Alkanes - Organic Chem I Flashcards
Hydrocarbon define:
Compounds made up of only hydrogen and carbon
What is the general formula for alkanes?
CnH2n+2
Define isomer:
Same molecular formula, different structural formula
What is each member of the homologous series represented by?
- General formula
- Adjacent members differ from each other by a CH2 group and they share the same functional group
Therefore have similar chemical properties - Show gradual trend in physical properties
What happens as the length of alkane chain increase?
Boiling point and melting point increases
What forces hold alkane molecules together?
Weak intermolecular forces as they are simple covalent molecules.
What happens to the IMF when the molecules increase in length?
IMF gets stronger therefore more energy is needed to overcome the IMF between the longer chains
How can London forces get stronger?
The greater the number of electrons per molecules, the stronger the London forces
How can London forces get stronger?
The greater the number of electrons per molecules, the stronger the London forces
What helps to reduce the bpt/mpt and how?
Branched alkanes because fewer IMF can develop between the branched chain molecule therefore less energy needed to overcome them
Why are alkanes less dense than water?
Water is also a simple covalent molecule, however, the intermolecular forces between water molecules are stronger than the IMF between alkane molecules. The stronger the forces the closer the molecules will pack to each other
Are alkanes soluble?
Alkanes are insoluble in water but soluble in all other hydrocarbons
Why are alkanes pretty unreactive molecules?
All the C-C bonds and C-H bonds are strong, sigma bonds.
Why do alkanes make great fuel?
They burn well
Complete combustion of alkanes:
Plentiful supply of oxygen
Eg/ C2H8(g) + 5O2(g)→ 3CO2(g) + 4H2O(l)
Complete combustion of alkanes:
Plentiful supply of oxygen
Eg/ C2H8(g) + 5O2(g)→ 3CO2(g) + 4H2O(l)
Incomplete combustion of alkanes:
Lack of oxygen
Eg/ C3H8(g) + 7/2O2(g) → 3CO(g) + 4H2O(l)
Incomplete combustion of alkanes:
Lack of oxygen
Eg/ C3H8(g) + 7/2O2(g) → 3CO(g) + 4H2O(l)
What is the initiation step?
The frequency of UV light has the right amount of energy to break the Cl-Cl bond homolytically. The highly reactive chlorine radicals then start attacking the methane molecules
What is the propagation step?
CH3CH3 + Cl · –> ·CH2CH3 + HCl
·CH2CH3 + Cl2 –> CH3CH2Cl + Cl ·
What is the termination step?
·CH2CH3 + ·Cl –> ClCH2CH3
What are some limitations of free radial substitution?
- Further substitution occurs, producing a variety of products
- Substitution can occur at different positions in the carbon chain
What is homolytic fission?
As the bond breaks, one electron from the shared pair goes on to each of the atoms involved in the bond. Highly reactive free radicals are produced. Homolytic fission occurs in non-polar bonds i.e. when the atoms have similar electronegativities
What s heterolytic fission?
As the bond breaks, both electrons go on to one atom and ions are produced. In this case the X-Y bond is polar i.e. there is a difference in electronegativity between X and Y.
How are deposits of crude oil and natural gas form?
Formed by the slow decay of marine animals and plants, under heat and pressure in the absence of air
What is the composition of crude oil?
Complex mixture consisting mainly of alkanes
How is crude oil separated?
Fractional distillation - compounds have different boiling points
What are the bonds in crude oil like?
C-H bonds are non-polar, so there are only weak London forces between the molecules. As the alkane chain gets longer, the number of electrons per molecule increases so the strength of the London forces increases resulting in an increase in the melting and boiling points
How does fractional distillation work?
Crude oil is vaporised at high temperature and passed into a column, which is hot at the bottom and cold at the top. As the vapour rises up the fractionating column it condenses at different levels. The fractions are collected at the different levels.
What is a fraction?
A fraction is a mixture of hydrocarbons with similar boiling points
As the C chain gets longer, the hydrocarbons:
- Become more viscous (thick)
- Harder to ignite
- Less volatile
- Higher boiling points
What are some fractions contaminated with?
Sulphur containing impurities
What do these impurities burn to form?
To form SO2 and SO3, toxic gases soluble in rainwater, leading to the formation of sulphurous acid (H2SO3) and sulfuric acid (H2SO4).
Which fractions are more in demand?
The fractions with shorter chains (eg/petrol and naphtha) are more in demand than larger fractions
What can the excess larger hydrocarbons be used for?
The longer hydrocarbons are cracked
Long chain alkane → shorter chain alkane(s) + alkene(s) (+ hydrogen)
Conditions needed for catalytic cracking:
Requires a catalyst (eg/Al2O3) and is carried out at a temperature of approximately 500
Conditions needed for thermal cracking:
Uses heat energy (approx. 800) to break C-C and C-H bonds. As the C-C bonds are weaker than the C-H bonds, they are more readily broken.
What is reforming?
The process which straight chain hydrocarbons are converted into the more desirable, branched chain hydrocarbons.
What are the conditions needed for reforming:
Heated to 500 in the presence of a platinum catalyst until they fragment. When the fragments join together, they reform as branched molecules
What is the effect of increasing the branching in a molecule?
Increases the efficiency with which they can burn in an engine
Why is CO2 not a pollutant?
It occurs naturally in the atmosphere
How is CO formed and what are ways to reduce it?
Formed: incomplete combustion of fuels
Problem: toxic
Ways to reduce: ensure a good supply of O2 when burning fuels
How is C (soot) formed and what are ways to reduce it?
Formed: Incomplete combustion of fuels
Problem: Blackens buildings, can cause respiratory problems, global dimming
Ways to reduce: Ensure a good supply of O2 when burning fuels
How is SO2 formed and what are ways to reduce it?
Formed: Combustion of S containing compounds in fuel (eg/fossil fuels)
Problem: Acid rain (SO2 + H2O → H2SO3)
Ways to reduce: Remove the sulphur from the fuel before burning, flue gas de-sulphurisation
How is NOx formed and what are ways to reduce it?
Formed: Reaction of N2 in the air with O2 in the air at very high temperature
Problem: Acid rain (2NO + H2O + 1/2O2 → 2HNO3)
Ways to reduce: Use catalytic converters in cars (NO2 + CO → N2 + CO2)
