Hydrocarbons Flashcards
(37 cards)
What is homolysis?
This is even splitting of a covalent bond and gives free radicals.
What is hetrolysis?
Hetrolysis is “uneven” splitting of a covalent bond and produces ions.
What is the first step in purifying hydrocarbons?
The hydrocarbons in crude oil are separated out into simpler mixtures through fractional distillation.
Outline the process of cracking.
Cracking is the process by which long chain hydrocarbons are broken up into smaller and unsaturated products which are more useful. This process can be carried out in a thermal cracker (run at 750-850°C) or catalytic cracker (run at 400-500°C).
What is the catylist used in a catalytic cracker?
A fluidized bed of zeolites (aluminosilicates).
What can the products of cracking be used for?
These may be used to make other chemicals or can be added to gasoline to increase the octane number (more branched chains).
Give a typical cracking equation.
C₁₀H₂₂ → C₅H₆ + C₇H₁₆
What are the conditions necessary for the photo chlorination of methane?
The presence of light. (U.V required)
What is the first step in the photochlorination of methane?
This is the initiation step.
Homolysis of the Cl-Cl bond occurs when a photon of light is absorbed. An attack on the methane by the Cl⋅ radicals formed then occurs.
What is the second step in the photochlorination of methane?
This is propagation and occurs through:
Cl⋅ +CH₄ → CH₃⋅ + HCl
CH₃⋅ + Cl₂ → CH₃Cl + Cl⋅
This step is a chain reaction so that once a chlorine molecule has been split light does not need to be absorbed for the reaction to continue so thousands of molecules can be formed for each photon of light absorbed.
What is the third and final stage in the photochlorination of methane?
This is the termination step. There are three possible versions:
CH₃⋅ + CH₃⋅ → C₂H₆
CH₃ + Cl⋅ → CH₃Cl
Cl⋅ + Cl⋅ → Cl₂
What are the three methods of producing Alkenes?
- ) Industrially - in the cracking process
- ) Dehydration of alcohols through an elimination reaction.
- ) from halogeno alkanes (elimination reaction)
Outline the possible process of creating an Alkene through the dehydration of alcohol.
This is the removal of the elements of water achieved in three ways:
- ) the alcohol vapour is passed over aluminum oxide at 300-350°C
- ) the alcohol vapour is passed over phosphoric acid at 200-250°C
- ) the alcohol is mixed with excess concentrated sulphuric acid and heated to 170°C
How would you produce an Alkene from a halogeno Alkene?
By heating the halogeno Alkene with potassium of sodium hydroxide dissolved in ethanol.
What do the bond energy terms of the C-C bond and the C=C bond show?
That the C=C(610KJmol⁻¹) double bond is less than twice as strong as the C-C (346KJmol⁻¹) single bond.
Why might it be expected that when carbon atoms bond, the four bonds will have slightly different values?
Because the valance electrons of carbon are made out of two 2S electrons and two 2P electrons. However the bonds are found to be all the same.
Why do the four Bonds for carbon in ethene have the same bond energy terms?
It is assumed that one of the 2S electrons is promoted into a 2P orbital to form four SP³ hybrid orbitals and all the bonds are the same.
Describe the strength of an SP³ hybrid orbital.
The SP³ orbital is more concentrated in direction than a P orbital or S orbital so it is able to overlap more and form a stronger bond.
How does each carbon atom produce three SP² orbitals in ethene?
They use a 2S orbital and two of the 2P orbitals to form three SP³ hybrid orbitals.
What bond type is formed when two P orbitals (I.e SP² orbitals in ethene) overlap endways?
They form a sigma bond.
Describe the bonding structure in ethene.
Each carbon forms 3 sigma bonds using SP² orbitals two with hydrogen atoms and one carbon atom, with each bond at an angle of 120°. These bonds are planar. The unhybridized P orbital of each carbon is at right angles to the plane of the SP² orbitals. These P orbitals on adjacent carbon atoms are close enough to overlap sideways. The overlapping takes place above and below the plane of the SP² bonds and is called a π bond. π bonds are not as strong as Σ bonds because there is less overlapping.
What are all the reactions of alkenes?
They are all electrophillic addition reactions and since there is a high concentration of electrons in the double bonds the attack will be by electrophiles.
What reaction can the second step of the first photochlorination of methane give rise to?
A “secondary chlorination”:
CH₃Cl⋅ → ⋅CH₂Cl + HCl
⋅CH₂Cl + Cl₂ → CH₂Cl₂ + Cl⋅
And so forth.
What is catalytic Hydrogenation (addition of hydrogen)?
This is when hydrogen adds across a double bond in an to form a saturated bond. It is an example of heterogenous catalysis, the hydrogen and double bond compound are adsorbed onto the surface of the catalyst using the “D” orbitals.
