12 alkenes

Question 1

Physical properties of the alkenes

Answer 1

• Alkenes are unsaturated hydrocarbons containing at least 1 C=C.
• They have the general formula CnH2n.
• They typically take part in addition reactions (to the C=C).
• They exhibit E/Z isomers including the cis / trans isomerism.
• The C=C is the functional group in an alkene and is responsible for is reactions
• Ethene is one of the most important alkenes. It is obtained from the catalytic cracking of long chain alkanes. Ethene is used to make polymers for the
  plastics industry.

Question 2

Definition: A pi-bond

Answer 2

(π-bond) is the reactive part of the double bond formed above and below the plane of the bonded atoms by the sideways overlap of p-orbitals. It’s an area of high electron density

Question 3

The C=C double bond

Answer 3

• The π-bond consists of 2 lobes one on each side of the sigma bond, the 2 lobes
  sideways overlap to produce a π-bond.
• To ensure maximum overlap, ‘intermittent ethane species’ must be a planar
  molecule.
• The asymmetric shape of the 2 bonds locks the molecule around the double bond.
  This means that there is no free rotation about these bonds.
• The π-bond changes the shape around the carbon atom to a trigonal planar with a
  bond angle of 120o:

Question 4

The shape of alkenes

Answer 4

Despite having a C=C - it is simply just one area of electron density.

So the explanation of the geometry is as follows:

Alkenes have three bonding ares in the same plane (3 areas of high electron density)
which repel equally to be as far apart as possible - TRIGONAL PLANAR (120o).

The π-bond changes the shape around the carbon atom to a trigonal planar with a
bond angle of 120o:

Question 5

Cahn-Ingold-Prelog rules

To have E/Z isomerism, a molecule must have:

Answer 5

A carbon-carbon double bond as there is no free rotation around a C=C which
means the geometry is maintained. (rigidity occurs because of the π-bond’s
electron density).

Each carbon in the double bond must be attached to two different groups.

The groups are given priority with reference to their atomic number.

Question 6

Definition: Stereoisomers

Answer 6

Stereoisomers have the same molecular formula, and arrangement of their
atoms. They differ only by spatial arrangement.

Question 7

E/Z stereosimerism aslo kown as

Answer 7

(geometric)

Question 8

priority for stereoisomersim is based of

Answer 8

atoic no

Answer 9

• The extra electrons in the C=C makes it more reactive as it is more electron rich (localised
  electron density).
• The C=C is it a lot stronger than just a C-C, the C=C is not however twice as strong as the C-C
  (due to orbital overlap)
• The sigma bond (σ) is a lot stronger than the pi bond (π) which means that the pi bond (π) breaks leaving the sigma bond (σ) intact when an alkene reacts. Pi bond has a low bond enthalpy
• The σ bond is stronger than the π bond the extent of overlap in sigma bonds is greaterthan that in pi bonds.
• Alkenes are more reactive than the alkanes due to the electron rich C=C.
• The C=C is the functional group made
  from a sigma, σ bond and a pi, π-bond.

Question 10

Addition reactions of alkenes:

Answer 10

Alkenes undergo addition reactions as they are unsaturated.

The reactant will add across the C=C forming a saturated product.

Two reactant molecules react to form one product.

Question 11

x4

the addition reactions of alkenes

Answer 11

1 Addition of hydrogen
2 Addition of a halogen
3 Addition of steam
4 Addition of hydrogen halides

Question 12

alkene

Addition of hydrogen

Answer 12

Each hydrogen joins to each carbon either side of the C=C.
This reaction is also called hydrogenation
It is also an example of a reduction reaction as hydrogen is added.

conditions:
- H2
- Ni catalyst
- 150 .c

Question 13

alkenes

Addition of a halogen

Answer 13

Each halogen joins to each carbon either side of the C=C.
This reaction is also called halogenation, it makes a dihalogeno alkane
It is also a test for unsaturation.

Answer 14

• The H joins to one of the carbons while the OH joins to the other carbon in the C=C
• This reaction is also called hydration as water is added to themolecule. This reaction makes an alcohol.
• conditons: steam , PHOSPHORIC
  ACID CATALYST

Question 15

alkenes

Addition of hydrogen halides

Answer 15

The hydrogen joins to one of the carbons while the halogen joins to the other
carbon in the C=C.

Any hydrogen halide will react, these are gases and are usually bubbled through
the alkene. This reaction makes a haloalkane.

Question 16

Definition: A carbocation

Answer 16

Definition: A carbocation is
an organic ion in which a
carbon atom has a positive
charge.

Question 17

alkenes

draw The electrophilic addition mechanism

cl-cl h-br

Answer 17

The C=C double bond is a region of high electron density due to
the π electrons. This means it will attract electrophiles to itself.

As all the reactions are addition reactions, the reactions are
described as electrophilic addition

Question 18

Addition to unsymmetrical alkenes

Answer 18

• forms two products one major one minor

Question 19

Markownikoff’s rule states…

Answer 19

‘A rule used to predict the major and minor product when adding a hydrogen halide to an unsymmetrical alkene’

Rule of thumb:

• Hydrogen adds to the C of the double bond with greatest number of hydrogenatoms already attached.
• Halide adds to other carbon.

Question 20

draw the major product

Forming the major product:

Answer 20

Once the C=C attacks the H with a partial positive charge, causing the heterolytic fission of the
H-Br bond, a negative bromide ion and positive carbocation remain.

The carbocation formed is positive on the middle C as the hydrogen adds to the first Carbon (this
is called a secondary carbocation)

Question 21

draw the minor product

Forming the minor product:

Answer 21

The carbocation formed is positive on the terminal C as the hydrogen adds to the second
carbon (this is called a primary carbocation)

Question 22

Carbocation stability from most ro least stable

Answer 22

Most stable
– tertiary carbocation (3 alkyl groups attached)

– secondary carbocation (2 alkyl groups attached)

– primary carbocation (0, or 1 alkyl group attached)
Least stable

Question 23

explain the trend in carbocation stability

Answer 23

WHY?

Carbocations are stabilised by alkyl groups.

Alkyl groups are considered to be electron donating – so they push electrons towards the
positive charge, thus spreading out the positive charge onto neighbouring carbons.

The more alkyl groups surrounding the positively charged carbon, the greater the spread of
charge – the more stable the carbocation.

Question 24

Definition: A polymer

Answer 24

a
long molecular chain made
up from monomer units.

Question 25

Definition: A monomer

Answer 25

a
small molecule that
combines with many other
monomers to form a
polymer.

Question 26

Polymerisation

Answer 26

is when an alkene undergoes an addition reaction to itself. Using high temp and pressure.

Question 27

addition polymerisation.

Answer 27

The majority of alkenes are used in the polymer industry
forming plastics. Polymerisation is when an alkene undergoes
an addition reaction to itself. Using high temp and pressure.

Question 28

non – biodegradable.

Answer 28

Polymers are basically alkanes so they are resistant to most chemicals and bacteria.
This makes them non – biodegradable.

Question 29

Recycling

Answer 29

Polymers are chopped, washed, dried and melted into pellets to make
new products

Question 30

Recycling PVC

Answer 30

PVC contains a large chlorine content so is hazardous. When burnt it
releases hydrogen chloride.

It is currently dissolved in a desirable solvent and recovered by
precipitation.

Question 31

Feedstock recycling

Answer 31

Reclaim monomers from waste polymers.

Polymers are cracked to produce a mixture of alkanes, alkenes and
aromatic compounds.

These are used further in the chemical industry. Alkenes can be
polymerised again.

Question 32

Bioplastics

Answer 32

Made from starch, cellulose, plant oils and proteins – renewable and
sustainable.

Question 33

Biodegradable.

Answer 33

Broken down by microorganisms into water, carbon dioxide and other
biological compounds.

Leave no toxic or visible residue.

Question 34

Photodegradable

Answer 34

• These polymers contain bonds that are weakened by the absorption of
  light