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Module 4 Chapter 12 (12.1 Properties of alkanes (Bonding (Saturated…
Module 4
Chapter 12
12.1 Properties of alkanes
Alkanes
Main components of natural gas and crude oil
Very stable and mainly used as fuels
General formula:
Bonding
Saturated hydrocarbons
- contain only hydrogen and carbon atoms joined by single covalent bonds
Each carbon atom is joined to four other atoms by single covalent bonds called a
Sigma Bond (σ- bond)
Sigma bond
- The result of two overlapping orbitals (each containing 1 electron), one from each bonding atom
Each alkane has 4 sigma bonds either between C-C or C-H
Positioned on a line directly between bonding atoms
Shape
Each carbon atom surrounded by four electron pairs in four σ-bonds- repulsion results in bond angle of 109.5 and tetrahedral arrangement
σ bond acts as a axes which the atoms a can freely rotate about
Variations in boiling point
As chain length increases boiling point increase, hence you can separate crude oil into fractions by fractional distillation
Chain length
As chain length increases, molecules have a larger surface area so there is more surface contact possible between molecules
This means the London forces between molecules are stronger and more energy is required to overcome the forces
Branching
Branched isomers have lower boiling points than straight chain isomers
Branched molecules have fewer surface points of contact than straight-chains so they have fewer London forces
Shape- branches get in the way and prevent the molecules getting as close as straight chain molecules decreasing intermolecular forces
11.2 Chemical reactions of alkanes
Reactivity
Not very reactive because
1)
C-C and C-H σ-bonds are very strong
2)
C-C bonds are non-polar
3)
Electronegativity of Carbon and Hydrogen is so similar that the C-H bond can be considered non-polar
Combustion
React in a plentiful supply of oxygen to produce Carbon dioxide and water
Combustion gives out heat- alkanes used as fuels as they are
1)
Readily available
2)
Easy to transport
3)
Burn in a plentiful supply of oxygen without releasing toxic gases
Complete
Methane (Natural gas)-
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
General equation-
CxHy + (x+ 0.25y)O2 -> xCO2 + 0.5yH2O
Incomplete
When Oxygen is limited incomplete combustion occurs
Forms toxic gas Carbon Monoxide and Carbon (soot)
Carbon monoxide formed-
CH4(g) + 1.5 O2(g) -> CO(g) + 2H2O(l)
Carbon formed- CH4(g) + O2(g) -> C(s) + 2H2O(l)
Halogens
Alkanes react with halogens in the presence of UV radiation (Sunlight)
Example- Methane + Bromine- CH4(g) + Br2(l) -> CH3Br(g) + HBr(g)
Known as a substitution reaction as a hydrogen atom has been substituted by a halogen atom
Bromination of Alkanes
- Radical substitution
Step 1
Initiation
1) Covalent bond of bromine molecule is broken by homolytic fission
2) Two highly reactive bromine radicals formed
Equations
- Br-Br -> 2Br :black_small_square:
Step 2
Propagation
1) Reaction propagate through two propagation steps-
Chain reaction
Equations
-1) CH4 + Br :black_small_square:-> CH3 :black_small_square: + HBr
2) CH3 :black_small_square:+ Br2 -> CH3Br + Br :black_small_square:
1st step- Bromine radical reacts with C-H bond in methane to form a methyl radical and HBr
2nd step- Methyl radical reacts with bromine molecule forming bromomethane and Br radical
Step 3
Termination
Two radicals collide forming a molecule with all electrons paired
Possible reaction
Equations
- 1) 2Br :black_small_square:-> Br2
2) 2CH3 :black_small_square:-> C2H6
3) CH3 :black_small_square:+Br :black_small_square:-> CH3Br
RADICALS
Molecule that contains at least one unpaired electron
Limitations
Further substitution- Another bromine radical could collide with a bromomethane molecule forming dibromomethane, could continue leading to a mixture of CH3Br, CH2Br2, CHBr3 and CBr4
Substitution at different points of carbon chain- When the carbon chain is longer, we could end up with a mixture of monosubstituted isomers by substitution at different points on the carbon chain
