C13 - Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons with at least 1 carbon to carbon double bond.
Aliphatic alkenes with a double bond have the formula CnH2n.
They can be branched, contain more than one double bond or be cyclic.
(Only branched will obey the general formula)

Question 2

How is a double bond formed?

Answer 2

For each of the carbon atoms belonging to the double bond, three of the four electrons are used in three sigma bonds (one to the other carbon atom of the double bond and the other two to two other atoms).

This leaves one electron on each carbon atom of the double bond not involved in the sigma bond.
This electron belongs in a p-orbital.

This forms a pi bond (the sideways overlap of 2 p-orbitals). The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms.

Question 3

Why are atoms joined by double bonds unable to rotate?

Answer 3

The pi bond locks the 2 carbon atoms in position and prevents them from rotating.
This makes the geometry of the alkenes different from that of alkanes where rotation is possible around every atom.

Question 4

What is the shape of atoms around a double bond?

Answer 4

Trigonal planar

This is because:
- there are 3 regions of electron density around each of the carbon atoms

• the three regions repel each other as far apart as possible so the bond angle around each carbon atom is 120°
• all of the atoms are in the same plane

Question 5

What are stereoisomers?

Answer 5

Isomers which have the same structural formula (as well as molecular formula) but in a different arrangement of the atoms in space.

Question 6

What are the two types of stereoisomers?

Answer 6

E/Z isomerism

Optical isomerism

Question 7

What is the difference between E/Z and optical isomerism?

Answer 7

E/Z isomerism only occurs in compounds with a C=C bond whereas optical isomerism can occur in a much wider range of compounds.

Question 8

Why do double bonds not allow free rotation?

Answer 8

The pi bond has regions of electron density above and below the plane of the sigma bond.

Question 9

What is necessary for E/Z isomerism to occur?

Answer 9

• a C=C double bond

- different groups attached to each carbon atom of the double bond

Question 10

Do but-1-ene and but-2-ene have E/Z isomerisms?

Answer 10

But-2-ene can has it has a methyl group and H atom on each of its binding carbon atoms however but-1-ene cannot as it has two H atoms in one side.

Question 11

What is an E and Z isomer?

Answer 11

E: the higher priority groups are on opposite sides of the bond.
(=)

Z: the higher priority groups are both on the same side.
(=/)

Question 12

What is cis-trans isomerism?

Answer 12

A special case of E/Z isomerism where one of the attached groups on each carbon of the double bond MUST be hydrogen

The cis isomer has the H atom on the same side of the molecule whereas the trans isomer has the H atoms on opposite sides of the isomer.

Question 13

How does cis-trans isomerism link to E/Z isomers?

Answer 13

The cis isomer is the Z isomer - the higher priority groups are both on the same side.

The trans isomer is the E isomer - the higher priority groups are on opposite sides of the

Question 14

When is cis-trans isomerism used?

Answer 14

When there is a C=C double bond with different groups on both carbons and a hydrogen atom bonded to both carbons.

They are determined by the priority of of atoms bonded to the carbons.

Question 15

How is priority of bonding atoms in isomerism determined?

Answer 15

Based on atomic number.
The greater the atomic number, the higher the priority.

High priority on same side = Z isomer

High priority on opposing sides = E isomer

Question 16

Why are lakebed more reactive than alkanes?

Answer 16

They have a pi bond (double bond) and the electron density of pi bonds are more exposed than sigma bonds so pi bonds break more readily.

Question 17

What is the hydrogenation of alkenes?

Answer 17

When an Allene is mixed with hydrogen and passed over a nickel catalyst as 423K.

An addition reaction occurs to form an alkane and hydrogen is added across the double bond.

Question 18

What is the halogenation of alkenes?

Answer 18

A rapid addition reaction with the halogens Cl and Br at room temp.
This can be used to test for unsaturation.

They also react with hydrogen halides e.g. HCl.

Question 19

What is the hydration of alkenes?

Answer 19

When alkenes are reacted with steam in the presence of a phosphoric acid catalyst to form alcohols.

Question 20

What are the different types of reaction mechanisms?

Answer 20

Electrophilic addition

Radical substitution

Nucleophilic substitution

Question 21

What is electrophilic addition?

Answer 21

A reaction mechanism for the saturation of alkenes to form saturated compounds.

Question 22

What is an electrophile?

Answer 22

An atom or group of atoms which is attracted to an electron-rich centre (e.g. Pi bond) and accepts an electron pair.

It’s usually a positive ion or molecule with slight charges.

Question 23

What occurs during electrophilic addition?

Answer 23

An electrophile comes into contact with an alkene, resulting in an induced dipole.

The less electronegative atom of the electrophile has a slight positive charge and is attracted to the negative charge of the pi bond.
The electron pair in the pi bond is attracted to the partial positive charge.

A bond forms between the positive atom and a carbon atom from the double bond, breaking the electrophile by heterolytic fission.
This forms a carbocation and halide ion which then react together and bond to form an addition product.

Question 24

What is Markownikoff’s rule?

Answer 24

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and smaller number of carbons.

Question 25

What are primary, secondary and tertiary alkenes?

Answer 25

Primary - the carbon is bonded to one R group/ 2 or 3 hydrogen atoms.

Secondary - the carbon is bonded to two R groups / 1 hydrogen atom

Tertiary - the carbon is bonded to 3 R groups, no hydrogen atoms.

Question 26

What’s addition polymerisation?

Answer 26

When alkenes react to form long saturated chains containing no double bonds, the properties of which depend on the monomers used.

Question 27

What does polymerisation require?

Answer 27

High temperature
High pressure
Catalysts

Addition polymers have high Mr
Synthetic polymers are named after their monomers.

Question 28

What are the methods of disposing polymers?

Answer 28

Recycle to make new products by recycling, PVC recycling, feedstock recycling etc.

Use waste as fuel

Produce biodegradable or photodegradable polymers

Question 29

What is feedstock recycling?

Answer 29

The chemical and thermal processes that can reclaim monomers, gases or oil from waste polymers.
The products resemble those from crude oil refiner Ines and it doesn’t require sorting and washing of polymers.

Question 30

What are photodegradable polymers?

Answer 30

Polymers with bonds that are weakened by absorbing light to start degradation.