6C: Alkenes Flashcards
What is the general formula of alkenes?
C(n)H(2n)
Why are alkenes unsaturated?
Not all bonds are with the max number of hydrogens
Double CC bond
How do covalent bonds form?
Atomic orbitals from different atoms overlap
Shares electrons
Bond forms from electrostatic attraction from atoms to the bonding electrons
What are single covalent bonds?
σ - sigma bonds
Overlap of two orbitals, high electron density
Strong electrostatic attraction, therefore strong bond
What is the double bond made of in hydrocarbons?
π - pi bond & σ - sigma bonds
π bond forms when two p orbitals overlap sideways
Electrons spread out, weaker electrostatic attraction
Therefore weaker bond
What is the effect on rotation by π bonds?
π electrons must overlap
Cannot rotate as easily
Leads to E-Z isomerism
What kind of isomerism is E/Z isomerism?
Geometric - rotation is prevented about a double bond
What is a cis isomer?
Same groups are on the same side of the CC double bond
Cis = Z-isomer
What is a trans isomer?
Same groups are on opposite sides of the CC double bond
Trans = E-isomer
What are the rules for E/Z isomerism?
The atom with the highest atomic number takes priority
If two atoms have the same atomic number and attached to same carbon, next bonded atom is taken into account
What 4 types of addition reactions do alkenes undergo?
Hydrogenation
Halogenation
Hydrogen halides
Addition polymerisation
What are the conditions for hydrogenation?
Nickle catalyst
150 C
What is the process of hydrogenation?
Alkene reacts with hydrogen gas to form alkane
Making margarine is an example
How does halogenation occur with alkenes?
Electrophilic addition
What occurs in electrophilic addition?
Heterolytic fission of halide molecule, electrons go to one atom and the other bonds to C atom
Causes positively charged carbocation
Bond forms between free halide ion and carbocation
Dihalogenoalkanes formed
How do alkenes react with bromine water?
Brown -> colourless if double CC bond
Forms a bromoalcohol and hydrogen bromide, e.g ethene forms 2-bromoethanol and hydrogen bromide
What is steam hydration?
Alkenes hydrated by steam to form an alcohol
300 C & 60-70 atm with phosphoric (V) acid catalyst
How do alkenes react with hydrogen halides?
Electrophillic addition
Forms halogenoalkane + hydrogen gas
What is markovnikoff’s rule?
When a compound HX is added to an unsymmetrical alkene, the halogen binds to the carbon with the most hydrogens bonded to it
What is a carbocation?
A positively charged carbon atom
What is the stability of a carbocation determined by?
The more carbons it is bonded to, the more stable the carbocation is
What is the process of the oxidation of alkenes?
Acidified potassium manganate (VII) added to alkene
Purple -> colourless
Produces alkene with two -OH alcohol groups (diol)
What is the reaction between an alkene and sulphuric acid and water?
Forms HSO4 group on alkane
Then add water to form an alcohol group
What is addition polymerisation?
Double bond in alkenes open up and join together to make polymers
How do you convert monomers into polymers?
Change double CC bond to single bond
Add single bonds to each end of the carbon, which go through brackets
How can waste plastic polymers be disposed?
Buried in landfill
Recycling - feedstock for cracking
Incineration- heat used for energy
How can polymers be made to be sustainable?
Use reactants that are as safe and environmentally friendly as possible Few other materials should be used Renewable raw materials should be used Energy use should be kept to a minimum Limit waste products
How are toxic gases removed when plastics are burned?
PVC produces HCl which is toxic
They must be neutralised in order to remove toxic gas
How do biodegradable polymers decompose?
Made of renewable raw materials
Produces CO2 and if plant- based it is carbon neutral
Naturally decompose
