ALKENES Flashcards
When does cis/trans isomerism occur
-When there is resricted rotation around the C=C bond
-There are two different atoms or molecules on each carbon of the carbons in the double bond. It is a form of stereoisomerism in which the groups have the same order of attachment but are ordered differently in space
E/Z isomerism
-Split the molecule in half, and the atoms directly attached to the carbons are ranked by increasing atomic number
-If the priorities on bth sides are on the same side, it exhibits E isomerism, and if they are opposite, Z isomerism
-If a double bond is present, count the atom twice (treat a double oxygen bond as two single bonds)
Formation of alkenes
Thermal cracking- breaking up large hydrocarbon molecules into smaller and more useful bits, achieved by using high pressures and temperatures without a catalyst, but cracking with a catalyst is known as catalytic cracking
DEHYDRATION OF ALCOHOLS
Reagent- H2SO4
Conditions-heat 170
Product- alkene + water
Reaction type- dehydration
REACTION OF HALOGENOALKANES WITH KOH IN ETHANOL
Reagent- KOH/NaOH in dry ethanol
Conditions- heat under reflux in ethanol
Product- alkene + water + KCl/NaCl
Reaction type- Elimination
Alkenes are more reactive than alkanes because…
The C=C bond is a very electron rich area and therefpre is attractive to electrophiles and the wesker pi bond is more susceptible to attack
Alkenes undergo addition reactions in which the pi bond breaks to release electrons to form bonds with reactants
REACTION WITH HYDROGEN
Reagent- hydrogen
Conditions- FINELY divided nickel catalyst
Product- alkane
Reaction type- electrophilic addition
TEST FOR ALKENES
Reagent- halogen/ bromine
Conditions- room temperature
Product- dihalogenoalkane
Reaction type- Electrophilic addition
(SOLUTION GOES FROM ORANGE TO COLOURLESS IF ALKENE IS PRESENT)
REACTION WITH BROMINE WATER
The unsaturated C=C will react with red-brown Br2 solution in the dark to form the saturated dibromoalkane
REACTION WITH HYDROGEN BROMIDE
Reagent- hydrogen halide in gas phase
Conditions- room temperature
Product- halogenoalkane
Reaction type- electrophilic addition
Predict major product using Markovnikoff’s rule
For assymetric alkenes, addition of hyfrogen bromide will result in hydrogen always adding to the carbon with the most hydrogens directly attached to it already
Explanation of major product
Carbocations are unstable species, but they can be made more stable if the charge on the carbocation is reduced. Alkyl groups are more electron pushing than hydrigen and so push electrons towards the C+ atom, reducing it’s charge
ADDITION POLYMERISATION
Conditions- heat under ressure 1000atm in the presence of a smallamount of oxygen
Why do plastics not have a sharp melting point
They melt over a range of temperatures because within any plastic the chain legnths of the polymers vary
What are the uses of polyethene
-Plastic bags
-Clingfilm
-Milk bottles
