Chapter 13: Alkenes

Question 1

Define the alkene homologous series?

Answer 1

A molecule containing at least One carbon-carbon double bond.

Question 2

Define the nature of the Carbon-carbon double bond?

Answer 2

A π-bond. The sideways overlap of p-orbitals as each carbon atom contributes one electron to the bond. High electron density occurs above and below the joining nuclei.

Question 3

What is the shape around each carbon atom in a double bonded pair?

Answer 3

Trigonal Planar; 120° bond angle.

Question 4

Why is the shape around a double bonded carbon atom trigonal planar?

Answer 4

• Three regions of high electron density
• Three regions equally repel
• All bonded atoms are on the same plane

Question 5

Why does stereoisomerism occur in alkenes?

Answer 5

The π-bond restricts rotation enabling variation across a chain due to a high electron density above and below the σ-bond.

Question 6

When are cis/trans definitions of stereoisomerism applicable?

Answer 6

When one of the attached groups to each carbon atom is hydrogen.

Question 7

When is the stereoisomer Cis?

Answer 7

When hydrogen is on the same side of the c=c bond. (z)

Question 8

When is the stereoisomer Trans?

Answer 8

When hydrogen is on different sides of the c=c bond. (E)

Question 9

What are the Cahn-Ingold-Prelog rules?

Answer 9

• assign a priority to each branch

- if Highest priority is on the same side (z) if on alternate (E)

Question 10

How do you assign Priority according to Cahn-Ingold-Prelog rules?

Answer 10

Assign each atom a value equal to the atomic number across one carbon in a c=c bond.continue down each chain to the first point of difference. The Branch with the highest priority at this point is the highest priority.

Question 11

What is the Hydrogenation of Alkenes?

Answer 11

The addition of hydrogen to an alkene to form an alkane. (Addition reaction)

Question 12

What are the conditions for the Hydrogenation of Alkenes?

Answer 12

Presence of a Nickel catalyst at 423K

Question 13

What is the Halogenation of Alkenes?

Answer 13

The addition of halogens to an alkene to form an haloalkane. (Addition reaction)

Question 14

What are the conditions for the Halogenation of Alkenes?

Answer 14

Room Temperature

Question 15

How do you test for a saturated hydrocarbon(aq)?

Answer 15

Addition of a halogen (Br) if colour remains then saturated; If colourless returns unsaturated.

Question 16

What are the reacting conditions for the addition of hydrogen halides to alkenes?

Answer 16

• Gaseous (RTP) reaction when mixed

- Hydrogen Halide bubbled through when alkene is a liquid.

Question 17

What is the Hydration of Alkenes?

Answer 17

Addition of water to an alkene to produce an alcohol (addition)

Question 18

What are the conditions for the Hydration of Alkenes?

Answer 18

Passing steam across the alkene in the presence of a phosphoric acid catalyst.

Question 19

What is the mechanism which usually takes place when an alkene is saturated?

Answer 19

Electrophilic addition.

Question 20

What is an Electrophile?

Answer 20

An atom or group which is strongly attracted to an area of electron-rich density.

Question 21

What should you include in a diagram for electrophilic addition?

Answer 21

ẟ+,ẟ-, Curly arrows (electron movement), Electrons, carbocations, charges.

Question 22

What is Markownikoff’s rule?

Answer 22

The hydrogen in a hydrogen halide will tend towards the carbocation with the greatest number of hydrogen atoms and fewest carbon atoms.

Question 23

What are primary, secondary…. carbocations?

Answer 23

Cations given an order directly proportional to the number of bonded carbon atoms.

Question 24

In the reaction of hydrogen halides with alkenes where will the halide tend towards?

Answer 24

The cation of highest order

Question 25

How and why does the stability of carbocations vary with order?

Answer 25

Greater order greater stability as thethe charge in the cation can be shared to the surrounding carbon atoms. More atoms, less charge difference, greater stability.

Question 26

What is addition polymerisation?

Answer 26

The addition of monomers over a break in c=c bond to form a chain

Question 27

What are the problems with PVC recycling?

Answer 27

Release of chlorine in hydrogen chloride, corrosive gasses and toxins.

Question 28

What are bioplastics?

Answer 28

Plastics formed from plant starch, cellulose, oils and proteins

Question 29

What are biodegradable polymers?

Answer 29

Polymers which can be digested by bacteria to leave no toxin

Question 30

What are photodegradable polymers?

Answer 30

Polymers which breakdown in response to light.