Chapter 13: Alkenes (13.1-13.5)

Question 1

general formula for alkenes

Answer 1

CnH2n

Question 2

properties of the C=C double bond

Answer 2

consists of a π-bond (pi-bond) and a σ-bond (sigma bond)
the π-bond prevents rotation around the C=C bond
trigonal planar shape around it, bond angle 120

Question 3

properties of the π-bond

Answer 3

pi-bond is the sideways overlap of adjacent P orbitals above and below the bonding atoms
In the middle of the overlapping orbitals is the sigma bond (overlap of orbitals directly between the bonding atoms)

Question 4

why are alkenes more reactive than alkanes

Answer 4

because alkenes have a π-bond
The pi-bond induces a region of high electron density above and below the plane of atoms
Alkenes are therefore more susceptible to attack from electrophiles causing the alkene to react.

Question 5

what are the two types of stereoisomerism

Answer 5

• E/Z isomerism (only occurs in some alkenes)

- optical isomerism (occurs in a range of different compounds)

Question 6

what must a molecule have in order for it to show E/Z isomerism?

Answer 6

• must have a C=C double bond

- must have 2 different groups attached to each carbon atom of the C=C bond

Question 7

What is the difference between cis-trans isomerism

Answer 7

cis: the repeated group is on the same side of both C atoms
trans: the repeated group is on the opposite side of both C atoms

Question 8

what gives an atom high priority (Cahn-Ingold-Prelog priority rules)

Answer 8

the atom with the greater atomic number has the higher priority

Question 9

what makes an isomer a Z isomer

Answer 9

a Z isomer has both of the higher priority groups on the same side pf the C=C bond
(Zame Zide)

Question 10

what makes an isomer an E isomer

Answer 10

an E isomer has the higher priority groups on the opposite side of the C=C bond

Question 11

what happens if the the first atoms attached to a C in a C=C double bond have the same priority?

Answer 11

keep going along the chains until you find a difference

Question 12

what happens when alkenes undergo addition reactions

Answer 12

a small molecule adds across the double bond, causing the π-bond to break and new σ-bonds to form
there is only 1 product
the double bond is lost, product is saturated

Question 13

alkene + hydrogen –>

Answer 13

alkane

addition reaction, needs a nickel catalyst

Question 14

alkene + halogen –>

Answer 14

dihaloalkane

addition reaction, no catalyst

Question 15

test for unsaturation (shows that an addition reaction has occurred)

Answer 15

unsaturated hydrocarbons decolourise bromine water:

• orange bromine water is added to an excess of aqueous organic compound (mixture is shaken)
• in the presence of a double bond, the mixture becomes colourless (bromine adds across the double bond)
• if the organic compound is saturated, no reaction will occur and bromine water will not decolourise

Question 16

alkene + hydrogen-halide –>

Answer 16

haloalkane
(addition reaction, no catalyst)
if the alkene has a carbon chain of 3 or more, there are 2 possible products, depending on which carbon the halogen adds to (e.g. 1-bromo vs 2-bromo)

Question 17

alkene + steam –>

Answer 17

alcohol
(addition reaction, acid catalyst e.g. H3PO4)
if the alkene has a carbon chain of 3 or more, there are 2 possible products, depending on which carbon the -OH adds to

Question 18

mechanism for the reaction of an alkene with an electrophile

Answer 18

electrophilic addition

Question 19

steps of the electrophilic addition mechanism (polar molecule)

Answer 19

step 1:

• a molecule (e.g. H-Br) acts as an electrophile, accepting the pair of electrons from the pi-bond of C=C and double bond breaks
• the electrophile bond (e.g. H-Br) breaks by heterolytic fission

step 2:
- a carbocation and negative ion (e.g. Br-) are formed

step 3:
- the negative ion (Br-) is attracted to the carbocation, forming a bond (C-Br). The addition product is formed

Question 20

how electrophilic addiction works in non-polar molecules

Answer 20

the high electron density of the pi-bond induces a dipole in the non-polar molecule so it can now act as an electrophile

Question 21

how to predict which halo-alkane will be the major product when electrophilic addition happens (unsymmetrical alkenes)

Answer 21

Markownikoff’s rule states that when an electrophile H-X reacts with an unsymmetrical alkene, the H atom is more likely to attach to the carbon atom with the greatest number of H atoms attached to it
i.e. secondary carbocation will form the major product

Question 22

why do alkene monomers polymerise?

Answer 22

they are much more stable without the C=C double bond

Question 23

environmental issues with putting waste polymers into landfill

Answer 23

addition polymers are unreactive and take a long time to biodegrade

Question 24

how can waste polymers be processed in a non-evironmentally damaging way?

Answer 24

• recycling
• combustion: polymers are burnt and the heat used to produce electricity
• used for chemical feedstocks

Question 25

problems with halogenated plastics

Answer 25

combustion of halogenated plastics can from toxic waste products (e.g. HCl gas) which must be removed to prevent discharge to the environment

Question 26

biodegradable polymers

Answer 26

polymers that are plant-based and break down naturally to form CO2, H2O, and other biodegradable compounds (non-toxic therefore doesn’t harm environment)

Question 27

photodegradable polymers

Answer 27

polymers that contain bonds that weaken in the presence of light, initiating the breakdown of polymers

Question 28

Electrophile

Answer 28

Electron pair acceptor

Question 29

E/Z isomerism

Answer 29

A type of stereoisomerism in which different groups attached to each carbon of a C=C of a double bond may be arranged differently in space because of the restricted rotation of the C=C bond.

Question 30

Cis- trans isomerism

Answer 30

• A type of E/Z isomerism
• One of the groups attached to each carbon atom is the same
• If the group is on the same side = cis and opposite side = trans

Question 31

Stereoisomers

Answer 31

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

Question 32

How can H-Br act as an electrophile?

Answer 32

The H accepts a pair of electrons