Alkenes Flashcards

1
Q

What are alkenes and their formula?

A

Unsaturated hydrocarbons

Contain C=C double bonds

Formula CnH2n

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2
Q

What are the bonds formed in an alkene?

A

C-H (sigma) bonds

Formed by the overlap of electron clouds belonging to the carbon atoms

C=C (Pi bond)

Formed by the overlap of adjacent P orbitals and has a high electron density.

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3
Q

What are the shapes of alkenes?

A

The Pi bond restricts rotation around the double bond and hence alkenes have flat shapes (planar)

Ethene (trigonal planar)

Alkenes can produce E/Z isomers

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4
Q

Why are alkenes more reactive than alkanes?

A

Alkenes have a C=C double bond which is a highly electron dense area and hence attracts electrophiles.

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5
Q

What is an electrophile?

A

A species which is electron deficient and hence is attracted to an electron dense region.

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6
Q

What is an electrophilic addition reaction?

A

Where an electrophile is attracted to the electron dense Pi bond in a molecule (alkene) which is then broken and the molecule is added in across the double bond

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7
Q

What is the reaction and conditions for hydrogenation of alkenes?

A

Hydrogen is added across the double bond.

Conditions:
60 degrees
Buckle catalyst

What happens:
Gases passed over catalyst
Alkene becomes saturated
Forms an alkane

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8
Q

What is the reaction and conditions for the hydration of alkenes?

A

Water molecule is added across the double bond.

Conditions:
High pressure
Phosphoric acid catalyst

What happens:
Steam and gaseous alkene are mixed
Forms an alcohol

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9
Q

Why are alkenes hydrogenated to be used in industry?

A

Vegetable and animal oils contain a high proportion of unsaturated fats (C=C bonds do not fit well together so are liquid at room temperature)
And are difficult to spread.

Hydrogenation of these fats raises their boiling points and produces products such as margarine

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10
Q

What is the halogenation reaction involving alkenes?

Give the mechanism and conditions

A

A halogen molecule is added across the double bond. (Cl-Cl)

Conditions:
Room temperature

What happens:
Halogen and alkene are mixed
Alkene becomes saturated as the electron dense region accepts one of the halogen atoms.
Other halogen atom is attracted to the carbocation
Forms a dihaloalkane

Used to test for C=C bonds by using Br-Br (bromine water)

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11
Q

What is the addition reaction involving hydrogen halides?

A

Hydrogen halide is added across the double bond

Forms a haloalkane

The Pi bond is attracted to the slightly positive hydrogen of the hydrogen halide (due to electronegativity) forming a halide ion and a carbocations. They then react forming a haloalkane.

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12
Q

What is formed when a hydrogen halide is added to an unsymmetrical alkene (eg propene)?

A

2 products can be formed depending on where the halogen joins to the molecule.

Use markovnikovs rule to identify the most likely product

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13
Q

What is markovnikovs rule?

A

When a hydrogen halide is added to an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom with the most hydrogen atoms initially attached.

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14
Q

What is a carbocation and how does their stability change with structure

A

When a carbon atom carries a positive charge often as an intermediate in a reaction.

The hydrogen atoms attached affects the stability of the carbo cation. The more alkyl groups attached (less hydrogen’s) the more the alkyl groups release electrons towards to carbocation allowing for a great spread of the overall charge (inductive effect)

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15
Q

How are alkenes converted to diols?

A

First the alkene is oxidised and then an addition reaction occurs.

Conditions:
Acidic (usually dilute H2SO4)
Potassium manganate VII (oxidising agent)
Water

The 2 OH groups are added across the C=C double bond.

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16
Q

What kind of polymers do alkenes form?

How are they formed?

A

Addition polymers as the C=C double bond is broken to allow multiple monomers to join

The Pi bond is broken allowing the electrons to then be used to form new sigma bonds between neighbouring carbon atoms.

17
Q

What are some uses of addition polymers?

A

Used in plastics and clothing

18
Q

What are the problems with addition polymers?

A

Made from materials derived from crude oil and are not renewable

Addition polymers are very stable and will remain chemically unchanged for years leading to problems in disposal.