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1

Addition Reaction

One of which two or more molecules react together to form a single molecule

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Example of an addition reaction

addition of hydrogen to ethene to form ethane.
Involved hydrogen reacting w/ an unsaturated substance (hydrogenation reaction)

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Ethene

Prod. on an industrial scale, often referred to as 'industry's most important organic chemical'

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products of addition reactions of ethene

1, 2 - dibromoethane

1,2 - dichloroethane

chloroethane

ethanol

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Hydrogenation of vegetable oils

Hydrogenation of C=C double bonds used in manufacture of margarine

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polyunsatuated

Edible oils such as sunflower oil, palm oil, corn oil, are refered to as polyunsaturated

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saturated vs unsaturated

Unsaturated compounds are thought to be less damaging to our health than saturated fats found in dairy products.
Animal fats - usually saturated, veg. oils - unsaturated

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margarine

By controlling degree of hydrogenation, margerine can be made as hard or soft as needed

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Mechanism of the reaction between ethene + bromine

Reaction between ethene + bromine can occur in dark. Suggests mechanism is diff from that between methane + chlorine (free radical substitution)

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Mechanism of the reaction between ethene + bromine -
steps

1. Polarisation

2. Heterolytic fission

3. Carbon Ion Formation

4. Ionic Addition

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Mechanism of the reaction between ethene + bromine -
Step 1: polarisation

1. C=C double bond in ethene is a region of high concentration of negative charge

2. Br₂ molecule, as it approaches C=C double bond, becomes polarised as result of repulsion by electrons in double bond

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Mechanism of the reaction between ethene + bromine -
Step 2: Heterolytic fission

1. Induced polarisation becomes so great that the Br₂ molecule splits into Br⁺ and Br⁻ ions

2. This breaking of bond so that the two electrons end up on one atom is called heterolysis or heterolytic fission bc two diff species are formed

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Mechanism of the reaction between ethene + bromine -
Step 3: Carbonium Ion Formation

1. Br⁺ species attacks C₂H₄ molecule. (C₂H₄ molecule has the req. two electrons in its pi bond)

2. Br⁺ ions forms a covalent bond w/ one of the carbon atoms using these two electrons, one of which belonged to the other carbon atom

3. Leaves this w/ a positive charge

4. Positively charged carbon atom is called carbonium ion. Often referred to as an intermediate

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Mechanism of the reaction between ethene + bromine -
Evidence for carbonium ion has cyclic structure

-Evidence to suggest that carbonium ion intermediae has cyclic structure
-Called a cyclic bromium ion

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Mechanism of the reaction between ethene + bromine -
Step 4: Ionic Addition

1. Carbonium ion attacked by Br⁻ ion. Results in formation of 1, 2-dibromoethane

2. Mechanism of this reaction is referred to as ioni addition since ions add across the C=C double bond

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evidence for ionic addition

- 1,2-dibromoethane formed but two toher compounds also formed. One of these compounds is 1-bromo-2-chloroethane

-Formed when the carbonium ion attacked by the Cl⁻ ion

Evidence for the existence of the carbonium ion as an intermediate

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other compound formed

2-bromoethanol,
formed when carbonium ion attacked by water

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three products formed when ethene is added to bromine water containing dissolved sodium chlroide

1,2-dibromoethane CH₂BrCH₂Br

1-bromo-2-chloroethane CH₂BrCH₂Cl

2-bromoethanol CH₂BrCH₂OH

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Mechanism for addition of Cl2 / HCl to ethene

-Same mechanism
-no evidence for formation of a cyclic intermediate in cases of these

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Polymerisation reactions

Possible for molecules of ethene to link w/ each other to form long chain of carbon atoms

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Polythene / poly(ethene)

Compound formed from polymerisation, it is an example of a polymer

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Polymers

Long chain molecules made by joining together mainly small molecules
-consists of repeating structure + can have a relative molecular mass of several thousand

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Monomers

Small molecules from which polymer is made

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Use of polythene

Used in manufacture of plastic bags, bowls, lunch boxes, buckets

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Example of polymers

poly(chloroethene) and poly(propene)

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Poly(chloroethene)

commonly called polyvinyl chloride (PVC)
Made by reacting together cloroethene molecules

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Poly(propene)

made by polymerisation of propene

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Use of chloroethene

used for clothes

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Use of poly(propene)

manufacture of toys, jugs, plastic beakers

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Repeating unit

The repeating unit of a polymer is that part of the polymer whose repetition produces the complete polymer chain except for the end groups