4.1.3 Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons with at least 1 C=C double bond.

Question 2

General formula of alkenes

Answer 2

CnH2n

Question 3

Reactivity of alkenes

Answer 3

High as the double bond is an area of high electron density

Question 4

Nature of the C=C double bond

Answer 4

Formed of 1 sigma bond made from the direct overlap of two orbitals and 1 pi bond formed from the sideways overlap of two orbitals. This pi bond means that the double bond cannot rotate.

Question 5

Geometry around C atom of ethene

Answer 5

Trigonal planar with H-C-H bond of 118 and H-C-C bond of 121. The double bond has higher electron density than the single bonds so repels slightly more giving a larger bond angle.

Question 6

Method to identify alkene

Answer 6

Addition of bromine water. The bromine water is decolourised. It reacts with the alkene to form a saturated dibromoalkane.

Question 7

Definition of stereoisomer

Answer 7

Compounds with the same structural formula but a different arrangement of atoms in space.

Question 8

Reason for E/Z isomerism

Answer 8

For a molecule to have E/Z isomers it must have a C=C double bond as, C=C bond cannot rotate. Each C in the double bond must have 2 different groups attached.

Question 9

How to identify E and Z isomers

Answer 9

Z has the priority groups on the Zame Zide.

E are Enemies on opposite sides

Question 10

CIP rules

Answer 10

The atom with the higher Ar is given the higher priority e.g. Br above Cl. A longer chain has priority over a shorter chain.

Question 11

Cis-trans isomerism

Answer 11

Special case of E/Z isomerism where H is attached to each C of the C=C. Cis for same side and trans for different sides.

Question 12

Type of reactions alkenes undergo

Answer 12

Electrophilic Addition

Question 13

Name of reactions with hydrogen, conditions and example

Answer 13

1. Hydrogenation
2. Nickel catalyst
   3a. manufacture of margarine from unsaturated vegetable oils
   3b. ethene + hydrogen –> ethane

Question 14

Product of reaction with hydrogen halide

Answer 14

haloalkane

Question 15

Product of reaction with halogen

Answer 15

di-substituted haloalkane

Question 16

Conditions and reaction for forming ethanol from ethene

Answer 16

1) steam and phosphoric acid catalyst.

2) ethene + steam –> ethanol

Question 17

Type of reaction, conditions for forming diols and equation for reaction on ethene

Answer 17

1) oxidation reaction
2) Cold, dilute and acidified potassium manganate
3) C2H4 + H2O + [O] –> CH2(OH)CH2(OH)

Question 18

Electrophile definition

Answer 18

electron pair acceptor

Question 19

Mechanism for reaction of propene with bromine

Answer 19

see notes

Question 20

Mechanism for reaction of propene with hydrogen bromide

Answer 20

see notes

Question 21

Markownikow’s rule

Answer 21

The major product in a reaction will always be the one formed via the more stable carbocation intermediate. Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations.

Question 22

Definition of addition polymers

Answer 22

When short chain monomers add together to form one larger polymer made of regularly repeating units.

Question 23

Problems associated with raw materials for addition polymerisation

Answer 23

large amounts of energy and resources needed.
polymers are made from alkenes, which are obtained from non-renewable crude oil.
Extraction and cracking of crude oil are both high energy processes and more energy is needed for polymerisation.

Question 24

Reason for low reactivity of addition polymers

Answer 24

Unreactive hydrocarbon chains due to multiple strong, non-polar covalent bonds in the chains.

Question 25

Are addition polymers biodegradable?

Answer 25

Addition polymers are non-biodegradable due to strong, non-polar, covalent bonds.

Question 26

How are addition polymers disposed of?

Answer 26

They mostly go to landfill.
Some can be recycled .
Some are used as organic feedstock for production of other plastics and organic chemicals.
Some are combusted to produce energy. Combustion can release toxic gases