Alkenes Flashcards
How do you show a double bond when naming an alkene
put the numbers of wherever a double bond occurs, in between the longest carbon chain, eg. “But”. And “ene”
Explain in terms of electron orbital overlap, how C-C single bonds differ from C=C double bonds?
Single bonds form from the end-on overlap of atomic orbitals, producing a sigma bond
The second bond in a double bond is produced by the sideways overlap of 2 p atomic orbitals, to make a pi bond
Single/sigma bonds are free to rotate, pi bonds cannot rotate, as this would break the overlap of the p orbitals
Where do the electrons exist in pi bonding?
Above and bellow the plane of the atoms
What’s stronger the sigma bond or pi bond?
sigma bond, because in a pi bond the electron density is bellow and above the plane of the molecule, whilst the electron density is directly between the 2 carbons in the sigma bond
Will the pi bond or the sigma bond break in a chemical reaction?
pi bond, it’s weaker
What type of bonds does a carbon to carbon triple bond have?
1 sigma bond
2 pi bonds
In what cases is the pi bond stronger than the sigma bond?
Carbon-Oxygen
Carbon-Nitrogen
Define stereoisomerism?
Stereoisomers are molecules that have the same structural formula, but have a different arrangement of atoms in space
How can E/Zisomerism occur?
There is a restricted rotation around a double bond
Each of the atoms of the double bond has 2 different groups attached
How to name with E/Z isomerism?
Functional groups on opposite sides= (E)
Functional groups on same side= (Z)
But these in front of the molecules names
To decide which is the functional group on each side calculate the Mr directly bonded around the carbon (might be a different element eg a halogen) including the carbon. The one with the highest is the functional group
When can Cis or Trans occur?
Is a special case of E/Z isomerism in which 1 of the groups attached to each carbon of the double bond is the same
If these groups on the same side eg higher end, = cis
If these are on opposite groups eg. higher and lower = trans
Explain the addition reaction of ethene with H2?
Requires Ni catalyst, and 150 degrees
Ethane is formed
Explain the addition reaction of ethene with a halogen eg. Cl2?
Room temp and pressure
1,2-dichloroethane formed
Explain the addition reaction of ethene with a halogen bonded to a hydrogen eg. HBR?
Room temp and pressure
Bromoethane formed
Explain the addition reaction of ethene with H2O?
Water is steam
Acid catalyst eg. phosphoric acid
Ethanol formed. OH and H bond to ethane, double bond broken
Colour and solubility of Bromine in water and cyclohexane?
In water:
Orange
Slightly soluble
In cyclohexane:
Orange
Very soluble
Colour and solubility of Potassium Bromide in water and cyclohexane?
In water:
Colourless
Very soluble
In cyclohexane:
Colourless
Insoluble
Colour and solubility of Bromoethane in water and cyclohexane?
In water:
Colourless
Slightly soluble
In cyclohexane:
Colourless
Very soluble
Describe what would happen if a small amount of Bromine water is shaken with a larger amount of cyclohexane?
Initially the colourless cyclohexane would form a separate layer on top of the orange bromine water. On shaking the 2 layers would remain, but majority of colour transferred into upper cyclohexane because bromine is more soluble in it
Describe what would happen if a equal amount of Bromine water is shaken with an equal amount of cyclohexane?
Initially the colourless cyclohexane would form a separate layer on top. On shaking the 2 layers would remain but the orange colour would disappear
An addition reaction is taking place
Describe why this reaction is done in the dark :equal amount of Bromine water is shaken with an equal amount of cyclohexane?
In the light there is enough UV light to allow free radical substitution of bromine with alkanes, slowing done the decolourisation
What mechanism occurs when on the addition reaction of an alkene with a halogen molecule or a hydrogen halide molecule?
Electrophilic addition
What’s an Electrophile?
An atom (or group of atoms), which is attracted to an electron rich centre or atom where it accepts a pair of electrons to form a new covalent bond
What’s an addition reaction?
A reaction in which a reactant molecule is added to an unsaturated molecule to make one saturated molecule
What does a curly arrow represent?
The movement of a pair of electrons
Describe how to draw the electrophilic addition reaction of ethene with Hydrogen Bromide?
- Draw ethene, and hydrogen bromide perpendicular to the double bond with the hydrogen being closer
Make the Hydrogen delta + and the Bromide delta -
Draw a curly arrow from the double bond to the hydrogen, and draw a curly arrow from the covalent bond in the Hydrogen bromide to the Bromide - Now draw ethane, except missing a hydrogen and where that occurs that hydrogen has a positive charge
Draw the bromide with a negative charge and draw it with an electron pair
Draw a curly arrow from the Bromide to the Positively charged carbon - Draw Bromoethane
What is the Hydrogen Halide attracted to in the Alkene?
The electron cloud of the pi bond
How do pi bond and hydrogen halide bond break In electrophilic addition?
heterolytically
What is the meaning of heterolytic fission?
The breaking of a covalent bond forming a cation and an anion, so when the covalent bond breaks both the electrons go to the same atom
In electrophilic addition why can the hydrogen halide ion be considered an electrophile in the first step?
It accepts a pair of electrons from the pi bond to form a new C-H bond
What is CH3CH2+ called?
An intermediate Carbocation: intermediate because it’s formed during the reaction but not the end, and it contains a carbon with a positive charge
In step 2 of In electrophilic addition why does one of the carbon atoms have a positive charge?
One of the electrons in the pi bond originally belonged to this carbon atom, and it’s now been lost to the new C-H bond
In step 2 of In electrophilic addition why does the bromine atom in the HBr molecule become a negative bromide ion?
It has received both electrons from the H-Br bond, one of which originally belonged to the H atom. The Br atom has thus gained one electron to form a Br-
When just a halogen is in an In electrophilic addition reaction how does the halogen become Delta positive and Delta Negative?
Eg. Br-Br bond isn’t naturally polar, temporary polarisation of the Br-Br bond occurs when the Br2 molecule approaches the pi electron cloud of the ethene molecule. The electrons in the bromine molecule are repelled away from the pi bond, meaning the far end becomes delta negative, so the closer end becomes delta positive creating an induced dipole
What does Markownikoff’s rule state?
When a hydrogen halide is added to an unsymmetrical alkene, the most likely product is the one in which the H bonds to the Carbon which already has the H’s bonded directly to it. The most likely product is called the major product, the less likely product is called the minor product
What’s a tertiary Carbocaiton?
C+ bonded to 3 C’s
What’s a secondary Carbocation?
C+ bonded to 2 C’s
What’s a primary Carbocation?
C+ bonded to 1 C
What’s the most stable Carbocation?
Tertiary
Explain why Markownikoff’s rule applies?
When the carbocation is formed the H is going to the less stable C ie. the one with the most Hydrogens/less carbonds