Alkenes Flashcards
Describe the structure of alkenes?
- unsaturated hydrocarbons
- their general formula is CnH2n
What is a sigma bond?
The overlap of two orbitals consisting of a pair of electrons
What is a pi bond?
- the overlap of two p orbitals
Explain the bonds in the double bond of alkenes
- three out of four electrons are used in three sigma bonds to other atoms, this leaves one electron in a p oribtal
- this forms a pi bond as the p orbitals from each carbons overlap
What does the P orbital make alkenes do
The pi bond prevents rotation around the double bond
What is the shape around a double bond
There are three areas of electron density, these repel causing a bond angle of 120 degrees
Define stereoisomers
Compounds with the same structural formula but with different arrangements of atoms in space
Why do stereoisomers exist?
You cant rotate around the carbon double bond
What conditions must an alkene meet to have stereoisomers?
- a double carbon to carbon bond
- different groups attached to each carbon atom of the double bond
What is an E isomer?
- if the atoms with the highest atomic number are on opposite sides
What is a Z isomer?
Used when atoms with the highest atomic number are attached on the same side of the bond
Who made these priority rules?
Cahn - Ingold - prelog rules
What are cis - trans isomers?
Special cases of E/Z isomers where the two carbon atoms within a double bond have the same group
What are cis/trans isomers equivalent to ?
E = trans
Z = Cis
What are the three reactions that alkenes undergo?
- Combustion
- electrophilic addition
- addition polymerisation
What are the different electrophilic addition reactions alkenes undergo?
-hydrogen
- halogens
- hydrogen halides
- steam
What are the conditions and products of the hydrogenation of alkenes?
- forms an alkane
- nickel catalyst
What are the products of an addition reaction of halogens with alkenes?
- a dihaloalkane
What is the test for alkenes
- add bromine water to an unsaturated compound
- the bromine reacts with the double bond, forming a haloalkane and causing a colour change from orange to colourless
What are the products of an addition reaction of hydrogen halides and alkenes?
- a haloalkane
What are the products and conditions of steam and alkenes
- an alcohol
- requires an acid catalyst and high temps and pressures
What is a electrophile?
an electron pair acceptor
Draw the mechanism of electrophilic addition for propene and Br2
What is Markownikoffs rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen attaches itself to the carbocation with the smallest amount of hydrogens and largest amount of carbons
Why does the halogen attach to the carbocation with the most carbons
- due to carbocation stability
- the more alkyl groups a carbocation is attached to, the more stable it is
describe a primary, secondary and tertiary carbocation?
Primary - 1 other carbon
Secondary - 2
Tertiary - 3
How can we determine what will be major product?
the carbocation with the most carbons attached will be the major product
