4.2 - Hydrocarbons Flashcards
What is an alkane ?
A saturated hydrocarbon with the general formula CnH2n+2
What is the general formula of alkanes ?
CnH2n+2
What are the physical properties of alkanes ?
- Tetrahedral shape around each carbon atom
- Bond angle = 109.5o
- Non-polar molecules and therefore only London forces between molecules
- Boiling point increases when more electrons are present and there is less branching
Explain why the boiling point of alkanes increases due to an increased number of electrons ?
- If there are more electrons present this means that London forces are stronger and therefore take more energy to overcome
- This causes the boiling point to increase
Explain why the boiling point of alkanes increases due to less branching ?
- If there is less branching, then there are more points of surface contact between molecules
- This means there are more London forces which take more energy to overcome
- This results in a higher boiling point
What are the chemical properties of alkanes increases due?
- Alkanes are saturated hydrocarbons with the general formula CnH2n+2
- They contain sigma bonds
Define the term ‘sigma bond’ ( σ bond ) ?
Sigma bond - The shared pair of electrons between the nuclei of two bonded atoms
Describe how a sigma bond forms ?
The overlap of orbitals directly between the nuclei of two atoms
What are the two types of combustion reactions for alkanes ?
- Complete combustion
- Incomplete combustion
What are the conditions and products for the complete combustion of an alkane ?
- Complete combustion requires plenty/ excess oxygen
- It produces COv2 and Hv20
What are the conditions and products of the incomplete combustion of alkanes ?
- Incomplete combustion occurs in limited supply of oxygen
- It products C or CO and Hv20
Do you know how to balance combustion equation of alkanes ?
Yes
What are the reagents for forming haloalkanes ?
- Alkane
- Halogen ( Fluorine, Chlorine, Bromine, etc.. )
What are the conditions for the formation of haloalkanes ?
UV light ( provides energy to break the bond in the halogen )
What type of reaction is the formation of haloalkanes ?
A substitution reaction
Define the term ‘substitution reaction’ ?
Substitution reaction - A reaction where an atom or group of atoms is replaced by another atom or group of atoms
Define the term ‘addition reaction’ ?
Addition reaction - A reaction in which a reactant is
added to an unsaturated molecule to make a one
saturated molecule
Define the term ‘elimination reaction’ ?
Elimination reaction - The removal of a molecule
from a saturated molecule to make an unsaturated molecule
What are the mechanisms of the formation of haloalkanes ?
1 ) Initiation
2 ) Propagation
3 ) Termination
What happens during ‘Initiation’ mechanism when forming haloalkanes ?
- Involves the homolytic fission of the covalent bond in the halogen
- This forms two highly reactive radicals
- The energy to break the bonds is provided by UV light
Explain how the formation of two chlorine radicals from a chlorine molecule during initiation is an example of homolytic fission ?
One electron from the bond pair goes to each chlorine atom to form two chlorine radicals
What happens during ‘Propagation’ mechanism when forming haloalkanes ?
- A chain reaction is propagated via two propagations steps
- The halogen radical first reacts with a C-H bond in the alkane forming an alkyl radical and a molecule of hydrogen halide
- The alkyl radical then reacts with another halogen molecule to form a haloalkane, while reforming a halogen radical
Explain how a chain reaction is is propagated by the ‘propagation’ mechanism ?
The new halogen radical produce is then able to attack another alkane molecule, so the china reaction can continue
What happens during ‘termination’ mechanism when forming haloalkanes ?
- Two radicals collide ( by chance ) forming a product that has no unpaired electrons
- By removing radicals from the reaction mixture in this way, the reaction is stopped
What are the limitations of forming haloalkanes ?
- You can get further substitution and the desired product made could react again
- Substitution can occur at different points on the carbon chain producing a mixture of chain isomers in your product
Why may radical substitution not be the best way to form a product ?
- This is because desired products may undergo further substitution
- Substitution may occur at different points in the carbon chain resulting in a mixture of position isomers that are costly/ time consuming to seperate
Define the term ‘homolytic fission’ ?
Homolytic fission - Each bonding atom receiving one electron from the bonded pair
Define the term ‘heterolytic fission’ ?
Heterolytic fission - One bonding atom receiving both electrons from bonded pair
Define the term ‘radical’ ?
Radical - A species with an unpaired electron
Write the initiation mechanism for the following reaction : CHv4 + CLv2 —> CHv3Cl + HCl ?
Write the propagation mechanism for the following reaction : CHv4 + CLv2 —> CHv3Cl + HCl ?
Write the propagation mechanism for the following reaction : CHv4 + CLv2 —> CHv3Cl + HCl ?
What are ‘curly arrows’ ?
Arrows that represent the movement of an electron pair
Draw curly arrows to represent heterolytic fission ?
Draw curly arrows to represent homolytic fission ?
Do you know how to draw ‘curly arrows’ to describe reaction mechanisms with relevant dipoles ?
yes
