Alkenes Flashcards
(15 cards)
What are alkenes and their general formula?
Unsaturated hydrocarbons
More reactive than alkanes.
CnH2n
Cycloalkenes CnH2n-2
What is the bonding in alkenes? Whats the reactivity of alkenes?
C=C made up of a sigma bond and a pi bonds. The sigma bond is the strongest type of covalent bond. 2 different bonds of different strengths. When a alkene reacts, the pi bonds breaks whereas the sigma bond remains in place as its stronger than the pi bonds breaks whereas so is harder to break .
( Learn Diagram)
More reactive than alkanes due to presence of pi bond, pi bond has a lower bond enthalpy than sigma bond and when they react the pi bond breaks and sigma bond remains.
What is the shape of alkenes?
Carbons is double bond : 3 bonding pairs, trigonal planar, 120’
Other Carbons: 4 bonding pairs, tetrahedral , 109.5’
What is strreoisomerism?
Compounds with the same structural formula but with a different arrangement of atoms in space.
What must E/Z isomerism have ?
- A carbon-carbon double bond that can’t rotate
- each carbon in the double bond must have 2 different groups attached to it.
(E highest priority/highest atomic number on opposite sides. )
(Z highest priority/ highest atomic number on zee same side.)
What must cis-trans isomerism have?
- carbon to carbon double bond
- each carbon in the double bond must be attached to 2 different atoms or group of atoms.
- two of the substituent groups must be the same.
(Z isomers = cis, same groups on zee same side)
(E isomers = trans, same groups on opposite sides)
What the reaction of alkenes with hydrogen (hydrogenation)?
Alkene + H2 —> saturated alkane.
Conditions: Ni catalyst, 150’
Whats the reaction where alkenes react with halogens (halogenation)?
Alkene + Halogen —> dihaloalkane
At room temp.
How do you test for the presence of s double bond (test for unsaturation)?
Add bromine change orange to colourless. If remains orange its an alkane.
What is the reaction with hydrogen halides?
Alkene + hydrogen halide —> haloalkane
Whats the reaction with steam (hydration of alkene)?
Alkene + H2O (g) —> Alcohol
Conditions: high temp, high pressure and H3PO4 catalyst.
What are electrophilic addition reactions of alkenes?
Electrophile is a electron pair acceptor.
Alkenes follow an electrophilic addition mechanism and use curly arrows to show it.
Curly arrows represent the movement of a pair of electrons to either break or make a bond.
The bond breaks by Heterolytic fissions which is the breaking of a covalent bond when 1 of the bonding atoms takes both electrons from the bonded pair.
What is the rule for unsymmetrical alkenes (Markownikoffs rule) ?
When alkene isn’t symmetrical (C=C isn’t in middle) and a hydrogen halide or steam is added across the double bond a mixture containing different structural isomers formed.
The more stable carbocation is more likely to form so is the major product. Tertiary is the most stable then secondary then primary.
E.g. … formed from a secondary carbocation which is more stable than the primary carbocation that … has been formed from.
What is addition polymerisation?
Many repeated monomers (all one type) added together to make a polymer.
Alkenes undergo addition polymerisation to produce saturated chains with no double bond.
Polymerisation carried out at high temps/pressure using catalysts
E.g. poly(ethene) is many ethene monomers.
Use an n to represent number of repeated units
What are the problems either addition polymerisation and how are they overcome?
Addition polymers are very stable and don’t break down naturally in environment/ are non biodegradable.
- 1. Sorting and recycling: reuse plastic and recycle
- 2. Combustion for energy production: burning polymers under controlled conditions produces heat energy thats harnessed to make electricity. However need to be under controlled conditions as can contain halogens that produce toxic gases when burnt.
- 3. Used as an organic feedstock: breaks down polymers into new monomers which can be converted into more useful materials or as fuel for oil refineries.
- 4. Using biodegradable plastics: made of natural materials and break down naturally
