3.3.4 Alkenes Flashcards
Alkenes are _________/_________
Alkenes are unsaturated hydrocarbons
General Formula For Alkenes:
General Formula For Alkenes: CnH2n
Alkenes contain a _____/_____ in their structure
Alkenes contain a double bond in their structure
The arrangement of bonds around C=C in alkenes is _______ and has the bond angle _____
The arrangement of bonds around C=C in alkenes is planar and has the bond angle 120^o
When do numbers need to be added to the name in alkenes?
when positional isomers could occur
Describe the structure of the C=C covalent bond
one sigma bond
one pi bond
GOOGLE DOC
Why are alkenes so vulnerable to attack?
Pi bonds are exposed
High electron density
Electrophiles attack
What is an electrophile?
an electron pair acceptor
Describe the reaction of bromine with alkenes
Change in FG:
Reagent:
Conditions:
Mechanism:
Type of reagent:
Change in FG: Alkene -> dihalogenoalkane
Reagent: Bromine
Conditions:Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile Brδ+
Describe the reaction of hydrogen halides with Alkenes
Change in FG:
Reagent:
Conditions:
Mechanism:
Type of reagent:
Change in FG: alkene -> halogenoalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile (H^D+)
Describe the reaction of sulfuric acid with alkenes (2 stages)
Change in FG:
Reagent:
Conditions:
Mechanism:
Type of reagent / reaction :
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
Draw the mechanism for the electrophilic addition of bromine with ethene
ON GOOGLE DOC
Describe qualitiatively the mechanism for electrophilic addition
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
What is Markownikoff’s rule?
In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it
How do addition reactions change if the alkene is unsymmetrical?
It can lead to 2 isomeric products
What is meant by the major product?
formed in a majority
formed via the more stable carbocation intermediate
Describe the order of stability for carbocations
tertiary > secondary > primary
(count the number of methyl groups on the carbon)
(3 = tertiary, 2 = secondary, 1 = primary)
Why are tertiary carbocations more stable than secondary carbocations?
Alkyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion, making it more stable.
How do we draw H2SO4 in mechanisms?
ON GOOGLE DOC
What is the role of H2SO4 in the reaction of sulfuric acid with alkenes?
It acts as a catalyst as it is generated
Write the equation for stage 1 of the reaction of ethene with sulfuric acid
CH2=CH2 + H2SO4 -> CH3CH2OSO2OH
Write the equation for stage 2 of the reaction of ethene with sulfuric acid
CH3CH2OSO2OH + H2O -> CH3CH2OH + H2SO4
Draw the mechanism for the reaction of sulfuric acid with propene
ON GOOGLE DOC
State the type of structural isomerism shown by but-1-ene and but-2-ene: MS [1]
Positional isomerism
State the type of polymerisation that alkenes undergo: MS [1]
Addition polymerisation
How do we test for alkenes?
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
Why are polyalkenes unreactive?
Strong C-C and C-H bonds
Draw a repeating unit of polypropene
ON GOOGLE DOC
How are addition polymers made?
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
How do longer chain polyalkenes compare to short chains?
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
Why are plasticisers used on polymers?
To make the polymer more flexible
How do plasticiser molecules work?
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
When shouldn’t you add an n to a repeating unit?
if asked to draw one repeating unit
Describe the industrial hydration of alkenes
essential conditions
what is formed?
equation for ethene + water
