Chapter 13: Alkenes (13.1-13.5) Flashcards

1
Q

what are alkenes

A

unsaturated hydrocarbons containing the C=C functional group
can have multiple double bonds
either straight chain, branched, or in rings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

general formula for alkenes

A

CnH2n

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

properties of the C=C double bond

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

properties of the π-bond

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

why are alkenes more reactive than alkanes

A

because alkenes have a π-bond
The pi-bond induces a region of high electron density above and below the plane bonding the C atoms in the double bond. Electrophiles can attack the high electron density of the π-bond, causing the alkene to react.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what are the two types of stereoisomerism

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

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

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

A
  • must have a C=C double bond

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the difference between cis-trans isomerism

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

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

A

the atom with the greater atomic number has the higher priority

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what makes an isomer a Z isomer

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what makes an isomer an E isomer

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

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

A

keep going along the chains until you find a difference

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what happens when alkenes undergo addition reactions

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

alkene + hydrogen –>

A

alkane

addition reaction, needs a nickel catalyst

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

alkene + halogen –>

A

dihaloalkane

addition reaction, no catalyst

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

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

A

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
17
Q

alkene + hydrogen-halide –>

A

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)

18
Q

alkene + steam –>

A

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

19
Q

mechanism for the reaction of an alkene with an electrophile

A

electrophilic addition

20
Q

steps of the electrophilic addition mechanism (polar molecule)

A

step 1:

  • a molecule (e.g. H-Br) acts as an electrophile, accepting the pair of electrons from the pi-bond of C=C
  • 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 product is formed

21
Q

how electrophilic addiction works in non-polar molecules

A

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

22
Q

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

A

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

23
Q

why do alkene monomers polymerise?

A

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

24
Q

environmental issues with putting waste polymers into landfill

A

addition polymers are unreactive and take a long time to biodegrade

25
Q

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

A
  • recycling
  • combustion: polymers are burnt and the heat used to produce electricity
  • used for chemical feedstocks
26
Q

problems with halogenated plastics

A

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

27
Q

biodegradable polymers

A

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

28
Q

photodegradable polymers

A

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

29
Q

Electrophile

A

An atom or a group of atoms which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond.

30
Q

E/Z isomerism

A

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.

31
Q

Cis- trans isomerism

A

A special type of E/Z isomerism in which there are two non-hydrogen groups and two hydrogen atoms around the C=C double bond

32
Q

Stereoisomers

A

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