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CH0003 > Chemistry of Alkenes > Flashcards

Chemistry of Alkenes Flashcards

1
Q

What are alkenes

A

Compounds with a C=C bond

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2
Q

What does -ene mean

A

One double bond

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3
Q

What does -diene mean

A

2 double bonds

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4
Q

What does -triene mean

A

3 double bonds

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5
Q

What is the electronic structure of double bonds (π bonds)

A

In a π bond p-orbitals of C atoms overlap. π electrons are further from the nucleus (less tightly held) so are involved in the chemistry

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6
Q

Why does cis/trans isomerism exist

A

Rotation would break the π bond

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7
Q

Describe cis isomers

A

Z-isomerism (on the same side)

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8
Q

Describe trans isomers

A

E-isomerism (on different sides)

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9
Q

Out of cis and trans-isomers which form exists more commonly and why

A

Trans-isomers as there is less steric strain meanign they are more stable

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10
Q

Why are cis isomers less stable

A

There is repulsion between the methyl groups in the cis isomer.

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11
Q

What types of geometric isomers must be present in (1-7) carbon rings

A

The double bond must be cis.

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12
Q

In which types of fatty acids do alkenes exist

A

Unsaturated fats and fatty acids (saturated fats have no C=C bonds)

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13
Q

What type of fatty acids are mammals unable to produce

A

Fatty acids with C=C double bond further away from carbonyl groups 9 and 10 (counting from the C=O end). These essential fatty acids must be obtained from the diet.

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14
Q

What does omega refer to

A

The position of the double bond from the CH3 end

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15
Q

What are food sources of linoleic acid

A

Oily fish, some plant oils

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16
Q

What are fats

A

Esters of fatty acids

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17
Q

What are terpenes

A

Hydrocarbons with multiples of 5 carbon atoms

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18
Q

What are terpenes produced by and what is their significance

A

Produced by many plants. The smaller ones (10 and 15 carbons) are often aromatic and used in flavours and fragrances

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19
Q

What are carotenoids

A

Brightly coloured compounds containing many conjugated alkenes

20
Q

Where are carotenoids found

A

In many plants

21
Q

What does cojugation mean

A

Alternating single and double bonds

22
Q

What can humans convert carotene into

A

Vitamin A

23
Q

What is vitamin A required for

A

Vision in the rod cells of the retina

24
Q

What do addition reactions of alkenes do

A

Converts the double bond into 2 single bonds (opposite of elimination)

25
Q

What happens in addition of hydrogen halides with alkenes

A

H+ is an electrophile. Electrophiles are attracted to electrons which are found in the double bond. So this is electrophilic addition.
E.g. ethene + HCl -> secondary alkyl chloride

26
Q

What is Markovnikov’s rule

A

When there is more than one possible produce, the more substituted halide is usually formed

27
Q

What happens in hydration of alkenes

A

The addition of water to give an alcohol. Ethene (+ water and acid catalyst for protons) -> ethanol. This is the reverse of elimination of an alcohol, is acid catalyst and follows “Markovnikov’s rule”

28
Q

Why is hydration of alkenes reversible

A

Aqueous acid (lots of water) favours hydration, but conc acid favours dehydration (=elimination). Application of Le Chatelier’s principle

29
Q

What happens in addition of halogens to alkenes reactions

A

Br2/ Cl2/ I2 + ethene -> dialkyl bromide/ chloride/ iodide

30
Q

What if the addition of bromine to alkenes takes place when bromie is dissolved in water

A

Product is called bromohydrin (CHBrCH2OH). Ethene and water and bromide go to bromohydrin and hydrogen bromide

31
Q

What is a test for alkenes

A

The disappearance of the orange/brown colour of the bromide indicated the presence of an alkene.

32
Q

What can the reaction of unsaturated fatty acids with iodine be used for

A

To quatify the number of C=C bonds. The “iodine value” is the number of grams of iodine reacting with 100g of oil

33
Q

What is hydrogenation

A

Addition of H2 (reduction) to an alkene. The hydrogen molecule contains two electrons. Alkene + hydrogen + metal catalyst -> alkane

34
Q

What catalyst is used in hydrogenation of alkenes

A

A solid metal such as Ni. Pd or Pt. The reaction takes place in the metal surface. Metal powder is very fine/ porous or distributed as small particles on carbon to give a large surface area for the reaction

35
Q

What kind of reaction is the hydrogenation of an alkene

A

Its a heterogenous reaction (more than one phase)

36
Q

Why is hydrogenation of alkenes industrially important

A

It is used for hydrogenating liquid vegetable oils to make solid fats

37
Q

What is a polymer

A

A large molecule made up of a repeating unit

38
Q

How are polymers produced

A

Through addition of an alkene many times to itself. They are produced by a chain reaction with initiation, propagation and termination steps. Monomers add on to the end of a growing polymer chain

39
Q

What does polymerisation of ethene produce

A

Polyethene

40
Q

What does polymerisation of vinyl chloride produceq

A

PVC poly(vinyl chloride). Used for pipes, flooring

41
Q

What does polymerisation of styrene produce

A

Polystyrene. Used for packaging, cartons, cups

42
Q

What does polymerisation of tetrafluoroethylene produce

A

Teflon. Used for non-stick surfaces, cable insulation

43
Q

What is important to note about polymers in terms of recyling

A

The code on the plastic packaging indicated the type of polymer

44
Q

What happens during initiation of polymerisation

A

Creates radicals from molecules with paired electrons

45
Q

What happens during propogation of polymerisation

A

Radical end of polymer chain reacts with alkene, increasing the length of the polymer

46
Q

What happens during termination of polymerisation

A

Reaction of two radicals to create a non-radical

CH0003 (12 decks)

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