Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons containing a C=C bond.

Question 2

Are alkenes more or less reactive than alkanes?

Answer 2

more reactive due to the high concentration of electrons in the C=C double bond

Question 3

What are the physical properties of alkenes?

Answer 3

-double bond does not greatly affect melting/boiling points
-van der waals forces are the only intermolecular forces that act between the alkene molecules
-this makes the physical properties of alkenes similar to those of the alkanes
-boiling point increases with number of carbons present
-not soluble in water

Question 4

How do alkenes react?

Answer 4

-alkenes are more reactive than alkanes
-C=C bond can easily be attacked by electrophiles and undergo electrophilic addition

Question 5

What is formed during electrophilic addition?

Answer 5

a carbocation

Question 6

What happens during electrophilic addition?

4

Answer 6

-electrophile is attracted to the double bond
-electrophiles are positively charged and accept a pair of electrons
-a positive ion, carbocation is formed
-a negatively charged ion forms a bond with the carbocation

Question 7

What are electrophiles?

Answer 7

electron pair acceptors

Question 8

Carbocation stability

Answer 8

Tertiary carbocations are the most stable, and primary the least, so when they form, if there is the option between 2 different types of carbocation, the most stable one will form the majority

Question 9

Combustion of alkenes

Answer 9

Alkenes produce a very smoky, sooty flame when burnt, in comparison to alkanes. (This can be a way to test if something is an alkene or alkane)

Question 10

Shape and Bonding of alkenes

4

Answer 10

• The planar nature of alkenes is due to the flat region around the double bond.
• Bond angles in alkenes are approximately 120 degrees, with slight variations due to electron density.
• Double bonds in alkenes exhibit restricted rotation, leading to fixed positions of attached groups.
• Alkenes have sigma bonds and pi bonds, with the latter preventing bond rotation.

Question 11

what are common electrophiles

Answer 11

Common electrophiles include hydrogen halides and sulfuric acid.

Question 12

Reaction with Sulfuric Acid

Answer 12

Exothermic reaction at room temperature.
Forms alkyl hydrogen sulfate as the product.
Sulfuric acid acts as a catalyst in subsequent alcohol formation.
Mechanism involves electrophile attacking the double bond.

Question 13

polymers

Answer 13

Polymers are large molecules from small monomers.
Natural polymers include starch, protein, cellulose, and DNA.
Synthetic polymers like plastics and fibers are human-made.

Question 14

Addition Polymerisation of Alkenes

Answer 14

Polyethene is an example of a saturated polymer.

Polyalkenes are unreactive due to the absence of double bonds.

Question 15

Polymer Properties and Structure

Answer 15

• Polyalkenes have high melting points due to long chains.
• Polymerisation involves opening double bonds to form single

Question 16

Modifying Polymer Properties

Answer 16

Plasticisers alter polymer properties like flexibility.
Polyvinyl chloride (PVC) is an example influenced by plasticisers.
Polyethylene (polythene) has high and low-density variations.

Question 17

making an addition polymer

3 processes

Answer 17

1 - fractional distillation to separate the mixyures of conounds
2 - thermal cracking to break down into smaller alkenes
3 - addition polymerisation to produce the longer chains from the alkene polymers

Question 18

why are polymers unreactive

Answer 18

no polar bonds