B3 - Alkanes And Alkane Reactions Flashcards
(18 cards)
Give 3 reasons why alkanes are fairly un-reactive:
- C—C and C—H sigma bonds are strong
- C—C bonds are non-polar
- The electro negativity of carbon and hydrogen is so similar that the C—H bond can be considered to be non-polar
What do alkanes produce during complete combustion?
Carbon dioxide and water
What do alkanes produce during incomplete combustion?
Carbon monoxide or carbon and water
What is the mechanism for converting an alkane to a haloalkane?
Radical substitution
What are the three stages of radical substitution?
Initiation, propagation and termination.
Using methane reacting with bromine as an example, what happens in the initiation stage?
- The covalent bond in a bromine molecule is broken by homolytic fission
- Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals
- Energy used for this bond fission is provided by UV radiation
What is the effect of chain length on boiling point (as chain length increases)?
As the chain length increases:
- The molecules have larger surface area
- So more surface contact is possible between molecules
- The London forces between the molecules will be greater
- So more energy is required to overcome these forces
So increasing chain length increases boiling point.
What is the effect of branching on the boiling point?
As branching increases:
- There are fewer surface points of contact between molecules of the branched alkanes
- Giving fewer London forces
- Branches also get in the way and prevent branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further
- Less energy required to overcome these London forces
Branched isomers have lower boiling points.
What bonding is in alkanes?
Single covalent bonds called sigma bonds.
What is a sigma bond like in alkanes?
The result of the overlap of two orbitals, one from each bonding atom. Each overlapping orbital contains one electron, so the sigma bond has two electrons that are shared between the bonding atoms.
How many sigma bond does each carbon atom in an alkane have?
4.
What is the bond angle around each carbon atom in an alkane?
109.5
What is the arrangement around each carbon atom in an alkane?
A 3D tetrahedral arrangement.
Using methane reacting with bromine as an example, what happens in the propagation stage?
The reaction propagates through two propagation steps, a chain reaction:
- In the first propagation steps, a bromine radical, Br▪️, reacts with a C—H bond in the methane, forming a methyl radical, ▪️CH3, and a molecule of hydrogen bromide, HBr
- In the second propagation steps, each methyl radical reacts with another bromine molecule, forming the organic product bromomethane, CH3Br, together with a new bromine radical (Br ▪️)
- The new bromine radical reacts with another CH4 molecule as in the first propagation steps, and the two steps can continue to cycle through in a chain reaction
How long do the propagation steps usually continue for?
Until all the reactants have been used up.
Using methane reacting with bromine as an example, what happens in the termination stage?
Two radicals collide, forming a molecule with all electrons pairs.
In the reaction in the questions, possible termination steps include:
- Br ▪️+ Br ▪️-> Br2
- ▪️CH3 + ▪️CH3 -> C2H6
- ▪️CH3 + ▪️Br -> CH3Br
When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction.
What organic compound does radical substitution of alkanes give us?
Haloalkanes
What are two limitations of radical substitution in organic synthesis?
- Further substitution - substitution can continue until all hydrogen atoms have been substituted if a halogen radical collides with a haloalkane molecule repeatedly
- Substitution at different positions in a carbon chain - in a longer carbon chain, you can get a mixture of monosubstituted isomers by substitution at different positions in the carbon chain
