3.3.4 Alkenes

Question 1

Alkenes are _________/_________

Answer 1

Alkenes are unsaturated hydrocarbons

Question 2

General Formula For Alkenes:

Answer 2

General Formula For Alkenes: CnH2n

Question 3

Alkenes contain a _____/_____ in their structure

Answer 3

Alkenes contain a double bond in their structure

Question 4

The arrangement of bonds around C=C in alkenes is _______ and has the bond angle _____

Answer 4

The arrangement of bonds around C=C in alkenes is planar and has the bond angle 120^o

Question 5

When do numbers need to be added to the name in alkenes?

Answer 5

when positional isomers could occur

Question 6

Describe the structure of the C=C covalent bond

Answer 6

one sigma bond
one pi bond

GOOGLE DOC

Question 7

Why are alkenes so vulnerable to attack?

Answer 7

Pi bonds are exposed
High electron density
Electrophiles attack

Question 8

What is an electrophile?

Answer 8

an electron pair acceptor

Question 9

Describe the reaction of bromine with alkenes

Change in FG:
Reagent:
Conditions:
Mechanism:
Type of reagent:

Answer 9

Change in FG: Alkene -> dihalogenoalkane
Reagent: Bromine
Conditions:Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile Brδ+

Question 10

Describe the reaction of hydrogen halides with Alkenes
Change in FG:
Reagent:
Conditions:
Mechanism:
Type of reagent:

Answer 10

Change in FG: alkene -> halogenoalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile (H^D+)

Question 11

Describe the reaction of sulfuric acid with alkenes (2 stages)
Change in FG:
Reagent:
Conditions:
Mechanism:
Type of reagent / reaction :

Answer 11

STAGE 1:

Change in FG: alkene -> alkyl hydrogen sulfate
Reagent: concentrated H2SO4
Conditions: room temperature
Mechanism: Electrophilic addition
Type of reagent: Electrophile, H2SO4

STAGE 2:

Change in FG: alkyl hydrogensulfate -> alcohol
Reagent: water
Conditions: warm mixture
Type of Reaction: hydrolysis

Question 12

Draw the mechanism for the electrophilic addition of bromine with ethene

Answer 12

ON GOOGLE DOC

Question 13

Describe qualitiatively the mechanism for electrophilic addition

Answer 13

Electrophile is attracted to C=C due to electron density
Electrophiles are +ve and accept a pair of electrons from the double bond
A positive ion is formed ( carbocation )
A negatively charged ion forms a bond with the carbocation

Question 14

What is Markownikoff’s rule?

Answer 14

In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it

Question 15

How do addition reactions change if the alkene is unsymmetrical?

Answer 15

It can lead to 2 isomeric products

Question 16

What is meant by the major product?

Answer 16

formed in a majority
formed via the more stable carbocation intermediate

Question 17

Describe the order of stability for carbocations

Answer 17

tertiary > secondary > primary

(count the number of methyl groups on the carbon)
(3 = tertiary, 2 = secondary, 1 = primary)

Question 18

Why are tertiary carbocations more stable than secondary carbocations?

Answer 18

Alkyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion, making it more stable.

Question 19

How do we draw H2SO4 in mechanisms?

Answer 19

ON GOOGLE DOC

Question 20

What is the role of H2SO4 in the reaction of sulfuric acid with alkenes?

Answer 20

It acts as a catalyst as it is generated

Question 21

Write the equation for stage 1 of the reaction of ethene with sulfuric acid

Answer 21

CH2=CH2 + H2SO4 -> CH3CH2OSO2OH

Question 22

Write the equation for stage 2 of the reaction of ethene with sulfuric acid

Answer 22

CH3CH2OSO2OH + H2O -> CH3CH2OH + H2SO4

Question 23

Draw the mechanism for the reaction of sulfuric acid with propene

Answer 23

ON GOOGLE DOC

Question 24

State the type of structural isomerism shown by but-1-ene and but-2-ene: MS [1]

Answer 24

Positional isomerism

Question 25

State the type of polymerisation that alkenes undergo: MS [1]

Answer 25

Addition polymerisation

Question 26

How do we test for alkenes?

Answer 26

Add orange bromine water
Shake
The bromine is added across the C=C bond by electrophilic addition
This turns the water orange to colourless
A dibromoalkane is formed

Question 27

Why are polyalkenes unreactive?

Answer 27

Strong C-C and C-H bonds

Question 28

Draw a repeating unit of polypropene

Answer 28

ON GOOGLE DOC

Question 29

How are addition polymers made?

Answer 29

Made from alkenes
The double bond opens up and the monomers join together
The polyalkene is a saturated molecule
The main carbon chain is non-polar, making polyalkenes unreactive

Question 30

How do longer chain polyalkenes compare to short chains?

Answer 30

Long straight chains will be strong and rigid due to the strong intermolecular van der Waals forces

Short branched chains will be weaker and more flexible

Question 31

Why are plasticisers used on polymers?

Answer 31

To make the polymer more flexible

Question 32

How do plasticiser molecules work?

Answer 32

The plasticiser molecules fit in the gaps between the polymer chains, pushing them apart

Intermolecular forces between chains are reduced in strength, and so the polymer gets easier to bend

Question 33

When shouldn’t you add an n to a repeating unit?

Answer 33

if asked to draw one repeating unit

Question 34

Describe the industrial hydration of alkenes

essential conditions
what is formed?
equation for ethene + water