Alkanes Flashcards
Why are alkanes unreactive?
1. C-C bonds and C-H bonds are very strong so cannot break easily yo form stronger bonds with other elements that easily
2. Have no polar side groups
What are the two main reactions that take place with alkanes?
1. Combustion
2. Reaction with halogens in the presence of UV light
Why are alkanes used as fuels?
They readily burn in air and are highly exothermic
What are the products of complete combustion?
Carbon dioxide and water
What steps are in the mechanism for Free Radical Substitution?
Initiation
Propagation
Termination
Do free radicals have a charge?
No
What is a free radical?
A highly reactive species which has one or more unpaired electrons
What kind of bond among many will be broken?
The weakest one
How do we terminate the presence of free radicals?
Reacting free radicals with other free radicals
What is a petroleum fraction?
A mixture of hydrocarbons of similar length within the same range of boiling point
How then are heavy residues from the fractionating column separated?
Distillation under a vacuum
Define cracking
The conversion of large hydrocarbon molecules into smaller hydrocarbon molecules by the breakage of C-C bonds.
What is produced from the cracking of alkanes?
Small chain alkane + Alkene + Hydrogen
What are the economic reasons for cracking?
1. Smaller chain alkanes are much more valuable than large chain alkanes
2. Smaller chain alkanes have more uses and are in higher demand as a result
What are the two types of cracking?
1. Thermal
2. Catalytic
What are the requirements of thermal cracking?
High pressure of about 7000kPa
High temperatures of 400 to 900
What are the requirements of catalytic cracking?
Low oressure
Low temperature of about 450
Zeolite catalyst
What are the products of thermal cracking?
Small chain alkanes and sometime hydrogen
What are the products of catalytic cracking?
Branched and cyclic alkanes
Aromatic hydrocarbons
What is the advantage of catalytic cracking over thermal cracking?
Less energy costs due to the use of low pressure and temperature
What is the molecular shape and bond angles of alkanes that have sp3 hybridisation?
Tetrahedral with a bond angle of 109.5 degrees
What is the trend in boiling point of alkanes?
General increase as the carbon chains get longer
Why is boiling point low with branched molecules?
Weaker intermolecular forces between chains so less energy is required to break them
What is the solubility of alkanes
Insoluble in water but soluble in some organic solvents
