Module 4.1.2 - Alkanes Flashcards
Are alkanes polar or non-polar molecules?
Non-polar
What type of hydrocarbon are alkanes?
Saturated hydrocarbons
What type of bonds occur between C-H and C-C?
Sigma bonds
What is a sigma bond?
Formed between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms, which allows free rotation of the sigma bond
What is the bonding in alkanes? (3)
- saturated hydrocarbons
- C-C and C-H made up of sigma bonds
- by the direct single overlap of orbitals directly between bonding atoms
For what energy needed to boil a liquid alkane?
Energy is needed to break the induced dipole-dipole attraction forces between the molecules
What happens to the boiling point as the carbon chain length increases? (4 points)
Boiling point increases,
as more surface contact between molecules (and more electrons),
so there are more induced dipole-dipole interactions between the molecules,
which need more energy to overcome
What happens to the boiling point of a branched isomer?
A branched isomer has a lower boiling point than the unbranched isomer
In a more branched alkane, less surface contact between molecules meaning fewer induced dipole-dipole interactions,
therefore need less energy to break the weaker London forces between molecules
What is the trend in boiling point as the number of branches increase?
As the number of branches increases, the boiling point decreases
How do you name alkanes?
- Find longest carbon chain (parent chain), name the stem
- Name any alkyl groups attached to parent chain, put in before stem
- Number alkyl groups from the end to give the lowest numbers possible
- With multiple alkyl groups, list in alphabetical ordered in the name
- Cyclic alkanes have the prefix -cyclo
What shape and bond angle do alkanes have?
- Tetrahedral
- 109.5 degrees
- 4 bond pairs around the C atom repel each other equally and get as far apart as possible
Why are alkanes relatively unreactive?
C-C and C-H sigma bonds are non-polar and strong (and similar electronegativities)
What two chemical reactions do alkanes undergo?
- Combustion
- Halogenation
What is combustion of alkanes?
An exothermic reaction as heat energy is released to the surroundings (alkanes good as fuels)
What is complete combustion of alkanes?
In plentiful supply of oxygen, alkanes burn completely to produce CO2 and H2O
What is incomplete combustion of alkanes?
In limited supply of oxygen, alkanes burn incompletely to produce CO and H2O
(CO is toxic gas and prevents haemoglobin binding with oxygen)
What reaction mechanism does halogenation of alkanes undergo?
Free radical substitution
What are the reagents of halogenation of alkanes?
- alkane
- halogen (F2, Cl2, Br2, I2)
What are the conditions for the halogenation of alkanes?
UV light/300 degrees celsius (+alkane in excess)
What happens in the initiation stage of free radical substitution (in halogenation of alkanes)?
The halogen splits into two radicals by homolytic fission
What must you show in free radical substitution reactions?
The species which has the radical, represented by using a dot
What occurs in the propagation stage of free radical substitution (of halogenation of alkanes)?
1st step: Halogen (X) radical attacks the alkane molecule and the alkane forms a radical as hydrogen from alkane forms HX (one less hydrogen atom in alkane molecules)
2nd step: diatomic halogen and alkane radical form the haloalkane and the halogen radical regenerates
What happens in termination stage of free radical substitution (of halogenation of alkanes)?
The reaction ends when two radicals combine forming a new molecule (add the radicals together)
What are the disadvantages of free radical substitution? (3)
- Low % yield of desired haloalkane
- Separation from the product mixture, by fractional distillation, is costly
- Excess of alkane needs to be bought in bulk
What are the limitations of free radical substitution? (3)
- Leads to formation of a mixture of products
- Further substitution means mixture of haloalkanes is produced (until all halogen atoms replaced)
- Reactions at different positions in carbon chain (different structural isomers - name examples in an exam of different isomers that can form)
