Organic - ALKANES Flashcards
Sigma Bonds Definition
A bond formed by the direct lengthways overlap of orbitals directly between 2 bonding atoms
Sigma bonds
All C-C and C-H (single bonds) bonds in alkanes are sigma bonds and can freely rotate
These are the strongest type of covalent bond
Boiling points of alkanes
Alkanes are non-polar
They only have London forces
More carbon atoms = More electrons = Stronger London forces between molecules = Higher boiling point
Length of carbon chain
Increased carbon chain:
More electrons because it’s a bigger molecule
The london forces are stronger and there’s more of them
Boiling point increases
Branching
Increased branching decreases the boiling point :
Less surface area contact
Less london forces between molecules
Complete combustion with alkanes
Alkane + Oxygen —> CO2 + Water
When balancing do in the order C,H,O
Incomplete Combustion
If there is a limited supply of oxygen then carbon monoxide or carbon is produced
This is called incomplete combustion
Carbon monoxide = CO
Carbon particulates = C
Reaction Mechanisms
A covalent bond can be broken by 2 types of fission :
- Homolytic Fission
- Heterolytic Fission
Homolytic Fission - Each bonding atom receives one electron from the covalent bond, forming 2 radicals
What is a radical?
A species (atom,ion or molecule) with an unpaired electron
Free radical substitution
A covalent bond is broken and each of the bonded atoms receives one electron to form 2 radicals
Free radical substitution Mechanism
What is a free radical?
A reactive species due to the presence of an unpaired electron
Condition - UV light
Reagent - Halogen molecule
Free Radical Substitution Mechanism
Stages
3 Stages
1 - Initiation : Creating free radicals
e.g Cl2 —> Cl° + Cl°
2 - Propagation : A molecule & radical create a molecule & radical
3 - Termination (2 radicals react to form a molecule)
Propagation
1st propagation :
The halogen radical reacts with the alkane
The radical removes a hydrogen from the alkane creating a hydrogen halide molecule
The alkane is left as an alkyl radical
CH4 + X° —> •CH3 + HX
Propagation
2nd Step
The alkyl radical reacts with a halogen molecule
This creates a halogenalkane
The remaining halogen atom from the molecule is left as a halogen radical
•CH3 + X2 —> CH3X + X•
The halogen radical created at the end of second propagation goes to to react with another alkane molecule
Then 1sr propagation is repeated
Chain reaction of 1st & 2nd propagation until u run out of halogen molecules or alkanes
Termination (2 radicals react to form a molecule)
Two halogen radicals combine
X• + X• —> X2
A halogen radical combines with an alkyl radical
•CH3 + X• —> CH3X
Two alkyl radicals combine
•CH3 + •CH3 —> CH3CH3
Free Radical Substitution Mechanism Example
1) Initiation
X2 —> •X + •X
2) 1st Propagation
CH4 + X• —> •CH3 + HX
3) 2nd propagation
•CH3 + X2 —> CH3X + X•
4) Termination
X• + X• —> X2
•CH3 + X• —> CH3X
•CH3 + •CH3 —> CH3CH3
Issues with free radical substitution
The yield is often low due to:
• Further substitution can occur forming a mixture of products
• Substitution can occur in different positions creating position isomers
