organic chemistry - alkanes, alkenes, alcohols, carboxylic acids, ester, macromolecules, polymer,...

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ORGANIC CHEMISTRY Alkanes What are the characteristics of a homologous series?

o They can be represented by a general formula. o There is a common difference of CH2 between successive members.

o The can be prepared by a general method.

o They show the same chemical reactions.

o Gradation in physical properties down a series as there an increase in the

size and mass of the molecules. Melting point, boiling point and viscosity increases whereas flammability decreases.

education.mu/chemistry Alkanes is a homologous series of saturated hydrocarbons with the general formula, CnH2n+2. They are obtained from petroleum (crude oil).

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Examples of Alkanes:

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Methane

Ethane

Propane

Butane

Pentane




Isomerism Compounds having the same molecular formula but different structural formulae are said to exhibit isomerism. education.mu/chemistry Isomers of C2H4Cl2

Isomers of C2H5OH

education.mu/chemistry Isomers of C4H10




education.mu/chemistry Isomers of C5H10

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Isomers of C4H8




Alkanes are generally unreactive except in terms of: (a) Burning or Combustion Alkanes burn in plentiful supply of air to form carbon dioxide and water. They burn with a blue flame.

CH4(g) + 2O2(g) � CO2(g) + 2H2O(l)

C5H12(g) + 8O2(g) � 5CO2(g) + 6H2O(l)

education.mu/chemistry (b) Substitution by Chlorine (presence of sunlight is required)





Alkenes Alkenes is a homologous series of unsaturated hydrocarbons (because of carbon double bonds) with the general formula, CnH2n. They are obtained from petroleum (crude oil). Ethene

education.mu/chemistry Butene

Isomers of Pentene

education.mu/chemistry Alkenes undergo addition reactions because of the double bond.

Propene

Pentene




1. Burning or Combustion

C2H4(g) + 3O2(g) � 2CO2(g) + 2H2O(l) They burn with a yellow flame. education.mu/chemistry 2. Addition of halogens (chlorine or bromine)

3. Addition of hydrogen

Conditions: Nickel catalyst 200oC education.mu/chemistry 4. Addition of steam/water

Conditions: Phosphoric Acid catalyst 300oC How would you distinguish between alkanes and alkenes? Alkenes decolourises bromine water (reddish-brown to colourless) whereas alkanes show no visible reaction with aqueous bromine.




What is cracking? Cracking is the process whereby long-chain hydrocarbon molecules are broken down into simpler and more useful molecules.

C22H46 � C6H14 + 8C2H4

Uses of cracking:

(a) Manufacture of alkenes (b) Manufacture of hydrogen (c) Manufacture of gasoline

education.mu/chemistry Manufacture of Margarine Margarine is made by hydrogenating unsaturated vegetables oils to form a solid product. If there aren't any carbon-carbon double bonds, the substance is said to be saturated. If there is only one carbon-carbon double bond in each of the hydrocarbon chains, it is called a mono-unsaturated. If there are two or more carbon-carbon double bonds in each chain, then it is said to be polyunsaturated.

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Alcohols Alcohols is a homologous series containing the –OH group. They have the general formula CnH2n+1OH. Methanol

education.mu/chemistry Propanol and its isomer

Butanol and its isomers

Ethanol




Pentanol and its isomers


(a) Combustion

C2H5OH + 3O2 � 2CO2 + 3H2O

(b) Oxidation to ethanoic acid Ethanol + oxidising agent � ethanoic acid + water Condition: Either

(i) heat with acidified potassium manganate (VII) OR (ii) heat with acidified potassium dichromate (VII) OR (iii) atmospheric oxygen (the reaction is slower)

(c) Production of ethanol by fermentation education.mu/chemistry Yeast is added to aqueous glucose. The temperature is kept around 35oC. Ethanol and carbon dioxide are produced. Ethanol is then obtained from this solution by fractional distillation.

C6H1206 � 2C2H5OH + 2CO2




(d) Production of ethanol from ethane

Conditions: Phosphoric Acid catalyst 300oC education.mu/chemistry Uses of Ethanol (a) As a constituent of alcoholic beverages (e.g. wine and beer) (b) As a renewable fuel in cars (c) As a solvent for aftershaves and nail varnishes education.mu/chemistry

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Carboxylic Acids Carboxylic Acids is a homologous series containing the –CO2H group. They have the general formula CnH2n+1COOH. They are weak acids. They turn damp blue litmus paper red. Methanoic acid Ethanoic acid

education.mu/chemistry Propanoic acid Butanoic acid

(a) Liberates hydrogen from reactive metals

2CH3COOH + 2Na � 2CH3COO–Na+ + H2 (b) Liberates carbon dioxide from carbonates

2CH3COOH + CaCO3 � (CH3COO)2Ca + CO2 + H2O education.mu/chemistry (c) React with bases and alkali to form salt and water

CH3COOH + NaOH � CH3COO–Na+ + H2O




(d) Esterification (i) Ethanol + ethanoic acid � ethyl ethanoate + water

C2H5OH + CH3COOH � CH3COOC2H5 + H2O

Conditions: heat with a little concentrated sulphuric acid education.mu/chemistry (ii) butanol + propanoic acid � butyl propanoate + water

C4H9OH + C2H5COOH � C2H5COOC4H9 + H2O (iii) propanol + butanoic acid � propyl butanoate + water

C3H7OH + C3H7COOH � C3H7COOC3H7 + H2O Uses of Esters (Esters are sweet smelling liquids)

(i) used as artificial flavour in sweets, cakes and fruit juices (ii) used a solvent for nail varnish


+ �




Macromolecules Macromolecules are large molecules built up from small units, different macromolecules having different units and/or different linkages. Addition polymerisation occurs when identical monomer molecules join together to form only one product (a polymer). The product has the same empirical formula as the monomer. Example 1: Poly(ethene) or polyethene

Poly(ethene) is used for the manufacture of plastic bags. education.mu/chemistry Example 2: Poly(chloroethene) or polyvinylchloride (PVC)

Uses of PVC 1. Manufacture of plastic water pipes 2. For coating fabrics to make them waterproof 3. Used as electrical insulator





Condensation polymerisation occurs when two or more different monomer molecules join together to produce a polymer and other small molecules. Example 1 Nylon (Polyamide) Polyamides such as nylon are condensation polymers. Both the amine and the acid monomer units each have two functional groups - one on each end of the molecule. In this polymer, every other repeating unit is identical. education.mu/chemistry

Nylon is very similar to the protein polyamides in silk and wool, but is stronger. more durable, more chemically inert, and cheaper to produce than the natural fibers. Nylon is often simply represented as:

education.mu/chemistry Uses of nylon (a) To make hard-wearing fabrics (b) To make strong ropes (Polyurethane)




Example 2 Terylene (Polyester)

Polyesters such as PET (polyethylene terephthalate) are condensation polymers. The formation of a polyester follows the same procedure as in the synthesis of a simple ester. The only difference is that both the alcohol and the acid monomer units each have two functional groups - one on each end of the molecule. In this polymer, every other repeating unit is identical.

PET is made from ethylene glycol and terephthalic acid by splitting out water molecules (-H from alcohol and -OH from acid as shown in red on the graphic). The units are joined to make the ester group shown in green.


Terylene is often simply represented as:

Uses of terylene (a) Used as substitutes for cotton (Dacron clothing) and wool in the textile industry education.mu/chemistry (b) Used as the bonding resin in glass fibre plastics (c) Made as tape (Mylar) which is magnetically coated for use in tape recorders and videotape machines




Synthetic fibres are said to be non-biodegrable i.e. they cannot be broken into simpler substances by bacteria. Thus, it is not easy to dispose (get rid of) them. One method is by burning them but here they produce harmful combustion products like CO, soot and HCl (PVC). The main macromolecules found in foodstuffs are:

(a) Proteins (b) Fats (c) Carbohydrates

Proteins

o Proteins possess the same amide linkages as NYLON but with different units.

education.mu/chemistry o Proteins are formed by the condensation of amino acids.

o Proteins can be broken back into amino acids by refluxing the protein with dilute hydrochloric acid.

o Amino acids can be separated by chromatography. Because amino acids

are colourless, the position of each amino acid is found using a ‘locating agent’. This is sprayed over the chromatogram and produces a coloured product. The colours are then compared to a standard chart to deduce the amino acids present.





Fats

o Fats are ester molecules possessing the same linkages as Terylene but with different units.

o Fats are formed by the condensation of glycerol and fatty acids.

o Fats can be broken back into glycerol and fatty acids by boiling with

aqueous NaOH.

Lard + NaOH � Soap + Glycerol

Fat + alkali �� carboxylic salt + alcohol education.mu/chemistry Structure of a fatty acid

Structure of a soap

education.mu/chemistry Structure of glycerol




Carbohydrates

o Starch is formed by the condensation polymerisation of smaller carbohydrate units called sugars (glucose).

o Starch can be broken down into sugars (glucose) by boiling with dilute

hydrochloric acid. o Starch can also be broken down by natural catalysts called Enzymes.

Amylase converts starch to maltose. education.mu/chemistry Starch is a macromolecule represented as:

Structure of a sugar (glucose)

education.mu/chemistry Fuels

� Petroleum, coal and natural gas are natural sources of fossil fuels.

� Methane is the main constituent of natural gas.

� Petroleum is a mixture of hydrocarbons. It can be separated into useful fractions by fractional distillation.

Fractions Uses Butane � Bottled gas for cooking

Petrol (Gasoline) � As fuel in motor vehicles engine

� Cigarette lighter fuel Kerosene (Paraffin) � Oil stoves for cooking

� Aircraft fuel Diesel oil � For fuel in diesel engine e.g. lorries, buses and

ships Lubricating oil � As lubricants in moving parts of machinery

� Making waxes and polishes Bitumen � For making roads


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