Alkenes Flashcards
Why are alkenes more reactive than alkanes?
Low bond enthalpy of pi bonds as electron density is concentrated above and below the plane of the molecule so it’s further away from the nuclei of the bonded atoms so attraction is weaker and easier to overcome.
What are the 4 alkenes complete addition reactions with?
Hydrogen
Halogens
Hydrogen halides
Water (steam)
What happens when an addition reaction takes place in an alkene?
Pi bond is broken and a small molecule is added on, forming an alkane.
What are the conditions for the reaction between an alkene and hydrogen? H2
Nickel and high temp and pressure
What are the conditions for the reaction between an alkene and a halogen? X2
RTP
What is the addition reaction between bromine and an alkene used for?
Detecting the presence of a double bond to see if hydrocarbon is saturated or not. eg. Hexane vs cyclohexene.
Orange -> colourless if C=C present originally
No colour change if no C=C present
What are the conditions for the reaction between an alkene and hydrogen halides? HX
Hear under reflux
What are the conditions for the reaction between an alkene and water? (Steam)
Phosphoric acid catalyst catalyst (H3PO4) High temp (so H2O is steam)
Alcohol formed
A double bond consists of
1 sigma bond and 1 pi bond
What is a sigma bond?
A covalent bond formed by the direct overlap of two orbitals. Electron density concentrated between two nuclei of bonded atoms. Free rotation around every atom.
What is a pi bond?
A covalent bond formed by the sideways overlap of two p orbitals. Electron density is concentrated above and below the plane of the molecule. Rotation restricted.
Why are sigma bonds stronger than pi bonds?
In a sigma bond electron density is concentrated between the nuclei of the bonded atoms but in a pi bond the electron density is above and below the plane of the molecule so there is less attraction to the nuclei
What is an electrophile?
A lone pair acceptor
What is a nucleophile?
A lone pair donor
Electrophiles are normally
Positive ion or delta +
