Alkenes Flashcards
Alkenes
A homologous series of hydrocarbon which contains a double bond
What type of hydrocarbon are alkenes?
Unsaturated because they contain a double bond thus don’t have the maximum number of hydrogen atoms bonded to it
Alkenes functional group
C=C
General formula of alkenes
C2H2n
Naming alkenes
Number of carbon atoms in longest chain
- number(s) of the carbon that the double bond is on
-ene (if 2 double bonds is diene etc)
How are double bonds formed?
When a sigma covalent bond forms
So a pi covalent bond can form
How are sigma bonds formed in carbon?
When the 2s orbitals in 2 carbon atoms which each contain a single electron overlap in a straight line
Where is the electron density in the sigma bond?
Between the 2 nuclei due to the overlap of orbitals side on in a straight line, on the molecular axis
Why is a sigma bond formed when 2 orbitals overlap?
Because the 2 electrons In an orbital are shared and give the highest possible electron density between 2 atoms
Thus the nucleus is attracted electrostatically to this
How do pi bonds form in a carbon atom?
After a sigma bond is formed, the 2p orbitals in each carbon atom, which only has 1 electron, overlaps side ways
Where is the electron density in a pi bond?
Above the molecular axis and below the molecular axis
Therefore spread out above and below the nuclei
Electrostatic attraction in a pi bond
Because the electron density is spread out above and below the nucleus, it is weaker than in sigma bonds where the electron density is in between
Why are sigma bonds stronger than pi bonds?
Because in sigma bonds the high electron density is between 2 nuclei thus stronger electrostatic attraction
Whereas in pi bonds the electron density is spread above and below the nucleus
Double bond strength vs single bond strength
Even though pi bonds are weaker than sigma bonds, having combined strength of both means they are stronger than single bonds
What reagent are alkenes considered?
Nucleophiles because they contain a double bond where there is high electron density thus an area of negative charge
What reagents do alkenes usually react with?
Electrophiles (electron-pair acceptors) which usually are attracted to high electron density areas
Reaction of alkenes + hydrogen halogens or hydrogen halides
Electrophilic addition because an electrophile is being added to the Alkene without substituting any atoms to make a larger molecule
Alkene + hydrogen reaction name
Hydrogenation
What does Alkene + hydrogen form
An alkane by adding 2 hydrogen and breaking the double bond
Conditions for hydrogenation of alkenes
Nickel catalyst
150°c
What happens to the melting point after hydrogenation of Alkene?
You remove the double bond by adding hydrogen to a carbon
Which raises the melting point
Therefore alkane is a solid at room temperature
Halogen + Alkene forms?
Dihalogenoalkane
So opening up the double bond to attach 2 halogen atoms = DI
How are halogen atoms added to the Alkene when undergoing electrophilic addition?
Adds across the double bond when it breaks
So ensure they are added to different carbons and not the same
Step 1 of electrophilic addition of Alkene and halogen
The high electron density (negative) area around double bond of Alkene repels electrons in the halogen molecule. Induces a dipole across halogen molecule where closest halogen atom is positively charged
Step 2 of electrophilic addition of Alkene with halogen
There is heterolytic fission of the halogen molecule
Where the 2 electrons in this bond are given to furthest halogen atom
And so the double bond in Alkene breaks because the 2 electrons in pi bond form bond with closest halogen atom
Arrow mechanisms in the heterolytic fission of halogen molecule
An arrow from the bond between halogens to the furthest halogen atom to show how 2 electrons in the bond go there
Arrow from double bond in Alkene to closest halogen to show how these electrons are shared thus bonded
Step 3 of electrophilic addition of halogen with Alkene
We are left with a halogen atom which has 2 electrons from the heterolytic fission thus has lone pair of electrons
And also a positively charged carbocation (the carbon on Alkene which 1sthalogen not bonded to)
How are charges shown in the 2nd halogen atom and carbocation in electrophilic addition
Lone pair of electrons with a negative symbol on the 2nd halogen atom
A + sign on the empty carbocation that isn’t d+
Step 4 of electrophilic addition of halogen atom to Alkene
The lone pair of electrons on the halogen atom is attracted to carbocation so they form a bond
Thus the double bond is broken and a dihalogenoalkane has been formed
Test for alkenes
Shake Alkene with bromine water (brown)
Which will form dibromoalkane which is colourless via electrophilic addition
This bromine water tests for C=C bonds
Colour change in Alkene + bromine water test
Orange ——> colourless
Formation of alcohols by reacting water vapour and alkenes?
Steam hydration
Forms alcohols
Conditions for steam hydration of alkenes
300°c
60atm-70atm
Phosphoric (V) acid catalyst
H3PO4
Formation of diols from reacting alkenes?
By oxidising alkenes using acidified potassium manganite (VII)
What is acidified potassium manganite (VII)?
The oxidising agent used to form diols from alkenes
Has symbol H+/MnO₄⁻
How do you write the oxidation of alkenes by acidified potassium manganite (VII)?
The Alkene
Then an arrow with H+/MnO₄⁻ over it
Then forms a diol
What colour is H+/MnO₄⁻?
Purple
What colour does the solution with Alkene reacted with H+/MnO₄⁻ form (colour of diol)
Purple to
COLOURLESS
Reaction of hydrogen halides and alkenes forms? And why
Forms halogenoalkanes
Because the double bond in Alkene breaks and a halogen atom and hydrogen atom is added across the double bond
But what is important about the reaction between hydrogen halides and alkenes?
There are 2 possible products:
Minor
Major
If the Alkene is unsymmetrical eg double bond not in middle of carbon chain
What determines if the product formed in hydrogen halide + Alkene is major or minor?
How stable the carbocation formed is
Because the carbocation formed (carbon the hydrogen DOESNT BOND TO) can be stable or not
Types of carbocations formed in order of least stable
Primary carbocation
Secondary carbocation
Tertiary carbocation
What determines the type of carbocation thus stability?
How many alkyl groups each carbocation is bonded to
Primary carbocation
Bonded to 1 alkyl group
Least stable
Secondary carbocation
Bonded to 2 alkyl groups
Medium stability
Tertiary carbocation
Bonded to 3 alkyl groups
Most stable
Why does the carbocation being bonded to the most alkyl groups make it most stable?
Because the alkyl groups can feed electrons to positively charged carbocation
Step 1 of reaction between alkenes and hydrogen halides
High electron density area in alkene’s double bond attracts the slightly positive charged hydrogen (because hydrogen halides are polar molecules)
Step 2 of reaction between alkenes and hydrogen halides
Heterolytic fission of hydrogen halide where 2 electrons in bond are given to halogen and hydrogen joins to a carbon because the double bond breaks up
How is the addition of hydrogen from hydrogen halide to an Alkene shown in mechanism?
Arrow from bond in hydrogen halide to the halogen molecule to show heterolytic fission
Arrow from double bond to d+ hydrogen to show how pi bond electrons form a bond with H d+
Which carbon will hydrogen bond to in major product?
The hydrogen will bond to the carbon with most hydrogen bonded to it so the carbocation formed will be the one with most alkyl groups attached to it = stable
Which carbon will hydrogen bond to in the minor product?
The carbon with the least hydrogen bonded to it already so has more alkyl groups
Thus forms carbocation with least number of alkyl groups to it = less stable
Step 3 of reaction between hydrogen halide and Alkene
We now have a halogen atom with a lone pair of electrons from the heterolytic fission
And a carbocation formed
These are attracted to each other thus bond to form a halogenoalkane
Which carbon does the halogen bond to in the major product?
The carbon with the most alkyl groups attached to it because this one is the carbocation
Thus the more stable carbocation
Which carbon does the halogen bond to in the minor product?
The carbon with the least alkyl groups attached to it because this is a less stable carbocation
Markowikoff’s rule
The major product from the addition of a hydrogen halide to an unsymmetrical Alkene is the one where hydrogen bonds to the carbon with the most hydrogen already bonded to it
