Organic Chemistry- alkanes, alkenes and alcohols Flashcards
What is cracking?
Long chained alkanes breaking into shorter chained alkenes and alkanes.
use of high temperatures /thermal decomposition
Either
to break large molecules / to form smaller molecules / to break bonds in large molecules / to break carbon-carbon bonds (1)
OR
producing alkenes / producing carbon-carbon double bonds (1)
Define catenate
Reacting with itself.
What is a different name for cracking ?
Pyrolysis.
Why does aluminium oxide help with reacting alkanes?
It reduces the strength of the bonds.
What does half an arrow mean in diagrams?
One electron moving.
What is a free radical ?
An atom or molecule with a single unpaired electron.
What is halogenation?
Substituting a hydrogen with a halogen.
What is the initiation step of halogenation of bromine and methane ?
UV Light causes the Br(2) molecule to split into 2 free radicals.
This is homolytic fission.
What is the propagation step of halogenation of bromine and methane ?
One of the bromine free radicals will react with the bond between a hydrogen and carbon in a methane molecule. The bromine gains an electron and so does the carbon. The result is bromine bonded to hydrogen and a CH(3) free radical.
What is the second propagation step of halogenation of bromine and methane ?
A CH(3) free radical will react with a Br(2) molecule. Electrons will move around resulting in a bromine free radical and a carbon bonded to 3 hydrogens and a bromine.
Explain the structure of a double bond?
It is a sigma (σ) bond in the middle with a pi bonds on the top and bottom. The pi bonds are straight and extend so they are over both carbons.
The σ bond is stronger than a single pi bond but together it is stronger than a single one.
What type of bond is a single bond?
A sigma (σ) bond
Which has the higher boiling point? A cis or trans isomer?
The cis isomer. This is because the shape of the cis isomer means more can fit together in a smaller space. This increases the surface area and so it increases the forces of attraction, thus more energy is needed to break them.
Explain the process of addition with hydrogen bromine and ethene.
The bromine will have a slightly negative charge while the hydrogen has a positive one. The hydrogen gains an electron from the double bond and it gives one to the bromine. The result is a bromine free radical and C(2)H(5) with a open positive bond which the bromine is later attracted to.
What is electro negativity ?
The ability of atoms in a bond to pull electrons of the bond, closer to the atom.
What happens when butene comes into contact with ICl?
The electronegative chlorine will gain an electron from the iodine and the iodine gains an electron from the butenes double bond. This results in the double bond braking open meaning the iodine can join and there is an open positive bond and a chlorine free radical. This latter joins the to butene.
When ICl reacts with butene, which two molecules can form?
CH”3”CH”2”CHICH”2”Cl and CH”3”CH”2”CHClCH”2”I. The latter will form more.
What is a Z isomer?
Same as a cis isomer.
What is the same as an E isomer?
Same as a trans isomer.
What happens if a alkane is burnt incompletely?
There won’t be enough oxygen and carbon monoxide will be produced as well as the other products.
Give the halogenation of methane and Cl”2”
CH”4” + Cl”2” = HCl + CH”3”Cl
After a halogenation you may get CH”3”Br and a Br free radical. What can happen now?
The bromine may continue to react with the hydrogen bonds resulting in a carbon bonded to 4 bromines or less.
What is the termination step of halogenation?
Any two free radicals can meet. E.g. A CH”3” and Br free radical may meet producing CH”3”Br which won’t react with anything else.
What type of reaction are most addition reactions?
Exothermic.
What do larger alkanes have greater boiling points?
There are greater intermolecular forces.
Define empirical formula.
The simplest ratio of the atoms in each element in a compound.
Define skeletal formula
Shows the bonds of the carbon skeleton only.
What is a cycoalkane?
A an alkane that is in a ring. Each carbon atom is bonded to 2 hydrogen atoms.
Why are alkenes more reactive?
The double bond they have has a high electron density. Thus there are more electrons to be reacted with.
Also the pi bond sticks out top and bottom so electrophiles will be attracted.
What would be the name of a three carbon alkane with a carbon coming from the middle, top and bottom?
Dimethylpropane
How are side chains numbered?
Number it so you have the smallest number.
What chain do you use to create the suffix of an alkenes name?
The longest chain with the double bond.
How is the number of the location of the double bond determined?
Number it so it is the lowest number. Only count carbon carbon bonds.
What is a homologous series?
A group of organic compounds that all have the same general formula.
What is the general formula of alcohols?
C”n”H”2n+1”OH
What is a chain isomer?
Isomers with different arrangements of the carbon Skelton.
What are structural isomers?
The atoms are connected in different ways but they still have the same molecular formula.
What are the different types of isomers?
Chain, position, functional group
What is a position isomer?
An isomer where the carbon skeleton remains the same but important groups are moved around on the skeleton.
For example there are two position isomers of C”3”H”7”Br. The bromine could go on the end or in the middle of the chain.
What are functional group isomers?
Isomers with a different functional group.
What type of reaction is one where a halogen replaces a hydrogen on an alkane?
A free radical substation reaction.
What are the two types of cracking ?
Thermal and catalytic cracking.
Explain thermal cracking.
Is takes place in high temperatures and pressures. (1000C and 70atm)
It produces lots of alkenes.
The alkenes produced are good at making polymers.
Explain catalytic cracking.
It makes motor fuels and aromatic hydrocarbons.
It uses a zeolite catalyst, slight pressure and high temperature (450C)
It cuts costs as it uses less energy and it speeds up the reaction.
What is reforming an alkane?
It is done to long chained alkanes to make cycloalkanes and aromatic hydrocarbons.
It uses a catalyst.
What is the problem with sulphur oxides?
It is produced through burning alkanes. Power stations produce the most and scrubber are used to get rid of most.
They are poisonous and cause problems for people with asthma.
They cause acid rain.
What is the problem with carbon monoxide and hydrocarbon particles.
Caused by incomplete combustion.
Carbon monoxide is poisonous.
Sometimes in cars, some of the fuel is burnt and goes out the exhaust pipe. These hydrocarbons can cause health and breathing problems.
What is the problem with nitrogen oxides?
Cars make them.
They cause acid rain.
Nitrogen dioxide causes breathing problems.
In sunlight, nitrogen dioxide reacts to produce ground level ozone with is another air pollutant.
What is the name of the reaction where the double bond of an alkene is broken open to bond with other things?
An addition reaction.
What are compounds with benzene ring structures called ?
Arenes or aromatic compounds. All other organic compounds are called aliphatic particles.
What is benzene?
C”6”H”6”. It as a cyclic alkene with 6 carbons and 3 double bonds.
It is stable as the double bond electrons are delocalised around the carbon ring.
What is the symbol for benzene ?
A uniform hexagon with a circle inside it. The circle means the electrons are delocalised along the carbon ring.
What does the reactivity of double bonds mean they are good at making ?
Petrochemicals.
How can an alkene become an alkane? Give an example.
Ethene will react with hydrogen with a nickel catalyst and a temperature of 150 C. This will break open the double bond and the hydrogen will join.
What is an electrophile?
A particle that is attracted to electrons.
They could be positively charged ions or polar molecules where the slightly positive end is attracted.
What is a nucleophile?
A chemical species that donates an electron pair to an electrophile to form a chemical bond.
It is attracted to places where there aren’t enough electrons.
A double bond is nucleophilic.
What is a hydrogen halide?
A halide atom bonded to a hydrogen atom. They take part in electrophilic addition.
What happens when hydrogen halides react with unsymmetrical alkenes?
Two products can form. One will form in larger amounts as it is more stable.
Explain the reaction with hydrogen bromide and propene.
The hydrogen will be slightly positive. Heterolytic fission occurs meaning It will bond with a carbon atom from the double bond leaving an open positive bond. The positive bond is on the carbon that is attached to two other carbons, the strength of the bond will decrease as electrons from the carbons are pulled towards the positive bond making it weaker. This makes the molecule more stable and the 1- bromine (with 2 free electrons) has more of a chance to bond to the positive.
If the positive bond is on the carbon bonded to only one carbon, the positive bond will be stronger meaning the molecule is more likely to decompose so the bromine can’t react.
What two products can form when hydrogen bromide reacts with propene?
1- bromopropane (in smaller amounts)
2-bromopropane (in larger amounts)
What happens when bromine water meets an alkene?
Bromine water is dilute so it contains more water then bromine. The carbocation is more likely to react with water. This means a OH group is more likely to add to the second carbon than another Br.
This means a bromoalcohol will probably form.
Bromoethanol may form. ( a two carbon chain with 2 hydrogen bonded to each. One has a Br atom, the other OH)
What is a carbocation?
An organic ion containing a positively charged carbon atom.
Other than bromine water, what can be used to test for alkenes?
Acidified potassium manganate. (VII)
It will go from purple to colourless if there is an alkene.
The alkene has been oxidised and a diol( an alcohol with two OH groups) has been made.
What would be produced with ethene reacted with Acidified potassium manganate. (VII)?
Ethane 1,2 diol would be made.
A 2 carbon chain with each carbon bonded to 2 hydrogens and a OH.
What does [0] mean?
It is often used to show an oxidising agent in an organic equation.
What are stereoisomers ?
They have the same structural formula but a different arrangement in space.
What is the ionic equation of acids reacting with carbonates ?
2H+ and CO”3”2- forms CO”2” and H”2”O
How do you remember which is an E and which is a Z isomer?
Think E for enemy. On opposite sides.
Z for on ze zame zide.
How are priorities chosen for e and z isomers?
Look at the atom with the highest atomic number. This has priority.
If it is a group of atoms attached to the carbon double bond, look at the atom directly attached double bond.
If they are the same, look at the atoms directly attached to them.
Count double bonds twice.
When bromine water reacts with an alkene, what type of reaction is it?
Electrophilic addition.
What is the process when a bromine molecule reacts with an alkene?
The molecule will approach the double bond. The electrons of the double bond will cause the electrons of the Br₂ to be repelled and move to one end. This means the end closest to the double bond doesn’t have enough electrons and acts as an electrophile.
Heterolytic fission occurs meaning the double bond donates an electron to the sigma positive bromine and the bonding electrons go to the sigma - Br meaning it has 2 unpaired electrons and is minus.
This then reacts with the open positive bond of the other carbon caused by opening the double bond.
What does Draw the structural formula of the repeat unit mean in an exam?
Draw the molecule in brackets with the double bond going out at each side. Also have an n at the bottom.
When a halogen reacts with an alkane, what are the conditions needed?
uv light.
When acidified potassium manganate reacts with a double bond what type of a reaction is it?
Free radical substition and oxidation.
