Alkanes Flashcards
1
Q
What is alkanes
A
- saturated hydrocarbons
- each C atom is sp3 hybridised, tetrahedrally surrounded by H and C atoms
- general formula: CnH2n+2
2
Q
Preparation of alkanes
A
- Thermal cracking
- Catalytic cracking
3
Q
What is Free radical
A
- atom or group of atoms that has an unpaired electron in its outershell
- highly reactive and readily form bonds with other atoms to regain their full octet of electrons
4
Q
What is Homolytic Fission
A
- type of bond breaking process where each of the atom involved in forming the bond acquires one of the two bonding electrons
- forming free radicals
5
Q
What is Heterolytic Fission
A
- type of bond breaking process where splitting of bond gives and unequal share of bonding electron to each particle
6
Q
Free-radical substitution in organic synthesis
A
- not a good method as it is hard to control the reaction resulting in a mixture containing other by-products
7
Q
Reactivity of halogens with alkanes
A
- F2(g): reacts explosively
- Cl2(g), Br2(l): react with alkanes in prescence of UV light or heat
- I2(s): no reaction with alkanes
8
Q
Relative stability or radicals
A
- depends on number of electron donating alkyl groups attached to the carbon with unpaired electrons
9
Q
Reactions of alkanes
A
- Combustion
- Free-radical substitution(Halogenation)
10
Q
What are the steps in free radical substitution mechanism
A
- initiation: under UV light, homolytic fission of Cl-Cl bond takes place and two chlorine free radicals are formed
- propagation: highly reactive chlorine free radicals collide with alkanes abstracting a hydrogen atom to produce HCl and a methyl radical. Methyl radical reacts further with another chlorine molecule to form chloromethane and chlorine radical is regenerated. Leads to a chain reaction
- termination: when two free radicals collide the unpaired electrons pair up to form a stable product which terminates the chain reaction. Highly exothermic as it only has bond forming
