Chapter 13 - Properties of Alkenes

Question 1

What does a Carbon to carbon double bond consist of?

Answer 1

• A sigma bond which is the direct overlap of orbitals of. bonding atoms
• A pi bond which is the sideways overlap of adjacent p orbitals above and below the bonding C atoms. Prevents rotation of the bond.

Question 2

Shape around a carbon to carbon double bond?

Answer 2

• Three regions of electron densities
• Repel as far apart forming a trigonal planar
• Bond angle = 120degrees

Question 3

Why are alkenes more reactive than alkanes?

Answer 3

• They consist of a pi bond
• Pi bond introduces a region of high electron density above and below the plane of the bonding c atoms.
• Electrophiles (electron - deficient species) attack the high electron density and cause the alkenes to react.
• Pi bond has smaller bond enthalpy and therefore is broken more easily

Question 4

What is a stereoisomer?

Answer 4

same structural formula but a different arrangement of atoms in space.

Question 5

What shows if a molecule has E/Z isomers?

Answer 5

• Has C=C double bond

- There are two different groups attached to each carbon atom of the C=C double bonds

Question 6

What are cis and trans isomers?

Answer 6

Cis - Both hydrogen atoms on the same side

Trans - Both hydrogen atoms on opposite sides

Question 7

How to classify E/Z isomers?

Answer 7

E isomers - has both higher priority groups on opposite sides of the C=C bond
Z isomers - has both higher priority groups on the same side of the C=C bond

Question 8

What is the test for unsaturated hydrocarbons?

Answer 8

Bromine water test in which unsaturated hydrocarbons decolourise bromine water:

• Orange bromine water
• In the presence of a double bond, the mixture becomes colourless. The bromine adds across the C=C double bond, forming a dibrominated organic product which is colourless
• Addition reaction

Question 9

For an addition reaction with steam to occur what is needed?

Answer 9

An acid catalyst such as phosphoric acid

Question 10

What is an electrophile?

Answer 10

An electrophile is an atom or group of atoms that accepts an electron pair from electron rich centre. Electron deficient species and has a partial positive charge.

Question 11

Electrophilic addition steps?

Answer 11

H-Br acts as an electrophile, accepting the pair of electrons from the pi bond of C=C
- H-Br bonds breaks by heterolytic fission
- A carbocation and Br- ion are formed
- Br- ion attracted to carbocation forming a C-Br bond
Product formed

Question 12

How Bromine is an electrophile

Answer 12

A bromine molecule is non polar.

• Initially, the bromine molecule approaches the alkene.. The high electron density of the pi bond induces a dipole on the Br2 molecule giving one Br molecule a positive charge while the other has a negative charge
• Br2 can now act as an electrophile

Question 13

Markownikoff’s rule can be used to predict the major product, what is the rule?

Answer 13

The H atom of the H-X attaches to the carbon atom with the greater number of H atoms.

Question 14

How are waste polymers processed?

Answer 14

• Recycling
• Energy production (Polymers burnt and heat used to generate electricity
• Chemical feedstocks (Used as organic feedstock for the production of plastics)
• Biodegradable polymers (break down naturally to form CO2, H2O and biological compounds)
• Photodegradable polymers (bonds that weaken in the presence of light and initiate the breakdown of the polymer)