Chapter 12 Flashcards
How reactive are alkanes? Why?
Alkanes are very unreactive due to the strong and non-polar C-C and C-H bonds
Why are alkanes a good fuel?
The combustion of alkanes is very exothermic
What is produced in incomplete combustion of alkanes?
CO(g) or C(s)
Give 3 reasons why incomplete combustion is bad?
- Less energy per mole is released
- Carbon monoxide is toxic
- Carbon particles are respiratory irritants/ carcinogenic and cause global dimming
When do Alkanes react with halogens?
Under UV light
What occurs during a reaction between a alkane and a halogen?
The hydrogen atoms on the alkane is replaced with Halogen atoms
What type of reaction is the reaction between an alkane and a halogen?
A substitution reaction
What is formed during a reaction between an alkane and a halogen?
A haloalkane and a hydrogen halide
What does the reaction mechanism involve for the halogenation of alkane?
Radical substitution
What are the tree stages of the mechanism for the halogenation of alkanes?
The mechanism takes place in three stages: initiation, propagation and termination
What occurs during the mechanism Initiation step?
A molecules absorbs UV light, homolytic fission occurs and radicals form
Is Initiation endo or exothermic?
Endothermic as only bonds are breaking
What occurs during the mechanism Propagation step?
Radicals react with molecules forming new radicals and molecules
What type of reaction is the Propagation phase?
A chain reaction
What occurs during the mechanism Termination step?
Radicals combine, forming a stable molecule
Is the termination step exo or endothermic?
Exothermic as only bond making occurs
How do you create the final equation after a halogenation of alkanes reaction?
Cancel the same molecules on either side of the equation
What are the 3 limitations of radical substitution in organic synthesis?
- Further substitution
- Substitution at different positions in a carbon chain
- Reactions produce a mixture of products
What are alkanes?
Saturated Hydrocarbons
What is the general formula for alkanes?
CnH2n+2
When do alkanes show chain isomerism?
In a chain
When do alkane chains show isomerism?
When its a chain with 4 or more Carbon atoms
What bonds are present between the Carbon atoms in an alkane?
A single SIGMA bond. (Covalent bond)
What is a SIGMA bond?
A sigma bond is a head on overlap of the p-orbitals, one from each carbon atom.
What is the shape of an alkane and why does it adopt this shape?
A tetrahedral shape with a bond angle of 109.5°because around each central C-atom are 4 regions of electron density, which repel each other as far apart as possible in order to minimise repulsion.
What are cycloalkanes?
Alicyclic hydrocarbons
What is the general formula for a cycloalkane?
CnH2n
What is the bond angle, reactivity and stability of Cyclopropane?
60° bond angle, very reactive, strained ring
What is the bond angle, reactivity and stability of Cyclobutane?
90°bond angle, very reactive, strained ring
What is the bond angle and stability of Cyclopentane?
108° bond angle, less strained
What is the bond angle and stability of Cyclohexane?
109.5° bond angle, stable
How are alkanes found?
They are extracted from crude oil by fractional distilation
How does chain length of alkanes effect boiling point?
The longer the chain the higher the boiling point
What state are alkanes with 1-4 carbon atoms?
Colourless gases
What state are alkanes with 5-17 carbon atoms?
Colourless liquids
What state are alkanes with 18+ carbon atoms?
White waxy solids
Why does the boiling point of alkanes increase as chain length does?
More electrons means stronger induced dipole-dipole interactions (London Forces) so more energy is needed to overcome them resulting in a higher boiling point.
If branching on alkanes increases what happens to boiling point?
decreases
Why does branching of alkanes decrease boiling point?
Branched alkanes have less surface area in contact so weaker/ fewer intermolecular forces hence less energy is needed to overcome them so lower boiling point.
How reactive are alkanes? Why?
Alkanes are very unreactive due to the strong and non-polar C-C and C-H bonds
What are the 2 reactions alkanes can carry out?
- Combustion
2. Halogen substitution
What makes alkanes very good fuels?
Combustion of alkanes is very exothermic hence making them good fuels.
What state is an alkane with a C1-C4 chain length?
Gas
What state is an alkane with a C5-C17 chain length?
Liquid
What state is an alkane with a C18+ chain length?
Solid
What is produced in the incomplete combustion of an alkane in a limited air supply of oxygen?
CO(g) or C (s)
Why is the incomplete combustion of alkanes an issue?
- Less energy released per mole
- Carbon monoxide is poisonous
- Carbon particles are respiratory irritants/ carcinogenic and add to global warming.
What causes alkanes to react with halogens like Bromine and Chlorine?
The presence of UV light
What reaction occurs between a alkane and a halogen under UV light?
A substitution reaction where the halogen replaces the hydrogen atoms in the alkane molecule
What is the product of a substitution reaction between a halogen and a alkane?
A haloalkane and a hydrogen halide are formed
What is the reaction mechanism for halogen substitution of an alkane involve?
The reaction mechanism involves a radical substitution
What are the three stages of a halogen substitution reaction with an alkane?
- Initiation
- Propagation
- Termination
What is a definition for the Initiation stage of a halogen substitution reaction?
A molecule adsorbs UV light, homolytic fission occurs and radicals form.
What is a definition for the Propagation stage of a halogen substitution reaction?
Radicals react with a molecule forming new radicals and molecules - chain reaction
What is a definition for the Termination stage of a halogen substitution reaction?
Radicals combine forming a stable molecule
Is the Initiation stage of a halogen substitution reaction endo or exothermic?
ENDOTHERMIC
What type of reaction is the propagation stage of halogen substitution?
Chain reaction
Is the Termination stage of a halogen substitution reaction endo or exothermic?
EXOTHERMIC
What does further substitution lead to ?
Results in unpredicted compounds being made
What does substitution at different positions in a carbon chain lead to?
With longer chained alkanes, halogens can occur in different positions , these then require separation which is expensive.
