DF6: Alkenes and Addition Reactions Flashcards
What shape does a single carbon bond have?
Tetrahedral
What is the angle between two carbon atoms in a single covalent bond?
109.5 degrees
What shape does a carbon double bond have?
Trigonal Planar
What is the angle between two carbon atoms in a double bond?
120 degrees
Do all the carbon atoms in an alkene compound lie in a planar shape?
No only those in the double bond e.g ethene is completely planar but in propene only the C=C unit is
What type of bond is a single covalent bond?
A sigma bond
How is a sigma bond formed?
When two s orbitals overlap in the space between two atoms
Why are sigma bonds very strong?
There is the highest possible electron density between the two positive nucleui as the two s orbitals overlap
How is a pi bond formed?
When two p orbitals overlap sideways
What does a pi bond consist of?
2 areas of negative charge, one above and one below the line of atoms
What is a carbon - carbon double bond made up of?
One sigma bond and one pi bond
Why is a double bond not twice as strong as a single bond?
Because pi bonds are weaker than sigma bonds
Define an “electrophile”
A positive ion/molecule with a positive partial charge which accepts a lone pair of electrons to form a covalent bond
Define an “addition reaction”
Two or more molecules reacting to form a larger single molecule
Why does the carbon-carbon double bond have a highly negative region?
The four electrons in the double bond of ethene give the region between the two atoms a higher than normal negative charge density
What can be used as a test for the saturation of a hydrocarbon?
Shaking the compound with bromine water
What occurs with an alkene is shaken with bromine water?
It goes from orange to colourless
What occurs with an alkane is shaken with bromine water?
It stays orange
Write a formula for the electrophilic addition of bromine to an alkene
H2C=CH2 + Br2 ====> CH2BrCH2Br
Describe the first step of electrophilic addition with bromine (polarisation)
The bromine molecule moves closer to the negative C=C bond which repels electrons to the bottom of the br-br bond, polarising the molecule and creating a postively charged (an electrophile) bromine atom
Describe the second step of electrophilic addition with bromine (carbocation)
An electron pair from the C=C bond attacks the positive bromine atom, forming a dative covalent bond, the carbon atom the electron pair came from becomes a carbocation
Describe the third step of electrophilic addition with bromine (new covalent bond)
A pair of electrons moves from the now negative bromide ion to the carbocation to form a new carbon-bromine covalent bond
What product is formed from the electrophilic addition of ethene with bromine?
1,2-dibromoethane
How does ethene react with hydrogen bromide solution and what is formed?
Readily at a room temperature in a polar solvent to form bromoethane
What occurs if HBr is added to an unsymmetrical alkene?
Two products are formed e.g with propene: 2-bromopropane and 1-bromopropane is formed
How do alkenes create alcohols?
By hydrating them in the presence of an acid catalyst
Give the overall equation for the hydrolysis reaction of an alkene with water
H2C=CH2 + H20 ===H2SO4===> C2H5OH
Give the steps of the hydrolysis reaction of alkene with water and their formulas (4)
1) Cold concentrated sulfuric acid reacts with an alkene in an electrophilic addition reaction
H2C=CH2 + H2SO4 ====> CH3CH2OSO2OH
(ethene) + (sulfuric acid) ====> (ethyl hydrogen sulfate)
2) Cold water is added and the product is warmed, it is hydrolysed to form an alcohol
CH3CH2OSO2OH + H2O ====> CH3CH2OH + H2SO4
(ethanol)
How can ethene be turned into ethanol?
Through the process of steam hydration
What temperature must the steam hydration of ethene occur at?
300 degrees c
What pressure must the steam hydration of ethene occur at?
600atm
What type of reaction is the steam hydration of ethene?
A reversible one
Give the equation for the steam hydration of ethene
H2C=CH2 + H2O ⇌ C2H5OH
What kind of catalyst is used during the steam hydration of ethene?
A solid phosphoric acid catalyst absorbed onto solid silica
How low is the reaction yield of the steam hydration of ethene?
5%
How can is the overall yield of the steam hydration of ethene increased from 5% to 95%?
By recycling unreacted ethene gas
How does ethene form ethane?
By reacting with hydrogen gas
Why is a catalyst needed in the hydrogenation of ethene?
To break the strong hydrogen - hydrogen bonds and form hydrogen atoms that can react with the alkane
What are the two catalysts that can be used in the hydrogenation of ethene?
- Platinum
- Nickel
What is the advantage of using nickel over platinum as a catalyst?
It is cheaper
What is the disadvantage of using nickel over platinum as a catalyst?
It is less efficient
In what conditions is platinum used as a catalyst in the hydrogenation of ethene?
Standard laboratory conditions
Under what temperature is nickel used as a catalyst in the hydrogenation of ethene
150 degrees c
Under what pressure is nickel used as a catalyst in the hydrogenation of ethene?
5atm
Give the formula for the reaction of ethene with hydrogen?
H2C=CH2 + H2 ====> CH3CH3
Give one usage of hydrogenation
To make unsaturated fats and oils more saturated and therefore thicker
