Chemistry of Alkenes

Question 1

What are alkenes

Answer 1

Compounds with a C=C bond

Question 2

What does -ene mean

Answer 2

One double bond

Question 3

What does -diene mean

Answer 3

2 double bonds

Question 4

What does -triene mean

Answer 4

3 double bonds

Question 5

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

Answer 5

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

Question 6

Why does cis/trans isomerism exist

Answer 6

Rotation would break the π bond

Question 7

Describe cis isomers

Answer 7

Z-isomerism (on the same side)

Question 8

Describe trans isomers

Answer 8

E-isomerism (on different sides)

Question 9

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

Answer 9

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

Question 10

Why are cis isomers less stable

Answer 10

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

Question 11

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

Answer 11

The double bond must be cis.

Question 12

In which types of fatty acids do alkenes exist

Answer 12

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

Question 13

What type of fatty acids are mammals unable to produce

Answer 13

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.

Question 14

What does omega refer to

Answer 14

The position of the double bond from the CH3 end

Question 15

What are food sources of linoleic acid

Answer 15

Oily fish, some plant oils

Question 16

What are fats

Answer 16

Esters of fatty acids

Question 17

What are terpenes

Answer 17

Hydrocarbons with multiples of 5 carbon atoms

Question 18

What are terpenes produced by and what is their significance

Answer 18

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

Question 19

What are carotenoids

Answer 19

Brightly coloured compounds containing many conjugated alkenes

Question 20

Where are carotenoids found

Answer 20

In many plants

Question 21

What does cojugation mean

Answer 21

Alternating single and double bonds

Question 22

What can humans convert carotene into

Answer 22

Vitamin A

Question 23

What is vitamin A required for

Answer 23

Vision in the rod cells of the retina

Question 24

What do addition reactions of alkenes do

Answer 24

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

Question 25

What happens in addition of hydrogen halides with alkenes

Answer 25

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

Question 26

What is Markovnikov's rule

Answer 26

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

Question 27

What happens in hydration of alkenes

Answer 27

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"

Question 28

Why is hydration of alkenes reversible

Answer 28

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

Question 29

What happens in addition of halogens to alkenes reactions

Answer 29

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

Question 30

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

Answer 30

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

Question 31

What is a test for alkenes

Answer 31

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

Question 32

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

Answer 32

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

Question 33

What is hydrogenation

Answer 33

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

Question 34

What catalyst is used in hydrogenation of alkenes

Answer 34

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

Question 35

What kind of reaction is the hydrogenation of an alkene

Answer 35

Its a heterogenous reaction (more than one phase)

Question 36

Why is hydrogenation of alkenes industrially important

Answer 36

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

Question 37

What is a polymer

Answer 37

A large molecule made up of a repeating unit

Question 38

How are polymers produced

Answer 38

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

Question 39

What does polymerisation of ethene produce

Answer 39

Polyethene

Question 40

What does polymerisation of vinyl chloride produceq

Answer 40

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

Question 41

What does polymerisation of styrene produce

Answer 41

Polystyrene. Used for packaging, cartons, cups

Question 42

What does polymerisation of tetrafluoroethylene produce

Answer 42

Teflon. Used for non-stick surfaces, cable insulation

Question 43

What is important to note about polymers in terms of recyling

Answer 43

The code on the plastic packaging indicated the type of polymer

Question 44

What happens during initiation of polymerisation

Answer 44

Creates radicals from molecules with paired electrons

Question 45

What happens during propogation of polymerisation

Answer 45

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

Question 46

What happens during termination of polymerisation

Answer 46

Reaction of two radicals to create a non-radical