol + HF

• Butan-2-ol + HCℓ

Tertiary alcohols

• Occurs easily and fastest

• The tertiary alcohol is converted to a tertiary

halo-alkane

• Occurs at room temperature

• Only works with hydrogen bromide and

hydrogen chloride.

Practice

SUBSTITUTION IN HALO-ALKANES with BASES

• Halo-alkanes react with an aqueous

solution of a DILUTED metal hydroxide to

form an alcohol.Concentrated

hydroxide

solutions will

cause a different

type of reaction

EXAMPLE

Chloro methane + KOH

2 Bromo-2,3 dimethyl butane +

NaOH

ADDITION REACTIONS

• Unsaturated compounds can undergo

addition

• Double bonds become single bonds and

triple bonds become double bonds and

an extra “something” is added.

4 types of addition reactions

1. Hydrohalogenation

2. Halogenation

3. Hydration

4. Hydrogenation

1. HYDROHALOGENATION

• Addition of a hydrogen halide to an alkene

Conditions and process

• HCℓ, HBr or HI must be added to an alkene

(NOT HF)

• No water must be present

• During the reaction the double bond breaks and the

hydrogen attaches to one side and the halogen to the

other side. The hydrogen will always attach to the most

“primary” carbon of the two.

Practice:

• Propene + HBr

• 2 methyl-1-butene + HCℓ

• 3 ethyl-2-pentene + HI

• 3,3,4 trimethyl-1-pentene + HBr

2. HALOGENATION

• The addition of a halogen

• No catalyst is necessary

• Only with chlorine and bromine (DIATOMIC

MOLECULES!!!!!) One gets added each side of the

“broken” double bond.

• Iodine is to slow and fluorine is to volatile

Example

Practice

• Propene + Chlorine gas

• 1-butene + Bromine gas

• 2,3 dimethyl-2-hexene + Cℓ2

• 4,5,6 triethyl-2-octene + Br2

3. Hydration

• Addition of excess water to an alkene to produce

an alcohol.

• A strong acid is required as a catalyst generally

H3PO4 + H2SO4

EASY EXAMPLE:

Markovnikov’s rule for asymmetrical alkenes

• When the H2O splits during the addition reaction

the single hydrogen atom will go to the carbon

with the most hydrogens and the hydroxyl

functional group (O-H) will go to become tertiary

over secondary over primary.

• (worked the same way in hydrohalogenation)

Example

Practice

• 2-butane + H2O

• 1-butane + H2O

• 2 methyl-2-pentene + H2O

• 3,4 diethyl-2-heptane + H2O

4. Hydrogenation

• The addition of hydrogen gas in a hydrogen

atmosphere (NO OXYGEN) to an unsaturated

hydrocarbon which then results in a more saturated

hydrocarbon.

• The alkene must be dissolved in a non-polar solvent

like hexane or benzene

• Needs a catalyst: Pt, Ni or Pd

• This is sometimes called catalytic hydrogenation and

is used in making margarine.

Example

Practice

• propene + hydrogen

• 2 methyl-propene + hydrogen

• 3 methyl-but-1-ene + hydrogen

APPLICATIONS OF ADDITION REACTIONS

• Hydrogenation of plant oils – plant oils are

unsaturated and due to multiple bonds are in

liquid form.

• Hydrogen is added in the presence of nickel as a

catalyst and the plant oil hardens and produces

margarine.

ELIMINATION

1. Dehydrohalogenation

2. Dehydration (of alcohols)

3. Thermal cracking and catalytic

cracking

Dehydrohalogenation

A hydrogen halide is eliminated from a

haloalkane to for an alkene.

The reaction must be strongly heated

during reflux – the vapours must condense

and return to the reaction vessel during

heating.

What you need for dehydrohalogenation?

• A strong base dissolved in pure ethanol

• This is known as warm ethanolic NaOH (or

KOH)

• The product is an alkene, water and a

halide salt.

Example

Practice

• 1 Bromo-butane + NaOH

• 2 Chloro-pentane + KOH

• 1 Bromo-propane + KOH

• 3 Chloro-hexane + NaOH

DEHYDRATION (of alcohols)

• H2O is removed from an alcohol.

• It is a catalysed acid reaction.

• Concentrated sulfuric acid is used as a

dehydrating agent. (OR phosphoric acid)

• The product is an alkene and water

Example

ASYMMETRICAL alcohols

• We can focus on the main

product which is that the “H” will

come off the more secondary

carbon.

• AKA The one with the least H’s ->

This will form the main product (by-

products can also be formed)

Practice

1. 2 methyl-1-propanol

2. 3,3 dimethyl-2-hexanol

3. 3 ethyl-2-pentanol

Cracking

• Hydrocarbons can be cracked to make smaller

more useful molecules.

• Thermal cracking occurs at a high temperature

and pressure.

• Catalytic cracking occurs at a lower temperature

but requires a catalyst.

EXAMPLE – The product of thermal cracking is multiple short chains – at least ONE must be an alkene

Complete Ex 9 Page 122