Chapter 13: Alkenes (13.1-13.5) Flashcards Preview

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Flashcards in Chapter 13: Alkenes (13.1-13.5) Deck (32)
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1
Q

general formula for alkenes

A

CnH2n

2
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

3
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)

4
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 of atoms
Alkenes are therefore more susceptible to attack from electrophiles causing the alkene to react.

5
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)

6
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

7
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

8
Q

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

A

the atom with the greater atomic number has the higher priority

9
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)

10
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

11
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

12
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

13
Q

alkene + hydrogen –>

A

alkane

addition reaction, needs a nickel catalyst

14
Q

alkene + halogen –>

A

dihaloalkane

addition reaction, no catalyst

15
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
16
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)

17
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

18
Q

mechanism for the reaction of an alkene with an electrophile

A

electrophilic addition

19
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 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

20
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

21
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

22
Q

why do alkene monomers polymerise?

A

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

23
Q

environmental issues with putting waste polymers into landfill

A

addition polymers are unreactive and take a long time to biodegrade

24
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
25
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

26
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)

27
Q

photodegradable polymers

A

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

28
Q

Electrophile

A

Electron pair acceptor

29
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.

30
Q

Cis- trans isomerism

A
  • 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
31
Q

Stereoisomers

A

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

32
Q

How can H-Br act as an electrophile?

A

The H accepts a pair of electrons