2) Structures, reactions and properties of commercially important organic compounds Flashcards
What is molecular formulae?
Tells you type and number of each atom present in a molecule. E.g. C4H10 has 4 carbon atoms and 10 hydrogen atoms
What is displayed formulae?
Shows all the bonds present, so you can see how each atom joints to others
What is structural formulae?
Shows how all atoms are arranged, but is not required to show all the bonds
What is a skeletal formula?
Where hydrogen atoms are removed and carbon chains are reduced to a skeletal line, where each corner represents a carbon.
What is a 3D bond?
Dashed line means the bond is facing away from you, wedged line means the body is facing towards you, and the line represents the bond in the plane of the paper
What are alkanes?
A series of hydrocarbons with the general formula CnH2n+2
What are straight-chain alkanes?
Alkanes with a straight chain of carbon atoms
What are branched alkanes?
Alkanes with one or more carob atoms attached to a carbon in the main chain
What are cyclic alkanes?
Alkanes jointed together in a ring
How are sigma bonds formed in alkanes?
The overlap of orbitals in alkanes results in sigma bonds
Where are sigma bonds found in alkanes?
Between carbons and hydrogens and between adjacent carbons, these single bonds are free to rotate
What are the carbon orbitals involved in the bonding in alkanes called?
Sp3 hybrid orbitals
How are sp3 orbitals formed?
When the 2s orbital and three 2p orbitals in carbons outer shell rearrange themselves into four identical orbitals, each containing one unpaired electron, this is called hybridisation
What determines the boiling points of alkanes?
The longer the carbon chain of a straight chain alkane, the higher the boiling point.
Why do longer carbon chain have stronger London forces?
They have more electrons
Why is more energy required to break the London forces when there is more electrons?
Having more electrons means that the electron density fluctuate more easily, so instantaneous dipoles are stronger in size, so more energy is required to break them.
What are structural isomers?
Molecules with the same molecular formula but different structural formula.
Why do hydrocarbons e.g. alkanes have a variety of isomers?
Due to a range of possible branches on a carbon chain.
What is the same and what differs with isomers?
They have the same chemical properties, but may have different physical properties, e.g. boiling points
What are alkenes?
A series of hydrocarbons with the general formula CnH2n
How are sigma bonds formed in alkenes?
The overlap of orbitals in alkenes results in the formation of sigma bonds.
Where are sigma bonds found in alkenes?
Between carbon and hydrogens and one of the bonds between adjacent carbon atoms
What are the carbon orbitals involved in alkene bonding called?
Sp2 hybrid orbitals
How are sp2 orbitals formed?
When the 2s and two of 2p orbitals in the carbons outer shell rearrange themselves into three identical orbitals each containing one unpaired electron
What happens to the remaining p orbitals in alkens?
The remaining p orbitals in each carbon overlap sideways to form a pi bond.
What is the significant effects of the pi bond
1) Restricts rotation around double bond this means that groups attached to the carbons in the double bond are locked in position, which can lead to the formation of stereoisomers.
2) The high electron density influences many of the chemical reactions in alkenes. Electrophiles will be attracted to an often react with the double bond in addition reactions.
What are stereoisomers in alkenes?
Compounds with the same structural formula but their atoms have a different arrangement in space
How are stereoisomers caused in alkenes?
By the restricted rotation of the carbon-carbon double bond
How is double carbon-carbon bonds different than the carbon-carbon single bonds?
The bonds are shorter and stronger, as the attraction between the nucleus and the shared pair of electrons is greater.
Are double carbon-carbon bonds twice as strong as carbon-carbon single bonds?
No, as the p orbital overlap is less effective than overlap by hybrid orbitals
What type of electrons are in the orbitals of a benzene ring?
Delocalised electrons
Why are alkenes more reactive than alkanes?
There is a greater electron density in the carbon-carbon double bonds in alkenes so most likely to be attacked by reactants e.g. electrophiles. And the pi bond is weaker than the sigma bond in alkanes so is more likely to break
What are the most important reactions of alkanes?
Combustion and reaction with halogens
What is an unsaturated carbon?
Contains carbon-carbon double or triple bonds
What is a saturated carbon?
Contains carbon-carbon single bonds
What is combustion?
Reaction with oxygen
Give examples of hydrocarbon fuels
Petrol or diesel
How many carbons does petrol contain?
4-12 carbons
How many carbons does diesel contain?
8-21 carbons
What does complete combustion produce?
CO2 +H2O
What does incomplete combustion produce?
CO + C
When does complete combustion occur?
Complete combustion occurs in excess oxygen
When does incomplete combustion occur?
Incomplete combustion occurs in limited oxygen
What is the reaction with halogens also called?
Free radical substitution
What is a mechanism?
A mechanism shows what occurs at each stage of a process
What is a free radical mechanism?
It is a free radical substitution, it is a chain reaction.
What is the three stages of free radical substitution?
Initiation
Propagation
Termination
What occurs in inititation?
Homolytic bond fission of a halogen to form halogen free radicals, uses UV light or heat (300C)
What is homolytic bond fission?
When a bond breaks and each fragment formed retains one of the electrons from the bond pair
What occurs in propagation?
Steps where one free radical reacts and forms a different free radical
What occurs in termination?
Combination of any two free radicals to form a stable product
What is a free radical?
A free radical is a reactive atom or group of atoms, that contains an unpaired electron, often formed by homolytic bond fission
Show the reaction when methane is reacted with bromine in the presence of UV light.
(Initiation)
Br2 → Br* + Br*
(Propagation)
Step 1: CH4 + Br* → CH3* + HBr
Step 2: CH3* + Br2 → CH3Br + Br*
(Termination)
Br* + Br* → Br2
Br* + CH3* → CH3Br
CH3* + CH3* → C2H6
Show the reaction when butane is reacted with chlorine in the presence of UV light.
(Initiation)
Cl2 → Cl* + Cl*
(Propagation)
Step 1: C4H10+ Cl* →C4H9* + HCl
Step 2: C4H9* + Cl2 → C4H9Cl + Cl*
(Termination)
Cl* + Cl* → Cl2
Cl* + C4H9* → C4H9Cl
C4H9* + C4H9* → C8H18
What does’*’ mean
The radical
Alkenes are a lot more reactive than alkanes, but can you explain why?
Alkenes have a double bond – which means they have a stable sigma bond and a reactive pi bond. Because of this combination, the pi bond can be easily broken in the presence of an electrophile.
What is an electrophile?
a species that can form a slightly positive and a slightly negative atom
Why can you can react alkenes to form a range of products?
Due to an electrophile present
What do you get when Alkene + Halogen, and what are the conditions needed?
Halogenoalkane, no conditions needed
What do you get when Alkene + Hydrogen Halide and what are the conditions needed?
Halogenoalkane, no conditions needed
What do you get when Alkene + Hydrogen and what are the conditions needed?
Alkane.
Ni catalyst, 150 °C
What do you get when Alkene + Water and what are the conditions needed?
Alcohol
H3PO4, 300 °C, 60-70 atm
What do you get when Alkene + Sulphuric Acid and what are the conditions needed?
Alkyl Hydrogen Sulphate
No conditions needed
What does a curly arrow represent?
The use of a curly arrow represents the movement of a pair of electrons. It moves from negative to positive.
What happens to the double bond during electrophilic addition?
During Electrophilic Addition, the electrophile becomes polar when it approaches the double bond. The part that gets attacked becomes δ+ and the part that gains the electrons from the bond becomes δ-.
What occurs to the carbocation?
At the intermediate stage, the carbon that only has 3 bonds becomes positive (known as a carbocation). The species that gained a pair of electrons in the first stage then attacks the carbocation. After this the product is made.
What is an asymmetrical alkene?
An asymmetrical alkene is any alkene where the number of carbon atoms on each side of the double bond are DIFFERENT.
What are the 3 possible types of carbocations?
Primary carbocation
Secondary carbocation
Tertiary carbocation
What is a primary carbocation?
Carbocation is at the end of the carbon chain.
What is a secondary carbocation?
Carbocation is in the middle of the carbon chain.
What is a tertiary carbocation?
Carbocation is in the middle of the carbon chain with a branch attached to the carbon.
How is stability determined in a carbocation?
The more carbons that are attached to the carbocation the more stable the carbocation is.
What is the most stable carbocation?
Tertiary carbocation
What is the least stable carbocation?
Primary carbocation
What 2 things do the stability of a carbocation determine?
1)Which carbocation is more likely to be formed from the reaction of an asymmetrical alkane and a hydrogen halide
2)What product you are more likely to get.
Why is the major product more likely to form?
Because the carbocation formed is more stable (attached to more carbons)
How would you describe a polymer?
A polymer is a long repeating chain of units. Can be same/different units.
What are two main ways in which we can make a polymer?
Addition Polymerisation
Condensation Polymerisation
Give an example of addition polymerisation
Free radical polymerisation
What is addition polymerisation?
In addition polymerisation, a monomer is added together to produce one product.
What occurs in free radical polymerisation of alkenes?
The double bond breaks and the alkene monomers join together where the double bond used to be.
What does square brackets represent?
That it is a polymer
What do radicals do in free radical polymerisation?
Radicals are will ‘attack’ the alkene monomer, allowing the bond to break and the monomers to join and make the polymer.
What are the 3 conditions needed for free radical polymerisation?
200 °C
2000 atm pressure
A small amount of oxygen
What are 3 uses of poly(ethene) ?
Cling film
Grocery bags
Black bin bags
Why is poly(ethene) useful?
Flexible and cheap to make
What are 3 uses of poly(propene)?
Food containers
Bottle tops
Drinks cups
Why is poly(propene) useful?
Semirigid, tough and good heat resistance
What are 3 uses of poly(styrene)?
Packaging
Insulation
Sound dampening
Why is poly(styrene) useful?
dampening properties,
being lightweight (when expanded)
being moisture resistant and inert.
What are 3 uses of PVC?
Window frames
Drainage pipes
Flooring
Why is PVC useful?
dense
rigid
readily available
cheap
What are 3 uses of PTFE?
Non-stick coating (Teflon)
Bearings
Lubricants
Why is PTFE useful?
Good chemical resistance,
resistant to heat, low temperatures, light, UV, weathering
Low friction
What is combustion?
One mole of fuel being burned in oxygen under standard conditions.
Why do we need combustion?
Combustion is an important chemical reaction, used in industry, heating, cooking and even the car engine
What is cracking?
Cracking is the process where a long chained hydrocarbon is broken down into two smaller chained hydrocarbons.
What is a disadvantage of cracking?
Where the break occurs is not consistent which means you can make a mixture of products.
What do you produce when you crack a hydrocarbon
Alkane and alkene
What are the 2 types of cracking?
Catalytic Cracking
Steam Cracking
What are the conditions needed for catalytic cracking?
Catalytic cracking requires 550 °C and a zeolite catalyst (aluminium or silicon oxide).
What are the conditions needed for steam cracking?
This requires 800 °C and a pressure of 70 atms but does not need a catalyst.
