4.1.3 Alkenes Flashcards
Describe the basic structure of alkenes
- **unsaturated **- at least one double bond
- three sigma bonds around a carbon
- fourth electron is not involved in sigma bonds - in the p orbital and the overlap of two p-orbitals forms a** pi bond **
Define Pi bond and describe what it is
a Pi bond is formed by the sideways overlap of two adjacent p-orbitals
-electron density is concentrated above and below the bonding line
-Pi bond locks the carbon atoms in position and stops them rotating
Compare the strength of sigma bonds and Pi bonds
- Pi bonds are weaker than sigma bonds
- as electron density is spread out
- reduced electrostatic attraction between nuclei and shared pair of electrons
- reducing the bond enthalpy
What shape do alkenes have?
trigonal planar
-three regions of electron density around each carbon atom (double bond is the same as the single bond)
-equal repulsion so each bond angle is 120 degrees
Define stereoisomerism
- What are the two types ?
same structural formula but a different arrangement of atoms in space
-E/Z isomerism = only occurs in compounds with a C=C double bond
-Optical isomerism = wider range of compounds
What two conditions must be satisfied for E/Z isomerism to occur?
- C=C double bond
- two DIFFERENT groups attached to each carbon atom
Describe the shape of the Z-isomer
same groups both ABOVE or BELOW the double bond (so on the same side)
Describe the shape of the E-isomer
same groups on opposite sides of the double bond (across it)
What are the rules for naming E/Z isomers when there are multiple different groups?
- look at the atoms directly bonded to the carbon atoms - the atom with the higher atomic number is given priority
- Decide which two atoms have highest priority
- If the atoms on one side are the same look for the **first point of difference **and use this
- Decide if it is a Z or E isomer
When is the Cis/trans naming system used?
one of the attached groups on each carbon atom must be the same
-Cis (same side) isomer is the Z isomer
-Trans (opposite sides) isomer is the E isomer
Are alkenes more or less reactive than alkanes?
Why?
More reactive
-Pi bond is more exposed so it is more likely to be atttacked by electrophiles and undergo electrophillic addition reactions
-The Pi bond also has a low bond enthalpy
Describe the hydrogenation reaction of alkenes
-What conditions are needed?
-What is produced?
- Needs a nickel catalyst and 150 degrees heat
- produces an **alkane **
Describe the halogenation reaction of alkenes
-What conditions are needed?
-What is produced?
- Electrophillic addition reaction
- needs gaseous chlorine or bromine at room temp
- produces dihaloalkanes
What is the test for an unsaturated hydrocarbon (alkene)?
add bromine water - should go from orange to colourless
Describe the reaction of alkenes with hydrogen halides?
- needs gaseous hydrogen halides
- produces haloalkanes
(same reaction occurs with hydrochloric or hydrobromic acid) - TWO possible products
Describe the hydration reaction of alkenes
- needs steam and a phosphoric acid catalyst
- steam adds across the double bond to form an alcohol - TWO possible products
Define electrophile
partially or fully positively charged atom or group of atoms which is** attracted to an electron rich centre** and** attracts an electron pair**
Where is the electron density located in an alkene?
double bond is a region of high electron density due to the Pi electrons - this area attracts electrophiles
Describe the mechanism for the electrophillic addition of but-2-ene with hydrogen bromide (or another POLAR molecule)
- hydrogen bromide is polar and** hydrogen has a partial positive charge** (acts as an electrophile)
- H-Br accepts a pair of electrons from the Pi bond of C=C
- bond forms between carbon and hydrogen (from hydrogen bromide)
- H-Br bond breaks - via heterolytic fission - electron pair goes to the bromine atom
- bromide ion and carbocation are formed - these react to form the addition product
Describe the mechanism for the electrophillic addition of propene with bromine (or another NON POLAR molecule)
*** Bromine molecule approaches the alkene and the high electron density of the Pi bond induces a dipole on the C=C bond
* electron pair in Pi bond is attracted to the partially positive bromine (acts as an electrophile) - double bond breaks
* bond forms between carbon atom and bromine atom
* Br-Br bond breaks via heterolytic fission - electron pair moves to Br (slight positive) end
* Br - ion reacts with carbocation to form the addition product
What is the rule when a hydrogen halide reacts with an **unsymmetrical alkene **?
hydrogen halide will attach to the carbon with the greatest number of hydrogen atoms and smallest number of carbon atoms/alkyl groups
How does the number of alkyl groups impact carbocation stability?
- What impact does the size of the alkyl group have?
- more alkyl groups means **charge is more spread out **(as each alkyl group donates and pushes electrons towards the positive charge of the carbocation) - this is the inductive effect
- this makes the** ion more stable** - THE MOST STABLE CARBOCATION IS THE MOST LIKELY TO FORM AND BE THE MAJOR PRODUCT
Size - larger alkyl groups = increased stability
So… tertiary carbocations are more stable than secondary carbocations which are more stable then primary carbocations
Define addition polymerisation
unsaturated alkene molecules are joined together to produce saturated alkane chains (no double bonds)
Give three ways of processing waste polymers
- recycling
- can be used as fuels - due to high energy content (but must be carefully controlled to prevent the release of toxic gases)
- feedstock recycling (eg they can be cracked into monomers) - can be used as raw materials to produce new polymers
Give the benefits of biodegradable and photodegradable polymers
Bio - can be broken down by microorganisms so leave no toxic residue
Photo - bonds absorb light which causes them to start to degrade
