O-level organic chemistry

Organic chemistry is the study of compounds of carbon

Uniqueness of carbon

• Carbon forms very many useful compounds with different physical and chemical properties.

Importance of organic compounds

Organic compounds are used as

• Drugs
• Perfumes
• Clothes
• Shoes
• Dyes
• Detergents for washing
• Packing materials
• Herbicide to dry weeds

Terminologies

Hydrocarbons are compounds that contain carbon and hydrogen only

Functional groups are reactive parts of organic compounds for example

Double bonds (C=C) for alkenes

Triple bonds for C ≡C) alkynes

Hydroxyl group (-OH) for alcohols

Carboxylic groups (-COOH) for carboxylic acids

Homologous series

These are a group of compounds  with members related as follows

• Members have similar general formula e.g. the general formula of alkanes is C_nH_2n+2.
• Have similar functional groups and thus same chemical properties
• Have similar methods of preparation
• Show gradual change in physical properties for instance alkanes range from gases to liquids to solids

Alkanes

This is the simplest homologous series with saturated hydrocarbons with a genera

l formula C_nH_2n+2.

 

Compounds with the same molecular formula but different structural formulae like butane and methylpropane are called isomers

Physical properties of alkane

• they are insoluble in water
• they are soluble in organic solvents
• they range from gases to liquids to waxy solids

Chemical properties

1. They burn in air to produce carbon dioxide, water, and heat. Due to the production of heat, they are used as fuel.

Example 

CH₄ + 2O₂        →   CO₂ + 2H₂O + heat

• Chlorination: they react with chlorine in the presence of sunlight or u.v-light to produce chlorinated alkanes.

Example

_{Preparation of alkanes}

1. By distillation of petroleum oil
2. By cracking: cracking is the breakdown of long-chain hydrocarbons into useful short-chain hydrocarbons by heat (thermos-cracking) or by a catalyst (catalytic cracking)
3. From Biogas: methane is the main component of biogas. Biogas is produced by anaerobic decomposition of organic matter (cow dung, feces, plant remains) in the presence of water

Alkenes

These are hydrocarbons that contain aa double bond

The general formula is C_nH_2n n ≥ 2

Examples are

Ethene          CH₂=CH₂

Propene           CH₃CH=CH₂

But-1-ene         CH₃CH₂CH=CH₂

But-2-ene         CH₃CH=CHCH₂

Preparation of ethene

By dehydration (removal of a water molecule from) of ethanol with hot concentrated sulphuric acid

Testing or ethene

1. Ethene decolorizes bromine water

2. Ethene decolorized bromine tetrachloromethane

3. Ethene decolorizes acidified potassium permanganate (VII)

4. Alkenes form polymers called polyalkenes

A polymer is a molecule with a high molecular mass formed combination of very many small molecules called monomers.

Examples

(ii) Propene polymerize to form polypropene

 

(iii) Vinyl chloride polymerize to form polyvinylchloride (PVC)

Uses of polyethene

• Insulator
• Water pipes
• Packing mate

Polymerization of dienes

Alkenes with conjugated double bond undergo polymerization to form polyalkenes with double bonds

Example

Polymers from conjugated dienes have double bonds, they are elastic and constitute different forms of rubber.

Vulcanization of rubber

It is heating rubber with Sulphur to make it

• Less elastic,
• More resistant to heat
• More durable
• Easier to dye

Uses of vulcanized rubber

• For raincoats
• Boots
• Shoe soles
• Rubber bands

Natural and artificial polymers

Natural polymers are polymers made plant or animal bodies

Examples are

Polymers	monomers
Starch	glucose
Cellulose	glucose
Protein	Amino acids
Cotton
Wool
silk
sisal

Artificial polymers are polymers that are man-made

Examples

Nylon a

Polyester

Polyethene

Advantage of natural polymers

• Cheap
• biodegradable

The disadvantages of natural polymers

• not durable
• have low property value such as low tensile strength

Advantage of synthetic polymers/plastic

• light and portable
• resistant most chemicals
• they are durable
• they are thermos insulators

The disadvantage of synthetic polymers

• they are nonbiodegradable
• fire hazards

Thermoplastic and thermosetting polymers

Thermoplastic polymers are those that soften and can be remoulded into new shapes. E.g. polyethene

Thermosetting polymers are those that decompose on heating and cannot be remoulded on heating for example vulcanized rubber, melamine, Bakelite.

Alkyne

They are hydrocarbons with a general formula C_nH_2n-2, n. They contain a triple bond

Examples

Ethyne HC≡CH

Propyne, CH₃C≡ CH

But-1-yne CH₃CH₂C≡ CH

But-2-yne CH₃C≡ CCH₃

Like alkenes they decolorize bromine water

Alcohols

They are compounds that contain hydroxyl (-OH) group.

Example

Ethanol or CH₃CH₂OH

Preparation of ethanol

By fermentation of glucose or starch-containing food (cassava, maize, and bananas)

Uses of ethanol

Social drink

Antiseptic in soft drinks

Production of ethene

Fuel

Extraction of sugar

• sugar canes are crushed and juice extracted
• Sulphur dioxide is bubbled through to breach the juice
• Lime is added to clarify the juice and adjust pH and filtered
• The juice is boiled to increase its concentration from 15% to 60%.
• Sugar crystals in ethanol are added to crystalize sugar
• The sugar crystals are washed with water and dried

The reaction of sugar with sulphuric acid

Sugar is dehydrated with concentrated sulphuric acid with the evolution of heat to a black mass of carbon

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Compiled by Dr. Bbosa Science