Alkanes Flashcards
What is the general formula for alkanes?
CnH2n+2
What are the physical states of alkanes based on their carbon chain length?
- C1 - C4: gases
- C5 - C17: liquids
- C18 and above: solids
What happens to the boiling point of alkanes as the carbon chain increases?
Increases
Are alkanes soluble in water?
Insoluble in water, soluble in nonpolar or slightly polar solvents
What is a common industrial method for preparing alkanes?
Cracking of large hydrocarbon molecules (from crude oil)
What are the conditions for non catalytic steam cracking of alkanes?
temperature over 800 °C
What are the conditions for catalytic cracking of alkanes?
- temperature approximately 500°C
- catalyst: zeolite (aluminium oxide and silicon oxide)
What are the two main types of chemical reactions that alkanes undergo?
- Combustion
- Substitution with Br2 and Cl2 in the presence of sunlight or high temperature (300 - 400 °C)
Write the balanced equation for the combustion of ethane (C2H6).
2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l)
What is the general reaction for the halogenation of alkanes?
CH4(g) + X2(g) → CH3X(g) + HX(g)
What provides the energy needed for the halogenation of alkanes?
UV light or sunlight
What is the observation when alkanes react with Br2?
Brown color of bromine is discharged with white fumes of HBr(g)
What is the observation when alkanes react with Cl2?
Yellow color of chlorine is discharged with white fumes of HCl(g)
What are the three stages of the Free Radical Substitution (FRS) mechanism?
- Initiation
- Propagation
- Termination
What occurs during the initiation step of the FRS mechanism?
Homolytic fission occurs to produce two radical species
What happens during the propagation step of the FRS mechanism?
Free radicals react to produce more free radicals
What occurs during the termination step of the FRS mechanism?
Two free radicals combine, producing a molecule and no free radical is generated
Which type of free radical is the most stable?
Tertiary (3°) radicals
Fill in the blank: The order of free radical stability from least to most stable is: _______.
Methyl < Primary (1°) < Secondary (2°) < Tertiary (3°)
True or False: The more stable radical will produce the major product in reactions.
True
What is the mechanism for the halogenation process of alkanes?
Free radical substitution
What can be observed when complete combustion or incomplete combustion occurs ?
Complete : blue flames
Incomplete : Yellow flames
What is the product when complete combustion occurs?
CO2 and H20
What is the purpose of sunlight/uv in the halogenation reaction?
provide energy to break the bond of homolytic cleavage to produce free radical
