Module 4: Alkenes (Chapter 13)

Question 1

Define a pi bond

Answer 1

The sideways overlap of adjacent p-orbitals above and below the bonding C atoms. Electron density is localised above and below the plain of the sigma bond. Restricted bond rotation as a result

Question 2

Bond angle for a double C bond

Answer 2

120 = 3 regions of electron density / 3 bond pairs. Symmetrical molecule giving a trigonal planar shape. Electrons repel as far apart as possible.

Question 3

Define stereoisomers

Answer 3

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

Question 4

Define E/Z isomerism

Answer 4

A molecule has a double bond ( no bond rotation) and two different groups are attached to each carbon atom in the double bond.

Question 5

E isomerism

Answer 5

Opposite sides

Question 6

Z isomerism

Answer 6

Same side

Question 7

What is cis/trans isomerism

Answer 7

Special case of E/Z isomerism. One of the groups attached must be hydrogen.

Question 8

Cis is the same as…

Answer 8

Z isomerism

Question 9

Trans is the same as…

Answer 9

E isomerism

Question 10

Describe the Cahn-Ingold-Prelog

Answer 10

Priority given to atom with the highest atomic number on each carbon atom. Z if highest on the same side and E if on different sides.

Question 11

Why is a sigma bond stronger than a pi bond?

Answer 11

Overlap of orbitals in sigma bonds are end to end and along the line between the two bonding nuclei, whilst pi bond is sideways adjacent overlap, which is poor overlap.

Question 12

Alkene reactivity

Answer 12

more reactive due to low bond enthalpy of the pi bond as electrons more exposed being on the outside. More readily broken pi bond compared to a sigma bond.

Question 13

Reactants for hydrogenation

Answer 13

Ni catalyst
423K
H2
Forms an alkane

Question 14

Halogenation reactants

Answer 14

Halogen added e.g. bromine
Forms a haloalkane
Bromine used to test for double bond = orange to colourless

Question 15

Hydrogen Halide reactants

Answer 15

Gaseous hydrogen halide e.g. HCl
RTP = 20 and 101kPa
Forms a haloalkane

Question 16

Hydration reactants

Answer 16

Steam
H3PO4 catalyst
Alcohol forms

Question 17

Define electrophile

Answer 17

Electron pair acceptor

Question 18

What type of reaction are addition of molecules to an alkene called?

Answer 18

Electrophilic addition

Question 19

Markownikoff’s rule?

Answer 19

Primary - tertiary carbocation stability. Determines the major product in a reaction. Increasing number of alkyl groups meaning positive charge more spread over alkyl groups so more stable.

Question 20

Define monomer

Answer 20

Repeat unit from which a polymer is made

Question 21

Define polymer

Answer 21

Long chain molecule made up of repeating units of a monomer

Question 22

Define addition polymer

Answer 22

Long chain molecule formed from many monomer units (containing a double bond) joining end to end. The only product from the reaction.

Question 23

Benefits to sustainability in polymer disposal?

Answer 23

Combustion for energy production
Use as an organic feedstock for production of plastics and other organic chemicals
Removal of toxic waste products e.g. HCl formed in disposal of PVC