Electrophilic addition Flashcards
Why are alkenes far more reactive than alkanes?
due to the presence of the C=C bond- when it breaks the alkenes can undergo addition reactions to form saturated products
Why can an electrophilic addition reaction take place?
-as the C=C bond is an area of high electron density, it can attack electrophiles
-electrophiles usually have an area of partial positive charge, and can accept a pair of electrons
What is step one in an electrophilic addition reaction?
-a pair of electrons from the C=C bond forms a covalent bond with the slightly positive atom
-a negative ion forms
-the addition of this atom also means the other carbon is now a carbocation
What is step two in an electrophilic addition reaction?
the lone pair of electrons on the negative ion formed form a covalent bond with the carbocation
What are the three main electrophilic addition reactions?
-halogenoalkane
-dihalogenoalkane
-adding cold concentrated H-OSO3H to form alkyl sulphates
What happens when you heat alkenes with water in the presence of concentrated sulphuric acid?
an alcohol is produced
Is ethanol produced via this reaction with H2SO4?
not industrially, in industry it is produced either by fermentation of sugars or by the direct hydration of ethene
Describe the two steps of the reaction of alkenes with concentrated sulphuric acid
-the concentrated sulphuric acid reacts with the alkene in an electrophilic addition reaction, forming an alkyl hydrogensulphate
-cold water is added, then the product is warmed, when it hydrolyses to form an alcohol
What does the sulphuric acid act as?
a catalyst
In the reaction involving dihydrogenoalkanes, why is there an area of slightly positive and negative charges on the halogen molecule?
the halogen molecules are non-polar, but the high electron density of the C=C bond induces a temporary dipole in the diatomic molecule
What is the test for alkenes?
bromine water will go from orange/brown to colourless in the presence of alkenes
What can the bromine water test be used for?
working out the degree of unsaturation in margarines and butter
What other product can be formed when bromine water is added?
could have a Br attacking the carbocation to form a dibromoalkane OR an OH due to water molecules being able to attack it as well
What happens if an alkene is unsymmetrical?
more than one product can form (a major and a minor product)
What is the major product always formed from?
the most stable carbocation- in order of stability is tertiary-secondary-primary
LOOK FOR THE CARBON WITH THE MOST HYDROGENS ATTACHED
