3.4 Alkenes Flashcards
What are some properties about alkenes?
Unsaturated hydrocarbons Homologous series Functional group of a double bond General formula CnH2n (only applies to one double bond) Insoluble in the water
Are alkenes more or less reactive than alkanes? Why?
More
They have a double bond
Why are alkenes insoluble in water?
Water uses hydrogen bonding and alkenes use van der vaals
Therefore they have incompatible intermolecular forces
What is the trend in boiling points relating to carbon chain length? Why?
As the carbon chain length increases the boiling point increases
It has a larger surface area therefore stronger intermolecular forces are which are harder to overcome
What does the bonding in alkenes include?
Sigma bond
Pi bond
What is stereoisomerism?
When molecules have the same molecular and structural formulae but a different arrangement of the atoms in space
Why does stereoisomerism occur in alkenes?
Because the double bond in alkenes does not allow rotation
What type of isomerism is stereoisomerism?
Geometrical isomerism or E-Z isomerism
In E-Z isomerism how do you determine which molecule is E or Z?
Z - the one with the larger atomic mass on one side
E - the molecule with a smaller difference in masses on each side
What are the reactions of alkenes?
Combustion
Electrophilic addition
Electrophilic addition reaction with concentrated sulphuric acid
Reaction with water
What are some points about combustion of alkenes?
They don’t burn very well often incomplete forming carbon
Alkenes are made by cracking making them valuable and expensive for plastics
In electrophilic reactions what is the reagent?
Electrophile
What is an electrophile?
Attracted to negative charge (electrons)
Therefore the less EN charged atom in the molecule (the positive delta one) acts as the electrophile
What are some examples of electrophiles?
HCl HBr HI Cl2 Br2 I2 H2SO4 H2O H2
What is an addition reaction?
Where 2 or more molecules join together by breaking the π bond to form a single product
What is the first part of the mechanism for electrophilic addition reactions?
Put the electrophile closest to the alkene in the vertical molecule
An arrow from the double bond to the electrophile
An arrow from the middle of the bond of the electrophile to the more EN atom attached (the delta negative one)
What is the second part of the mechanism for electrophilic addition reactions?
A temporary intermediate is formed called a carbocation
An arrow goes from the free radical of the negatively charged atom to the carbon on the carbocation
The carbon with only 3 bonds has a positive charge on it
What is the third part of the mechanism for electrophilic addition reactions?
Draw the (probably halogenoalkane) by putting the free radical in the gap on the carbon
What takes place in the first part of electrophilic addition reactions?
Heterolytic fission
Meaning an atom takes both electrons as opposed to one to each
What is the carbocation scale?
Tertiary - secondary - primary
Explain the carbocations?
Tertiary - 3 carbons attached to the carbon with only 3 bonds
Secondary - 2 carbons attached to the carbon with only 3 bonds
Primary - 1 carbon attached to the carbon with only 3 bonds
What is important about the stability of carbocations?
When the alkene double bond breaks it has two possible outcomes
Therefore you get more of the product with a more stable intermediate/carbocation
What is the stability of the carbocations?
The inductive effect of the alyl groups as they cancel out the positive charge
The electrons are being pulled towards the positive charge
How can Br2 Cl2 etc… be used as electrophiles in electrophilic addition reactions?
Because of fluctuating electron density
This causes a temporary induced dipole giving rise to Van der Waal forces
The delta postive end will be attracted to the double bond making a permanent dipole as the double bonds electrons are available
What is the display formula of cH2SO4 (Sulphuric acid)?
H +
|
O -
|
O=S=O
|
O
|
HWhat is different about electrophilic addition reaction with sulphuric acid?
Nothing it’s the exact same mechanism
What can be created after electrophilic addition of sulphuric acid?
Alcohols
Where H2O is added
And H2SO4 (sulphuric acid) or H3PO4 (phosphoric acid) can act as a catalyst
What are the conditions for making alcohols from alkenes?
High heat
High pressure
What is the test for alkenes?
Add bromine water
Because an alkene is unsaturated it will discolour the bromine water
What is a polymer?
Many repeating subunits joining to form a larger molecule
What is polymerisation?
Joining many monomers together into a long chain
What are some problems with plastic?
Non-biodegradable
Addition polymers are un reactive and can’t be broken down by bacteria etc
Therefore landfill sites are becoming full
What type of polymers do we know?
Addition polymers
How do you show polymerisation?
Draw the monomer with n on the left
Draw an arrow with pressure above it and catalyst below it
Draw the single bond monomer inside square brackets with the bonds going out the square brackets
And a small n on the outside of the brackets
How do we name polymers?
Add poly in-front of the monomer
What is high density polythene?
Straight chains = chains are closer together leading to more effective Van der Waals forces
Higher surface area of contact
Larger SA:V ratio
Higher melting point
Conditions for forming high density polymers?
Low temperature and pressure
Using a Ziegler-Natta catalyst
What is low density polythene?
Branched chains = chains further apart leading to weaker Van der Waals forces
Conditions for forming low density polymers?
High temperature and pressure
What is cross linking?
Where polymer chains are joined by covalent bonds and not intermolecular forces
Therefore the polymer molecules can’t slide over each other easily and become rigid
What is a thermosoftening polymer?
Consists of individual tangled polymer chains
When heated intermolecular forces become weak and it softens
It can harden when cool
Therefore it can be remolded
What are plasticers?
Additives that change the properties of plastics
Small molecules eg esters that fit between the polymer chains allowing them to slide over each other easily (behaving like molecular lubricants)
This makes plastic more flexible
How can we recycle plastics?
Mechanical recycling - sort, wash and grind into pellets then melt to be remoulded
But the quality of the polymer reduces
Feedstock recycling - heat to produce new monomers to make new polymers
What is a thermosetting polymer?
Consists of polymer chains with cross links - covalent bonds between chains
When heated it doesn’t melt or soften as the bonds are strong and rigid
You can’t recycle it
