What type of reaction are typical of the alkenes?

Electrophilic addition

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Describe the halogenation of alkenes.

Addition of halogens such as chlorine or bromine across the C=C double bond at room temperature to form haloalkanes.


For example, the reaction of ethene with bromine produces 1,2-dibromoethane:

C2H4 + Br2 ➔ C2H4Br2

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State the reagent and explain the positive result in the test for the alkene C=C double bond.

Bromine water turns from orange to colourless in the presence of an alkene. 


This colour change occurs because orange bromine is used up in the reaction, as it adds across the C=C bond of the alkene, and a colourless bromoalkane is produced.

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What type of bond fission (homolytic or heterolytic) occurs in the reaction of alkenes with hydrogen halides?

Heterolytic fission

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What causes the Br-Br bond to break in the reaction mechanism of Br2 with an alkene?

The high electron density of the π-electrons which polarises the Br-Br bond causing heterolytic fission.

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How does an unsymmetrical molecule like HBr act as an electrophile?

The H-Br bond is polar, so the partially positive hydrogen atom acts as an electrophile because it is attracted to the electron rich double bond.

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What is the reaction mechanism for the reaction of ethene with bromine?

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What is the reaction mechanism for the reaction of ethene with hydrogen bromide?

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What is the reaction mechanism for the reaction of ethene with concentrated sulfuric acid?

When water is added to the product, ethanol is formed and sulfuric acid reforms.

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How does carbocation stability determine the major product when hydrogen halides are added to unsymmetrical alkenes?

The major product is one which is formed via the most stable carbocation intermediate. 


The more substituted a carbocation is, the greater its stability.

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What is the definition of an electrophile?

An electrophile is an electron-pair acceptor.


An electrophile is an electron-deficient species that accepts a pair of electrons to form a new covalent bond. Examples of electrophiles include AlCl3, Br+ and H3O+.

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What is a carbocation?

A carbocation is an ion that contains a positively charged carbon atom.


For example, the structure of the C(CH3)H2+ carbocation is:

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What is an addition reaction?

An addition reaction is a reaction in which two reactants combine to form one larger product.


C2H4 + Br2 ➔ C2H4Br2 is an example of an addition reaction.

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Describe the hydration of alkenes.

Addition of steam across the C=C double bond in the presence of a phosphoric acid catalyst to form an alcohol.


For example, the reaction of ethene with steam produces ethanol:

C2H4 + H2O ➔ C2H5OH

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State the order of stability of primary, secondary and tertiary carbocations.

The stability of carbocations increases in the order: primary < secondary < tertiary.

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