Chapter 15: Hydrocarbons Flashcards
Why do alkanes and cycloalkanes not have any polarities?
There is no electronegativity difference between carbon and hydrogen. Many a-time, the symmetrical nature of alkanes like methane also contribute to their lack of bond polarity.
What is the trend of boiling points in an alkane?
The boiling points increase with the size of the molecule.
What is the explanation for the trend in the boiling points of the alkanes.
The attractions between alkane molecules is the Van Der Waals dispersion forces, which will increase as the molecules increase in size.
What is the relationship between isomers of alkanes and their boiling points
The more branched the chain, the lower the boiling point as the dispersion forces are much smaller for shorter molecules. The molecules also cannot come very close to each other due to the excessive branching natures.
Explain the solubility of alkanes
Alkanes can dissolve in water but they cant dissolve in organic solvents.
What is the mechanism behind the solubility of alkanes?
The energy released from the breakage of the hydrogen bonds is water is less than the energy released from breaking of the vander waals forces. The energy release does not compensate.
Why can alkanes dissolve in organic solvents
Organic solvents only have van der waals forces which can easily be compensated for when an energy breaks.
Why is cylcopropane more reactive than the other cycloalkanes?
The 60o bond angle is much smaller than 109.5o which causes a great deal of electron repulsion that make sit much more reactive.
Explain the cis-trans structure of a cyclic hydrocarbon.
Cyclic compounds always have to have a cis stricture in order for everything to join up.
Describe the trend between the combustion of an alkane and their lengths
The longer alkanes are usually harder to ignite than the smaller hydrocarbons due the stronger dispersion forces.
Describe the colour of the flames between the complete combustion and an incomplete combustion.
Complete combustion burns with a blue flame and incomplete combustion burns with a yellow flames and tends to have soot particles.
Describe the reaction of an alkane with fluoride.
It is an explosive reaction that takes place in the cold and dark. And you get hydrogen fluoride.
What are free radicals?
they are atoms or group of atoms that have a single unpaired electron
How do free radicals form?
From a homolytic fission
What happens when methane and chlorine is exposed to a flame?
It explodes producing carbon and hydrogen chloride.
Explain the multiple substitution of halogen free radicals to a given hydrocarbon
The hydrocarbon collides with a halogen free radical and then replaces one of the hydrogen with the free radicals of the halogen. Then eventually, following more collisions, the all of the hydrogens can be removed to form the halogenocarbon
What are aromatic hydrocarbons?
They are the hydrocarbons that have benzene rings.
List out the products in their layers in the fractional distillation of crude oil.
- Refinery gases
- Gasoline
- naphtha
- Kerosene/Paraffin
- Diesel Oil
- Fuel Oil
- Lubricating oil
- Bitumen
What is the catalyst that is used during cracking? What is the other options?
Zeolites (Aluminosilicates)
Others: Aluminium Oxide and Silicon dioxide.
What is reforming?
It is a process that improves the octane rating of a a hydrocarbon and also is a useful source of aromatic compounds.
How does reforming work?
Isomerisation reaction occurs and chain molecules get converted into rings and loses hydrogen.
Explain the conditions and the catalyst that is used in the process of reforming.
Uses a platinum catalyst that is suspended in aluminium oxide. The vapour is passed over the solid catalyst at a temperature of 500 degrees.
How does the zeolite catalyst work?
it has a site that removes a hydrogen that can remove the hydrogen along with the electrons attached to it, resulting in a carbocation.
What are the conditions under which thermal cracking takes place?
450 - 7Summer 50 degrees and upto 70 atm
How does thermal cracking differ from catalytic cracking?
Catalytic cracking goes through ionic intermediates while thermal cracking only generates the free radicals.
What is the objective of dehydrating an alcohol?
It allows you to make gaseous alkenes like ethene
What is the catalyst used in the dehydration of ethanol.
Aluminium oxide
The vapour is just passed over the heated aluminium oxide.
What are the two acid used mainly in the dehydration of an alcohol using an acid?
Concentrated sulphuric acid Concentrated phosphoric(V) acid
Why is sulphuric acid not preferred in the dehydration of alcohols?
- It is a strong oxidising agent: it makes carbon dioxide and reduces to sulphur dioxide. It is very difficult to remove these gases from the ethene gas.
- It reacts to produce a mass of carbon.
What is the process of dehydrating ethanol to give ethene?
The Ethanol is heated with excess concentrated sulphuric acid at a high temp of 170 C.
The gases produced are passed through sodium hydroxide to remove the carbon dioxide and the silicon dioxide.
Describe the dehydration of a cycloalcohol to give a cycloalkene.
It is a similar process to the dehydration of alcohols producing the cyclo-alkene while releasing water.
What happens when a more complicated alcohol is dehydrated
More complex alcohols have a chance of forming isomers, depending on the position of the double bond.
Also, the isomers may have a Cis-trans isomerism.
Describe the state that alkenes ar at in rtp.
Ethenes, Propenes and some butenes: Gases
The rest are liquids.
What is the relationship between the boiling point if an alkene and its corresponding alkane?
Alkenes depend on their van der waals forces for their boiling points
alkenes have 2 less electrons than the corresponding alkane
Thus, their boiling point is a little lower.
Explain the bonding of alkenes.
Alkenes have a double covalent bond.
Therefore, they have a sigma bond and pi bond
Why is the pi bond of an alkene weaker than than the sigma bond?
The pi bond is a region of highly negative charge above and below the plane of the molecule, outside the control of the nucleus, which leads it to be more vulnerable to attack.
Why can ethene only exist in one plane?
The double bond gives it a restricted rotation because if the molecule were to rotate, then the pi bonds will no longer line up and it would break.
What is the bond angle of the sp2 hybridisation of ethene?
120 degrees
It is slightly distorted from 120 because 2 hydrogens and a carbon are bonded rather than 3 identical groups.
What are the type of reactions that alkenes may partake in?
- Combustion (Not preferred)
2. Addition reactions
What is an electrophile?
it is something that is attracted to an electron rich region, therefore having a full or slight positive charge.
Why are alkenes easily attacked by electrophiles?
Because they have a region of negative charge by the pi bonds that attracts electrophiles.
Why might Na+ not react with ethene?
Na+ likes to form ionic bonds while the carbocation likes to form covalent bonds.
Describe the formation of a carbocation and the eventual formation of the dative bond.
- A molecule with a small dipole approaches the alkene pi bond by the positive side
- A covalent bond is formed
- The electrons are repelled making a negative ion
- The Alkene loses the pi bond and now makes a carbocation with a postive charge.
- the Negative ion is now attracted to the positive ion and makes a dative bond, by donating both the electrons.
Describe the hydrogenation of an alkene.
You can hydrogenate a carbon-carbon double bond by reacting it with hydrogen gas in the presence of a nickel catalyst at about 150 oC
How is ethanol manufactured?
By reacting ethene with steam
what is the nature of the ethene - steam reaction?
It is reversible
What is Markovnikov’s rule?
It states that when a molecule HX is added across a carbon-carbon double bond, the hydrogen atom which already has the more hydrogen atoms attach to it
Based on Markovnikov’s Rule, what forms when propene is hydrated
Propan-2-ol
Because the hydrogen goes to the carbon with 2 hydrogens
And the OH is left to only go to the second carbon
What are the conditions for the conversion of ethene to ethanol?
A catalyst of silicon dioxide coated with phosphoric (V) acid
300 oC
60 to 70 atmospheres
How does the carbocation form when ethene reacts with phosphoric (V) catalyst?
- Ethene’s pi bond breaks and then the moves down to react with PO4-
- The hydrogen breaks away from PO4- and bonds with the carbon forming a positive carbocation
- The catalyst becomes negative.
How does the carbocation react with water and what is the product?
The carbocation has a positive charge
The water has lone pairs
They bond together to form CH3CH2OHH+
How does CH3CH2OHH react with H2PO4- to regenerate the PO4 catalyst?
The hydrogen gives its electron to oxygen and neutralises the positive charge
The hydrogen bonds with H2PO4- to regenerate the PO4- catalyst
In the reaction of ethene and steam, why is an excess of ethene used an not an excess of steam?
It is because of the catalyst
The phosphoric (V) catalyst is coated on an SiO2 support
Too much steam can dilute the catalyst and it will wash away.
Considering that the forward reaction in the production ethanol is exothermic, why is a high temperature of 300 used?
The rate of reaction will become too low
What is the difference between primary, tertiary and secondary carbocations?
Primary: has one alkyl group attached
Secondary: has two alkyl group attached
Tertiary: has three alkyl groups attached
What is the negative inductive effect?
The higher electronegativity of a halogen will bring the electron close to it in an electron pushing effect.
What happens to the alkyl group when attached to a carbon?
What is this affect called?
They tend to push the electrons away from them
The carbon atom pulls on the electrons more strongly
It is the positive inductive effect.
Why is the tertiary carbocation more stable than the primary carbocation
The electric charge is spread around the atom more evenly and is more stable
