4.1.3 Alkenes

Question 1

What are alkenes and cycloalkenes

Answer 1

They are unsaturated hydrocarbons

Question 2

What is the general fromula for alkenes

Answer 2

CnH₂n

Question 3

What variations can you have of alkenes

Answer 3

Alkenes can be straight chained or branched. They can also contain more than one double bond or be cyclic

Question 4

What variations of alkenes obey the general formula CnH₂n

Answer 4

straight-chained and branched chain alkenes obey the general formula (of CnH₂n) but cyclic alkenes and alkenes with more than one double bond do not.

Question 5

explain the structure of alkenes

Answer 5

Each carbon atom in an alkene has four electrons in its outershell and uses those electrons to form bonds.
For each cabon atom of the double bond, 3 of the 4 electrons are used in 3 sigma (𝜎-)bonds, one to the carbon atom of the double bond and the other 2 electrons to the other atoms (C or H).
The π-bond is formed by the sideways overlap of the 2, 4th electrons that are in a p-orbital. The π-bond locks the two carbon atoms in position and prevents them from rotating around the double bond.

Question 6

Explain how π-bonds are formed and their properties

Answer 6

A π-bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond. Each carbon atom contributes one electron to the electron pair in the π-bond. The π-electron density is concentrated above and below the line joining the nuclei of the bonding atoms. This means that the π-bond locks the two carbon atoms in position and prevents them from rotating around the double bond. This restricted rotation is what makes stereoisomers possible.

Question 7

explain the structure of the double bond e.g. C=C

Answer 7

The = can be thought of as two different bonds a 𝜎-bond and π-bond.
Sigma bonds are formed by the head on overlap of orbitals and their electron densty is on the ‘line’ between the atoms. e.g. →←
Pi-bonds are formed by sideways overlap of p-orbitals. The electron density is above and below the plane containing the atoms e.g. ↑↑ overlaping and ↓↓overlapping

arrows representing the direction of overlap

Question 8

Explain the shape around a double bond

Answer 8

The shape around each of the carbon atoms in a double bond is trigonal planar with a bond angle of 120°.
This is because there are 3 bonding regions which each repel equally as there are no lone pairs.

Question 9

compare the reactivity of Alkenes to alkanes and explain any difference

Answer 9

Alkenes are more reactive than alkanes because of the presence of the π-bond in the C=C. The π-electron density is concentrated above and below the the plane of the 𝜎-bond. This means that the π-bond is on the outside of the double bond, the electrons are therefore more exposed and the bond breaks more easily. Whoch results in aklenes undergoing addition reactions relatively easily.

Question 10

Define addition reaction

Answer 10

An addition reaction is a reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule

Question 11

List the addition reactions of alkenes

Answer 11

Alkenes undergo many addition reactions with:
- hydrogen in the presence of a nickle catalyst → alkanes
- halogens → dihaloalkanes
- hydrogen halides → haloalkanes
- steam in the presence of an acid catalyst e.g. H₃PO₄ → alcohols

Bromine can be used to detect the presence of a double C=C bond as a test for unsaturation in a carbon chain turning from orange to colourless if they are present

Question 12

Explain hydrogenation of alkenes

Answer 12

When an alkene is mixed with hydrogen and passed over a nickle catalyst at 150℃ an addition reaction takes place to form an alkane.

Question 13

Explain halogenation of alkenes

Answer 13

Alkenes undergo a rapid addition reaction with the _halogens _chlorine and bromine at room temperature.
Bromine can be used to test for unsaturation to detect if there is a C=C bond present. When bromine water (an orange solution) is added dropwise to a sample of an alkene, bromine adds across the double bond. The orange colour disappears - indicating the presence of a C=C bond.

If no C=C bonds are presence there is no reaction and no colour change

Any compound containing C=C will decolourise bromine water.

Question 14

Explain hydrohalogenation of alkenes

Answer 14

alkenes react gaseous hydrogen halides at room temperature to form haloalkanes.
If the alkene is a gas e.g. ethene, then the reaction happens when the gases mix. If the alkene is a liquid the gaseous hydrogen halide is bubbled through it.
Alkenes also react with concentrated hydrochloric and hydrobromic acid which are solutions of hydrogen halides in water.

Question 15

What happens when an unsymmetrical alkene reacts with an unsymmetrical compound

Answer 15

When an unsymmetrical alkene reacts with an unsymmetrical compound two products are possible.

Question 16

explain hydration of alkenes

Answer 16

alcohols are formed when alkenes react with steam, H₂O(g), in the presence of an acid catalyst, such as H₃PO₄ or sometimes H₂SO₄ at 500℃

This addition reaction is used widely in industry to produce ethanol from ethene

Question 17

what is used to test for unsaturation

Answer 17

The reaction of alkenes with bromine can be used to identify if there is a C=C bond present (and the organic compound is unsaturated).

Question 18

How is bromine water used to test for unsaturation

Answer 18

When bromine water (orange solution) is added dropwise to a sample of an alkene, bromine is added across the double bond and the orange colour disappears, indicating the presence of a C=C bond.
If the same test is carried out with a saturated comound, there is no addition reaction and no colour change.

Question 19

define unsaturated

Answer 19

containing a multiple carbon-carbon bond

Question 20

define alkenes

Answer 20

the hydrogen homologuous series with one double carbon-carbon bond and the general formula CnH2n+2